D. Michielsen

A test suite for quantitative comparison of hydrodynamic codes in astrophysics

We test four commonly used astrophysical simulation codes, enzo, flash, gadget and hydra, using a suite of numerical problems with analytic initial and final states. Situations similar to the conditions of these tests, a Sod shock, a Sedov blast, and both a static and translating King sphere, occur commonly in astrophysics, where the accurate treatment of shocks, sound waves, supernovae explosions and collapsed haloes is a key condition for obtaining reliable validated simulations. We demonstrate that comparable results can be obtained for Lagrangian and Eulerian codes by requiring that approximately one particle exists per grid cell in the region of interest. We conclude that adaptive Eulerian codes, with their ability to place refinements in regions of rapidly changing density, are well suited to problems where physical processes are related to such changes. Lagrangian methods, on the other hand, are well suited to problems where large density contrasts occur and the physics are related to the local density itself rather than the local density gradient.

The puzzlingly large Ca II triplet absorption in dwarf elliptical galaxies

We present central CaT, PaT, and CaT* indices for a sample of 15 dwarf elliptical galaxies (dEs). Twelve of these have CaT* ~ 7 A and extend the negative correlation between the CaT* index and the central velocity dispersion σ, which was derived for bright elliptical galaxies (Es), down to 20 km s-1 < σ < 55 km s-1. For five dEs, we have independent age and metallicity estimates. Four of these have CaT* ~ 7 A, much higher than expected from their low metallicities (-1.5 < [Z/H] < -0.5). The observed anticorrelation of CaT* as a function of σ or Z is in flagrant disagreement with theory. We discuss some of the amendments that have been proposed to bring the theoretical predictions into agreement with the observed CaT* values of bright Es and how they can be extended to incorporate the observed CaT* values of dEs as well. Moreover, three dEs in our sample have CaT* ~ 5 A, as would be expected for metal-poor stellar systems. Any theory for dE evolution will have to be able to explain the coexistence of low-CaT* and high-CaT* dEs at a given mean metallicity. This could be the first direct evidence that the dE population is not homogeneous and that different evolutionary paths led to morphologically and kinematically similar but chemically distinct objects.

Formation and evolution of dwarf elliptical galaxies – II. Spatially resolved star formation histories

We present optical Very Large Telescope spectroscopy of 16 dwarf elliptical galaxies (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 one to four epochs. The statistical significance of these reconstructions was carefully tested; the two programs give remarkably consistent results. The old stellar population of the dEs, which dominates their mass, is likely coeval with that of massive ellipticals or bulges, but the star formation efficiency is lower. Important intermediate age (1–5 Gyr) populations and frequently tails of star formation until recent times are detected. These histories are reminiscent of their lower mass dwarf spheroidal counterparts of the Local Group. Most galaxies (10/16) show significant metallicity gradients, with metallicity declining by 0.5 dex over one half-light radius on average. These gradients are already present in the old population. The flattened (or discy), rotating objects (6/16) have flat metallicity profiles. This may be consistent with a distinct origin for these galaxies or it may be due to their geometry. The central single stellar population equivalent age varies between 1 and 6 Gyr, with the age slowly increasing with radius in the vast majority of objects. The group and cluster galaxies have similar radial gradients and star formation histories. The strong and old metallicity gradients place important constraints on the possible formation scenarios of dEs. Numerical simulations of the formation of spherical low-mass galaxies reproduce these gradients, but they require a longer time for them to build up. A gentle depletion of the gas, by ram pressure stripping or starvation, could drive the gas-rich, star-forming progenitors to the present dEs.

The relation between stellar populations, structure and environment for dwarf elliptical galaxies from the MAGPOP-ITP

