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Probing the host galaxy of a luminous quasar

July 1, 2008

Probing the Host Galaxy of a Luminous Quasar with Gemini Laser Adaptive Optics

Figure 1. Sloan Digital Sky Survey (SDSS) image of the distant host galaxy/quasar PG 1426+015. The field shown is about 50 x 50 arsec2. See Figure 4 for an image of the same field taken with the Hubble Space Telescope.

Figure 2. Arbitrarily scaled copies of the NIFS spectrum of the host galaxy of PG 1426+015 (black lines) with model fits (red lines) overlaid. The models of a M5 Ia supergiant (top spectrum) and a K5 III giant (bottom spectrum) have been broadened to match the width of the host-galaxy absorption features. The better fit with the supergiant spectrum is indicative of a young stellar population in the host galaxy. 

Figure 3. The MBH - σ* relationship for AGNs. Filled squares represent AGNs with velocity dispersion measurements based on the Ca II triplet. The open star at the top represents the Watson et al. measurement for PG 1426+015, while the open square to its immediate left is the position of PG 1426+015 in a previous study. The superior spectrum obtained using NIFS and Altair led to a more precise measurement of the velocity dispersion.

HST imaging of the core of PG1426+015

Figure 4. Hubble Space Telescope WFPC2 optical image of PG1426. The SDSS image in Figure 1 shows more of the host galaxy structure. However, in the SDSS image the core of the source is saturated. Here we see the central few arcseconds of the system where the central supermassive black hole powering the quasar is surmised to be. The 3" x 3" NIFS field-of-view covered the bright central source. The total size of the field shown is 22" x 17".

A team led by Linda C. Watson, Ph. D. student at the Department of Astronomy, Ohio State University, has made the first bulge stellar velocity dispersion measurement of a luminous quasar host with the Gemini North Laser Guide Star adaptive optics system. The target quasar was PG1426+015, about 1.36 billion light-years away (Figure 1). This result is also the basis for the 500th refereed paper based on Gemini observations.

They employed the spectro-imaging spectrograph NIFS fed by the Gemini facility adaptive optics system Altair to obtain high signal-to-noise ratio measurements of near-infrared Si I and Mg I lines and also several CO bandheads in the quasar host galaxy spectrum of PG 1426+15 (Figure 2). Using these measurements, the team found a stellar velocity dispersion of 217 +/- 15 kilometers/second in the region of the galaxy between 0.1'' and 1'' (520 light-years and 5,200 light-years) from the galaxy's center.

The NIFS integral field unit and the laser guide star adaptive optics system are well suited for host studies of luminous quasars. The bright quasar at the center of the galaxy outshines the stars in the host. But with adaptive optics, the authors could confine that diluting quasar light to the very center of the image and simply exclude it from their spectrum. The integral field unit then allowed them to gather more stellar light from the host galaxy than would have been possible with a normal single-slit spectral observation.

Using their derived velocity dispersion (σ*), the authors found that PG 1426+015 lies significantly above the MBH - σ* relation for active galactic nuclei (Figure 3), where MBH is the mass of the black hole powering the quasar. One possible reason for this is that high-luminosity quasars—like PG1426+015—might have disks of gas around their central black holes that have different inclinations compared to lower luminosity quasars. To determine whether all high-luminosity quasars lie above the MBH - σ* relation and why, the authors propose observations of more hosts of high-luminosity quasars with NIFS and Altair.

Finally, not only did the team make a precise measurement of the central bulge velocity dispersion, but they also found a surprisingly young stellar population in this quasar host.

For more details, see the article "First stellar velocity dispersion measurement of a luminous quasar host with Gemini North Laser Guide Star adaptive optics," by Linda C. Watson, Paul Martini, Kalliopi M. Dasyra, Misty C. Bentz, Laura Ferrarese, Bradley M. Peterson, Richard W. Pogge and Linda Tacconi, The Astrophysical Journal, 2008, Accepted. As noted earlier, this article is the 500th refereed Gemini paper. A pre-print of the article can be found on astro-ph at: http://arxiv.org/abs/0806.3271