Home About us Contact
|
Home About us Contact | ||
|
|
||
Dwarf Elliptical Galaxies (dwarf + elliptical_galaxy)
Selected AbstractsDwarf elliptical galaxies: structure, star formation and colour,magnitude diagramsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Giovanni Carraro The aim of this paper is to cast light on the formation and evolution of elliptical galaxies by means of N -body hydrodynamical simulations that include star formation, feedback and chemical evolution. Particular attention is paid to the case of dwarf spheroidals of the Local Group which, thanks to their proximity and modern ground-based and space instrumentation, can be resolved into single stars so that independent determinations of their age and star formation history can be derived. Indeed, the analysis of the colour,magnitude diagram of their stellar content allows us to infer the past history of star formation and chemical enrichment, thus setting important constraints on galactic models. Dwarf galaxies are known to exhibit complicated histories of star formation ranging from a single very old episode to a series of bursts over most of the Hubble time. By understanding the physical process driving star formation in these objects, we might be able to infer the mechanism governing star formation in more massive elliptical galaxies. Given these premises, we start from virialized haloes of dark matter, and follow the infall of gas into the potential wells and the formation of stars. We find that in objects of the same total mass, different star formation histories are possible, if the collapse phase started at different initial densities. We predict the final structure of dwarf spheroidal galaxies, their kinematics, their large-scale distribution of gas and stars, and their detailed histories of the star formation and metal enrichment. Using a population synthesis technique, star formation and metal enrichment rates are then adopted to generate the present colour,magnitude diagrams of the stellar populations hosted by dwarf spheroidal galaxies. The simulations are made assuming the redshift of galaxy formation and varying the cosmological parameters H0 and q0. The resulting colour,magnitude diagrams are then compared with the observational ones for some dwarf spheroidals of the Local Group. [source] Formation and evolution of dwarf elliptical galaxies , II.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2009Spatially resolved star formation histories ABSTRACT 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. [source] Metallicity gradients: Mass dependency in dwarf elliptical galaxies,ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009M. Koleva Abstract 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/re. The two samples contain the same types of objects and the reason of the disagreement is still not understood (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Stellar populations of seven early-type dwarf galaxies and their nuclei,ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009S. Paudel Abstract Dwarf galaxies are the numerically dominating population in the dense regions of the universe. Although they seem to be simple systems at first view, the stellar populations of dwarf elliptical galaxies (dEs) might be fairly complex. Nucleated dEs are of particular interest, since a number of objects exhibit different stellar populations in their nuclei and host galaxy. We present stellar population parameters obtained from integrated optical spectra using a Lick index analysis of seven nucleated dwarf elliptical galaxies and their nuclei. After subtracting the scaled galaxy spectra from the nucleus spectra, we compared them with one another and explore their stellar populations. As a preliminary result, we find that the luminosity weighted ages of the nuclei slightly lower than those of galaxies, however, we do not see any significant difference in metallicity of the host galaxies and their nuclei (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Smooth, undisturbed dwarf spheroidal galaxies in the Perseus cluster core: Implications for dark matter contentASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009Sj. Penny Abstract Using deep HST/ACS observations of the core of the Perseus Cluster, we identify a large population of dwarf elliptical galaxies down to MV = ,12. All these dwarfs are remarkably smooth in appearance, showing no evidence for internal features that could be the result of tidal processes or star formation induced by the cluster potential. Based on these observations and the relatively large sizes of these dwarfs, we argue that at least some must have a large dark matter component to prevent their disruption by the cluster potential. We further derive a new method to quantify the dark matter content of cluster dSphs without the use of kinematics, which are impossible to obtain at these distances. We find that mass-to-light ratios for dwarfs in the core of the Perseus Cluster are comparable to those found for Local Group dSphs, ranging between M,/L, , 1 and 120. This is evidence that dwarf spheroidals reside in dark matter subhalos that protect them from tidal processes in the cores of dense clusters (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] How to disentangle the group of dwarf elliptical galaxies in the Virgo clusterASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009T. Gotthart Abstract A sample of early-type dwarf galaxies of the Virgo cluster extracted from the Sloan Digital Sky Survey is analyzed to search for two different populations: galaxies which were formed in the cluster and galaxies which were accreted at a later stage and which were stripped during this process. Photometric and spectroscopic data has been used for the analysis. Due to the low signal-to-noise ratios of the data, the results are however inconclusive (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||