Dwarf Galaxies

Deep extragalactic surveys with ISOCAM revealed the presence of a large density of faint mid-infrared (MIR) sources. We have computed the 15 µm integrated galaxy light produced by these galaxies above a sensitivity limit of 50 µJy. It sets a lower limit to the 15 µm extragalactic background light of (2.4 ± 0.5) nW m −2 sr −1. The redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). It peaks around z ∼ 0.8 in agreement with studies in other fields. The rest-frame 15 µm and bolometric infrared (8-1000 µm) luminosities of ISOCAM galaxies are computed using the correlations that we establish between the 6.75, 12, 15 µm and infrared (IR) luminosities of local galaxies. The resulting IR luminosities were double-checked using radio (1.4 GHz) flux densities from the ultra-deep VLA and WSRT surveys of the HDFN on a sample of 24 galaxies as well as on a sample of 109 local galaxies in common between ISOCAM and the NRAO VLA Sky Survey (NVSS). This comparison shows for the first time that MIR and radio luminosities correlate up to z ∼ 1. This result validates the bolometric IR luminosities derived from MIR luminosities unless both the radio-far infrared (FIR) and the MIR-FIR correlations become invalid around z ∼ 1. The fraction of IR light produced by active nuclei was computed from the cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively. We find that at most 20% of the 15 µm integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. About 75% of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies (LIR ≥ 10 11 L). They exhibit star formation rates of the order of ∼100 M yr −1. The comoving density of infrared light due to these luminous IR galaxies was more than 40 times larger at z ∼ 1 than today. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140 µm was computed from the MIR-FIR correlations for star forming galaxies and from the spectral energy distribution of the Seyfert 2, NGC 1068, for AGNs. We find that the galaxies unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e. (16 ± 5) nW m −2 sr −1 as compared to the (25 ± 7) nW m −2 sr −1 measured with the COBE satellite, with less than 10% due to AGNs. Since the CIRB contains most of the light radiated over the history of star formation in the universe, this means that a large fraction of present-day stars must have formed during a dusty starburst event similar to those revealed by ISOCAM.

We present the first determination of the radial velocities and metallicities of 78 red giant stars in the isolated dwarf irregular galaxy WLM. Observations of the calcium II triplet in these stars were made with FORS2 at the VLT-UT2 in two separated fields of view in WLM, and the [Fe/H] values were conformed to the Carretta & Gratton ([Fe/H] CG97 ) metallicity scale. The mean metallicity is [Fe/H] = −1.27 ± 0.04 dex, with a standard deviation of σ = 0.37. We find that the stars in the inner field are more metal-rich by Δ[Fe/H]= 0.30 ± 0.06 dex. These results are in agreement with previous photometric studies that found a radial population gradient, as well as the expectation of higher metallicities in the central star-forming regions. Age estimates using Victoria-Regina stellar models show that the youngest stars in the sample (less than 6 Gyr) are more metal-rich by Δ[Fe/H]= 0.32 ± 0.08 dex. These stars also show a lower velocity dispersion at all elliptical radii compared to the metal-poor stars. Kinematics for the whole red giant sample suggest a velocity gradient approximately half that of the gas rotation curve, with the stellar component occupying a thicker disk decoupled from the H i rotation plane. Taken together, the kinematics, metallicities, and ages in our sample suggest a young metal-rich, and kinematically cold stellar population in the central gas-rich regions of WLM, surrounded by a separate dynamically hot halo of older, metal-poor stars.

We report on the observations of 14 dwarf spheroidal galaxies with the Fermi Gamma-Ray Space Telescope taken during the first 11 months of survey mode operations. The Fermi telescope, which is conducting an all-sky γ-ray survey in the 20 MeV to >300 GeV energy range, provides a new opportunity to test particle dark matter models through the expected γ-ray emission produced by pair annihilation of weakly interacting massive particles (WIMPs). Local Group dwarf spheroidal galaxies, the largest galactic substructures predicted by the cold dark

