Galaxy surveys have shown that luminous galaxies are mainly distributed in large filaments and galaxy clusters. The remaining large volumes are virtually devoid of luminous galaxies. This is in concordance with the formation of the large-scale structure in the universe as derived from cosmological simulations. However, the numerical results indicate that cosmological voids are abundantly populated with dark matter haloes which may in principle host dwarf galaxies. Observational efforts have in contrast revealed that voids are apparently devoid of dwarf galaxies. We investigate the formation of dwarf galaxies in voids by hydrodynamical cosmological simulations. Due to the cosmic ultraviolet background radiation low-mass haloes show generally a reduced baryon fraction. We determine the characteristic mass below which dwarf galaxies are baryon deficient. We show that the circular velocity below which the accretion of baryons is suppressed is approximately 40&#x2009;km<svg style="vertical-align:-0.12538pt;width:19.7875px;" id="M1" height="13.925" version="1.1" viewBox="0 0 19.7875 13.925" width="19.7875"  xmlns="http://www.w3.org/2000/svg">
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			<text transform="matrix(1,0,0,-1,-67.09,66.03)">
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</svg>. The suppressed baryon accretion is caused by the photo-heating due to the UV background. We set up a spherical halo model and show that the effective equation of the state of the gas in the periphery of dwarf galaxies determines the characteristic mass. This implies that any process which heats the gas around dwarf galaxies increases the characteristic mass and thus reduces the number of observable dwarf galaxies.

Matthias Hoeft

Stefan Gottl&#246;ber

Dwarf Galaxies in Voids: Dark Matter Halos and Gas Cooling

Regina Schulte-Ladbeck

Ulrich Hopp

Elias Brinks

Andrey Kravtsov

Dwarf-Galaxy Cosmology

The lower bound for the mass of a galaxy is unknown, as are the typical luminosity of the smallest galaxies and their numbers. The answers depend on the extent to which star formation in the first population of small mass halos may be suppressed by radiative feedback loops. If early populations of dwarf galaxies did form in significant number before reionization, their &#8220;fossils&#8221; should be found today in the Local Group. This paper reviews our ongoing efforts to simulate and identify fossil dwarfs in the Local Group. It is widely believed that reionization stopped star formation in fossil dwarfs. However, here we dispute this idea and discuss a physical mechanism whereby recent episodes of star formation would be produced in some fossil dwarfs that, hence, may characterized by a bimodal star formation history. The same mechanism could turn dark halos that failed to form stars before reionization into gas-rich &#8220;dark galaxies&#8221;. We believe that current observational data supports the thesis that a fraction of the new ultra-faint dwarfs discovered in the Local Group are fossil dwarfs and we predict the existence of a population of ultra-faint dwarfs with lower surface brightness than currently observed.

Massimo Ricotti

The First Galaxies and the Likely Discovery of Their Fossils in the Local Group


We review numerical works carried out over the last decade on the role of environmental mechanisms in shaping nature of the faintest galaxies known, dwarf spheroidals (dSphs). 
The combination of tidally induced morphological
transformation, termed tidal stirring, with mass loss due to tidal and ram-pressure stripping aided by heating due to the cosmic ionizing background can turn late-type dwarfs resembling present-day dIrrs into classic dSphs. 
The time of infall into the primary halo is shown to be a key parameter. Dwarfs accreting at <svg style="vertical-align:-1.76814pt;width:39.037498px;" id="M1" height="12.8875" version="1.1" viewBox="0 0 39.037498 12.8875" width="39.037498"  xmlns="http://www.w3.org/2000/svg">
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</svg> when the
cosmic ultraviolet ionizing flux was much higher than today, and was thus able to keep the gas in the dwarfs warm and diffuse, were rapidly stripped of their baryons via ram pressure and tidal forces, producing very dark-matter-dominated objects with truncated star-formation histories, such as the Draco dSph. The low star-formation efficiency expected in such low-metallicity objects prior to their infall was crucial for keeping their disks gas dominated until stripping took over. <i  >Therefore gas stripping along with inefficient star-formation provides a new feedback mechanism, alternative to photoevaporation or supernovae feedback, playing a crucial role in dwarf galaxy formation and evolution</i>. We also discuss how the ultra-faint dSphs belong to a different population of lower-mass dwarf satellites that were mostly shaped by reionization rather than by environmental mechanisms (&#x201c;reionization fossils&#x201d;). Finally, we scrutinize the various caveats in the current understanding of environmental effects as well as other recent ideas on the origin of Local Group dSphs. 
                  
