Symposium 344 - Abstracts


The Interstellar Medium and Star Formation in Dwarf Galaxies

Bolatto, Alberto

I will review our understanding of the properties of the interstellar medium (ISM) in dwarf galaxies in connection to their star formation activity. What are the dominant phases of the ISM in these objects? How do the properties of these phases depend on the galaxy properties? What do we know about their cold gas content and its link to star formation activity? Does star formation proceed differently in these galaxies? How does star formation feedback operate in dwarf galaxies? The availability of observations from space-based facilities such as FUSE, Spitzer, Herschel, and Fermi, as well as observatories such as SOFIA and ALMA, is allowing us to make significant strides in our understanding of these questions.

Dwarf Galaxies at Low and High Redshift

Kong, Xu

The overwhelming majority of galaxies in the Universe are dwarf galaxies. But although they are important components in understanding galaxy evolution, these systems are typically too faint to be observed at high redshifts. However, we are able to obtain an unobscured view of early star formation and chemical enrichment in these galaxies at low redshift. In this talk, I will review some progress about high redshift dwarf galaxies, and then I will introduce some work based on local analog of high redshift dwarf galaxies. At the end, I will talk about the spectroscopic survey for compact galaxies using the LAMOST.

Cosmic Web Research: Where are the Missing Dwarf Galaxies and Inter-Galactic HI Clouds

Carignan, Claude

We know very well that the observed HI content cannot explain the Star Formation Rate (SFR) observed in galaxies. The only way galaxies can sustain that SFR is by accreting HI-rich dwarf galaxies or Inter-Galactic HI clouds. However, no observation to detect those accretion events has been very conclusive so far. Instruments having the necessary sensitivity (e.g. GBT) lack the necessary spatial resolution and those with the proper resolution (e.g. VLA) lack the sensitivity. I will show, with an example, that both are necessary to detect those illusive HI clouds. The SKA precursor MeerKAT is starting its operation as we speak and will start the Large Survey Programs at the end of 2018. FAST has started its observations in “drift scan” mode with CRAFTS (Commensal Radio Astronomy Fast Survey) and should be able to start pointed observations in 2019. In the near future (2019-20), the best combination to study low column density HI will be to combine the sensitivity of FAST with the spatial resolution of MeerKAT. The combination of the data from those two telescopes will allow, 4-5 years before SKA1-MID, to do "cosmic web" research to levels < 5 X 1017 cm−2, close to 1016 cm−2, densities that would normally only be accessible to the full SKA around 2030. It is at those densities that we expect the galaxies to connect with the surrounding cosmic web that could feed the observed SFR in galaxies.

Local Group Dwarf Galaxies

Cole, Andrew

Local Group dwarf galaxies are a unique astrophysical laboratory because they are the only objects in which we can reliably and precisely characterize the star formation histories of low-mass galaxies going back to the epoch of reionization. There are of order 100 known galaxies less massive than the Small Magellanic Cloud within ~1 Megaparsec of the Milky Way, with a vide variety of star formation history, gas content, and mass to light ratios. In this overview the current understanding of the formation and evolution of low-mass galaxies across cosmic time will be presented, and the possibility of drawing links between the properties of individual systems and the broader Local Group and cosmological context will be discussed. Local Group dwarfs will remain a uniquely powerful testbed to constrain the properties of dark matter and to evaluate the performance of simulations for the foreseeable future.

The Future of Dwarf Galaxy Research: What Telescopes will Discover

Haynes, Martha

Dwarf galaxies make ideal laboratories to test galaxy evolution paradigms and comological models. Within the dwarf rubric, we find star-forming, star-bursting and fully quenched galaxies as well as objects only barely able to hold on to their gas and/or form stars. Detailed studies of dwarfs across the spectrum allow us to gauge the efficacy of astrophysical processes at play in the lowest mass halos such as gas accretion, feedback, turbulence and chemical enrichment. Dwarf galaxies also make excellent targets for direct detection dark matter searches. Future studies will deliver unprecedented insights on the orbits of dwarf companions around the Milky Way, on their star formation histories and on the 3-D internal motions of their stars. Over large volumes, we will assess the impact of local environment on baryon cycling and star formation laws, leading to a full picture of the evolution of dwarfs across cosmic time. In combination, future discoveries promise to trace the history of assembly within the Local Group and beyond, probe how stars form under pristine conditions, and test models of structure formation on small scales.

Massive stars as drivers for the chemical enrichment of dwarf galaxies

Ekström, Sylvia

After reviewing the main physical process governing the evolution of massive stars, we will consider the way the low-metallicity environment found in dwarf galaxies modifies these processes. The impact on their nucleosynthesis and thus on the expected chemical enrichment driven by massive stars is reviewed.

Overview of the ISM properties of Dwarf galaxies: consequences of the low metallicity and star formation activity

Madden, Suzanne

Recent space missions together with the wealth of optical, radio and millimetre observations of dwarf galaxies are allowing us to construct a picture of their InterStellar Medium (ISM).  Their low mass, often prominent star formation activity, and metal-poor ISM have a striking impact on the physical processes that take place to shape the structure of their ISM. The properties of the gas and dust associated with molecular clouds, photodissociation regions and ionised phases of dwarf galaxies bring a very different picture of the ISM, compared to their more metal rich counterparts. In spite of the fact that molecular gas is infamously difficult to detect in dwarf galaxies, star formation persists.I will review what we know to date of the properties and structure of the molecular clouds, photodissociation regions, ionised gas phases and star formation activities that are gleaned from various surveys at various size scales in dwarf galaxies, and how they compare/contrast to more massive, dusty galaxies, thus setting the stage for our session on ISM and star formation in dwarf galaxies.                        

What do planetary nebula and H II regions reveal about the chemical evolution of nearby dwarf galaxies?

Gonçalves, Denise R.

