SCRIPT LANGUAGE="JavaScript" type="text/javascript"> Dr. Gilbert's M31 Research

The Andromeda Galaxy

A Testbed for Theories of Galaxy Formation

The Andromeda Galaxy (otherwise known as M31): our nearest spiral galaxy neighbor.
Credit and Copyright Robert Gendler.

Why care about stellar halos?

Current theories of galaxy formation contend that larger galaxies are built up through the merging of smaller systems over the lifetime of the universe. Stellar halos are sparse places, with long dynamical times, and the imprints of galaxy collisions (in the form of tidal streams) can be be identified for billions of years after a collision. This makes them ideal starting points for studying the applicability of current models of galaxy formation to the actual physical universe.

Why study Andromeda?

Andromeda, like our own Milky Way, is a large spiral galaxy. To study our own stellar halo's global properties, astronomers must contend with our position within the galaxy, which requires disentangling the various constituent parts (thin/thick disk, bulge) and extremely large surveys covering vast portions of the sky. At only 780 kpc distant, Andromeda is far enough away to give us a convenient global view, while close enough that we can take spectra of individual stars. Its disk is also highly inclined, enabling us to observe the inner parts of the spheroid by studying regions along the minor axis of the galaxy.

Until relatively recently, the stellar halos of Andromeda and the Milky Way were thought to be very different. Andromeda's stellar halo was found to be more metal-rich than the Milky Way's halo, and its density was thought to fall off much more steeply. In fact, Andromeda's "stellar halo" looked much more like an extension of its bulge than a distinct spheroidal component, as we see in the Milky Way.

SPLASH: Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo

The SPLASH collaboration has undertaken an extensive photometric and spectroscopic survey of Andromeda's stellar halo using multiple telescopes and instruments, including the Mosaic camera on the Kitt Peak National Observatory 4 m telescope, the MegaCam camera on the 3.6 meter Canada-France-Hawaii Telescope, and the DEIMOS spectrograph on the 10 meter Keck II Telescope. In addition to obtaining photometry and spectra in stellar halo fields ranging from 8 to 180 kpc in projected distance from the center of Andromeda, we have also targeted many of Andromeda's dwarf galaxies and discovered a new dwarf spheroidal, Andromeda XIV (Majewski et al. 2007).

M31's extended, metal-poor halo

In order to be able to identify individual stars as red giant branch (RGB) stars in Andromeda's stellar halo (rather than Milky Way dwarf stars along our line of sight), I developed a series of spectroscopic and photometric diagnostics that allows us to give each star a probability of being an Andromeda red giant branch star. This has given us the sensitivity needed to probe the properties of Andromeda's stellar halo to such great distances. For example, in a field located 165 kpc in projection from Andromeda's center, we use this method to identify 3 (out of more than 150) stars that are secure RGB stars in M31 (Gilbert et al. 2006).

The ability to identify individual stars as M31 RGB stars or MW dwarf stars has led directly to our group's discovery of M31's extended, power-law, metal-poor halo (Guhathakurta et al. 2005 and Kalirai et al. 2006), and to the realization that it is the inner parts of the MW and M31 spheroids that differ, while the outer parts of the spheroids of the MW and M31 are quite similar (density proportional to ~r^-3 and metal-poor).

How big is M31's halo?

As we have found M31 RGB stars in every field in which we have taken spectra, we don't know the full extent of M31's halo. We can confidently trace it out to 180 kpc from the center of the galaxy, which corresponds to over 580,000 light-years. If you could see M31's stellar halo with your naked eyes, Andromeda would be at least as big as 50 full moons across! It would stretch 1/3 of the way from the horizon to the zenith, and be approximately as big as the Big Dipper.

The inner spheroid of M31

We have used the technique briefly discussed above to spectroscopically identify M31 RGB stars in the inner parts of the spheroid (8-30 kpc in projection from the center of the galaxy, along the minor axis) without using radial velocity as a selection criterion. This has given us a kinematically unbiased sample of M31 RGB stars. We use this sample to measure the velocity dispersion of M31's spheroid in this radial range (130 km/s). Our data also show evidence of a kinematically cold population, which we identify as the forward continuation of M31's Giant Southern Stream (see Gilbert et al. 2007). This material likely represents tidal debris which is approaching its fourth pericentric passage. The observational constraints available from tracing a single tidal debris feature through as many as four pericentric passages will enable detailed modeling of M31's mass distribution.

The Future

My long term goals are to establish the extent, shape, and global properties of M31's stellar halo, and to compare the observations to detailed numerical simulations of stellar halo formation in a cosmological context. Are stellar halos nothing but vast graveyards, composed of the superposition of trails of debris from numerous minor mergers over the lifetime of the galaxy, or do stellar halos have a smoother underlying component, formed in situ? The answer to this question will require detailed comparisons of large-scale observations with highly sophisticated simulations of stellar halo formation.

Andromeda in the News!

Extended Stellar Halo

Science Daily
The Santa Cruz Sentinel

Evidence of a Galactic Collision (Gilbert et al. 2007)

Science Daily
Featured in the Research Highlights section of Nature, 2007, vol. 449, 7163, p. 640
And on Public Radio (StarDate)!

Related Publications

For an up to date list of related refereed and non-refereed publications, visit ADS.