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Register a new userTowards a new classification of early-type galaxies: an integral-field view
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Added by
Jesus Falcon-Barroso
Publication date
2007
fields
Physics
and research's language is
English
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In this proceeding we make use of the two-dimensional stellar kinematics of a representative sample of E and S0 galaxies obtained with the SAURON integral-field spectrograph to reveal that early-type galaxies appear in two broad flavours, depending on whether they exhibit clear large-scale rotation or not. We measure the level of rotation via a new parameter LambdaR and use it as a basis for a new kinematic classification that separates early-type galaxies into slow and fast rotators. With the aid of broad-band imaging we will reinforce this finding by comparing our kinematic results to the photometric properties of these two classes.
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We present an analysis of deep WSRT observations of the HI in 33 nearby early-type galaxies selected from a sample studied earlier at optical wavelengths with the SAURON integral-field spectrograph. The sample covers both field environments and the Virgo cluster. Our analysis shows that gas accretion plays a role in the evolution of field early-type galaxies, but less so for those in clusters. For detection limits of a few times 10^6 Msun, HI is detected in about 2/3 of the field galaxies, while <10% of the Virgo objects are detected. In about half of the detections, the HI forms a regularly rotating disc or ring. All HI discs have counterparts of ionised gas and inner HI discs are also detected in molecular gas. The cold ISM is dominated by molecular gas (M_H2/M_HI ~ 10). We conclude that accretion of HI is common for field early-type galaxies, but the amount of material involved is usually small. Cluster galaxies appear not to accrete HI. The few galaxies with a significant young sub-population all have inner gas discs, but for the remaining galaxies there is no trend between stellar population and HI. Some early-type galaxies are very gas rich, but only have an old population. The stellar populations of field galaxies are typically younger than those in Virgo. This is likely related to differences in accretion history. In about 50% of the galaxies we detect a central continuum source. In many objects this emission is from a low-luminosity AGN, in some it is consistent with the observed star formation. Galaxies with HI in the central regions are more likely detected in continuum. This is due to a higher probability for star formation to occur in such galaxies and not to HI-related AGN fuelling. (Abridged)
Observations of galaxy isophotes, longs-slit kinematics and high-resolution photometry suggested a possible dichotomy between two distinct classes of E galaxies. But these methods are expensive for large galaxy samples. Instead, integral-field spectroscopic can efficiently recognize the shape, dynamics and stellar population of complete samples of early-type galaxies (ETGs). These studies showed that the two main classes, the fast and slow rotators, can be separated using stellar kinematics. We showed there is a dichotomy in the dynamics of the two classes. The slow rotators are weakly triaxial and dominate above $M_{rm crit}approx2times10^{11} M_odot$. Below $M_{rm crit}$, the structure of fast rotators parallels that of spiral galaxies. There is a smooth sequence along which, the metals content, the enhancement in $alpha$-elements, and the weight of the stellar initial mass function, all increase with the CENTRAL mass density slope, or bulge mass fraction, while the molecular gas fraction correspondingly decreases. The properties of ETGs on galaxy scaling relations, and in particular the $(M_{ast}, R_{rm e})$ diagram, and their dependence on environment, indicate two main independent channels for galaxy evolution. Fast rotators ETGs start as star forming disks and evolve trough a channel dominated by gas accretion, bulge growth and quenching. While slow rotators assemble near the center of massive halos via intense star formation at high redshift, and remain as such for the rest of their evolution via a channel dominated by gas poor mergers. This is consistent with independent studies of the galaxies redshift evolution.
(Abridged) We explore the evolution of field early-type galaxies on a sample extracted from GOODS/CDFS. The galaxies are selected by means of a non-parametric analysis followed by visual inspection. We exclude those galaxies which are not consistent with an evolution into the Kormendy relation. The final set comprises 249 galaxies with a median redshift z=0.7. The distribution of number counts versus apparent magnitude suggests a substantial decrease of the comoving number density with redshift. The majority of the galaxies feature passively evolving old stellar populations. One third of those in the upper half of the redshift distribution have blue colors, in contrast to only 10% in the lower redshift subsample. An adaptive binning of the color maps is performed to explore the internal color distribution. We find that most blue galaxies in our sample feature blue cores whereas most of the red early-types are passively evolving stellar populations with red cores. The color gradients and scatter do not evolve with redshift and are compatible with the observations at z=0 assuming a radial dependence of the metallicity within each galaxy. This work emphasizes the need for a careful sample selection, as we found that most of those galaxies which were visually classified as early types -- but then rejected based on the Kormendy relation -- feature blue colors characteristic of recent star formation.
As a demonstration of the capabilities of the new Oxford SWIFT integral field spectrograph, we present first observations for a set of 14 early-type galaxies in the core of the Coma cluster. Our data consist of I- and z-band spatially resolved spectroscopy obtained with the Oxford SWIFT spectrograph, combined with r-band photometry from the SDSS archive for 14 early- type galaxies. We derive spatially resolved kinematics for all objects from observations of the calcium triplet absorption features at sim 8500 {AA} . Using this kinematic information we classify galaxies as either Fast Rotators or Slow Rotators. We compare the fraction of fast and slow rotators in our sample, representing the densest environment in the nearby Universe, to results from the ATLAS3D survey, finding the slow rotator fraction is sim 50 per cent larger in the core of the Coma cluster than in the Virgo cluster or field, a 1.2 {sigma} increase given our selection criteria. Comparing our sample to the Virgo cluster core only (which is 24 times less dense than the Coma core) we find no evidence of an increase in the slow rotator fraction. Combining measurements of the effective velocity dispersion {sigma_e} with the photometric data we determine the Fundamental Plane for our sample of galaxies. We find the use of the average velocity dispersion within 1 effective radius, {sigma_e}, reduces the residuals by 13 per cent with respect to comparable studies using central velocity dispersions, consistent with other recent integral field Fundamental Plane determinations.
Many early-type galaxies are detected at 24 to 160 micron but the emission is usually dominated by an AGN or heating from the evolved stellar population. Here we present MIPS observations of a sample of elliptical and lenticular galaxies which are rich in cold molecular gas, and we investigate how much of the MIR to FIR emission could be due to star formation activity. The 24 micron images show a rich variety of structures, including nuclear point sources, rings, disks, and smooth extended emission, and comparisons to matched-resolution CO and radio continuum images suggest that the bulk of the 24 micron emission can be traced to star formation. The star formation efficiencies are comparable to those found in normal spirals. Some future directions for progress are also mentioned.
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