Michele Cappellari

Abstract:

We present observations of a very massive galaxy at z = 1.82 that show that its morphology, size, velocity dispersion, and stellar population properties are fully consistent with those expected for passively evolving progenitors of today's giant ellipticals. These findings are based on a deep optical rest-frame spectrum obtained with the Multi-Object InfraRed Camera and Spectrograph on the Subaru Telescope of a high-z passive galaxy candidate (pBzK) from the COSMOS field, for which we accurately measure its redshift of z = 1.8230 and obtain an upper limit on its velocity dispersion σ* < 326 km s^-1. By detailed stellar population modeling of both the galaxy broadband spectral energy distribution and the rest-frame optical spectrum, we derive a star formation-weighted age and formation redshift of tsf ≃ 1-2 Gyr and zform ≃ 2.5-4, and a stellar mass of M * ≃ (3-4) × 10¹¹ M⊙. This is in agreement with a virial mass limit of Mvir < 7 × 10¹¹ M⊙, derived from the measured σ* value and stellar half-light radius, as well as with the dynamical mass limit based on the Jeans equations. In contrast to previously reported super-dense passive galaxies at z ∼ 2, the present galaxy at z = 1.82 appears to have both size and velocity dispersion similar to early-type galaxies in the local universe with similar stellar mass. This suggests that z 2 massive and passive galaxies may exhibit a wide range of properties, then possibly following quite different evolutionary histories from z ∼ 2 to z = 0. © 2010 The American Astronomical Society. All rights reserved.

Abstract:

We present an analysis of deep Westerbork Synthesis Radio Telescope observations of the neutral hydrogen in 33 nearby early-type galaxies selected from a representative sample studied earlier at optical wavelengths with the SAURON integral-field spectrograph. This is the deepest homogeneous set of H-i imaging data available for this class of objects. 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.The H-i properties of SAURON early-type galaxies strongly depend on environment. For detection limits of a few times 10⁶- M-, H-i is detected in about 2/3 of the field galaxies, while <10 per cent of the Virgo objects are detected. In about half of the detections, the H-i forms a regularly rotating disc or ring. In many galaxies unsettled tails and clouds are seen. All H-i discs have counterparts of ionized gas, and inner H-i discs are also detected in molecular gas. The cold interstellar medium (ISM) in the central regions is dominated by molecular gas (). Assuming our sample is representative, we conclude that accretion of H-i is very common for field early-type galaxies, but the amount of material involved is usually small and the effects on the host galaxy are, at most, subtle. Cluster galaxies appear not to accrete H-i, or the accreted material gets removed quickly by environmental effects. The relation between H-i and stellar population is complex. 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 H-i properties. A number of 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. There is no obvious overall relation between gas H-i content and global dynamical characteristics except that the fastest rotators all have an H-i disc. This confirms that if fast and slow rotators are the result of different evolution paths, this is not strongly reflected in the current H-i content. In about 50 per cent of the galaxies we detect a central radio continuum source. In many objects this emission is from a low-luminosity active galactic nucleus (AGN), and in some it is consistent with the observed star formation. Galaxies with H-i 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 H-i-related AGN fuelling. © 2010 The Authors. Journal compilation © 2010 RAS.