The local Universe provides an opportunity to study the nearby examples of a wide variety of galaxy types. The nearby galaxies are of paramount importance because of the great detail with which we can resolve their stellar populations. In addition to the early- and late-type spirals, the Local Group contains a great number of dwarf irregulars, ellipticals and spheroidals, for which the spectroscopy of individual stars can be obtained. Thus, the chemical evolution of these galaxies can be traced, with the only need of finding populations spanning a large age range and for which we can accurately derive the composition. Planetary nebulae (PNe) are low- and intermediate-stellar mass remnants (so, of old- and intermediate-ages) for which chemical abundances can be obtained up to 3-4 Mpc. Coupled with H II regions, frequently brighter and much easily detected, which represent young stellar populations, they are among the best tracers of the chemical evolution of nearby galaxies, allowing to draw a chemical time line. Although representing different populations, PNe and H II regions share similar spectroscopic features and therefore can be observed and analysed in the same way. In this review I will discuss the main results for the nearby dwarf galaxies chemistry, as given by these two populations, in terms of constraints to chemical evolution models and the mass-metallicity relation of the local Universe.

How Do Galaxies Populate Low Mass Halos?

Brooks, Alyson

In just a few short years the Large Synoptic Survey Telescope will begin survey operations, leading to the discovery of hundreds of Local Group dwarf galaxies, including hundreds of dwarf galaxies in the untra-faint regime.  These tiny galaxies provide a test of both our understanding of dark matter and of star formation.  But are we ready for these discoveries?  Will be able to easily interpret the information that they hold?  In this talk, I will highlight uncertainties in our theoretical modeling that lead to degenerate predictions that will complicate our interpretation of even pure dwarf galaxy number counts.  I will also discuss potential paths toward breaking degeneracies, and identifying the clues that dwarfs will provide about the physical conditions necessary for the earliest star formation.

The role of dwarf galaxies from the peak epoch of cosmic star formation to the epoch of reionization

Atek, Hakim

Thanks to a major progress in observing facilities and numerical simulations, there is now a growing interest in the population of dwarf galaxies at high redshift, which play a central role in various studies across cosmic time, since they are the most representative objects of the galaxy population. Such importance is based on their contribution to the star formation history of the Universe, their different physical properties and time evolution compared to their massive counterparts, and their dominant contribution to the ionizing background at the epoch of reionization. In this talk, I will summarize our current views on the low-mass galaxies at redshifts z>1 based mainly on NIR spectroscopy from ground- and space-based observations and gravitational lensing that allowed to study galaxies down to stellar masses of 10^7 - 10^8 solar masses. At very high redshift, the very steep faint-end of the UV luminosity function observed at z>6 suggest that faint galaxies probably played a major role in the reionization process. I will discuss the great progress made during the past few years thanks to gravitational lensing, and the routes to move forward to detect a potential turnover of the UV LF, where cosmological simulations predict suppression of the star formation activity in the smallest dark matter halos.

The Cosmological Formation of Dwarf Galaxies

Navarro, Julio

I will review our understanding of the formation of dwarf galaxies in the current Lambda-Cold Dark Matter (LCDM) cosmological paradigm. I will focus on the relation between the baryonic mass of a dwarf and that of its dark matter halo host as predicted by LCDM simulations; how it relates to a number of alleged problems for LCDM on small scales, such as the "missing satellites", "too-big-to-fail", and "cusp-core" problems; as well as their potential resolution. I will also touch upon the role of cosmic reionization in shaping the baryon content and star formation histories of faint dwarfs, as well as on the effect of Galactic tides on the structural parameters of satellite galaxies. I will also comment on how dwarfs contribute to the buildup of the outer stellar halos of galaxies like the Milky Way and, time permitting, on how dwarf galaxies may help to rule out some alternative theories of gravity. 

The merger debris of dwarf galaxies in the local stellar halo

Du, Cuihua

In the standard hierarchical model of galaxy formation, the halos of galaxies like the Milky Way, are thought to form hierarchically via the accumulation of stars from dwarf galaxies and this merging process left behind many stellar streams and substructures in the Galactic halo. A number of distinctive characteristics of halo stars provide information about dwarf galaxies. Although the accumulated debris from old accretion events rapidly disperses in space, a significant amount of recognizable substructure should be visible both spatially and in phase-space distribution. Based on the first Gaia data release and spectroscopy from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 5, we are able to obtain the required full phase-space information of stars. The halo stars are identified by kinematics and metallicity. We fit a three-component Gaussian Mixture Model to the Cartesian velocities of the stars and use different metallicity to pick out halo stars. Then we perform a comprehensive analysis of the halo sample stars in the space of energy and angular momentum (E Vs. L_z) to determine the significant structures. The watershed algorithm is used to estimate extent of possible substructures and determine their constituent stars. Finally, we find 10 statistically significant substructures and these substructures are not reported by previous works. The retrograde stars with less bound energy may be all correlative to the progenitor galaxy of ω Cen. These structures may be the debris of dwarf galaxies accretion event and their dynamical and chemical informations can help to understand the history of the Milky Way.

Dwarf galaxies during reionization

Wyithe, Stuart

In this talk I describe insights into the modelling and astrophysics of dwarf galaxies during reionization from the DRAGONS suite of hydrodynamic and semi-analytic simulations. Comparison of our N-body and hydrodynamic simulations shows that the hydrostatic suppression of baryonic accretion affects the growth rate of dwarf galaxy dark matter haloes at z>6, lowering the amplitude of the halo mass function by factors of ~2-4, and so dramatically reducing the number of halos available to host galaxies in the reionization era. Our hydrodynamical simulations further show that unlike the case of self-regulated low redshift galaxies, the bottleneck for growth of early dwarf galaxies is in converting infalling gas to cold star-forming gas, which uniformly occurs over a time-scale of 300 Myr. We have incorporated these properties into the MERAXES semi-analytic model, and used it to study the role of dwarf galaxies during reionization. The steep faint-end slope of Ly-break galaxies at z>6 is predicted to extend beyond absolute magnitudes of -12, implying that approximately 50% of the ionizing photons needed for reionization have been observed at z<7. I will show how the resulting reionisation structure is sensitive to the high-z dwarf galaxy population, and that its measurement with SKA-low will help discriminate between different galaxy formation scenarios.

