51ÁÔÆæÈë¿Ú

Abstract:

We use interferometric 12CO(1-0) observations to compare and contrast the extent, surface brightness profiles and kinematics of the molecular gas in CO-rich ATLAS3D early-type galaxies (ETGs) and spiral galaxies. We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scalelengths. Amongst ETGs, we find that the extent of the gas is independent of its kinematic misalignment (with respect to the stars), but does depend on the environment, with Virgo cluster ETGs having less extended molecular gas reservoirs, further emphasizing that cluster ETGs follow different evolutionary pathways from those in the field. Approximately half of ETGs have molecular gas surface brightness profiles that follow the stellar light profile. These systems often have relaxed gas out to large radii, suggesting they are unlikely to have had recent merger/accretion events. A third of the sample galaxies show molecular gas surface brightness profiles that fall off slower than the light, and sometimes show a truncation. These galaxies often have a low mass, and eitherhave disturbed molecular gas or are in the Virgo cluster, suggesting that recent mergers, ram pressure stripping and/or the presence of hot gas can compress/truncate the gas. The remaining galaxies have rings, or composite profiles, that we argue can be caused by the effects of bars. We investigated the kinematics of the molecular gas using position-velocity diagrams, and compared the observed kinematics with dynamical model predictions, and the observed stellar and ionized gas velocities. We confirm that the molecular gas reaches beyond the turnover of the circular velocity curve in~70 per cent of our CO-rich ATLAS3D ETGs, validating previous work on the CO Tully-Fisher relation. In general we find that in most galaxies the molecular gas is dynamically cold, and the observed CO rotation matches well model predictions of the circular velocity. In the galaxies with the largest molecular masses, dust obscuration and/or population gradients can cause model predictions of the circular velocity to disagree with observations of the molecular gas rotation; however, these effects are confined to the most star forming systems. Bars and non-equilibrium conditions can also make the gas deviate from circular orbits. In both these cases, one expects the model circular velocity to be higher than the observed CO velocity, in agreement with our observations. Molecular gas is a better direct tracer of the circular velocity than the ionized gas, justifying its use as a kinematic tracer for Tully-Fisher and similar analyses.

Abstract:

We report the discovery of a giant HI tail in the intragroup medium of HCG 44 as part of the ATLAS3D survey. The tail is ~300 kpc long in projection and contains ~5 × 108 M ⊙of HI. We detect no diffuse stellar light at the location of the tail down to ~28.5 mag arcsec-2 in g band. We speculate that the tail might have formed as gas was stripped from the outer regions of NGC 3187 (a member of HCG 44) by the group tidal field. In this case, a simple model indicates that about 1/3 of the galaxy's HI was stripped during a time interval of <1 Gyr. Alternatively, the tail may be the remnant of an interaction between HCG 44 and NGC 3162, a spiral galaxy now ~650 kpc away from the group. Regardless of the precise formation mechanism, the detected HI tail shows for the first time direct evidence of gas stripping in HCG 44. It also highlights that deep HI observations over a large field are needed to gather a complete census of this kind of events inthe local Universe.©2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

In order to try and understand its origins, we present high-quality long-slit spectral observations of the counter-rotating stellar discs in the strange S0 galaxy NGC 4550. We kinematically decompose the spectra into two counter-rotating stellar components (plus a gaseous component), in order to study both their kinematics and their populations. The derived kinematics largely confirm what was known previously about the stellar discs, but trace them to larger radii with smaller errors; the fitted gaseous component allows us to trace the hydrogen emission lines for the first time, which are found to follow the same rather strange kinematics previously seen in the [OIII] line. Analysis of the populations of the two separate stellar components shows that the secondary disc has a significantly younger mean age than the primary disc, consistent with later star formation from the associated gaseous material. In addition, the secondary disc is somewhat brighter, also consistent with such additional star formation. However, these measurements cannot be self-consistently modelled by a scenario in which extra stars have been added to initially identical counter-rotating stellar discs, which rules out the Evans & Collett's elegant 'separatrix-crossing' model for the formation of such massive counter-rotating discs from a single galaxy, leaving some form of unusual gas accretion history as the most likely formation mechanism. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.