Dwarf galaxies, as the most numerous type of galaxy, offer the potential to study galaxy formation and evolution in detail in the nearby universe. Although they seem to be simple systems at first view, they remain poorly understood. In an attempt to alleviate this situation, the MAGPOP EU Research and Training Network embarked on a study of dwarf galaxies named MAGPOP-ITP. In this paper, we present the analysis of a sample of 24 dwarf elliptical galaxies (dEs) in the Virgo cluster and in the field, using optical long-slit spectroscopy. We examine their stellar populations in combination with their light distribution and environment. We confirm and strengthen previous results that dEs are, on average, younger and more metal-poor than normal elliptical galaxies, and that their [alpha/Fe] abundance ratios scatter around solar. This is in accordance with the downsizing picture of galaxy formation where mass is the main driver for the star formation history. We also find new correlations between the luminosity-weighted mean age, the large-scale asymmetry, and the projected Virgocentric distance. We find that environment plays an important role in the termination of the star formation activity by ram-pressure stripping of the gas in short time-scales, and in the transformation of discy dwarfs to more spheroidal objects by harassment over longer time-scales. This points towards a continuing infalling scenario for the evolution of dEs.

The H I Content of E+A Galaxies

We present deep single-dish H I observations of a sample of six nearby E+A galaxies (0.05 < z < 0.1). A nonnegligible fraction of a local sample of E+As are detected in H I. In four galaxies, we have detected up to a few times 109 M☉ of neutral gas. These E+A galaxies are almost as gas-rich as spiral galaxies with comparable luminosities. There appears to exist no direct correlation between the amount of H I present in an E+A galaxy and its star formation rate as traced by radio continuum emission. Moreover, the end of the starburst does not necessarily require the complete exhaustion of the neutral gas reservoir. Most likely, an intense burst of star formation consumed the dense molecular clouds, which are the sites of massive star formation. This effectively stops star formation, even though copious amounts of diffuse neutral gas remain. The remaining H I reservoir may eventually lead to further episodes of star formation. This may indicate that some E+As are observed in the inactive phase of the star formation duty cycle.

TOWARD A SOLUTION FOR THE Ca II TRIPLET PUZZLE : RESULTS FROM DWARF ELLIPTICAL GALAXIES

We present new estimates of ages and metallicities, based on VLT FORS optical (4400-5500 A) spectroscopy, of 16 dwarf elliptical galaxies (dEs) in the Fornax Cluster and in southern groups. These dEs are more metal-rich and younger than previous estimates based on narrowband photometry and low-resolution spectroscopy. For our sample we find a mean metallicity [Z/H] = -0.33 dex and mean age 3.5 Gyr, consistent with similar samples of dEs in other environments (Local Group, Virgo). Three dEs in our sample show emission lines and very young ages. This suggests that some dEs formed stars until a very recent epoch and were self-enriched by a long star formation history. Previous observations of large near-infrared (~8500 A) Ca II absorption strengths in these dEs are in good agreement with the new metallicity estimates, solving part of the so-called calcium puzzle.

The CO content of the Local Group dwarf irregular galaxies IC 5152, UGCA 438 and the Phoenix dwarf

We present a search for CO(1 → 0) emission in three Local Group dwarf irregular galaxies: IC 5152, the Phoenix dwarf and UGCA 438, using the ATNF Mopra radio telescope. Our scans largely cover the optical extent of the galaxies and the stripped H I cloud west of the Phoenix dwarf. Apart from a tentative but non-significant emission peak at one position in the Phoenix dwarf, no significant emission was detected in the CO spectra of these galaxies. For a velocity width of 6km s -1 , we derive 4σ upper limits of 0.03, 0.04 and 0.06 K km s -1 for IC 5152, the Phoenix dwarf and UGCA 438, respectively. This is an improvement of over a factor of 10 compared with previous observations of IC 5152; the other two galaxies had not yet been observed at millimetre wavelengths. Assuming a Galactic CO-to-H 2 conversion factor, we derive upper limits on the molecular gas mass of 6.2 x 10 4 , 3.7 x 10 3 and 1.4 × 10 5 M ⊙ for IC 5152, the Phoenix dwarf and UGCA 438, respectively. We investigate two possible causes for the lack of CO emission in these galaxies. On the one hand, there may be a genuine lack of molecular gas in these systems, in spite of the presence of large amounts of neutral gas. However, in the case of IC 5152 which is actively forming stars, molecular gas is at least expected to be present in the star-forming regions. On the other hand, there may be a large increase in the CO-to-H 2 conversion factor in very low-metallicity dwarfs (-2 ≤ [Fe/H] < - 1), making CO a poor tracer of the molecular gas content in dwarf galaxies.