Stars and gas in the most metal-poor galaxies: SBS 0335-052E

Wofford, Aida

Understanding the properties of stars and gas at low metallicity is of large relevance for a variety of  fields in astrophysics, since it relates to multiple topical questions which range from understanding the properties of galaxies that contributed to cosmic reionization to the evolution of metal-poor massive stars that give rise to the formation of heavy binary black holes. Crucial are observational constraints for the theoretical predictions. These can be obtained from rest-frame ultraviolet and optical spectra of the most metal-poor galaxies known. Among these, SBS 0335-052E (z=0.0135, 12+log(O/H)=7.3) is one of the most well-studied. For this galaxy, we present Hubble Space Telescope}(HST) / Cosmic Origins Spectrograph (COS) detections of the C IV 1549, 1551,  He II 1640, O III] 1661, 1666, and [C III] 1907 + C III] 1909 UV emission lines; and a co-spatial Very Large Telescope (VLT) / Multi Unit Spectroscopic Explorer (MUSE) spectrum covering from 4600 to 9400 Ang. Using these and archival datasets we investigate plausible sources of hard ionising photons and test: a) the latest Charlot & Bruzal spectral synthesis models with very massive (300 Msun) single non-rotating stars; b) the performance of the spectral analysis tool, BayEsian Analysis of GaLaxy sEds (BEAGLE); and c) the extent to which physical properties of the gas and dust derived independently from the UV and optical are constrained.

A whirling plane of satellite galaxies around Centaurus A challenges cold dark matter cosmology

Müller, Oliver

Dwarf galaxies are tracers of the fine-structure of the large-scale structure of the universe, but their predicted distribution from simulations is in serious conflict with observations in the Local Group, the best studied group of galaxies today. Recently, a plane of dwarf galaxy satellites was discovered around Centaurus A, providing a unique opportunity to test cosmological predictions beyond the Local Group. We surveyed the complete Centaurus Group with the Dark Energy Camera, doubling the census of known dwarf galaxies in the group. Furthermore, we found strong evidence that this satellite plane is corotating, posing a major challenge to the Lambda+CDM cosmology (Müller et al. 2018, Science, 359, 534). Comparison to high-resolution dark matter simulations show that this finding is highly significant, with less than 0.5% of Centaurus A analogues hosting such phase-space correlated structures in the Millennium-II and Illustris simulations.

Angular momentum of dwarf galaxies

Kurapati, Sushma

We use high-resolution HI observations and broad band photometry to measure the baryonic mass (M) and baryonic specific  angular momentum (j) for 11 dwarf galaxies that lie in the Lynx-Cancer void. We find that the locus of void dwarf galaxies in the j-M plane is the same as that of dwarf galaxies in average density environments.  However, all dwarf galaxies (i.e. regardless of environment) have significantly higher specific angular momentum than expected from the relation obtained for larger spiral galaxies. We find that this elevation in specific angular momentum occurs for dwarf galaxies with masses lower than 10$^{9.1}$  M$_{\odot}$. As the mass of the galaxy increases beyond 10$^{9.1}$  M$_{\odot}$, the baryonic specific angular momentum decreases and they tend to follow the relation obtained for the massive galaxies with zero bulge fraction. Interestingly, the mass threshold that we find, viz, 10$^{9.1} M$_{\odot}$ is very similar to the mass threshold below which galaxy discs start to become systematically thicker. We examine the possibility that both these effects, viz. the thickening of disks and the increase in specific angular momentum are results of feedback from star formation. Such feedback would preferentially remove the low angular momentum gas from the central parts of dwarfs (thus increasing the specific angular momentum of the system) and also inject mechanical energy into the system, leading to thicker discs. We find however, that the observed amount of observed star formation in our sample galaxies is insufficient to produce the observed increase in the specific angular momentum. It hence appears that other, as of yet not identified mechanisms, play a role in producing the observed enhancement in specific angular momentum. We speculate that cold accretion may be one possible mechanism.

The Dwarf Galaxy Host of a Repeating Fast Radio Burst

Chatterjee, Shami

Fast radio bursts (FRBs) are enigmatic dispersed flashes of radio waves of extragalctic origin. FRB 121102 is the first (and so far, only) source that is known to produce repeat bursts, and the first to be precisely localized to a host galaxy. The host of FRB 121102 is an irregular dwarf galaxy at a redshift z = 0.193, with low metallicity, prominent emission lines, and a star formation rate of about 0.4 solar masses per year - properties comparable to those of extreme emission line galaxies that are also known to host hydrogen-poor superluminous supernovae and long-duration gamma-ray bursts. The burst source is further localized to a bright knot of star formation within the host galaxy, and is coincident with a persistent, variable, unresolved radio source. The high and variable rotation measure of the bursts demonstrates that the burst source is embedded in an extreme magneto-ionic environment, suggestive of the Milky Way galactic center. We do not yet understand whether FRB 121102 is representative of the wider FRB population, or how the host galaxy properties influence its repeating nature and detectability.

Evolution of star-forming dwarf galaxies in different environments

Grossi, Marco

The ubiquity of star-forming dwarf galaxies (SFDG) in the local Universe allows us to trace their evolution in all type of environments, from voids to rich clusters. SFDGs in low-density regions are still assembling their mass, they often show peculiar gas morphology and kinematics, likely associated to external gas accretion or minor mergers, and they can experience strong bursts of star formation. The most metal-poor SFDGs are found in the field and they are unique laboratories to investigate the star formation process in the low-metallicity regime, at conditions similar to their high-redshift analogues. On the other hand, SFDGs in intermediate- and high-density environments provide a key to understand the processes that remove their interstellar medium (ISM) and suppress star formation, leading to the different types of gas-poor early-type dwarfs. We review the most recent results on the properties of SFDGs at low and high galaxy densities focusing in particular on the impact of a cluster environment on their ISM components (dust, molecular, atomic and ionised gas). We compare the population of SFDGs in the nearest rich clusters: Virgo, which is still in the process of assembly, and Fornax, which is more dynamically evolved, more compact and denser. We discuss how the different evolutionary stage of the two structures affects the properties of SFDGs.

