Michele Cappellari

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.

Abstract:

We study resolution effects in numerical simulations of gas-rich and gas-poor major mergers, and show that the formation of slowly rotating elliptical galaxies often requires a resolution that is beyond the present-day standards to be properly modelled. Our sample of equal-mass merger models encompasses various masses and spatial resolutions, ranging from about 200 pc and 10⁵ particles per component (stars, gas and dark matter), i.e. a gas mass resolution of ∼10⁵ M⊙, typical of some recently published major merger simulations, to up to 32 pc and ∼10³ M⊙ in simulations using 2.4 × 10⁷ collisionless particles and 1.2 × 10⁷ gas particles, among the highest resolutions reached so far for gas-rich major merger of massive disc galaxies. We find that the formation of fast-rotating early-type galaxies, that are flattened by a significant residual rotation, is overall correctly reproduced at all such resolutions. However, the formation of slow-rotating early-type galaxies, which have a low-residual angular momentum and are supported mostly by anisotropic velocity dispersions, is strongly resolution-dependent. The evacuation of angular momentum from the main stellar body is largely missed at standard resolution, and systems that should be slow rotators are then found to be fast rotators. The effect is most important for gas-rich mergers, but is also witnessed in mergers with an absent or modest gas component (0-10 per cent in mass). The effect is robust with respect to our initial conditions and interaction orbits, and originates in the physical treatment of the relaxation process during the coalescence of the galaxies. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of gas-rich mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated. Moreover, the effect of gas in a galaxy merger is not limited to helping the survival/rebuilding of rotating disc components: at high resolution, gas actively participates in the relaxation process and the formation of slowly rotating stellar systems. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

We study resolution effects in numerical simulations of gas-rich (20% of the total baryonic mass) major mergers, and show that the formation of slowly-rotating elliptical galaxies requires a resolution that is beyond the present-day standards to be properly modelled. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of wet mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated. © 2010 American Institute of Physics.

Abstract:

In this short paper, we present a few preliminary results from the ambitious ATLAS^3D project, which intends to probe the first volume-limited sample of early-type galaxies observed via multi-band photometry, integral-field spectroscopy, radio and millimeter observations, and supported by a large library of numerical simulations and models. We more specifically address the existence of two main families of early-type galaxies, the slow and fast rotators. © 2010 American Institute of Physics.

Abstract:

We examine the nuclear morphology, kinematics, and stellar populations in nearby S0 galaxy NGC 404 using a combination of adaptive optics assisted near-IR integral-field spectroscopy, optical spectroscopy, and Hubble Space Telescope imaging. These observations enable study of the NGC 404 nucleus at a level of detail possible only in the nearest galaxies. The surface brightness profile suggests the presence of three components: a bulge, a nuclear star cluster (NSC), and a central light excess within the cluster at radii < 3 pc. These components have distinct kinematics with modest rotation seen in the NSC and counter-rotation seen in the central excess. Molecular hydrogen emission traces a disk with rotation nearly orthogonal to that of the stars. The stellar populations of the three components are also distinct, with half of the mass of the NSC having ages of ∼ 1 Gyr (perhaps resulting from a galaxy merger), while the bulge is dominated by much older stars. Dynamical modeling of the stellar kinematics gives a total NSC mass of 1.1 × 10⁷ M ⊙. Dynamical detection of a possible intermediate-mass black hole (BH) is hindered by uncertainties in the central stellar mass profile. Assuming a constant mass-to-light ratio, the stellar dynamical modeling suggests a BH mass of <1 × 10⁵ M ⊙, while the molecular hydrogen gas kinematics are best fitted by a BH with a mass of 4.5^+3.5 -2.0 × 10⁵ M ⊙. Unresolved and possibly variable dust emission in the near-infrared and active galactic nucleus-like molecular hydrogen emission-line ratios do suggest the presence of an accreting BH in this nearby LINER galaxy. © 2010. The American Astronomical Society. All rights reserved.