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Dwarf spheroidal galaxies are promising targets for the indirect detection of dark matter through gamma-ray emission due to their proximity, lack of astrophysical backgrounds and high dark matter density. They are often used to place restrictive bounds on the dark matter annihilation cross section. In this report, we analyze six years of Fermi-LAT gamma-ray data from Dwarf Spheroidal galaxies that are satellites of the Milky Way, and constrain the parameter space that explain the Fermi-LAT Galactic center gamma-ray emission in 4 body final states, neutralino dark matter annihilation cross-section, and decaying dark matter lifetime for several decay channels.

Galaxies grow hierarchically by accreting smaller objects. The abundance of dark matter substructure in simulated Milky Way like galaxies shows evidence for this assembly process. The luminous satellites of our Galaxy are believed to be the visible counterparts to some of these substructures. Existing high-resolution galactic halo simulations show that the variations in substructure distributions and other properties among Milky Way like halos today are significant (a factor of at least 2-3). This, coupled with a few number of high resolution simulated halos, makes the interpretation of many results cosmic variance limited. In order to quantify the scatter in halo properties in a statistically meaningful way, we have initiated the CaterPillar project to perform zoom-In simulations of at least 60 highly resolved galactic size halos. Here we present our overall science goals. We aim to quantify the uniqueness of the Milky Way, to study the role that dwarf galaxies play in shaping large galaxies, to determine whether there really is a missing satellite problem, and to examine the origin of the old, metal-poor stellar halo of the Galaxy.

Here we present three adaptive mesh refinement radiation hydrodynamics simulations that illustrate the impact of momentum transfer from ionising radiation to the absorbing gas on star formation in high-redshift dwarf galaxies. Momentum transfer is calculated by solving the radiative transfer equation with a ray tracing algorithm that is adaptive in spatial and angular coordinates. We find that momentum input partially affects star formation by increasing the turbulent support to a three-dimensional rms velocity equal to the circular velocity of early haloes. Compared to a calculation that neglects radiation pressure, the star formation rate is decreased by a factor of five to 1.8 × 10−2 M⊙ yr−1 in a dwarf galaxy with a dark matter and stellar mass of 2.0 × 108 M⊙ and 4.5 × 105 M⊙, respectively, when radiation pressure is included. Its mean metallicity of 10−2.1 Z⊙ is consistent with the observed dwarf galaxy luminosity-metallicity relation. In addition to photo-heating in H II regions, radiation pressure ...

Using N‐body simulations of the interaction between an accreting satellite and M31, we have successfully reproduced observed distribution of the Andromeda stellar stream. We predict the spatial distribution of the metal in the progenitor dwarf galaxy by the comparison with the recent observations and the simulations.

The recent extraordinarily deep WFC3/IR data from the HUDF09 program has dramatically expanded our insights to the galaxy population at z∼7–9, and given us hints as to the population at z∼10. In particular, as part of our program of identifying and characterizing the galaxy population at high redshift (z>6), we have been undertaking a thorough analysis of the luminosity functions to much lower luminosities than previous studies. We have used very deep fields (the HUDF, HUDF09 and the HUDF05 datasets), as well as extremely faint objects from highly‐magnified sources from cluster lensing surveys, to derive the contribution from lower luminosity galaxies to as faint as 0.03–0.06 L* from z∼7 to z∼2. Recent results on the contribution of ULIRGS to star‐forming galaxies indicate that in the first 1–2 Gyr the dominant contributors to the star formation rate density in the universe are the lower luminosity and lower mass objects that are fully represented in the Lyman Break population at z∼4–7 and at earlier time...

Dwarf spheroidal (dSph) galaxies are the smallest galactic systems known. Also, under standard gravity, they are the most dark matter dominated systems known in astrophysics. The local sample of dSph satellites of the Milky Way comprise some two dozen systems, ranging from a few million, down to just a few thousand solar masses in stellar content, for the recently discovered ones. High quality observations of their stellar kinematics now allow detailed inferences of their structural and dynamical properties. We perform a modeling of the gravitational equilibrium configurations for a sample of he best studied dSphs under MOND, and find that a satisfactory description is obtained, without the need to invoke the presence of dark matter or any tidal effects, for a particular interpolation function, valid at this regime. We find also that the now well established scalings of the structural properties of these systems are naturally accounted for, while they require fine tuning under the dark matter hypothesis. In particular we find a previously unrecognized scaling between the inferred mass to light ratio of a particular dSph, with the age of most recent star formation episode for the system. Again, a trait which emerges naturally under any modified gravity scheme, and which appears as an unexplained coincidence under the dark matter hypothesis.

