Hubble Space Telescope Images of Post-Starburst QuasarsHubble Space Telescope Images of Post-Starburst QuasarsMichael S. BrothertonMichael S. Brotherton, S. Cales, R. Ganguly, Z. Shang (, S. Cales, R. Ganguly, Z. Shang (University of WyomingUniversity of Wyoming))

G. Canalizo (G. Canalizo (University of California at Riverside)University of California at Riverside) ABSTRACT: We report the results of a Hubble Space Telescope snapshot survey of 29 post-ABSTRACT: We report the results of a Hubble Space Telescope snapshot survey of 29 post-starburst quasars at redshifts between 0.25 and 0.4. The broadband F606W ACS images starburst quasars at redshifts between 0.25 and 0.4. The broadband F606W ACS images resolve morphological detail in nearly all targets and indicate that interactions and merger resolve morphological detail in nearly all targets and indicate that interactions and merger activity are common for this class. Some spectacular images include double nuclei, activity are common for this class. Some spectacular images include double nuclei, starbursting ring systems, and nuclear dust rings. The images will be used to constrain starbursting ring systems, and nuclear dust rings. The images will be used to constrain spectral modeling to separate AGN and stellar components. We are investigating the spectral modeling to separate AGN and stellar components. We are investigating the hypothesis that post-starburst quasars represent the stage of massive galaxy evolution hypothesis that post-starburst quasars represent the stage of massive galaxy evolution known as "blow out."known as "blow out."

Relevance

Recently evidence for an ``AGN-starburst'' connection has become quantitative and compelling in the form of the MBH-sigma correlation (e.g. Tremaine et al. 2002). Stated more physically, essentially every large galaxy harbors a black hole some 0.15% of the mass of its spheroidal component.

Theorists have proposed various models, with those involving hierarchical galaxy evolution via merger activity, and feedback from the central AGN to control star formation becoming popular. Sophisticated simulations of galaxy mergers now exist (e.g. di Matteo et al. 2005) and the process proceeds through observable phases. Hopkins et al. (2008) shows this evolutionary sequence: isolated systems, small group, interaction/merger, coalescence/(U)LIRG, “Blowout,” Quasar, Decay (post-starburst galaxy), ending with a dead elliptical. Hopkins identifies IRAS-selected quasars as objects at blow out, and some do have suggestive properties. We’re investigating the alternative notion that post-starburst quasars are objects at blow out.

Results & Discussion

The first striking result for this spectroscopically selected sample is that interactions and merging are extremely common. Close inspection reveals that most of the isolated systems show asymmetries, tidal tails, shells, or other features indicative of mergers.

These objects represent the most extreme examples of recent star formation and black hole growth in the semi-local universe. They appear consistent with a hierarchical evolutionary scheme, but indicate that the simplest versions require modification. The AGN activity can appear at different epochs, all the way from pre-merger to post-merger.

There are a few isolated and apparently undisturbed systems (e.g., a few barred spirals) that may represent the transition to “Seyfert mode” fueling that creates both stars and nuclear activity, but at the lower levels more consistent with the local neighborhood.

We acknowledge support from the US National Science Foundation through grant AST 05-07781 and NASA through grant NNG05GE84G.

References

Brotherton et al. 1999, ApJ, 540, L87

Brotherton et al. 2002, PASP, 114, 593

Canalizo et al. 2000, AJ, 119, 59

Di Matteo et al. 2005, Nature, 433, 604

Hopkins et al. 2008, ApJS, 175, 356

Schneider et al. 2002, AJ, 123, 567

Tremaine et al. 2002, ApJ, 574, 740

Zabludoff et al. 1996, ApJ, 466, 104

Figure 2: Our ACS F606W snapshot images, 10 arcseconds on a side (which is about 50 kpc at these redshifts). Note double nuclei, star-forming knots, and other structures too small to resolve from the ground such as nuclear dust lanes. Faint tidal tails and shells are apparent in many images, too.

Fig.3. A comparison of ground-based SDSS imaging (left) with HST imaging (right). The SDSS rest-frame spectrum is shown below.

Fig.1. UN J1025-0040 from Brotherton et al. (1999). The Keck spectrum is modeled as an instantaneous starburst plus an AGN. The Keck K-band image shows extended, asymmetric fuzz with a nearby companion (verified also at z=0.634).

Introduction and Observations

The rare class of post-starburst quasars show composite spectra with both the broad lines of an AGN and the Balmer jumps and absorption lines that are the signatures of a massive luminous starburst a few 100 Myr old,

A striking case is UN J1025-0040 at z=0.634 (Fig. 1; Brotherton et al. 1999). The starburst component has an age of 400 Myr, and a bolometric luminosity of 1011.6 solar, equal to that of the quasar. Keck and HST imaging shows a merger remnant (Brotherton et al. 1999, 2002). A nearby companion galaxy is also in a post-starburst phase (Canalizo et al. 2000). UN J1025-0040 is consistent being an evolved Ultra Luminous IR Galaxy (ULIRG).

The new large quasar surveys like the Sloan Digital Sky Survey (SDSS, e.g., Schneider et al. 2002), are revealing post-starburst quasars in significant numbers for the first time. We have spectroscopically selected some 600 post-starburst quasars in SDSS data release 3 (DR3) using an automated algorithm based on one that Zabludoff et al. (1996) used to select post-starburst galaxies based on Balmer jumps and high-order absorption lines.

SDSS images show that about 40% have close companions, and many single sources show fuzz or tidal features, reminiscent of the post-starburst galaxies which are uniformly interacting systems.

We recently completed an ACS SNAP program obtaining F606W images of 29 post-starburst SDSS DR3 quasars, a subsample of a catalog selected with r < 18.6, H-delta absorption equivalent widths > 1A, 0.25 < z < 0.4 (ensuring high luminosity and similar size scales, resolving structures a half kpc across). Using the F606W filter longward of the Balmer jump maximizes the contrast of the stellar component against the AGN core, and reveals the morphology of the host galaxies.

Fig. 2 shows our HST images. Fig. 3 compares HST and SDSS images for one example post-starburst quasar.