We consider the Universe deep inside the cell of uniformity. At these scales, the Universe is filled with inhomogeneously distributed discrete structures (galaxies, groups and clusters of galaxies), which perturb the background Friedmann model. Here, the mechanical approach (Eingorn & Zhuk, 2012) is the most appropriate to describe the dynamics of the inhomogeneities which is defined, on the one hand, by gravitational potentials of inhomogeneities and, on the other hand, by the cosmological expansion of the Universe. In this paper, we present additional arguments in favor of this approach. First, we estimate the size of the cell of uniformity. With the help of the standard methods of statistical physics and for the galaxies of the type of the Milky Way and Andromeda, we get that it is of the order of 190 Mpc which is rather close to observations. Then, we show that the nonrelativistic approximation (with respect to the peculiar velocities) is valid for $z \lesssim 10$, i.e. approximately for 13 billion years from the present moment. We consider scalar perturbations and, within the $\Lambda$CDM model, justify the main equations. Moreover, we demonstrate that radiation can be naturally incorporated into our scheme. This emphasizes the viability of our approach. This approach gives a possibility to analyze different cosmological models and compare them with the observable Universe. For example, we indicate some problematic aspects of the spatially flat models. Such models require a rather specific distribution of the inhomogeneities to get a finite potential at any points outside gravitating masses. We also criticize the application of the Schwarzschild-de Sitter solution to the description of the motion of test bodies on the cosmological background.

The present work is devoted to studying the dynamical evolution of galaxies in scalar-Gauss-Bonnet gravity and its relationship with the MOND paradigm. This study is useful for giving meaning to the presence of a new gravitational constant. The stability of dark matter is strongly dependent on matter density. We are interested in calculating the maximum rotational velocity of galaxies. We show that rotating galaxies can be described by a new parameter that depends both on the minimum value of scalar fields and on the effective mass of this field. According to observational data, we have shown that this parameter is a constant.

We present a stellar population study of three H II galaxies (Mrk 36, UM 408, and UM 461) based on the analysis of new ground-based high-resolution near-infrared J, H, and Kp broadband and Brγ narrowband images obtained with Gemini/NIRI. We identify and determine the relative ages and masses of the elementary star clusters and/or star cluster complexes of the starburst regions in each of these galaxies by comparing the colors with evolutionary synthesis models that include the contribution of stellar continuum, nebular continuum, and emission lines. We found that the current star cluster formation efficiency in our sample of low-luminosity H II galaxies is ~10%. Therefore, most of the recent star formation is not in massive clusters. Our findings seem to indicate that the star formation mode in our sample of galaxies is clumpy, and that these complexes are formed by a few massive star clusters with masses gsim104 M sun. The age distribution of these star cluster complexes shows that the current burst started recently and likely simultaneously over short timescales in their host galaxies, triggered by some internal mechanism. Finally, the fraction of the total cluster mass with respect to the low surface brightness (or host galaxy) mass, considering our complete range in ages, is less than 1%.

The conventional cold, particle interpretation of dark matter (CDM) still lacks laboratory support and struggles with the basic properties of common dwarf galaxies, which have surprisingly uniform central masses and shallow density profiles\cite{Moore1994,Gilmore2007,Strigari2008,WP2011,AAE2013}. In contrast, galaxies predicted by CDM extend to much lower masses, with steeper, singular profiles\cite{DC1991,Kauffmann1993,Klypin1999,Moore1999}. This tension motivates cold, wavelike dark matter ({\psiDM}) composed of a non-relativistic Bose-Einstein condensate, so the uncertainty principle counters gravity below a Jeans scale\cite{Peebles2000,Hu2000,MS2014}. Here we achieve
the first cosmological simulations of this quantum state at unprecedentedly high resolution capable of resolving dwarf galaxies, with only one free parameter, $\bf{m_B}$, the boson mass. We demonstrate the large scale structure is indistinguishable from CDM, as desired, but differs radically inside galaxies where quantum interference forms
solitonic cores surrounded by extended haloes of fluctuating density granules. These results allow us to determine
$\bf{m_B=(8.0^{+1.8}_{-2.0})\times 10^{-23}~eV}$ using stellar
phase-space distributions in dwarf spheroidal galaxies. Denser, more massive solitons are predicted for Milky Way sized galaxies, providing a substantial seed to help explain early spheroid formation. The onset of galaxy formation is found substantially delayed relative to CDM, appearing at $\bf{z\lesssim 13}$ in our simulations.

