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Gas in Galaxies at the End of their Lives - Food for New Stars

Our knowledge and understanding of the cold gas in galaxies remains incomplete. Scientists at the Max Planck Institute for Astrophysics have now developed a new method to stack data from several hundred galaxies so that information about gas content can be extracted for galaxies that are difficult to detect individually. In their analysis, the researchers found that even old, elliptical galaxies still contain a disk of gas that occasionally lights up with young stars.

Fig. 1: A schematic representation of an ALFALFA 3-dimensional data-cube. Each pixel represents a position on the sky (defined by RA and Dec) and a velocity, where each data cube is 2.4 by 2.4 degrees in size and about 5500 km/s in velocity range (25 MHz in frequency). For each pixel a value of the flux density is recorded. For each target, we extract a spectrum over the velocity range from the data-cube at a given position of the sky. Two examples of extracted spectra are shown on the right, illustrating an HI detection (bottom) and an HI non-detection (top).
Image credit: Fabello, S. et al, 2011, MNRAS, 411, 993

Fig. 2: Example of stacked spectrum. Dotted lines show the boundaries of the signal, inside which we integrate the flux to compute the mass in atomic gas.
Image credit: Fabello, S. et al, 2011, MNRAS, 411, 993

Fig. 3: Examples of nearby elliptical galaxies that have been found to possess outer rings that are actively forming stars. In these images taken with the Hubble space telescope, ultraviolet light from young hot stars has been rendered in blue, while green and red light from the galaxies is shown in their natural colours.
Image credit: NASA/ESA /JPL-Caltech/STScI /UCLA

Fig. 4: A close-up of a simulation of a galaxy that is losing its gas through the process of ram-pressure stripping. In physics, ram pressure is the pressure exerted on a body which is moving through a fluid medium. It causes a strong drag force to be exerted on that body. In the case of a galaxy moving through the intergalactic gas, the ram pressure may be capable of stripping the galaxy of much of its interstellar gas. This will depend on the density of the ambient gas and the velocity of the body.
Image credit: University of Zurich

For technological reasons, studies of the cold neutral gas in galaxies have lagged far behind studies of the stars in galaxies. Stars emit radiation at optical wavelengths, but cold hydrogen gas in atomic form (HI) emits radiation at a wavelength of 21 cm and is only detectable at radio wavelengths.

The last decade has seen renewed efforts to carry out HI surveys over large areas of the sky using existing radio telescopes. The Arecibo Legacy Fast ALFA (ALFALFA) survey is the most advanced of these. When complete, ALFALFA will have detected more than 30,000 extragalactic HI line sources. However, ALFALFA is still shallow compared to state-of-the-art optical galaxy surveys, such as the Sloan Digital Sky Survey. As a result, most of the galaxies detected in such surveys are late-type spiral and irregular galaxies. Rather little is known about cold gas in early-type elliptical and lenticular galaxies.

In collaboration with researchers at Cornell University, MPA scientists developed a stacking tool that enabled them to measure the average atomic gas content of early-type galaxies. Spectra were extracted from the ALFALFA data "cubes" (see Figure 1) at the positions of elliptical galaxies identified in the Sloan Digital Sky Survey. Although the HI line was usually not detected in individual spectra, it did appear when the spectra were carefully superposed (see Figure 2). This allowed the MPA scientists to extract a measurement of the mean HI gas mass for the combined sample of galaxies.

Interestingly, the mean HI mass fraction does not drop below two per cent, even in the most massive and red elliptical galaxies. One question is whether this gas is associated with the central spheroidal component of the galaxy, or is spread over large radii in a disk or a ring. The 4 arc minute Arecibo beam means that the location of the gas is not known, but the MPA scientists used statistical techniques to argue that the gas is not associated with the central spheroid, but must be located in a disk. Interestingly, a population of elliptical galaxies with outer rings of star formation has now been identified by the Hubble Space Telescope (see Figure 3). Taken together, these observations suggest that elliptical galaxies do occasionally accrete gas from the external environment and form new stars.

MPA scientists also used their stacking tool to investigate atomic gas in galaxies residing in galaxy groups. It has long been known that galaxies residing in dense environments, such as groups and clusters, form stars less actively than isolated galaxies. There has been on-going debate about why this is so. One popular theory is that in dense environments, galaxies can no longer accrete gas (the fuel for star formation) from the external environment, and they simply starve to death over a timescale of a few billion years. New results from MPA scientists indicate that groups may be even more deadly to some galaxies than previously believed. Low mass galaxies appear to actually lose their cold atomic gas if they are in groups. This process is termed "ram-pressure stripping" (see Figure 4) and was previously believed to be effective only in very big clusters. If the MPA results are to be believed, small galaxies are advised to live in the countryside if they want to hang on to their food!

Future radio surveys will be hundreds of times more sensitive than ALFALFA and will map the gas in hundreds of thousands of galaxies over much of the sky. The scientists will thus obtain much more complete information about how galaxies acquire and lose their gas.

Silvia Fabello, Barbara Catinella, Guinevere Kauffmann

Relevant science papers:

Fabello, Silvia; Catinella, Barbara; Giovanelli, Riccardo; Kauffmann, Guinevere; Haynes, Martha P.; Heckman, Timothy M.; Schiminovich, David; "ALFALFA H I data stacking - I. Does the bulge quench ongoing star formation in early-type galaxies?", MNRAS, 411, Issue 2, pp. 993-1012 linkPfeilExtern.gif

Related links:

linkPfeilExtern.gifThe ALFALFA survey
linkPfeilExtern.gifThe Sloan Digital Sky Survey

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last modified: 2012-3-29