We revisit the issue of whether the basic properties of satellite galaxies around the Milky Way, such as their number density, radial distribution, and central mass, can be reproduced within current cosmological ΛCDM‐based models. For this purpose we combine merger trees extracted from very high resolution N‐body simulations with three different semi‐analytic model (SAM) codes. These merger trees describe the hierarchical assembly of a Milky Way‐like halo, while the SAMs are used to predict the relationship between the dark matter (sub)haloes and observable galaxy properties, allowing us to make a direct and detailed comparison with observational data. We conclude that both the luminosity function and the observed relation between luminosity and central mass can be naturally accounted for within the (Λ) Cold Dark Matter paradigm.

We study the formation histories and present‐day structure of galactic subsystems formed in a high resolution hydrodynamic simulation of a Milky Way‐sized galaxy. The simulated satellites span nearly 4 orders of magnitude in luminosity but have a very similar mass within their inner 600 pc with very little scatter. This is in agreement with the recent measurements for dwarf spheroidal galaxies (dSphs). Interestingly, subhaloes with circular velocity around 20 km/s sometimes host satellites that are as bright as the classical satellites and sometimes they are found as completely dark objects. We find that there is a critical circular velocity at the epoch of reionization, below which haloes fail to host star formation and found as “dark satellites” today.

We present the results from a new set of CDM simulations suggesting that a fraction of the recently discovered ultra‐faint dwarf population consists of the fossil remnants of galaxies that formed the majority of their stars before reionization. Our new simulations allow us to map the distribution of fossil and non‐fossils dwarfs in the Local group and compare the simulated maps to observations. We show good agreement between our simulations and observations of the ultra‐faints in both their radial distribution within the Milky Way virial radius and their stellar properties. However, the properties of the faintest ultra‐faint satellites found within 50 kpc of the Galactic center do not agree with being well‐preserved fossils of the first galaxies, perhaps because they are modified by tidal effects. We also find some discrepancies with respect to the number of dwarfs in the regions outside the Milky Way virial radius.

Extremely metal‐poor stars are believed to provide a unique opportunity to investigate the detailed nucleosynthesis by first generations of stars as well as the early stage of the Milky Way formation. Such objects have been found by recent surveys of dwarf galaxies around the Milky Way. Chemical abundance studies for stars in dwarf galaxies revealed the diversity of the abundance ratios (e.g. α/Fe ratios) and their metallicity dependence, suggesting that these dwarf galaxies are not simply remnants of the building blocks of the Milky Way halo, but have original enrichment histories. More recent studies for fainter galaxies (so‐called ultra‐faint dwarf galaxies) suggest a higher fraction of extremely metal‐poor stars in these galaxies than in classical dwarf galaxies, and the similarity of metallicity distribution to the field stars. However, chemical abundance ratios show large scatter, suggesting inhomogeneous chemical evolution. Comparisons of chemical abundance ratios between dwarf galaxies and the Milky Way halo are discussed.

We have constructed a series of models to investigate the influence of tidal forces on the formation and evolution of dwarf spheroidal galaxies. These Nbody/SPH models consist of a model for an isolated dwarf spheroidal galaxy which is placed on an orbit in a Milky‐Way sized—analytical—dark matter halo. We have varied orbital parameters, time of entrance into the potential and the dwarf progenitor total mass, totalling sixty models.The results indicate that dynamically hot dwarf spheroidal galaxies are robust against tides, unless they are on very close and eccentric orbits. In general there is an increase in star formation, caused by additional gas being funneled in to the galaxy due to tidal compression. The morphology of the galaxies (again, unless on very close and eccentric orbits) is however virtually unaltered.

Recent observations with large telescopes have revealed that stars in nearby dwarf spheroidal galaxies (dSph stars) have elemental abundances distinct from metal poor stars in our galaxy [7, 8, 9]. This indicates that the chemical evolution of these dwarf galaxies was dominated by supernovae that have not mainly contributed to the chemical evolution of our galaxy. On the other hand, the identification of host galaxies of gamma‐ray bursts (GRBs) suggests that GRBs may preferentially occur in low luminosity metal‐poor galaxies [6]. Supernovae associated with GRBs might have exclusively imprinted their nucleosynthesis on the elemental abundances of stars in dwarf galaxies while these supernovae were too rare to contribute to the chemical evolution of our galaxy. To test this conjecture, we have planned to perform nucleosynthesis calculations in aspherical supernovae and investigate the elemental abundance patterns of stars inheriting the nucleosynthesis products by taking into account ejecta with different a...