We address the issue of accuracy in recovering density profiles from observations of rotation curves of galaxies. We ``observe'' and analyze our models in much the same way as observers do the real galaxies. We find that the tilted ring model analysis produces an underestimate of the central rotational velocity. In some cases the galaxy halo density profile seems to have a flat core, while in reality it does not. We identify three effects, which explain the systematic biases: (1) inclination (2), small bulge, and (3) bar. The presence of even a small non-rotating bulge component reduces the rotation velocity. In the case of a disk with a bar, the underestimate of the circular velocity is larger due to a combination of non-circular motions and random velocities. Signatures of bars can be difficult to detect in the surface brightness profiles of the model galaxies. The variation of inclination angle and isophote position angle with radius are more reliable indicators of bar pr...

The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales of galaxies. Indeed, N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark-matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some...

Using photometric redshifts we determine the galaxy population of the clusters of galaxies Cl0016+16 at z = 0.55, Cl1600+41 at z = 0.54, Cl1601+42 at z = 0.54 and MS1008-1224 at z = 0.31. Comparing the clusters, we find no evidence for a universal shape of the total luminosity function (LF) at these redshifts. When dividing the LFs into spectral types, we find that the LF of the early-type galaxies alone can be described by a Gaussian, while the LF of the late-type galaxies is well fitted by a Schechter function, suggesting that the separate LFs for different populations may be universal. The difference in the total LFs can mainly be attributed to the varying relative normalisation of these populations, implying that clusters with an abundant population of late-type galaxies also have steeper faint-end slopes. In MS1008-1224 we detect a faint blue population that dominates over a population with colours consistent with dwarf ellipticals, opposite to clusters at lower redshift.

excellent at most wavelengths. The plateau between 1,700 Å and 3,000 Å is due to a local minimum between two broad absorption features caused by amorphous carbon grains, with the added flat contribution from Fe 3 O 4 grains. Silicate grains contribute to the rise at shorter wavelengths.

We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using full-spectrum fitting, we derive radial profiles of the SSP-equivalent ages and metallicities. We make a detailed analysis with ulyss and steckmap of the star-formation history in the core of the galaxies and in an aperture of one effective radius. We resolved the history into 1 to 4 epochs. The statistical significance of these reconstructions were carefuly tested; the two programs give remarkably consistent results.

We propose an extension of the GC3 streamer finding method of that can be applied to the future Gaia database. The original method looks for streamers along great circles in the sky, our extension adds the kinematical restriction that velocity vectors should also be constrained to lie along these great circles, as seen by a Galactocentric observer. We show how to use these combined criteria starting from heliocentric observables. We test it by using the mock Gaia catalogue of Brown et al. (2005), which includes a realistic Galactic background and observational errors, but with the addition of detailed star formation histories for the simulated satellites. We investigate its success rate as a function of initial satellite luminosity, star formation history and orbit. We find that the inclusion of the kinematical restriction vastly enhances the contrast between a streamer and the background, even in the presence of observational errors, provided we use only data with good astrometric quality (fractional errors of 30 per cent or better). The global nature of the method diminishes the erasing effect of phase mixing and permits the recovery of merger events of reasonable dynamical age. Satellites with a star formation history different to that of the Galactic background are also better isolated. We find that satellites in the range of 10 8 − 10 9 L ⊙ can be recovered even for events as old as ∼ 10 Gyr. Even satellites with 4−5×10 7 L ⊙ can be recovered for certain combinations of dynamical ages and orbits.

We revisit the XMM-Newton observation of M87 to make a new and more detailed measurement of the metal abundance profile. After having verified that spectral fits with single temperature models show a dramatic abundance drop within 1 arcmin of the cluster core, we show that more appropriate models, which include a multi-temperature component to account for the strong temperature gradient observed in M87, and a power-law component to account for the emission of the nucleus and knot A, give a substantially flat abundance profile. The drastic abundance decrement found by fitting a single temperature component to the data is an artifact known as "Fe-bias" (Buote 2000a,b) following from the application of an oversimplified spectral model to the data.