Star formation histories from color-magnitude diagrams reaching the oldest main sequence turnoffs: from ultra-faint dwarfs to the Magellanic Clouds.

monelli, matteo

Color-Magnitude Diagrams reaching the oldest main sequence turnoffs are essential for a reliable and accurate determination of the star formation history (SFH) of a stellar system from its formation to the present time. Our team has used the HST and wide field ground based imagers (such as VIMOS@VLT orDECam) to obtain the necessary data to determine the SFH of Local Group galaxies, from ultra-faint dwarfs such as EriII and LeoT, Isolated dwarfs (LCID project) and M31 satellites, to the Magellanic Clouds (SMASH project). This has allowed us to show that even these ultra-faint dwarfs have complex SFHs (Gallart et al. 2018, Monelli et al. 2018, in preparation), provided insight on the origin of the dwarf galaxy types (Gallart et al. 2015), informed about the origin of the LMC stellar bar (Monteagudo et al. 2018) and the LMC outside-in formation scenario (Meschin et al. 2014). In this talk I will concentrate on the most recent and exciting results selected among the above, considering other contributions to the conference for balance (I could provide an updated abstract closer to the conference).     

Stellar feedback in dwarf irregular galaxies traced with radio continuum observations

Heesen, Volker

Low mass dwarf irregular galaxies are subject to outflows, in which cosmic rays have since long been suspected to play an important role; they can be traced via their electron component, the cosmic ray electrons (CREs), in the radio continuum as non-thermal synchrotron emission. Thus far, spectral ageing has hampered the study of CREs far away from star formation sites. This is possibly now about to change with the advent of sensitive low-frequency observations such as with the Low-Frequency Aray (LOFAR). They can be combined with new broadband, high-frequency observations such as with the refurbished Very Large Array (VLA), in order to study spatially resolved radio continuum spectra at matched angular resolution and sensitivity. Here, we present results from our pilot study of the nearby starburst dwarf irregular galaxy IC 10, using 140-MHz LOFAR and 1.6- and 6.2-GHz VLA observations in order to measure non-thermal radio spectral indices. We show that spectral ageing occurs at two distinct locations over a wide range of frequencies. First, in the non-thermal superbubble, close to a massive HII region with on-going star-formation. With a CRE spectral age of ~1 Myr, this is a high-frequency phenomenon; the bubble is probably the result of a few recent supernovae or one, hypothetical, hypernova. Second, at distances of >0.5 kpc away from the star-forming disc in the halo. This low-frequency radio halo is likely connected to a galactic wind, which has set in during the current starburst, as hinted by the good agreement between the advective time-scale of 30 Myr with the age of the oldest stellar cluster. We discuss the relationship of the non-thermal ingredients of the interstellar medium – magnetic fields and cosmic rays – with other components, such as the warm neutral and warm ionised medium, and place outflows in dwarf galaxies into the context of galactic winds that we also find in more massive Magellanic-type and late-type spiral galaxies.

The true nature of ultra-faint stellar systems in the Milky Way halo

Jerjen, Helmut

The true nature of ultra-faint stellar systems in the Milky Way haloUltra-faint dwarf galaxies are intriguing stellar systems. Unlike large, luminous galaxies, they are completely dominated by the ubiquitous, cosmologically important non-baryonic dark matter. Stars contribute less than 1 percent to the total mass. At the same time these galaxies host some of the most pristine, least chemically evolved stars. These unique properties make dwarf galaxies the most sought-after laboratories in a number of astrophysical research areas. The latest all-sky imaging surveys have delivered a few dozen ultra-faint dwarf galaxy candidates in the Milky Way halo. As these systems are often found at the detection limits it is fundamentally important to establish their status as ultra-faint dwarf galaxies, stellar clusters, stream features or even false-positives. Incorrectly classified objects can seriously affect conclusions drawn by studies in near-field cosmology, galaxy formation, and stellar evolution, thereby hampering progress in these areas. I will present recent results from our ongoing deep follow-up observations as part of the Stromlo Milky Way Satellite Survey that has the goal to scrutinize candidates and physically/chemically characterize ultra-faint stellar systems. One immediate application is testing the validity of the size-luminosity and metallicity-luminosity relations in the ultra-faint regime. These crucial informations determine what sort of structures we are probing in the Milky Way halo.

Molecular gas at extremely low metallicity

Shi, Yong

Extremely metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe. In spite of the ongoing formation of stars in these galaxies, the presence of molecular gas (which is known to provide the material reservoir for star formation in galaxies such as our Milky Way) and its physical properties remains unclear. With IRAM 30m, we have achieved CO detection  in a galaxy at 7% Solar metallicity. Combined with archived infrared data, it is shown that the molecular gas mass per CO luminosity at extremely low metallicity is much larger than the Milky Way value. Our recent ALMA observation further reveals the spatial and kinematic properties of individual CO clumps in this galaxy. These observations help to advance the understanding of molecular ISM and star formation at very low metallicity.

Star formation at low rates: how a lack of massive stars may impact stellar feedback and the evolution of dwarf galaxies

Steyrleithner, Patrick

In recent years dedicated observations have uncovered star formation at extremely low rates in dwarf galaxies, tidal tails, ram-pressure stripped gas clouds, and the outskirts of galactic disks. At the same time, numerical simulations of galaxy evolution have advanced to higher spatial and mass resolutions, but have yet to account for the underfilling of the uppermost mass bins of stellar initial mass function (IMF) at low star-formation rates.  In such situations, simulations may simply scale down the IMF, without realizing that this unrealistically results in fractions of massive stars, along with fractions of massive star feedback energy (e.g., radiation and SNII explosions). Not properly accounting for such parameters has consequences for the self-regulation of star formation, the energetics of galaxies, as well as for the evolution of chemical abundances.Here we present numerical simulations of dwarf galaxies with low star-formation rates allowing for two extreme cases of the IMF: a "filled" case with fractional massive stars vs. a truncated IMF, at which the IMF is built bottom-up until the gas reservoir allows the formation of a last single star at an uppermost mass. The aim of the study is to demonstrate the different effects on galaxy evolution with respect to self-regulation, feedback, and chemistry. The case of a stochastic sampled IMF is situated somewhere in between these extremes.