                  

Lucio Mayer

Environmental Mechanisms Shaping the Nature of Dwarf Spheroidal Galaxies: The View of Computer Simulations

We use N-body simulations to study the effects of tides on the kinematical structure of satellite galaxies orbiting a Milky Way-like
potential. Here we focus on the evolution of a spherical, dark matter dominated satellite, which is modelled with two stellar components
set ab initio to be spatially and kinematically segregated, in a way that resembles the configuration of the metal poor and metal rich
stellar populations in several dwarf spheroidals of the Local Group. We find that an important attenuation of the initial differences in
the distribution of the two stellar components occurs for orbits with small pericentric radii. This is mainly due to (i)
the loss of the gravitational support provided by the dark matter component after tidal stripping takes place and (ii) tides preferentially
affect the more extended stellar component, leading to a net decrease in its velocity dispersion as a response for the mass loss, which
thus shrinks the kinematical gap. We apply these ideas to the Sculptor and Carina dwarf spheroidals. Differences in their orbits might
help to explain why in the former a clear kinematical separation between metal poor and metal rich stars is apparent, while in Carina
this segregation is significantly more subtle.

Laura V. Sales

Amina Helmi

Giuseppina Battaglia

The Effect of Tidal Stripping on Composite Stellar Populations in Dwarf Spheroidal Galaxies

We examine the past and current work on the star formation (SF) histories of dwarf galaxies in cosmological hydrodynamic simulations. The results obtained from different numerical methods are still somewhat mixed, but the differences are understandable if we consider the numerical and resolution effects. It remains a challenge to simulate the episodic nature of SF history in dwarf galaxies at late times within the cosmological context of a cold dark matter model. More work is needed to solve the mysteries of SF history of dwarf galaxies employing large-scale hydrodynamic simulations on the next generation of supercomputers.

Kentaro Nagamine

Star Formation History of Dwarf Galaxies in Cosmological Hydrodynamic Simulations