NGC 2915 is a blue compact dwarf galaxy embedded in an extended, low surface brightness H i disk exhibiting a two-armed spiral structure and a central barlike component. Commonly accepted mechanisms are unable to explain the existence of these patterns, and Bureau et al. proposed disk dark matter (scaling with the H i distribution) or a rotating triaxial dark halo as alternative solutions. In an attempt to explore these mechanisms, hydrodynamic simulations were run for each case and compared to observations using custom-ized column density and kinematic constraints. The spiral structure can be accounted for by both an unseen bar or triaxial halo, the former fitting the observations slightly better. However, the large bar mass or halo pattern frequency required make it unlikely that the spiral wave is driven by an external perturber. In particular , the spin parameter is much higher than predicted by current cold dark matter structure formation scenarios. The massive disk models show that when the observed gas surface density is scaled up by a factor about 10, the disk develops a spiral structure resembling closely the observed one, in perturbed density as well as perturbed velocity. This is consistent with more limited studies in other galaxies and suggests that the disk of NGC 2915 contains much more mass than is visible, tightly linked to the neutral hydrogen. A classic (quasi-)spherical halo is nevertheless still required, as increasing the disk mass further to fit the circular velocity curve would make the disk violently unstable. Scaling the observed surface density profile by an order of magnitude brings the disk and halo masses to comparable values within the disk radius. The surface density remains under Kennicutt's star formation threshold for a gaseous disk, and no stars are expected to form, as required by observations. Subject headings: dark matter — galaxies: evolution — galaxies: halos — galaxies: individual (NGC 2915) — galaxies: kinematics and dynamics — ISM: kinematics and dynamics

Because dwarf galaxies are the most abundant type of galaxy, they are crucial for our understanding of the formation and evolution of galaxies. Abundance ratios and their variations as a result of star formation are key constraints in chemical evolution models. Thus, the determination of these abundances in the dwarf galaxies of the Local Universe is extremely important. However, these objects are intrinsically faint, and observational constraints to their evolution can be obtained only for very nearby galaxies. NGC 185 is one of the four brightest dwarf companions of M31. However, unlike the other three -NGC 147, 205 and 221 (M32) -it has an important content of gas and dust. We have obtained deep spectroscopic observations of the Hα emitting population of NGC 185 using the Gemini multi-object spectrograph at the Gemini North telescope. As a result, in addition to the bright planetary nebulae (PNe) previously found in the galaxy and reported in the literature, we have found other, much fainter, PNe. We have then recalculated the electron temperatures and chemical abundances of the brightest PNe and, for the first time, we have derived their electron densities. Our characterization of the population properties of the PNe is interpreted in terms of the chemical evolution of NGC 185, which suggests that it has suffered a significant chemical enrichment within the last ∼8 Gyr. We have also discovered the first symbiotic star in the galaxy and we have determined the properties of a known supernova remnant located close to the centre of NGC 185.

We report the discovery of a dwarf Seyfert 1 active galactic nucleus (AGN) with a candidate intermediate-mass black hole hosted by the dwarf galaxy SDSS J160531.84+174826.1 at z = 0.032. A broad component of the Hα line with FWHM=781km s −1 is detected in its optical spectrum, and a bright, point-like nucleus is evident from a HST imaging observation. Non-thermal X-ray emission is also detected from the nucleus. The black hole mass, as estimated from the luminosity and width of the broad Hα component, is about 7 × 10 4 M ⊙ . The host galaxy appears to be a disk galaxy with a boxy bulge or nuclear bar; with an absolute magnitude of M R = −17.8, it is among the least luminous host galaxies ever identified for a Seyfert 1.

We present the results of N-body simulations of the accretion of high-density dwarf galaxies by low-density giant galaxies. Both galaxies contain power-law central density cusps and point masses representing supermassive black holes; the ratio of galaxy masses is 3:1. The cusp of the dwarf galaxy is always disrupted during the merger, leading to a remnant with a weak power law in the intrinsic density and a ``core'' in the projected density. Removing both black holes from the giant and dwarf galaxies allows the dwarf galaxy to remain intact and leads to a remnant with a high central density, contrary to what is observed. Our results support the hypothesis than the persistence of low-density cores in giant galaxies following mergers is a consequence of the existence of supermassive central black holes.

Deep long-slit spectroscopic observations of the blue compact galaxy IZw 18 obtained with the CFH 3.6 m Telescope are presented. The very low value of oxygen abundance previously reported is confirmed and a very homogeneous abundance distribution is found (no variation larger than 0.05 dex) over the whole ionized region. We concur with Tenorio-Tagle (1996) and Devost et al. (1997)

We present results on integral-field optical spectroscopy of five luminous Blue Compact Dwarf galaxies. The data were obtained using the fiber system integral attached at the William Herschel telescope. The galaxies Mrk 370, Mrk 35, Mrk 297, Mrk 314 and III Zw 102 were observed. The central 33. ′′ 6 × 29. ′′ 4 regions of the galaxies were mapped with a spatial resolution of 2. ′′ 7 spaxel −1 , except for Mrk 314, in which we observed the central 16 ′′ ×12 ′′ region with a resolution of 0. ′′ 9 spaxel −1 . We use high-resolution optical images to isolate the star-forming knots in the objects; line ratios, electron densities and oxygen abundances in each of these regions are computed. We build continuum and emission-line intensity maps as well as maps of the most relevant line ratios: [O III] λ5007/Hβ, [N II] λ6584/Hα, and Hα/Hβ, which allow us to obtain spatial information on the ionization structure and mechanisms. We also derive the gas velocity field from the Hα and [O III] λ5007 emission lines.