We present a large sample of 166 fully self‐consistent hydrodynamical N‐body/Tree‐SPH simulations of isolated dwarf spheroidal galaxies [1]. It has enabled us to identify the key physical parameters and mechanisms at the origin of the observed variety in the Local Group dSph properties. Using the recent data of the ESO Large Programme DART, we have constrained the star formation history of four Milky Way dSphs, Sextans, Carina, Sculptor and Fornax. For the first time, [Mg/Fe] vs [Fe/H] diagrams derived from high‐resolution spectroscopy of hundreds of individual stars are confronted with model predictions. Global relations of dSph are successfully reproduced. Our study shows that the total initial mass of these systems is the main driver of their evolution and explains the diversity in luminosity and metallicity observed in the Local Group dSphs.

We present results of a project devoted to the extensive survey of the ultra‐faint Milky Way dwarf spheroidal satellites recently discovered by the SDSS (Bootes I, Ursa Major II, Coma, Hercules, Canes Venatici I, Canes Venatici II, Leo IV), that we are monitoring for variability using time‐series multiband observations collected at a variety of telescopes from the 1.5 to the 4.3 m size, and reaching each galaxy’s main sequence turnoff In particular, we present results on the pulsation properties and “Oosterhoff type” of the RR Lyrae stars detected in two of the newly discovered dwarf spheroidals, namely Leo IV and Ursa Major II. The variable stars and the stellar populations of these two galaxies are discussed in light of the insight they can provide to reconstruct the star formation history and the merging episodes that led to the early assembly of the Galactic halo.

We present results of our search for X‐ray line emission associated with the radiative decay of the sterile neutrino, a well‐motivated dark matter candidate, in Suzaku Observatory spectra of the Ursa Minor dwarf spheroidal galaxy. These data represent the first deep observation of one of these extreme mass‐to‐light systems and the first dedicated dark matter search using an X‐ray telescope. No such emission line is positively detected; and, we derive new upper limits that match or approach the best previous results over the entire 1–20 keV mass range from a single Suzaku observation. These are used to place constraints on the existence of sterile neutrinos with given combinations of mass and active‐sterile neutrino oscillation mixing angle in the general case, and in the case where they are assumed to constitute all of the dark matter. The allowed range implies that sterile neutrinos remain a viable candidate to make up some—or all—of the dark matter and also explain pulsar kicks and various other astrophysical phenomena.

We derive ages, metallicities and [α/Fe] ratios for dwarf early‐type galaxies (dE/dS0) in the center and a region believed to be infalling into the Coma cluster. We find that a set of metallic indices cannot be described by linear relations with velocity dispersion (σ), while another group of indices exhibits linear index‐σ relations. dE/dS0 galaxies in the core of the cluster are on average younger, and have lower metallicities and lower α‐ratios (solar to sub‐solar) than the more luminous elliptical galaxies. On the other hand, dEs/dS0s in the infall region of Coma form a less homogeneous population. We find an unusually high fraction of dE/dS0s with high [α/Fe] ratios, suggesting short time scales for the star formation histories for these galaxies. Furthermore, the dE/dS0s with high α‐ratios have a range of ages and metallicities implying multiple formation scenarios where some galaxies have experienced their short star formation bursts at more recent epochs.

Globular clusters and low‐ellipticity early‐type galaxies can be treated as systems populated by a large number of stars and whose structures can be schematized as spherically symmetric. Their studies profit from the synthesis of stellar populations. The computation of synthetic models makes use of various contributions from star evolution and stellar dynamics. In the first sections of the paper we present a short review of our results on the occurrence of galactic winds in star systems ranging from globular clusters to elliptical galaxies, and the dynamical evolution of a typical massive globular cluster. In the subsequent sections we describe our approach to the problem of the stellar populations in elliptical galaxies. The projected radial behaviours of spectro‐photometric indices for a sample of eleven galaxies are compared with preliminary model results. The best agreement between observation and theory shows that our galaxies share a certain degree of heterogeneity. The gas energy dissipation varies...