Simulations at the Dwarf Scale: From Violent Dwarfs at Cosmic Dawn and Cosmic Noon to Quiet Discs today.

Ceverino, Daniel

Dwarf galaxies with stellar masses around 10^9 Msun can be explored at high and low redshifts and they give a glimpse of the different conditions of galaxies formation at different epochs. Using a large sample of about 300 zoom-in cosmological hydrodynamical simulations of galaxy formation I will briefly describe the formation of dwarfs at this mass scale at 3 different epochs: cosmic dawn (Ceverino, Klessen & Glover 2018), cosmic noon (Ceverino et al. 2015b), and today (Ceverino et al. 2017a). I will describe the FirstLight simulations of first galaxies at redshifts 5-15. These first dwarfs have extremely high star formation efficiencies (SSFR=SFR/Ms = 10-60 Gyr^-1) due to high gas fractions and high gas accretion rates. These simulations will make predictions that will be tested for the first time with the James Webb Space Telescope (JWST).  In particular, we look at gas kinematics through rest-frame visible nebular lines at redshifts higher than 6. At cosmic noon (z=2), galaxy formation is still a very violent and dynamic process and the gas kinematics at the dwarf scale is dominated by turbulence. The VELA simulations have generated a set of dispersion-dominated dwarfs  that show an elongated morphology due to their prolate dark-matter halos. Between z=1 to z=0, The AGORA simulation shows the formation of a low-mass disc due to slow gas accretion. The disc is initially dynamically hot (high sigma/v) and stable against gravitational instabilities. During several Gyrs, the disc slowly settles down into a low-mass galaxy that agrees with many local scaling relations, such as the stellar-mass-halo-mass and the baryonic Tully-Fisher relation.

The dwarf galaxy population as revealed by ALFALFA

Adams, Elizabeth

The ALFALFA HI survey is a blind neutral hydrogen survey of nearly 7000 square degrees of extragalactic sky with ~31,000 extragalactic detections. The combination of sensitivity and large sky coverage of ALFALFA has enabled the detection of difficult to observe low mass galaxies in large numbers, including dwarf galaxies overlooked in optical surveys. Three different, but connected, studies of dwarf galaxies from the ALFALFA survey are of particular interest: SHIELD (Survey of HI in Extremely Low-mass Dwarfs), candidate gas-rich ultra-faint dwarf galaxies, and field gas-rich ultra-diffuse galaxies.SHIELD is a systematic multiwavelength study of all dwarf galaxies from ALFALFA with M_HI < 10^7.2 Msun. The star formation, gas, and dark matter properties of the first 12 galaxies have been studied in detail, and an investigation of the full sample of 82 galaxies is underway. The low metallicity star-forming galaxy, Leoncino, was identified through this study. Candidate gas-rich ultra-faint dwarf galaxies extend the dwarf galaxy population to even lower masses. These galaxies are identified as isolated HI clouds with no discernible optical counterpart but subsequent high resolution HI imaging with WSRT and deep optical imaging with WIYN/(p)ODI reveal that many are extremely faint, gas-dominated galaxies. Leo P, discovered first as an HI detection, and then found to be an actively star-forming galaxy, bridges the gap between these candidate galaxies and the SHIELD sample.A population of field gas-rich ultra-diffuse galaxies was identified in the ALFALFA survey; their optical counterparts in SDSS are too faint to be trusted without the additional information of the HI detection. The HI kinematics indicate these are hosted by dwarf galaxy dark matter halos, possibly with a higher than average spin. Coma P, with a peak surface brightness of ~26.4 mag arcsec^-2 in g', demonstrates the sort of extreme low surface brightness galaxy that can be discovered in an HI survey.

Multiple Feedback in Low-Metallicity Massive Star Formation

Tanaka, Kei

To glimpse the variation of the high-mass end of IMF throughout the cosmic history, we theoretically investigate the impact of feedback and its metallicity dependence in massive star formation from prestellar cores at all metallicity range of Z=0-1Zsun (Tanaka et al. 2017, ApJ, 835, 32; Tanaka et al., in prep.). We include the feedback by MHD disk winds, radiation pressure, and photoevaporation solving the evolution of protostars and accretion flows self-consistently. Interestingly, we find that the feedback does not set the upper-mass limit of stellar birth mass at any metallicity, which is consistent with recent observations in the Large Magellanic Cloud (Crowther et al. 2010, 2016; Schneider et al. 2018). At the solar metallicity, we show that the MHD disk wind is the dominant feedback to set the star formation efficiency (SFE) from the prestellar core similar to low-mass star formation. The radiation pressure, which had been believed to be crucial for a long time, has a minor contribution even in the formation of over-100Msun stars. On the other hand, at the low metallicity of Z<1e-2Zsun, the photoevaporation becomes significant because the absorption of ionizing photons is weaker. Due to this efficient photoevaporation, the SFE gets lower at lower metallicity environments. Considering this SFE change and prestellar-core mass-function at low metallicity (Omukai & Tsuribe 2005), we conclude that the IMF slope is steeper, i.e., massive stars are rarer especially at 1e-5 - 1e-3Zsun compared to the local environment. Our study raises a question on the common assumption of the universal IMF with 100Msun cutoff. Since the total feedback strength in the cluster/galaxy scale is sensitive to the number fraction of massive stars, the re-evaluations of IMF at various environments are necessary.

Star-forming dwarf field galaxies found in the MUSE-Wide survey

Urrutia, Tanya

The exquisite depth of the HST deep fields allow us, in principle, to find dwarf field galaxies to study their evolution through cosmic time. However, a severe bottleneck is the inefficiency of spectroscopic surveys to identify and classify these usually faint systems. Here we present the MUSE-Wide survey, a full 3d spectroscopic survey covering 100 arcmin2 fields over the CANDELS-DEEP/GOODS-S and CANDELS-COSMOS regions. Through their emission lines we were able to find very low stellar mass, star-forming galaxies both at high and low redshift. Approximately 60 systems at z < 1 with log(M*) < 7.5 log(Msun) and another 60 at z > 2.8 with log(M*) < 8.0 log(Msun) found through their Lyman Alpha emission. For the systems at low redshift we will present the Mass-Star-Formation and Mass-Metallicity relations and their deviations from cluster, low star formation dwarf galaxies. In particular, we will present extreme O32-emitters, thought to be responsible for strong Lyman continuum leakage. At high redshift we will present the Lyman-Alpha luminosity function and cases of extreme equivalent width emission, both of which are important for pinning down the numbers and strength of dwarf galaxies contributing to the reionization of the Universe.

