HI in Early-type galaxies – HIPASS, Sauron and ATLAS3D projects
In recent years, the analysis of the structure, stellar populations and gas content of early-type galaxies has revealed that these objects are much less “boring” than previously thought. For example, they harbor multiple kinematical structure and show signs of recent star formation. Furthermore, while traditionally considered to be gas-poor, early-type galaxies have been discovered to host a significant amount of gas in a variety of phases. This gas makes up an important reservoir of fuel for star formation, at odds with the old idea that early-type galaxies evolve passively.
Our work started with early-type galaxies in the southern hemisphere, including objects part of the HIPASS survey. See:
Extended, regular HI structures around early-type galaxies, 2007 A&A 465, 787 Oosterloo, T. A.; Morganti, R.; Sadler, E. M.; van der Hulst, T.; Serra, P.
The H I-Rich Elliptical Galaxy NGC 5266 Morganti R., Sadler E., Oosterloo T., Pizzella A., Bertola F., 1997, AJ, 113,937
Tidal Remnants and Intergalactic HII Regions, Oosterloo et al. 2004 (astro-ph/0310632)
Extended HI disks in dust-lane elliptical galaxies, T.A. Oosterloo, R. Morganti, E.M. Sadler, D. Vergani, N. Caldwell, 2002, AJ 123, 729
HI in four star-forming E/S0 galaxies Sadler E.M., Morganti R., Oosterloo T.A., Karakas A. 2000, , AJ, 119, 1180
NGC5266 is an E4 elliptical galaxy (green in figure) with a prominent dust lane seen almost edge-on (not visible in the image). Observations with ATCA show that it has a disk of neutral hydrogen, perpendicular to the dust lane, extending to almost 10 R_e (red in figure). The estimated HI mass in NGC 5266 is M_HI ~ 2 x 10^10 Mo, a very large amount for an elliptical galaxy (for details see the full paper Morganti et al. 1997, AJ, 113,937).
The work on HI in early-type galaxies has continued in the northern hemisphere using the Westerbork Synthesis Radio Telescope. The work on the SAURON sample shows that the HI detection rate of early-type galaxies is a strong function of environment, with cluster galaxies being virtually always HI-poor. On the contrary, more than half of the galaxies outside the cluster environment host some HI, with a much larger variety of gas configurations than in disc galaxies. For more details see Morganti et al. (2006).
Right: Images representing the motion of gas and stars in the galaxy NGC4278. The red regions are moving away from us, the blue are approaching us. The figure represent a nice example of complementary data based on dutch instruments. On the left the neutral hydrogen as measured by the WSRT, on the right the ionized gas (bottom) and stars (top) as measured by the integral field spectrograph SAURON on the William Herschel Telescope on La Palma. Note the different scales that the two instruments can image: the SAURON data cover the very inner part of the HI image (black box). The results of this study are presented in a paper accepted for publication in MNRAS, Morganti, de Zeeuw, Oosterloo et al.2006 Neutral hydrogen in nearby elliptical and lenticular galaxies: the continuing formation of early-type galaxies and Oosterloo et al. 2010 Early-type in different environments: an HI view.
This work is now being extended to a very large, complete sample of nearby early-type galaxies as part of the Atlas3D project. The Atlas3D WSRT HI survey is building the most complete, homogeneous database of deep neutral hydrogen synthesis imaging of early-type galaxies (the picture shows a few examples with HI contours in red overlaid on an optical image in blue). The scientific exploitation of these data is made yet more exciting by the possibility of combining them with optical imaging and spectroscopy, CO single-dish and interferometric obervations, as well as N-body and semi-analytic models of galaxy formation.
Atlas3D is an ambitious survey aimed at studying early-type galaxies (see The ATLAS3D project – I. A volume-limited sample of 260 nearby early-type galaxies: science goals and selection criteria, Cappellari et al. 2011). Within this project we find hints that the morphology-density relation is driven by the “local” environment of a galaxy — what really matters is the number of neighbours within a few 100’s kpc from it. One of these results comes from a large amount of Westerbork data taken in order to study the neutral-hydrogen (HI) gas content of early-type galaxies. The results can be found in The ATLAS3D project – XIII. Mass and morphology of H I in early-type galaxies as a function of environment ,Serra et al. 2012.
We find that early-type galaxies in poor “local” environments host typically giant HI discs with radius up to many tens of kpc. These systems are very regular, indicating that the host galaxy has enjoyed a quiet life for a very long time. The situation is very different in richer environments like galaxy groups and at the outskirts of galaxy clusters. Here HI exhibits typically a very disturbed morphology. In many cases long HI tails stretch from the host galaxy into the surrounding space, demonstrating that some gas may have recently been removed from (or accreted onto) the galaxy. Early-type galaxies in these environments are evolving because of the interaction with what is around them. At even denser environment densities, in the very centre of clusters, hardly any HI is found. Galaxies live close to each other and immersed in a hot medium, which makes it very easy for them to lose their HI and very hard to re-accrete some.