Roughly half of all stars are formed in galaxies with stellar masses greater than logM*>10. This same population contains “red and dead” galaxies where star formation has effectively shut down, in some cases for many gigayears. We briefly review recent observational constraints on the distribution of specific star formation and stellar mass of this population and discuss how measurements of cold gas properties might provide insight into its evolution. We highlight several new results and describe GASS—the GALEX Arecibo SDSS survey—an on‐going large survey designed to obtain deep measurements of the HI properties of massive galaxies, providing a crucial link between past, present and possible future evolution of galaxies straddling the “transition mass” in galaxy properties.

We present a chemical analysis of two metal‐poor carbon stars belonging to the Carina Dwarf Spheroidal galaxy using high resolution optical spectra obtained with the VLT/UVES instrument. Their derived properties and chemical composition are compatible with these stars being in the TP‐AGB phase undergoing third dredge‐up episodes, although their extrinsic nature cannot be definitively discarded. The analysis indicates that these stars have large s‐element enhancements, which are compatible with theoretical s‐process nucleosynthesis predictions in low‐mass AGB stars (∼1.5M⊙), and also large carbon enhancements (C/O⩾5), much larger than the values typically found in galactic AGB carbon stars (C/O∼1–2). This is also in agreement with the theoretical prediction that carbon stars are formed more easily through third dredge‐up episodes as the initial stellar metallicity drops. We speculate on the possibility that the abundance pattern found in the low‐mass s‐elements (Y, Zr) may be revealing the existence of som...

We have obtained infrared spectra of carbon stars in four nearby galaxies—the Large and Small Magellanic Clouds, and the Fornax dwarf spheroidal galaxy. Our primary aim is to investigate mass‐loss rate and gas compositions of these stars as a function of metallicity, by comparing AGB stars in several galaxies with different metallicities. These stars were observed using the Infrared Spectrometer (IRS) onboard the Spitzer Space Telescope which covers 5–35 μm region, and the Infrared Spectrometer And Array Camera (ISAAC) on the Very Large Telescope which covers the 2.9–4.1 μm region. HCN, CH and C2H2 molecular bands, as well as SiC and MgS dust features are identified in the spectra. We find no evidence that mass‐loss rates depend on metallicity. Carbon stars are strongly affected by carbon production during the AGB phase; primarily mass‐loss of carbon‐rich stars are driven by amorphous carbon dust grains, that explains the little metallicity dependence of mass‐loss rate for carbon‐rich stars. We found that...

The authors study the Local Group of Galaxies approximating it as a binary galaxy (Milky Way and Andromeda Nebula). They find that inside 100 kpc from the Milky‐Way centre the influence of the Andromeda Nebula is very weak and that for the satellites of the Milky Way, even for the outer ones, perigalactic distances of about 30–40 kpc are likely.

We have detected several new HII regions in two Local Group galaxies (IC10 and Sex B). The hotstars method, was used to define the HII regions. We also present preliminary results about some physical parameters of these galaxies.

Dwarf galaxies are important to study because they potentially provide a direct connection to the earliest epochs of galaxy formation. They also allow us to study the star formation process at relatively low abundances. Our understanding of the origins and evolution of dwarf galaxies has been changing very rapidly. New observations are giving better insight into the relationship between the two main families of dwarf galaxies, the dwarf ellipticals and the dwarf irregulars. Here I review a broad range of topics related to the formation and evolution of dwarf galaxies.

The recent starburst in the nearby dwarf galaxy NGC 1569 has had a deep impact on the interstellar medium of the whole galaxy. In this paper, we discuss the consequences of this event for the distribution and kinematics of the neutral atomic hydrogen and demonstrate how strong star formation influences the properties of the surrounding molecular gas. The detection of a complex halo structure leads us to propose the accretion of an intergalactic cloud as the trigger and the gas reservoir of the remarkably strong starburst.

Dwarf galaxies with active star formation play a significant role in the family of starburst galaxies. They share a number of properties with their giant brethren, while their lower mass leads to some charateristics that are specific to star forming dwarfs. To see both similarities and differences, in this brief review, we focus on the different phases of the interstellar medium in dwarfs. The hot gas, typically observed in X‐rays, can, least in some cases, blow out of the galaxies. This phenomenon, still visible today, also provides an important mechanism for the metal enrichment of the intergalactic medium (IGM) in the early universe. The neutral gas, HI, sometimes extends into huge halos surrounding the optical galaxies. HI companions seem to be frequent, possibly pointing to infall and interaction as necessary means of feeding the starburst. On small scales, some dwarf galaxies show numerous HI holes, implying propagating star formation and allowing the investigation of early phases of what later may become a blow‐out of the gas. Molecular gas, the site of actual star formation, is notoriously difficult to detect in dwarfs. The reasons for this are well understood and not related to a dearth of molecular material, but rather the low metallicity environment chracteristic for dwarf galaxies. Sensitive observations have provided a number of maps of molecular gas in dwarfs and the determination of gas properties is slowly becoming possible.

