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Register a new userALFALFA HI Content and Star Formation in Virgo Cluster Early-Type Dwarfs
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Added by
Rebecca A. Koopmann
Publication date
2009
fields
Physics
and research's language is
English
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The ALFALFA (Arecibo Legacy Fast ALFA) blind survey is providing a census of HI in galaxies of all types in a range of environments. Here we report on ALFALFA results for Virgo Cluster early-type dwarfs between declinations of 4 and 16 degrees. Less than 2% of the Virgo early-type dwarf population is detected, compared to 70-80% of the Im/BCD dwarf population. Most of the dwarfs detected in HI show evidence for ongoing or recent star formation. Early-type galaxies with HI tend to be located in the outer regions of the cluster and to be brighter. Early-type dwarfs with HI may be undergoing morphological transition due to cluster environmental effects.
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Early-type dwarf galaxies dominate cluster populations, but their formation and evolutionary histories are poorly understood. The ALFALFA (Arecibo Legacy Fast ALFA) survey has completed observations of the Virgo Cluster in the declination range of 6 - 16 degrees. Less than 2% of the early-type dwarf population is detected, a significantly lower fraction than reported in previous papers based on more limited samples. In contrast ~30 of the irregular/BCD dwarf population is detected. The detected early-type galaxies tend to be located in the outer regions of the cluster, with a concentration in the direction of the M Cloud. Many show evidence for ongoing/recent star formation. Galaxies such as these may be undergoing morphological transition due to cluster environmental effects.
Aims: We are using the Arecibo Legacy Fast ALFA survey (ALFALFA), which is covering 17% of the sky at 21 cm, to study the HI content of Early-Type galaxies (ETG) in an unbiased way. The aim is to get an overall picture of the hot, warm and cold ISM of ETG, as a function of galaxy mass and environment, to understand its origin and fate, and to relate it to the formation and evolution history of these objects. Methods: This paper deals with the first part of our study, which is devoted to the 8-16 deg. declination strip in the Virgo cluster. In this sky region, using the Virgo Cluster Catalogue (VCC), we have defined an optical sample of 939 ETG, 457 of which are brighter than the VCC completeness limit at B_T=18.0. We have correlated this optical sample with the catalogue of detected HI sources from ALFALFA. Results: Out of the 389 ETG from the VCC with B_T<=18.0, outside the 1 deg. region of poor HI detection around M87, and corrected for background contamination of VCC galaxies without a known radial velocity, only 9 galaxies (2.3%) are detected in HI with a completeness limit of 3.5 and 7.6 x 10^7 Mo of HI for dwarf and giant ETG, respectively. In addition 4 VCC ETG with fainter magnitudes are also detected. Our HI detection rate is lower than previously claimed. The majority of the detected ETG appear to have peculiar morphology and to be located near the edges of the Virgo cluster. Conclusions: Our preliminary conclusion is that cluster ETG contain very little neutral gas, with the exceptions of a few peculiar dwarf galaxies at the edge of the ETG classification and of very few larger ETG, where the cold gas could have a recent external origin.
Based on the wealth of multiwavelength imaging data from the SDSS, we investigate whether dwarf and giant early-type galaxies in the Virgo cluster follow a continuum in their structural parameters and their stellar population characteristics. More specifically we study the relation between size and brightness for the galaxies and their color magnitude relation. In both cases, we find noticeable deviations from a simple joint behavior of dwarfs and giants. We discuss these findings in the light of the different formation mechanisms commonly assumed for dwarf and giant early types, thereby taking into account the existence of several distinct early-type dwarf subclasses. By comparing our results to a semianalytic model of galaxy formation, we argue that the analyzed relations might be reproduced by processes that form dwarfs and giants altogether. The work presented here is based on Janz & Lisker 2008, 2009.
We have carried out an HI stacking analysis of a volume-limited sample of ~5000 galaxies with imaging and spectroscopic data from GALEX and the Sloan Digital Sky Survey, which lie within the current footprint of the Arecibo Legacy Fast ALFA (ALFALFA) Survey. Our galaxies are selected to have stellar masses greater than 10^10 Msun and redshifts in the range 0.025<z<0.05. We extract a sub-sample of 1833 early-type galaxies with inclinations less than 70deg, with concentration indices C>2.6 and with light profiles that are well fit by a De Vaucouleurs model. We then stack HI line spectra extracted from the ALFALFA data cubes at the 3-D positions of the galaxies from these two samples in bins of stellar mass, stellar mass surface density, central velocity dispersion, and NUV-r colour. We use the stacked spectra to estimate the average HI gas fractions M_HI/M_* of the galaxies in each bin. Our main result is that the HI content of a galaxy is not influenced by its bulge. The average HI gas fractions of galaxies in both our samples correlate most strongly with NUV-r colour and with stellar surface density. The relation between average HI fraction and these two parameters is independent of concentration index C. We have tested whether the average HI gas content of bulge-dominated galaxies on the red sequence, differs from that of late-type galaxies on the red sequence. We find no evidence that galaxies with a significant bulge component are less efficient at turning their available gas reservoirs into stars. This result is in contradiction with the morphological quenching scenario proposed by Martig et al. (2009).
We present the analysis of Halpha3, an Halpha imaging survey of 409 galaxies selected from the HI Arecibo ALFALFA Survey in the Local Supercluster, including the Virgo cluster. We explore the relations between the stellar mass, the HI mass and the current, massive SFR of nearby galaxies in the Virgo cluster and we compare them with those of isolated galaxies in the Local Supercluster, disentangling the role of the environment in shaping the star formation properties of galaxies at the present cosmological epoch. We investigate the relationships between atomic neutral gas and newly formed stars in different environments, across many morphological types, and over a wide range of stellar masses adopting an updated calibration of the HI deficiency parameter. Studying the mean properties of late-type galaxies in the Local Supercluster, we find that galaxies in increasing local galaxy density conditions (or decreasing projected angular separation from M87) show a significant decrease in the HI content and in the mean specific star formation rate, along with a progressive reddening of their stellar populations. The gradual quenching of the star formation occurs outside-in, consistently with the predictions of the ram pressure model. Once considered as a whole, the Virgo cluster is effective in removing neutral hydrogen from galaxies, and this perturbation is strong enough to appreciably reduce the SFR of its entire galaxy population. An estimate of the present infall rate of 300-400 galaxies per Gyr in the Virgo cluster is obtained from the number of existing HI-rich late-type systems, assuming 200-300 Myr as the time scale for HI ablation. If the infall process has been acting at constant rate this would imply that the Virgo cluster has formed approximately 2 Gyr ago, consistently with the idea that Virgo is in a young state of dynamical evolution.
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