Friedmannian cosmology with its homogeneous expansion cannot explain the missing expansion redshifts in the local group. It can also not explain why the expansion is perfectly entropical (accelerates like a simple quadratic function). It uses the baseless assumption of the expansion-doppler duality to explain away the first problem, and dark energy to try and "explain" the accelerated expansion. In reality the expansion is entropic, which is why the rate of expansion is lower in the local group, and why an acceleration is taking place.

The first step in all of this is to understand that Newton was right when he said that a universe that has no center cannot collapse.

We present a detailed study of the star formation history (SFH) of the Tucana dwarf spheroidal galaxy. High quality, deep HST/ACS data, allowed us to obtain the deepest color-magnitude diagram to date, reaching the old main sequence turnoff (F814 ~ 29) with good photometric accuracy. Our analysis, based on three different SFH codes, shows that Tucana is an old and

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z /2. Such tests are particularly important for interpreting the surprising high-ionization UV line emission detected at z > 6 in recent years. We present HST /COS ultraviolet spectra of ten nearby metal-poor star-forming galaxies selected to show He ii emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + log O/H < 8.5) and present uniformly large specific star formation rates (sSFR ∼ 10 2 Gyr −1). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C iii] equivalent widths comparable to those seen in systems at z ∼ 6 − 7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + log O/H 8.0 (Z/Z 1/5) presenting minimal stellar wind features and prominent nebular emission in He ii and C iv. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He +-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C iv and He ii powered by these stars may provide useful metallicity constraints in the reionization era.

We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ∼ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGN) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA and ROSAT data where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to -2errors as large as a factor of 2 for individual quasars. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio.

The power spectrum of weak lensing shear caused by large-scale structure is an emerging tool for precision cosmology, in particular for measuring the effects of dark energy on the growth of structure at low redshift. One potential source of systematic error is intrinsic alignments of ellipticities of neighbouring galaxies (II correlation) that could mimic the correlations due to lensing. A related possibility pointed out by Hirata and Seljak (2004) is correlation between the intrinsic ellipticities of galaxies and the density field responsible for gravitational lensing shear (GI correlation). We present constraints on both the II and GI correlations using 265 908 spectroscopic galaxies from the SDSS, and using galaxies as tracers of the mass in the case of the GI analysis. The availability of redshifts in the SDSS allows us to select galaxies at small radial separations, which both reduces noise in the intrinsic alignment measurement and suppresses galaxy- galaxy lensing (which otherwise swamps the GI correlation). While we find no detection of the II correlation, our results are nonetheless statistically consistent with recent detections found using the SuperCOSMOS survey. In contrast, we have a clear detection of GI correlation in galaxies brighter than L* that persists to the largest scales probed (60 Mpc/h) and with a sign predicted by theoretical models. This correlation could cause the existing lensing surveys at z~1 to underestimate the linear amplitude of fluctuations by as much as 20% depending on the source sample used, while for surveys at z~0.5 the underestimation may reach 30%. (Abridged.)