Uncovering dwarf elliptical evolution through spatially resolved spectroscopy

Penny, Samantha

Despite being the dominant galaxy population by number in the nearby Universe, the formation timescale and mechanism of dwarf galaxies, including dwarf ellipticals, remains unknown. Are quenched dwarfs the remnants of hierarchical galaxy assembly, or are they formed at later times via the morphological transformation of disk galaxies? Spatially resolved spectroscopy allows us to study the origin of dEs through their stellar kinematics and star formation histories. Using data from the SDSS-IV MaNGA IFU survey, we identify quenched dwarf galaxies in the nearby Universe fainter than Mr = -19, selected independently of morphology and environment. I will show the majority of quenched dwarfs exhibit coherent rotation in their stellar kinematics, and several host disc or spiral features. Others exhibit kinematically distinct cores which must form via gas infall and accretion, or outflows. I will discuss an origin for these dwarfs as quenched low-mass disc/spiral galaxies, supporting the hypothesis that galaxy-galaxy or galaxy-group interactions quench star formation in low mass galaxies. I will also show that a number of bright dEs with signatures of gas accretion host AGN, which are likely maintaining their quiescence. 

Supernova-driven winds in simulated dwarf galaxies

Hu, Chia-Yu

We investigate galactic outflows/winds driven by supernova (SN) explosions in an isolated dwarf galaxy using high-resolution (1M_sol) hydrodynamical simulations that include cooling, star formation, stellar feedback and metal enrichment. We find that the system reaches a quasi-steady state in a Gyr timescale with strong temporal fluctuations.Clustered SNe lead to the formation of superbubbles which break the disk and vent out the hot gas, launching the winds. At the virial radius, the time-averaged loading factors of mass, momentum and energy are 3, 1 and 0.05, respectively. We identify two different modes for the winds to escape the halo. Hot gas acquires velocities higher than the escape velocity v_esc within the disk and escapes rapidly. In contrast, warm gas generally has velocities smaller than v_esc within the disk, but is afterwards accelerated multiple times by subsequent episodes of bursts of hot winds and slowly escape the halo. The strong interactions between different temperature phases highlight the caveat of extrapolating wind properties to large distances based on its local properties (e.g. the Bernoulli parameter). We conduct convergence study and find that wind properties converge at 5M_sol, and the winds diminish dramatically once the cooling masses of individual SNe become unresolved. We demonstrate that injecting the terminal momentum of SNe,a sub-grid recipe widely adopted in the literature, does not lead to more efficient winds compared to pure thermal energy injection. The failure of this recipe is owing to its incorrect assumption that most thermal energy has already been radiated away right after the injection even for very clustered SNe.

Molecular composition of local dwarf galaxies: Astrochemistry in low-metallicity environments

Nishimura, Yuri

Dwarf galaxies generally have low metallicity. Especially, nitrogen is less abundant by a factor of >10 compared with solar abundance. Metallicity is an important parameter which could have significant impacts on atomic and molecular composition of molecular clouds via change in the gas-phase and grain surface chemistry. To investigate molecular composition of low-metallicity environments, we conducted spectral line survey observations in the 3 mm band (85-116 GHz) toward three dwarf galaxies, the Large Magellanic Cloud (LMC), IC 10, and NGC 6822 with Mopra 22 m, Nobeyama 45 m and IRAM 30 m, respectively. As a result, the lines of CCH, HCN, HCO+, HNC, CS, SO, 13CO, and 12CO were detected in all three galaxies (except for HNC and SO in NGC 6822 due to the insufficient sensitivity). On the other hand, N2H+, CH3OH, C18O and CN were not detected in any galaxies. We found that the spectral intensity patterns are similar to one another regardless of star formation activities. In other words, the spectra observed with a several-ten pc resolution do not reflect the effect of local star formation activities, but rather represent the molecular composition specific to low-metallicity environments. Compared with solar-metallicity environments, the molecular compositions of dwarf galaxies are characterized by as follows. (1) Nitrogen-bearing molecules are deficient as a direct consequence of the lower elemental abundance of nitrogen. The HCN/HCO+ and HNC/HCO+ ratios are lower in dwarf galaxies by factors of 2 to >10 than solar-metallicity galaxies such as our Galaxy and M51. (2) Formation of CCH is enhanced, while that of CH3OH (methanol) is suppressed. Both are interpreted as a consequence of extended photon dominated regions in cloud peripheries due to the lower abundance of dust grains. We will also present latest ALMA results of the LMC and NGC 6822 to show the spatially-resolved physical and chemical properties of the ISM in low-metallicity environments. 

Understanding the emergence of a plane of satellites around the Milky Way

Santos-Santos, Isabel

Most of the satellites of the Milky Way and Andromeda galaxies occupy a narrow, elongated region across the sky as if located on a plane. Recent studies have suggested similar alignments in other galactic systems beyond the Local Group, like CenA. In the case of the MW -for which reliable phase-space data is available-, the plane consists of the 11 brightest satellites, it is almost perpendicular to the MW’s disk, and ~6-8 out of 11 show coherent kinematics as if their orbits were contained within the plane.Such an extremely anisotropic distribution of satellites is rare to reproduce within DM-only cosmological simulations, despite the preferential accretion of substructures onto halos through the filaments of the cosmic web. Subsequent studies using high resolution hydrodynamical simulations of MW-type galaxies have shown that the inclusion of baryonic physics plays a crucial role, however the few studies on this issue have not been able to recover planes of satellites with as high significance and kinematical coherence as the one observed around the MW.We have explored this problem with a set of high-resolution zoom-in cosmological hydro-simulations of disc galaxies, run with different codes and subgrid-physics implementations. All of them present a clear non-random distribution of their satellites, which we follow as it evolves with cosmic time. By looking at the Aitoff projection of their orbital poles and through careful statistical analyses, we identify groups of satellites with coherent kinematics to which we fit planes of small minor-to-major axis ratio. We search for the physical processes driving the appearance and disruption of planes of satellites. A clue may reside in their merger/assembly history as well as in secular instability processes.

