NRL Press Release 51-10r - 19 Apr 2010
Scientists at the Naval Research Laboratory have solved a long-standing dilemma about the mass of infrared bright merging galaxies. Because galaxies are the largest directly observable objects in the universe, learning more about their formation is key to understanding how the universe works.
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Galaxies in the Universe generally come in two shapes, spiral, like our own Milky Way, and elliptical, in which the stars move in random orbits, Rothberg explains. The largest galaxies in the Universe are elliptical in shape and how they formed is central to our understanding how the Universe has evolved over the last 15 billion years. The long-standing theory has been that spiral galaxies merge with each other forming most of the elliptical galaxies in the Universe. Spiral galaxies contain significant amounts of cold hydrogen gas. When they merge, the beautiful spiral patterns are destroyed and the gas is converted into new stars. The more gas present in the spiral galaxies, the more stars are formed and with it, large amounts of dust. The dust is heated by the young stars and radiates energy at infrared wavelengths.
Until recently scientists thought that these infrared bright merging galaxies were not massive enough to be the precursors of most elliptical galaxies in the Universe. The problem lay in the method of measuring their mass. The conventional method of measuring mass in dusty IR-bright galaxies uses near-infrared light to measure the random motions of old-stars. The larger the random motions, the more mass is present. Using near-infrared light makes it possible to penetrate the dust and see as many of the old stars as possible. However, a complication occurs when spiral galaxies merge, because most of their gas is funneled to the gravitational center of the system and forms a rotating disk. This rotating disk of gas is transformed into a rotating disk of young stars that is also very bright at near-infrared wavelengths. The rotating disk of young stars both outshines the old stars and makes it appear as if the old stars have significantly less random motion. In contrast to this conventional method, Rothberg and Fischer instead observed the random motions of old stars at shorter wavelengths effectively using the dust to their advantage to block the light from the young stars. Their new results showed that the old stars in merging galaxies have large random motions, which means they will eventually become very massive elliptical galaxies.
The next step for NRL researches is to directly observe the stellar disks in IR luminous mergers using three-dimensional spectroscopy. Each pixel is a spectrum, and from this the researchers can make two-dimensional maps of stellar motion and stellar age. This will allow them to measure the size, rotation, luminosity, mass and age of the central disk.
Optical (left) and Infrared (right) image of an IR-bright merging galaxy. The center of the merger is obscured in the left image, but in the infrared (right) the central disk is very bright and easily seen. (Optical: Hubble Space Telescope / IR: University of Hawaii 88" telescope)
UNVEILING THE σ-DISCREPANCY IN INFRARED-LUMINOUS MERGERS. I. DUST AND DYNAMICS
- The Astrophysical Journal, Volume 712, Number 1, 2010 Mar 02, DOI: 10.1088/0004-637X/712/1/318
Mergers in the local universe present a unique opportunity for studying the transformations of galaxies in detail. Presented here are recent results, based on multi-wavelength, high-resolution imaging and medium resolution spectroscopy, which demonstrate how star formation and the presence of red supergiants and/or asymptotic giant branch stars have led to a serious underestimation of the dynamical masses of infrared-bright galaxies. The dominance of a nuclear disk of young stars in the near-infrared bands, where dust obscuration does not block their signatures, can severely bias the global properties measured in a galaxy, including mass. This explains why past studies of gas-rich luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies, which have measured dynamical masses using the 1.62 or 2.29 μm CO band heads, have found that these galaxies are forming m < m* ellipticals. On the other hand, precisely because of dust obscuration, I-band photometry and velocity dispersions obtained with the calcium II triplet at 0.85 μm reflect the global properties of the mergers and suggest that all types of merger remnants, including infrared-bright ones, will form m > m* ellipticals. Moreover, merger remnants, including LIRGs, are placed on the I-band fundamental plane for the first time and appear to be virtually indistinguishable from elliptical galaxies.