Context. Evidence has grown over the past few years that the neutral phase of blue compact dwarf (BCD) galaxies may be metaldeficient as compared to the ionized gas of their H ii regions. These results have strong implications for our understanding of the chemical evolution of galaxies, and it is essential to strengthen the method, as well as to find possible explanations. Aims. We present the analysis of the interstellar spectrum of Pox 36 with the Far Ultraviolet Spectroscopic Explorer (FUSE). Pox 36 was selected because of the relatively low foreground gas content that makes it possible to detect absorption-lines weak enough that unseen components should not be saturated. Methods. Interstellar lines of H i, N i, O i, Si ii, P ii, Ar i, and Fe ii are detected. Column densities are derived directly from the observed line profiles except for H i, whose lines are contaminated by stellar absorption, thus needing the stellar continuum to be removed. We used the TLUSTY models to remove the stellar continuum and isolate the interstellar component. The best fit indicates that the dominant stellar population is B0. The observed far-UV flux agrees with an equivalent number of ∼ 300 B0 stars. The fit of the interstellar H i line gives a column density of 10 20.3±0.4 cm −2 . Chemical abundances were then computed from the column densities using the dominant ionization stage in the neutral gas. Our abundances are compared to those measured from emission-line spectra in the optical, probing the ionized gas of the H ii regions. Results. Our results suggest that the neutral gas of Pox 36 is metal-deficient by a factor ∼ 7 as compared to the ionized gas, and they agree with a metallicity of ≈ 1/35 Z ⊙ . Elemental depletion is not problematic because of the low dust content along the selected lines of sight. In contrast, the ionized gas shows a clear depletion pattern, with iron being strongly depleted. Conclusions. The abundance discontinuity between the neutral and ionized phases implies that most of the metals released by consecutive star-formation episodes mixes with the H i gas. The volume extent of the enrichment is so large that the metallicity of the neutral gas increases only slightly. The star-forming regions could be enriched only by a small fraction (∼ 1%), but it would greatly enhance its metallicity. Our results are compared to those of other BCDs. We confirm the overall underabundance of metals in their neutral gas, with perhaps only the lowest metallicity BCDs showing no discontinuity.

The small and elusive Carina dSph still presents many open questions concerning its stellar populations and dynamical evolution. It presents a unique star formation history, with well separated episodes over a time of ˜10 Gyr. However, the complex star formation history does not seem to have caused a strong chemical evolution of its stellar content. Moreover, the occurrence of extra-tidal

Context. Outflows powered by the injection of kinetic energy from massive stars can strongly affect the chemical evolution of galaxies, in particular of dwarf galaxies, as their lower gravitational potentials enhance the chance of a galactic wind. Aims. We therefore performed a detailed kinematic analysis of the neutral and ionised gas components in the nearby star-forming irregular dwarf galaxy NGC 4861. Similar to a recently published study of NGC 2366, we want to make predictions about the fate of the gas and to discuss some general issues about this galaxy. Methods. Fabry-Perot interferometric data centred on the Hα line were obtained with the 1.93m telescope at the Observatoire de Haute-Provence. They were complemented by H i synthesis data from the VLA. We performed a Gaussian decomposition of both the Hα and the H i emission lines in order to search for multiple components indicating outflowing gas. The expansion velocities of the detected outflows were compared to the escape velocity of NGC 4861, which was modelled with a pseudo-isothermal halo.

Aims. We aim to understand the real nature of the stellar overdensity at southern galactic latitudes in the region of CMa. Methods. We perform a critical re-analysis and discussion of recent results presented in the literature which interpret the CMa overdensity as the signature of an accreting dwarf galaxy or a new substructure within the Galaxy. Several issues are addressed. Results. We show that arguments against the "warp" interpretation are based on an erroneous perception of the Milky Way. There is nothing anomalous with colour-magnitude diagrams on opposite sides of the average warp mid-plane being different. We witnessed the rise and fall of the blue plume population, first attributed to young stars in a disrupting dwarf galaxy and now discarded as a normal disc population. Similarly, there is nothing anomalous in the outer thin+thick disc metallicities being low (−1 < [Fe/H] < −0.5), and spiral arms (as part of the thin disc) should, and do, warp. Most importantly, we show unambiguously that, contrary to previous claims, the warp produces a stellar overdensity that is distance-compatible with that observed in CMa. Conclusions. The CMa over-density remains fully accounted for in a first order approach by Galactic models without new substructures. Given the intrinsic uncertainties (concerning the properties of the warp, flare and disc cutoff, the role of extinction and degeneracy), minor deviations with respect to these models are not enough to support the hypothesis of an accreted dwarf galaxy or new substructure within the Milky Way disc.

We present a search for gamma-ray emission from the direction of the newly discovered dwarf galaxy Reticulum 2. Using Fermi-LAT data, we detect a signal that exceeds expected backgrounds between ~2-10 GeV and is consistent with annihilation of dark matter for particle masses less than a few x 10^2 GeV. Modeling the background as a Poisson process based on Fermi-LAT diffuse models, and taking into account trials factors, we detect emission with p-value less than 9.8 x 10^-5 (>3.7 sigma). An alternative, model-independent treatment of background reduces the significance, raising the p-value to 9.7 x 10^-3 (2.3 sigma). Even in this case, however, Reticulum 2 has the most significant gamma-ray signal of any known dwarf galaxy. If Reticulum 2 has a dark matter halo that is similar to those inferred for other nearby dwarfs, the signal is consistent with the s-wave relic abundance cross section for annihilation.

