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Abstract:

We propose a simple model for the origin of fast and slow rotator early-type galaxies (ETG) within the hierarchical Λcold dark matter (ΛCDM) scenario, that is based on the assumption that the mass fraction of stellar discs in ETGs is a proxy for the specific angular momentum expressed via λR. Within our model we reproduce the fraction of fast and slow rotators as a function of magnitude in the ATLAS3D survey, assuming that fast-rotating ETGs have at least 10 per cent of their total stellar mass in a disc component. In agreement with ATLAS3D observations we find that slow rotators are predominantly galaxies with M* > 1010.5M⊙ contributing ~20 per cent to the overall ETG population. We show in detail that the growth histories of fast and slow rotators are different, supporting the classification of ETGs into these two categories. Slow rotators accrete between ~50 and 90 per cent of their stellar mass from satellites and their most massive progenitors have on average up to three major mergers during their evolution. Fast rotators in contrast accrete less than 50 per cent and have on average less than one major merger in their past. We find that the underlying physical reason for the different growth histories is the slowing down and ultimately complete shut-down of gas cooling in massive galaxies. Once cooling and associated star formation in disc stop, galaxies grow via infall from satellites. Frequent minor mergers thereby destroy existing stellar discs via violent relaxation and also tend to lower the specific angular momentum of the main stellar body, lowering λR into the slow rotator regime. On average, the last gas-rich major merger interaction in slow rotators happens at z > 1.5, followed by a series of minor mergers. These results support the idea that kinematically decoupled cores (KDC) form during gas-rich major mergers at high z followed by minor mergers, which build-up the outer layers of the remnant, and make remnants that are initially too flat compared to observations become rounder. Fast rotators are less likely to form such KDCs due to the fact that they have on average less than one major merger in their past. Fast rotators in our model have different formation paths. The majority, 78 per cent, has bulge-to-total stellar mass ratios (B/T) > 0.5 and managed to grow stellar discs due to continued gas cooling or bulges due to frequent minor mergers. The remaining 22 per cent live in high-density environments and consist of low B/T galaxies with gas fractions below 15 per cent, that have exhausted their cold gas reservoir and have no hot halo from which gas can cool. These fast rotators most likely resemble the flattened disc-like fast rotators in the ATLAS3D survey. Our results predict that ETGs can change their state from fast to slow rotator and vice versa, while the former is taking place predominantly at low z (z < 2), the latter is occurring during cosmic epochs when cooling times are short and galaxies gas-rich. We predict that the ratio of the number density of slow to fast rotators is a strong function of redshift, with massive (>1010M⊙) fast rotators being more than one order of magnitude more frequent at z~ 2. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

We present an exploration of the integrated stellar populations of early-type galaxies (ETGs) from the ATLAS3D survey. We use two approaches: firstly the application of line-indices interpreted through single stellar population (SSP) models, which provide a single value of age, metallicity and abundance ratio. And secondly, by fitting a linear combination of SSP spectra to our data, smoothly weighted in the free parameters of age and metallicity, thereby inferring a star-formation history of these galaxies. Despite the significant differences in these approaches, we obtain generally consistent results, such that galaxies that are more massive appear older with enhanced abundance ratios using line indices, and have shorter star-formation histories weighted to early times. We highlight two limitations of the index-SSP approach. Firstly the SSP-equivalent ages belie the fact that ETGs are overwhelmingly composed of ancient stars. Secondly, the young stellar contributions implied in our star formation histories are required to obtain realistic UV-optical colours. We remark that, even fitting solar-abundance models, we can recover a star-formation duration that correlates with the measured alpha-enhancement, in agreement with other recent work. © 2012 International Astronomical Union.

Abstract:

We report the discovery of a powerful molecular wind from the nucleus of the non-interacting nearby S0 field galaxy NGC 1266. The single-dish CO profile exhibits emission to 400kms-1 and requires a nested Gaussian fit to be properly described. Interferometric observations reveal a massive, centrally concentrated molecular component with a mass of 1.1 × 109 M and a molecular outflow with a molecular mass of 2.4 × 107 M . The molecular gas close to the systemic velocity consists of a rotating, compact nucleus with a mass of about 4.1 × 108 M within a radius of 60pc. This compact molecular nucleus has a surface density of 2.7 × 104 M pc-2, more than two orders of magnitude larger than that of giant molecular clouds in the disk of the Milky Way, and it appears to sit on the Kennicutt-Schmidt relation despite its extreme kinematics and energetic activity. We interpret this nucleus as a disk that confines the outflowing wind. A mass outflow rate of 13 M yr-1 leads to a depletion timescale of ≲85 Myr. The star formation in NGC 1266 is insufficient to drive the outflow, and thus it is likely driven by the active galactic nucleus. The concentration of the majority of the molecular gas in the central 100pc requires an extraordinary loss of angular momentum, but no obvious companion or interacting galaxy is present to enable the transfer. NGC 1266 is the first known outflowing molecular system that does not show any evidence of a recent interaction. © 2011. The American Astronomical Society. All rights reserved..