In the past several years kinematic data sets from Milky Way satellite galaxies have greatly improved, furthering the evidence that these systems are the most dark matter dominated objects known. This paper discusses a maximum likelihood formalism that extracts important quantities from these kinematic data sets, including the amplitude of a rotational signal, proper motions, and the mass distributions. Using a simple model for galaxy rotation it is shown that the expected error on the amplitude of a rotational signal is <svg style="vertical-align:-0.0pt;width:10.1px;" id="M1" height="5.7375002" version="1.1" viewBox="0 0 10.1 5.7375002" width="10.1" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,5.675)"><path id="x223C" d="M548 311q-21 -57 -59 -94t-89 -37q-53 0 -128 52q-55 38 -84 38q-39 0 -70.5 -23t-45.5 -60l-32 12q21 57 59 94t89 37q53 0 128 -52q55 -38 84 -38q39 0 70.5 23t45.5 60z" /></g></svg>0.5 km <svg style="vertical-align:-0.16379pt;width:19.6875px;" id="M2" height="16.049999" version="1.1" viewBox="0 0 19.6875 16.049999" width="19.6875" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,15.775)"><path id="x73" d="M319 325l-25 -7q-33 99 -103 99q-29 0 -47 -19.5t-18 -49.5t22 -49.5t62 -36.5q63 -26 95 -57t32 -79q0 -64 -50 -101t-115 -37q-35 0 -67.5 10.5t-46.5 23.5q-5 11 -11 51t-6 67l27 5q14 -53 46.5 -88.5t75.5 -35.5q29 0 50 19.5t21 50.5t-19.5 51.5t-59.5 39.5&#xA;q-28 12 -46 22.5t-38.5 27t-30.5 38.5t-10 49q0 54 42.5 92t109.5 38q48 0 88 -18q6 -15 13 -50.5t9 -55.5z" /></g><g transform="matrix(.012,-0,0,-.012,6.3,7.613)"><path id="x2212" d="M535 230h-483v50h483v-50z" /></g><g transform="matrix(.012,-0,0,-.012,13.285,7.613)"><path id="x31" d="M384 0h-275v27q67 5 81.5 18.5t14.5 68.5v385q0 38 -7.5 47.5t-40.5 10.5l-48 2v24q85 15 178 52v-521q0 -55 14.5 -68.5t82.5 -18.5v-27z" /></g></svg> with <svg style="vertical-align:-0.0pt;width:10.1px;" id="M3" height="5.7375002" version="1.1" viewBox="0 0 10.1 5.7375002" width="10.1" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,5.675)"><use xlink:href="#x223C" /></g></svg><svg style="vertical-align:-0.1638pt;width:22.775px;" id="M4" height="16.049999" version="1.1" viewBox="0 0 22.775 16.049999" width="22.775" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,15.775)"><use xlink:href="#x31" /></g><g transform="matrix(.017,-0,0,-.017,8.222,15.775)"><path id="x30" d="M241 635q53 0 94 -28.5t63.5 -76t33.5 -102.5t11 -116q0 -58 -11 -112.5t-34 -103.5t-63.5 -78.5t-94.5 -29.5t-95 28t-64.5 75t-34.5 102.5t-11 118.5q0 58 11.5 112.5t34.5 103t64.5 78t95.5 29.5zM238 602q-32 0 -55.5 -25t-35.5 -68t-17.5 -91t-5.5 -105&#xA;q0 -76 10 -138.5t37 -107.5t69 -45q32 0 55.5 25t35.5 68.5t17.5 91.5t5.5 105t-5.5 105.5t-18 92t-36 68t-56.5 24.5z" /></g><g transform="matrix(.012,-0,0,-.012,16.375,7.613)"><path id="x33" d="M285 378v-2q65 -13 102 -54.5t37 -97.5q0 -57 -30.5 -104.5t-74 -75t-85.5 -42t-72 -14.5q-31 0 -59.5 11t-40.5 23q-19 18 -16 36q1 16 23 33q13 10 24 0q58 -51 124 -51q55 0 88 40t33 112q0 64 -39 96.5t-88 32.5q-29 0 -64 -11l-6 29q77 25 118 57.5t41 84.5&#xA;q0 45 -26.5 69.5t-68.5 24.5q-67 0 -120 -79l-20 20l43 63q51 56 127 56h1q66 0 107 -37t41 -95q0 -42 -31 -71q-22 -23 -68 -54z" /></g></svg> stars from either classical or ultra-faint satellites. As an example Sculptor is analyzed for the presence of a rotational signal; no significant detection of rotation is found, with a 90% c.l. upper limit of <svg style="vertical-align:-0.0pt;width:10.1px;" id="M5" height="5.7375002" version="1.1" viewBox="0 0 10.1 5.7375002" width="10.1" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,5.675)"><use xlink:href="#x223C" /></g></svg>2 km <svg style="vertical-align:-0.16379pt;width:19.6875px;" id="M6" height="16.049999" version="1.1" viewBox="0 0 19.6875 16.049999" width="19.6875" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,15.775)"><use xlink:href="#x73" /></g><g transform="matrix(.012,-0,0,-.012,6.3,7.613)"><use xlink:href="#x2212" /></g><g transform="matrix(.012,-0,0,-.012,13.285,7.613)"><use xlink:href="#x31" /></g></svg>. A criterion for model selection is presented that determines the parameters required to describe the dark matter halo density profiles and the stellar velocity anisotropy. Applied to four data sets with a wide range of velocities, models with variable velocity anisotropy are preferred relative to those with constant velocity anisotropy, and that central dark matter profiles both less cuspy and more cuspy than Lambda-Cold Dark Matter-based fits are equally acceptable.