Integrating our knowledge of star formation traced by observations at different wavelengths is essential for correctly interpreting and comparing star formation activity in a variety of systems and environments. This study compares extinction corrected integrated ultraviolet (UV) emission from resolved galaxies with color-magnitude diagram (CMD) based star formation rates (SFRs) derived from resolved stellar populations and CMD fitting techniques in 19 nearby starburst and post-starburst dwarf galaxies. The datasets are from the panchromatic STARBurst IRregular Dwarf Survey (STAR-BIRDS) and include deep legacy GALEX UV imaging, HST optical imaging, and Spitzer MIPS imaging. For the majority of the sample, the integrated near UV fluxes predicted from the CMDbased SFRs -using four different models -agree with the measured, extinction corrected, integrated near UV fluxes from GALEX images, but the far UV predicted fluxes do not. Further, we find a systematic deviation between the SFRs based on integrated far UV luminosities and existing scaling relations, and the SFRs based on the resolved stellar populations. This offset is not driven by different star formation timescales, variations in SFRs, UV attenuation, nor stochastic effects. This first comparison between CMD-based SFRs and an integrated FUV emission SFR indicator suggests that the most likely cause of the discrepancy is the theoretical FUV-SFR calibration from stellar evolutionary libraries and/or stellar atmospheric models. We present an empirical calibration of the FUV-based SFR relation for dwarf galaxies, with uncertainties, which is ∼53% larger than previous relations.

The chemical composition of two stars in WLM have been determined from high quality UVES data obtained at the VLT UT2 1 . The model atmospheres analysis shows that they have the same metallicity, [Fe/H] = −0.38 ±0.20 (±0.29) 2 . Reliable magnesium abundances are determined from several lines of two ionization states in both stars resulting in [Mg/Fe] = −0.24 ±0.16 (±0.28). This result suggests that the [α(Mg)/Fe] ratio in WLM may be suppressed relative to solar abundances (also supported by differential abundances relative to similar stars in NGC 6822 and the SMC). The absolute Mg abundance, [Mg/H] = −0.62 is high relative to what is expected from the nebulae though, where two independent spectroscopic analyses of the H II regions in WLM yield [O/H] = −0.89. Intriguingly, the oxygen abundance determined from the O I λ6158 feature in one WLM star is [O/H] = −0.21 ±0.10 (±0.05)

As the first paper of a series devoted to study the old stellar population in clusters and fields in the Small Magellanic Cloud (SMC), we present deep observations of NGC 121 in the F555W and F814W filters, obtained with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The resulting color-magnitude diagram reaches ∼3.5 mag below the main-sequence turn-off, deeper than any previous data. We derive the age of NGC 121 using both absolute and relative age-dating methods. Fitting isochrones in the ACS photometric system to the observed ridge line of NGC 121 gives ages of 11.8 ± 0.5 Gyr (Teramo), 11.2 ± 0.5 Gyr (Padova), and 10.5 ± 0.5 Gyr (Dartmouth). The cluster ridgeline is best approximated by the α-enhanced Dartmouth isochrones. Placing our relative ages on an absolute age scale, we find ages of 10.9 ± 0.5 Gyr (from the magnitude difference between the main-sequence turn-off and the horizontal branch) and 11.5 ± 0.5 Gyr (from the absolute magnitude of the horizontal branch), respectively. These five different age determinations are all lower by 2-3 Gyr than the ages of the oldest Galactic globular clusters (GCs) of comparable metallicity. Therefore we confirm the earlier finding that the oldest globular cluster in the SMC, NGC 121, is a few Gyr younger than its oldest counterparts in the Milky Way and in other nearby dwarf galaxies such as the Large Magellanic Cloud, Fornax, and Sagittarius. If it were accreted into the Galactic halo, NGC 121 would resemble the "young halo globulars," although it is not as young as the youngest globular clusters associated with the Sagittarius dwarf. The young age of NGC 121 could result from delayed cluster formation in the SMC or result from the random survival of only one example of an initially small number of star clusters.