Galactic winds are expected to be nearly omnipresent in dwarf galaxies based on theoretical considerations and several indirect observational evidences. The direct observational proof for their presence is far more elusive, contrary to the widespread expectation that dwarf galaxies are very favourable places to observe galactic winds. Interstingly, there is much better direct observational evidence for galactic winds from massive galaxies. I review critically the different methods, their relative sucesses to detect galactic winds in dwarf galaxies, and also comment on the future observational possibilities.

The nine dwarf spheroidal galaxies (dSphs) that are companions of the Galaxy provide excellent laboratories for exploring the physical processes that regulate star formation and galaxy evolution. Although the dSphs are currently devoid of gas, recent investigations of their stellar populations have shown that their star formation histories are surprisingly complex. The Carina dSph is a prime example; it actively formed stars in four distinct episodes over the last 1 to 14 Gyr. Carina retained an interstellar medium over numerous generations of supernovae explosions, which proves that star formation can be self-regulated even in these very low mass galaxies. Explaining the observed star-formation rates and chemical-enrichment histories of the dSphs will be challenging, but in doing so we will gain insight into the balance between star formation and the negative feedback from massive stars, and how galactic winds and massive dark matter halos shape the evolution of galaxies.

Irregular galaxies are generally less massive and less evolved systems than spirals. They constitute a different sort of system in which to study the processes of star formation and galactic evolution. The star formation history of Irregular Galaxies may be affected by several different factors. However, detailed knowledge of the star-formation history of a significant number of objects is necessary before it becomes possible to determine which parameters are important. In the last few years, there has been significant progress on this front. For the first time, we begin to probe in some detail the star-formation history of a fair number of resolved Irregulars in the Local Group. In the present, these results are summarized.

We discuss plausible contributions from evolving galaxies to the cosmic infrared background (CIRB). These estimates are based on two alternative evolutionary paradigms, roughly consistent with currently available data. One scheme is a combination of luminosity and density evolution, and assumes that galaxy evolution from optical through radio wavelengths is driven by substantial merging events, increasing the average galactic mass with cosmic time and triggering star formation in the gas‐rich systems. The other extreme pattern assumes a constant mass function up to the formation epoch and pure luminosity evolution. In such a case, early‐formed dust is assumed to hide from optical searches the expected bright star‐formation phase of spheroidal galaxies. The corresponding dust re‐radiation in the far‐IR may have been, during these early phases, orders of magnitude larger than today, as possibly indicated by long‐wavelength observations of hyperluminous IRAS galaxies, high redshift radio galaxies and quasars. We show that the rates of luminosity/density evolution, which determine the amount of energy radiated by galaxies in the IR, are very roughly constrained by the IRAS galaxy counts (also complemented by faint radio and K‐band counts, and, hopefully, by the forthcoming deep ISO surveys). On the other hand, the precise shape of the CIRB spectrum depends on evolution effects in the average dust temperature, which is likely to increase during the past active star‐forming phases as a consequence of the increased radiation field. Galaxies are the likely dominant sources of the IR background. A further significant contribution is expected in the mid‐ and far‐IR from dust‐enshrouded AGNs, which in the framework of unified models are expected to be numerous and strongly evolving. Altogether, most of the observed background level, after subtraction of the obvious Galactic and Zodiacal foregrounds, appears to be due to the integrated contribution of known source populations. In the near‐IR cosmological window in particular, any residual signals ‐ e.g. due to radiation processes occurring at very high redshifts ‐ are bound not to exceed relatively low intensity levels.

In an attempt to discover regions of very low heavy element abundance, we have observed H II regions in 7 nearby, very low luminosity dwarf irregular galaxies. All yield oxygen abundances of less than one‐tenth of the solar value, and three are within a factor of two of the lowest oxygen abundance ever detected in an H II region. These new abundance measurements are combined with other abundance measurements in nearby irregular galaxies, and a strong correlation is found between the oxygen abundance and galaxy luminosity. This relationship is used to infer that infall is no more important in irregular galaxies with extremely large H I haloes than in typical irregular galaxies. Surprisingly, dwarf elliptical galaxies closely adhere to the relationship defined by irregulars.