Metallicity gradients: Mass dependency in dwarf elliptical galaxies

The formation and evolution of galaxies is imprinted on their stellar population radial gradients. Two recent articles present conflicting results concerning the mass dependence of the metallicity gradients for early-type dwarf galaxies. On one side, Spolaor et al. (2009) show a tight positive correlation between the total metallicity [Z/H] and the mass. On the other side, in a distinct sample, we do not find any trend involving [Fe/H] (Koleva et al. 2009). In order to investigate the origin of the discrepancy, we examine various factors that may affect the determination of the gradients: namely the sky subtraction and the signal-to-noise ratio. We conclude that our detection of gradients are well above the possible analysis biases. Then, we measured the [Mg/Fe] relative abundance profile and found moderate gradients. The derived [Z/H] gradients scatter around -0.4 dex/r_e. The two samples contain the same types of objects and the reason of the disagreement is still not understood. Based on observations made with ESO telescopes at La Silla Paranal observatory under program ID076.B-0196

Evidence for a warm interstellar medium in Fornax dwarf elliptical galaxies – II. FCC032, FCC206 and FCCB729

We present R-band and Hα+[N ii] narrow-band imaging of FCC032, FCC206 and FCCB729, three dwarf elliptical galaxies (dEs) in the Fornax Cluster. These dEs contain significant amounts of ionised gas. FCC032 harbours a large ionised gas complex, consisting of several individual clouds, a superbubble and a filament that extends away from the galaxy centre. The ionised gas structures observed in FCC032 bear a strong resemblance to those observed in more gas-rich and more fiercely star-forming dwarf galaxies. FCC206, a very low surface brightness dE, contains one faint extended emission region, and two compact clouds. In FCCB729, the only nucleated galaxy in this sample, one of the ionised gas clouds coincides with the stellar nucleus. We derive ionised gas masses of a few 10 to 10 M⊙ for these galaxies. This brings our sample of dEs with ionised gas with Hα+[N ii] narrow-band imaging to five objects. The ionised gas morphologies in these galaxies range from pure nuclear emission peaks to extended emission complexes. This morphological diversity could also indicate a diversity in ionising processes in dEs with a warm interstellar medium (active galactic nuclei, starbursts, post-AGB stars,. . . ). Inside FCC206, four star clusters appear to be on the verge of merging to form a nucleus in this presently non-nucleated dE. Understanding the formation of nuclei in dEs could give us important clues to the formation of super-massive black holes (SMBHs).

The H i content of Fornax dwarf elliptical galaxies: FCC032 and FCC336

We present H I21-cm line observations, obtained with the Australia Telescope Compact Array, of two dwarf elliptical galaxies (dEs) in the Fomax cluster: FCC032 and FCC336. The optical positions and velocities of these galaxies place them well within the Fornax cluster. FCC032 was detected at the 3σ significance level with a total HI flux density of 0.66 ± 0.22 Jy km s -1 or an HI mass of 5.0 ± 1.7 x 10 7 h -2 75 M ○. . Based on our deep Ha + [N II] narrow-band images, obtained with FORS2 mounted on the Very Large Telescope, this dE was already known to contain 600 -1800 h -2 75 M ○. of ionized hydrogen (depending on the relative strengths of the Hα and [N II] emission lines). Hence, this is the first study of the complex, multiphase interstellar medium of a dE outside the Local Group. FCC336 was detected at the same significance level: 0.37 ± 0.10 Jy km s -1 or a total H I mass of 2.8 ± 0.7 x 10 7 h -2 75 M ○. . Using a compilation of H I data of dwarf galaxies, we find that the observed high HI mass boundary of the distribution of dwarf irregulars, blue compact dwarfs and dwarf ellipticals in a log L B versus log M HI diagram is in good agreement with a simple chemical evolution model with continuous star formation. The existence of many gas-poor dEs (undetected at 21 cm) suggests that the environment (or, more particularly, a galaxys orbit within a cluster) also plays a crucial role in determining the amount of gas in present-day dEs; for example, FCC032 and FCC336 are located in the sparsely populated outskirts of the Fomax cluster. This is in agreement with H I surveys of dEs in the Virgo cluster, and an Ha survey of the Fornax cluster, which also tend to place gas-rich dwarf galaxies in the cluster periphery.