Louis E. Strigari

Kinematics of Milky Way Satellites: Mass Estimates, Rotation Limits, and Proper Motions

A decade ago cosmological simulations of 
               increasingly higher resolution were used to 
               demonstrate that virialized regions of Cold Dark 
               Matter (CDM) halos are filled with a multitude of 
               dense, gravitationally bound clumps. These dark 
               matter <i>subhalos</i> are central 
               regions of halos that survived strong gravitational 
               tidal forces and dynamical friction during the 
               hierarchical sequence of merging and accretion via 
               which the CDM halos form. Comparisons with 
               observations revealed that there is a glaring 
               discrepancy between abundance of subhalos and 
               luminous satellites of the Milky Way and Andromeda 
               as a function of their circular velocity or bound 
               mass within a fixed aperture. This large 
               discrepancy, which became known as the 
               “substructure” or the “missing 
               satellites” problem, begs for an explanation. 
               In this paper, the author reviews the progress made during the 
               last several years both in quantifying the problem 
               and in exploring possible scenarios in which it 
               could be accommodated and explained in the context 
               of galaxy formation in the framework of the CDM 
               paradigm of structure formation. In particular, he 
               shows that the observed luminosity function, radial 
               distribution, and the remarkable similarity of the 
               inner density profiles of luminous satellites can 
               be understood within hierarchical CDM framework 
               using a simple model in which efficiency of star 
               formation monotonically decreases with decreasing 
               virial mass satellites had before 
their accretion <i>without any actual sharp galaxy formation threshold</i>.

Dark Matter Substructure and Dwarf Galactic Satellites

In the cold dark matter scenario, dark matter halos are assembled hierarchically from smaller subunits. Some of these subunits are disrupted during the merging process, whereas others survive temporarily in the form of subhalos. A long-standing problem with this picture is that the number of subhalos predicted by simulations exceeds the number of luminous dwarf galaxies seen in the vicinity of large galaxies like the Milky Way. Many of the subhalos must therefore have remained dark or very faint. If cold dark matter subhalos are as common as predicted, gravitational lensing may in principle offer a promising route to detection. In this paper, we describe the many ways through which lensing by subhalos can manifest itself, and summarize the results from current efforts to constrain the properties of cold dark matter subhalos using such effects.

Erik Zackrisson

Teresa Riehm

Gravitational Lensing as a Probe of Cold Dark Matter Subhalos

This paper gives an overview of the attempts to determine the distribution of dark matter in low surface
brightness disk and gas-rich dwarf galaxies, both through observations and computer simulations. Observations
seem to indicate an approximately constant dark matter density in the inner parts of galaxies, while
cosmological computer simulations indicate a steep power-law-like behaviour. This difference has become
known as the “core/cusp problem,” and it remains one of the unsolved problems in small-scale cosmology.

W. J. G. de Blok

The Core-Cusp Problem

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Advances in Astronomy

Dwarf-Galaxy Cosmology

Dwarf-Galaxy Cosmology

Articles

Dwarf Galaxies in Voids: Dark Matter Halos and Gas Cooling

Dwarf-Galaxy Cosmology

The First Galaxies and the Likely Discovery of Their Fossils in the Local Group

Environmental Mechanisms Shaping the Nature of Dwarf Spheroidal Galaxies: The View of Computer Simulations

The Effect of Tidal Stripping on Composite Stellar Populations in Dwarf Spheroidal Galaxies

Star Formation History of Dwarf Galaxies in Cosmological Hydrodynamic Simulations

Kinematics of Milky Way Satellites: Mass Estimates, Rotation Limits, and Proper Motions

Dark Matter Substructure and Dwarf Galactic Satellites

Gravitational Lensing as a Probe of Cold Dark Matter Subhalos

The Core-Cusp Problem