Gas-rich Dwarfs as Ultra-diffuse Galaxy Progenitors: Constraining Formation Models with Deep HI Observations

Spekkens, Kristine

We constrain formation models for ultra-diffuse galaxies (UDGs) in dense environments -- red systems with stellar masses akin to dwarf galaxies but effective radii comparable to that of the Milky Way -- using deep HI observations of candidate progenitors in the field.  Given their faintness in the optical, distance and dynamical mass measurements for UDGs remain scarce; nonetheless, the available data suggest that many are low surface brightness dwarfs. How are UDGs related to gas-rich field dwarfs, and what are their evolutionary histories? Numerical simulations offer two distinct possibilities: UDGs may stem from the high-spin tail of the dark matter halo distribution, or their large effective radii may arise from gas outflows during past star formation episodes. These scenarios imply different physical properties for UDG progenitors in the field, which we constrain by characterizing their HI reservoirs. We have detected HI in five blue UDGs around Hickson Compact Groups using the GBT, finding low-mass objects with high-spin halos and gas richnesses that correlate with their effective radii. In this contribution, we will confront predictions from UDG formation models with these observations to test the hypothesis that gas-rich dwarfs are UDG progenitors. We will also report on our on-going campaign to follow up dozens of field UDG candidates from DESI pre-imaging data in the Coma cluster outskirts in HI with the GBT, yielding the first measurements of their gas content as a function of environment, colour, surface brightness and dynamical mass.

The Future of Dwarf Galaxy Research: What simulations will predict

Sales, Laura

Dwarf galaxies are the most numerous type of galaxies in the Universe and largely fascinating objects in their own right. Believed to be the most dark matter dominated objects within the LCDM model, dwarf galaxies pose the most significant challenges to our cosmological scenario. The large variety of shapes, rotation curves and star formation histories observed in dwarfs seem at odds with the self-similarity expected from naive interpretations of the LCDM model. Feedback, star formation and reionization are believed responsible, to large extent, of some of these variations. Numerical simulations have started to resolve  the relevant scales to address the fundamental physics of the inter-stellar medium behind dwarf galaxy formation. As such, simulations can now provide realistic predictions for observables with the potential of probing or rejecting our cosmological model. Some examples include the number density of faint galaxies, the inner dark matter content of dwarfs and their relative inefficiency to form stars. Future simulations will stride to reproduce the properties of dwarf galaxies in different environments, ranging from the field, to Local Group like environments, to massive galaxy clusters; with an eye kept on evolution across redshift. This wide census of properties, once compared to observations, will shed new light on our understanding of galaxy formation as well as on the necessity --or not-- to modify the nature of the dark matter to reproduce current and upcoming dwarf galaxy observations.

On the tidal stripping of Local Group dwarf galaxies and the origin of their stellar mass-velocity dispersion relation

Fattahi, Azadeh

I use APOSTLE high-resolution hydrodynamical simulations of Local Groups combined with tidal stripping models to discuss the origin and shape of the stellar mass-velocity dispersion (Mstar-sigma) relation observed for Local Group dwarfs. I show that the tidal stripping of dwarfs embedded in cuspy dark matter halos, combined with the steep stellar mass-halo mass relation in Lambda-CDM can naturally explain the observed Mstar-sigma relation.  On the other hand, tidal stripping of dwarfs embedded in cored dark matter halos would face challenges to explain the shape of the Mstar-sigma relation.Based on our prediction, the existence of low surface brightness, low mass satellites of the Milky Way and Andromeda (an extreme example is the recently discovered Crater 2 Milky Way satellite) can only be reconciled with LCDM models if they are the remnants of once massive objects heavily affected by tidal stripping. I infer their progenitor masses, radii, and velocity dispersions, and find them in remarkable agreement with those of isolated dwarfs. I predict that the extreme cases (including Crater 2) should have lost more than 99 percent of their original stellar mass, and should have left some imprints in the stellar halo of the Milky Way as tidal features.  

Dwarf Galaxies and the Most Energetic Stellar Explosions

Fruchter, Andrew

Long-duration gamma-ray bursts (LGRBs) and hydrogen poor superluminous supernovae (SLSNe) are found preferentially in dwarf galaxies.   However, that preference is even more extreme than it at first seems.  We find that LGRBs and SLSNe form about thirty times more frequently per unit of star formation below a metallicity of 1/3 solar than above, with the relative rate increasing to about a factor of one hundred between 1/3 solar and solar metallicity.   At the same time GRBs and even more strongly SLSNe are found in hosts with very high emission line equivalent widths.  Indeed, the only sample of galaxies known to resemble the emission line properties of nearby SLSNe hosts is an artificial sample of galaxies formed by selecting galaxies with [OIII] equivalent widths above 100 A.  Interestingly, the only known fast radio burst host is a dwarf galaxy with a spectrum identical to that of a SLSN host.   These results, along with further observations that will be discussed in the talk, imply that an extreme subset of star-forming galaxies are particularly efficient at forming compact stellar remnants with high angular momentum, and indeed dominate the production of these objects in the universe. 

