A Generalized Power-law Diagnostic for Infrared Galaxies at z > 1: Active Galactic Nuclei and Hot Interstellar Dust

K. I. Caputi*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

11 Citations (Scopus)

Abstract

5 I present a generalized power-law (PL) diagnostic which allows one to identify the presence of active galactic nuclei (AGNs) in infrared (IR) galaxies at z > 1, down to flux densities at which the extragalactic IR background is mostly resolved. I derive this diagnostic from the analysis of 174 galaxies with S-nu(24 mu m) > 80 mu Jy and spectroscopic redshifts z(spec) > 1 in the Chandra Deep Field South, for which I study the rest-frame UV/optical/near-IR spectral energy distributions (SEDs), after subtracting a hot-dust, PL component with three possible spectral indices alpha = 1.3, 2.0, and 3.0. I obtain that 35% of these 24 mu m sources are power-law composite galaxies (PLCGs), which I define as those galaxies for which the SED fitting with stellar templates, without any previous PL subtraction, can be rejected with >2 sigma confidence. Subtracting the PL component from the PLCG SEDs produces stellar mass correction factors 80% of cases. The PLCG incidence is especially high (47%) at 1.0 <z <1.5. To unveil which PLCGs host AGNs, I conduct a combined analysis of 4 Ms X-ray data, galaxy morphologies, and a graybody modeling of the hot dust. I find that (1) 77% of all the X-ray AGNs in my 24 mu m sample at 1.0 <z <1.5 are recognized by the PLCG criterion; (2) PLCGs with alpha = 1.3 or 2.0 have regular morphologies and T-dust greater than or similar to 1000 K, indicating nuclear activity. Instead, PLCGs with alpha = 3.0 are characterized by disturbed galaxy dynamics, and a hot interstellar medium can explain their dust temperatures T-dust similar to 700-800 K. Overall, my results indicate that the fraction of AGNs among 24 mu m sources is between similar to 30% and 52% at 1.0 <z <1.5.

Original languageEnglish
Pages (from-to)103-114
Number of pages12
JournalAstrophysical Journal
Volume768
Issue number2
DOIs
Publication statusPublished - 10-May-2013

Keywords

  • infrared: galaxies
  • galaxies: evolution
  • galaxies: high-redshift
  • SPITZER-SPACE-TELESCOPE
  • DEEP-FIELD-SOUTH
  • STAR-FORMING GALAXIES
  • SIMILAR-TO 3
  • X-RAY
  • MIDINFRARED SPECTROSCOPY
  • LUMINOUS GALAXIES
  • HOST GALAXIES
  • OPTICAL SPECTROSCOPY
  • FORMATION HISTORY

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