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  Current Research Highlight :: December 2006 all highlights

The Outer Disks of Galaxies Observed by GALEX

The Galaxy Evolution Explorer (GALEX) satellite was launched by NASA in April 2003. It is an orbiting space telescope that is observing galaxies in ultraviolet (UV) light across 10 billion years of cosmic history (Figure 1). UV light is emitted primarly by massive young stars and it can hence be used as a very sensitive probe of recent star formation in a galaxy. However, UV radiation is almost entirely absorbed by ozone in the upper atmosphere of the Earth so astronomers must obtain their observations from space.

Fig. 1: An artists impression of GALEX in orbit around the Earth. The Solar panels are deployed and the telescope cover is open, nominal operations are proceeding.

Fig. 2: This image highlights the hidden spiral arms (blue) that were discovered around the nearby galaxy NGC 4625 by the ultraviolet eyes of NASA's Galaxy Evolution Explorer.

Fig. 3: Correlations between NUV-r (an indicator of the age of stars in the outer disk of the galaxy), D$_n$(4000) (an indicator of stellar age in the inner galaxy), and L[OIII]/$M_{BH}$ (an indicator of accretion rate onto the black hole).

Guinevere Kauffmann, astronomer from the Max Planck Institute for Astrophysics, in collaboration with colleagues from the USA, France and Korea, has been analyzing GALEX data in order to learn about the recent star formation histories of galaxies in the local Universe. The analysis has been carried out with galaxies that were observed both by GALEX and by the linkPfeilExtern.gifSloan Digital Sky Survey, a very large optical survey of almost a million nearby galaxies. The galaxy spectra obtained by the SDSS yield distances, luminosities and other physical characteristics of the galaxy (see linkPfeil.gif Research Highlight April 2002), while the GALEX data are useful for locating young, hot stars.

Traditionally, galaxies in the nearby Universe have been divided into two major classes: 1) elliptical galaxies, which have massive spheroids and red optical colours, and 2) spiral galaxies, which have actively star-forming blue disks. One surprise discovery from GALEX is that many early-type galaxies that look "red and dead" when viewed in optical light, turn out to have very extended outer disks that are only visible in the ultra-violet. An example of such a galaxy is shown in Figure 2. The galaxy NGC 4265 only showed an oval-shaped ball of light when observed in visible light. In UV light one sees a disk with a beautiful set of spiral arms (see linkPfeilExtern.gif for a press release about this galaxy).

The work of Kauffmann and collaborators extends this result to the entire population of elliptical galaxies in the local Universe. The analysis demonstrates that UV disks are a common phenomenon among this population. Interestingly, all elliptical galaxies where the central supermassive black hole is growing at an appreciable rate (see linkPfeil.gif Research Highlight July 2004) have outer UV-bright disks (Figure 3). This suggests that the disks contain a reservoir of gas necessary for ongoing growth of the central spheroids and supermassive black holes. The disks are subject to dynamical instabilities such as warps and bars, and gas will then flow inwards towards the nucleus of the galaxy.

So how do the outer UV disks discovered by GALEX form? In the standard theoretical scenario, disks form by the condensation of gas in gravitationally dominant dark matter halos. If the gas initially has the same angular momentum as the dark matter and conserves this angular momentum during its contraction, the distribution of disk sizes can be predicted. These predictions are found to be in good agreement with the observed sizes of spiral galaxies. Future work will determine whether the same is true of the UV disks around ellipticals.

Guinevere Kauffmann


GALEX (Galaxy Evolution Explorer) is a NASA Small Explorer, launched in April 2003. We gratefully acknowledge NASA's support for construction, operation, and science analysis for the GALEX mission, developed in cooperation with the Centre National d'Etudes Spatiales (CNES) of France and the Korean Ministry of Science and Technology.


Guinevere Kauffmann, Timothy M. Heckman, Tamas Budavari, Stephane Charlot, Charles G. Hoopes, D. Christopher Martin, Mark Seibert, Tom A. Barlow, Luciana Bianchi, Tim Conrow, Jose Donas, Karl Forster, Peter G. Friedman, Young-Wook Lee, Barry F. Madore, Bruno Milliard, Patrick F. Morrissey, Susan G. Neff, R. Michael Rich, David Schiminovich, Todd Small, Alex S. Szalay, Ted K. Wyder, S.K. Yi:
"Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX", linkPfeilExtern.gifastro-ph/0609436

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last modified: 2006-11-27