Star Formation and Molecular Gas in Extremely Metal-poor Galaxies: Insights from the Thermal Balance in the Neutral Gas

Lebouteiller, Vianney

The lack of detection of cold molecular gas in blue compact dwarf (BCD) galaxies is at variance with their intense star-formation episode. In particular, CO, often used a tracer of H2 through a conversion function, is selectively photodissociated in dust-poor environments.  On the other hand, a potentially large fraction of H2 is expected to reside in the so-called CO-free gas, where it could be traced by neutral gas infrared cooling lines [CI], [CII], or [OI]. Although the fraction of CO-free gas to total molecular gas is expected to be relatively large in metal-poor galaxies, a definite evidence is still lacking because of the difficulty in associating cooling lines with any given heating mechanism. The main issue at stake is to understand the role of molecular gas in the star formation process.I will first show that the heating mechanism in the neutral gas cannot be dominated by the photoelectric effect on dust grains below a threshold metallicity due to a low abundance of dust and polycyclic aromatic hydrocarbons. I will then present results from a study on the dwarf galaxy IZw18 (~2% solar metallicity) recently published in Lebouteiller et al. (2017). Optical and infrared lines are used to constrain the physical conditions in the HII region + HI region within a consistent photoionization and photodissociation model. We show that the HI region is entirely heated by a single ultraluminous X-ray source with important consequences on the applicability of [CII] to trace the star-formation rate and to trace the CO-free gas. We derive stringent upper limits on the size of H2 clumps that may be detected in the future with JWST and IRAM/NOEMA. We also show how the nature of X-ray sources can be constrained through their signatures in the interstellar medium and through the use of state-of-the-art models. I will conclude by proposing that star formation may be quenched in extremely metal-poor dwarf galaxies due to X-ray photoionization.

Mass Distribution in Stellar Structures of Local Dwarfs

Menéndez-Delmestre, Karín

Galaxies in the local universe are a fossil record of events in the distant universe and present critical constraints for examining models of formation and evolution of galaxies. Based on the Spitzer Survey of Stellar Structure in Galaxies (S4G) database, we study the stellar light that is typically associated with different stellar structures (such as bulge, disk, bar). S4G entails deep mid-IR imaging of nearby galaxies with IRAC at 3.6/4.5 um.  With a surface brightness limit at 3.6 um of 27 mag/arcsec^2, this survey is unique in its capability of probing stellar surface densities down to  << 1 M_Sun/pc^2, which is of particular interest when studying faint extended objects. The choice of mid-IR is particularly important because the emission of low mass stars, that dominate stellar mass in galaxies, dominates the flux in these bands. The S4G collaboration makes the reduced images publicly available, as well as the 2D decomposition for each galaxy. Based on the mid-IR morphological classification of Buta et al. (2015), we now focus our stellar mass distribution study on ~40 S4G dwarf galaxies. The 2D decomposition suggests that dwarfs are mostly well-fit by disk profiles with either an additional bulge component in ~1/3 of the systems (typically contributing ~25% of the total light), a bar, a nucleus and/or even second-disk component. Bars are present in ~20% of  the sample and, although their typical contribution to the 3.6 um light is ~9%, they show a variation in prominence. The 2D decomposition identifies compact nuclei  in ~1/4 of the sample with a wide range of contributions to the mid-IR light (1-20%), suggesting compact regions of dust likely heated by young stellar clusters. We are extending this analysis to the full S4G sample in an effort to draw comparisons with the wider galaxy sample and use the diversity of stellar structures within the local universe to establish a local reference for stellar structure formation studies.

A wide angle chemical survey of the Sagittarius Dwarf Spheroidal galaxy

Sbordone, Luca

The Sagittarius Dwarf Spheroidal galaxy (Sgr dSph) is the subject of the most evident tidal disruption event taking place in the Milky Way, and is the closest among dSph, greatly facilitating the detailed study of its stellar populations. For these reasons it has been considered an ideal testbench of both tidal merging events and dSph stellar populations. However, it quickly became evident that Sgr dSph was a rather exceptional object. Associated with a high number of globular clusters (the largest of which, M54, sits in its center), with an estimated pre-disruption total mass of various 10^9 solar masses, and hosting a chemically distictive, solar- to super-solar-metallicity population in its core, Sgr dSph most likely resembled an object like the Small Magellanic Cloud, before the tidal disruption by the Milky Way began. We have collected high resolution spectra (from UVES@VLT, FLAMES@VLT, and MIKE@Magellan) for more than 300 stars across the Sgr dSph main body, associated globular clusters, and the Sagittarius stream. Detailed abundance for ~20 elements are available for about 150 stars, while for the rest metallicity and [alpha/Fe] ratios are available. The detailed chemical picture of Sgr appears extremely interesting: the populations extend over 3 orders of magnitude in metallicity, with distinctive low [alpha/Fe] ratio in the metal rich component, high [Y/Ba] ratio extending to about [Fe/H]=-2, and a number of other peculiarities. The study also led to the discovery of a C-rich giant in the galaxy main body with extreme r- and s-porcess enancement, the first of its kind in Sgr dSph. IMPORTANT: Please consider this abstract submission for a talk, since I missed the talk abstract submission deadline due to an illness. If this was not possible, a poster will do as well. Thanks a lot in advance. LS

A Newly Discovered Reservoir of Dwarf Galaxies in the Centaurus A Group

Taylor, Matthew

Recent years have seen an acceleration in the discovery rate of dwarf galaxies in the Local Universe. The subsequent unveiling of coherent satellite phase-space structures like groups and planes has led to a renaissance in the study of low-surface brightness dwarf galaxies, including their utility in near-field cosmological studies. In an effort to push this field further, optical u'g'r'i'z' imaging of 22 deg^2 centered on the nearby giant elliptical galaxy NGC5128, as part of the "Survey of Centaurus A's Baryonic Structures" (SCABS) campaign, has been searched for new dwarf galaxies in the Centaurus A group. We will present first results of the stellar mass and stellar population properties for several dozens of promising new candidates, including several dwarf pairs appearing within a few pc in projection that may indicate physical associations. These new dwarf galaxies extend the size-luminosity relation toward fainter total luminosities and smaller sizes for known dwarf galaxies outside the Local Group, and are broadly consistent with the properties of nearby dwarf spheroidal galaxies. Altogether, these new results show NGC5128 to be the host of a large reservoir of low-mass dwarf galaxies that is at least as rich as that of the Local Group and is ripe for detailed follow-up observations.

XXX IAU General Assembly | ACV - Austria Center Vienna  | Bruno-Kreisky-Platz 1  | 1220 Vienna