• Title: A Population of Dust-Rich Quasars at z ~ 1.5
• Authors:  Y. Sophia Dai, Jacqueline Bergeron, Martin Elvis, Alain Omont, Jia-Sheng Huang, Jamie Bock, Asantha Cooray, Giovanni Fazio, Evanthia Hatziminaoglou, Edo Ibar, Georgios E. Magdis, Seb J. Oliver, Mathew J. Page, Ismael Perez-Fournon, Dimitra Rigopoulou, Isaac G. Roseboom, Douglas Scott, Myrto Symeonidis, Markos Trichas, Joaquin D. Vieira, Christopher N. A. Willmer, Michael Zemcov
• First author’s institution: Harvard-Smithsonian Center for Astrophysics & Boston College

Summary
As we’ve mentioned in previous astrobites, astronomers are actively researching the connection between star formation, black hole accretion, and galaxy evolution. In this paper, Dai et al. investigate a sample of 32 quasars with redshifts between 0.5 – 3.6 to shed light on the situation. They find that 31 of the 32 quasars display an excess of emission at far-infrared wavelengths and have a relatively high dust mass. The far-infrared excess is due to emission from cool dust and indicates that the galaxies might be experiencing ongoing star formation in which baby stars are illuminating cocoons of surrounding dust. Alternatively, the far-infrared emission could originate from a disk or torus of dust surrounding the quasar. The data set does not allow Dai et al. to determine the relative contributions from star formation and quasar heating, but the authors are excited about the prospect of acquiring high-resolution imaging and near-infrared/millimeter spectroscopy to further probe the connection between star formation and active galaxy evolution.

The Sample
Dai et al. began their sample selection by selecting all of the targets in the Lockman Hole field of the Spitzer Wide-area Infrared Extragalactic Survey (SPIRE) with 24-micron flux above 0.4 milliJanskys (mJy). They then kept only those objects that had been observed at far-infrared wavelengths as part of the Herschel Multi-tiered Extragalactic Survey . The authors dropped all of the quasars that were confused with other sources or at redshifts below 0.5.

Modeling the Quasars

Rest-frame mean SEDs for the quasars with and without an observed far-infrared excess. The colored dots mark observations from GALEX (grey), SDSS (purple), UKIDSS (cyan), SWIRE-IRAC (green), SWIRE-MIPS (orange), and HerMES-SPIRE (red). The mean SEDs for each group are plotted in magneta and quasar templates from Elvis et al. 1994 (E94) and Richards et al. 2006 (R06) are shown as dark and light blue, respectively. The yellow regions mark the areas used to compute the optical, near-infrared, and far-infrared luminosities of the quasars. The far-infrared excess is the discrepancy between the magenta curve marking the mean SED of the FIR-detected sample and the dashed blue curves marking the R06 and E94 SEDs. Figure 3 from Dai et al. 2012.

The authors combined ultraviolet data from the Galaxy Evolution Explorer (GALEX) telescope, optical data from the Sloan Digital Sky Survey, near-infrared data from the UKIRT Infrared Deep Sky Survey (UKIDSS), mid-infrared data from Spitzer, and far-infrared data from Herschel to produce spectral energy distributions (SEDs) for all 32 quasars in their final sample. Prior to Dai et al., astronomers had constructed SEDs including far-infrared wavelengths for only 10 (sub)millimeter quasars, so Dai et al. have significantly increased the sample size.

The figure below displays the mean SED for the whole sample of quasars. Five of the quasars are well-matched by a model SED without a far-infrared excess, but the remaining 27 quasars are best-fit by a model including both an underlying blackbody and an excess at long wavelengths. To fit those objects, Dai et al. use the T-α–β model from Blain et al. (2003) to constrain the dust temperature and the quasar luminosity. They find that the dust temperature T varies between 18K and 80K (with a mean of 34K) and that the slope of the Wien tail (α) ranges between 0.68 and 2.44. A high value of α>2 corresponds to emission dominated by colder dust and a low value of α corresponds to emission dominated by warmer dust. Dai et al. find that most of the quasars are fit by a model with α between 1 and 2, so the galaxies likely contain a mixture of colder and warmer dust. The β in the T-α-β model refers to the emissivity of the dust; Dai et al. set β=2.0 for all models following Priddey et al. (2003).

Results & Conclusions

As discussed earlier, Dai et al. observe a far-infrared excess in 27 of the 32 quasars, indicating that those quasars are dust-rich. Dai et al. also note that quasars that are fainter in the near-infrared have larger far-infrared excesses. Given that the near-infrared flux is likely due to dust heated by an active galactic nucleus (AGN), the anti-correlation of the far-infrared/near-infrared flux ratio with near-infrared flux is consistent with the theory that intense AGN activity (which should result in high near-infrared flux) might hinder star formation.

Dai et al. also use the observed far-infrared flux to estimate the star formation rate in the quasar host galaxies. They find that the far-infrared flux of most of the sample could be explained by enhanced star formation rates, but that a few quasars require such high star formation rates (~5000 times higher than the rate observed in the Milky Way) that the far-infrared flux is more likely explained by a combination of star formation and AGN activity. Future observations at near-infrared and millimeter wavelengths will allow astronomers to place further constraints on the dust distribution and improve our understanding of the complex relationships between star formation, AGN activity, and galaxy evolution.

• About the Author

About Courtney Dressing

I am a fourth-year graduate student in the Astronomy Department at Harvard University. My research interests include exoplanets, habitability, and astrobiology. I received a master's degree in astronomy and astrophysics from Harvard University and a bachelor's degree in astrophysical sciences from Princeton University. At Princeton, I worked with Jill Knapp to study the magnetic activity of M dwarfs with white dwarf companions and with Dave Spiegel to model the habitability of terrestrial exoplanets. For my senior thesis, I worked with Ed Turner, Michael McElwain, and the SEEDS (Strategic Explorations of Exoplanets and Disks with Subaru) collaboration to directly image young Jovian exoplanets using the Subaru telescope. At Harvard, I am working with Dave Charbonneau to study the properties, frequency, and detectability of small planets orbiting small stars.

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