Infrared Selection of Obscured Active Galactic Nuclei in the COSMOS Field

Abstract

We present a study of the connection between black hole accretion, star formation, and galaxy morphology at z ≤ 2.5. We focus on active galactic nuclei (AGNs) selected by their mid-IR power-law emission. By fitting optical to far-IR photometry with state-of-the-art spectral energy distribution (SED) techniques, we derive stellar masses, star formation rates, dust properties, and AGN contributions in galaxies over the whole COSMOS field. We find that obscured AGNs lie within or slightly above the star-forming sequence. We confirm our previous finding about compact host galaxies of obscured AGNs at z ∼ 1, and find that galaxies with 20-50% AGN contributions tend to have smaller sizes, by ∼25-50%, compared to galaxies without AGNs. Furthermore, we find that a high merger fraction of up to 0.5 is appropriate for the most luminous (log(L IR /L ) ∼ 12.5) AGN hosts and non-AGN galaxies, but not for the whole obscured AGN sample. Moreover, merger fraction depends on the total and star-forming infrared luminosity, rather than the decomposed AGN infrared luminosity. Our results suggest that major mergers are not the main driver of AGN activity, and therefore obscured AGNs might be triggered by internal mechanisms, such as secular processes, disk instabilities, and compaction in a particular evolutionary stage. We make the SED modeling results publicly available.

A population of luminous accreting black holes with hidden mergers

“…Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black-holes 1 . However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars 2,3 and other studies showing no such association 4,5 . Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated [6][7][8] (5 to 40 kiloparsecs).…”

Black hole scaling relations of active and quiescent galaxies: Addressing selection effects and constraining virial factors

“…We have finally investigated into AGN samples at higher redshifts, with stellar masses/black hole masses derived from spectral energy decomposition analysis and single epoch spectroscopy (e.g., Chang et al 2017;Delvecchio et al 2017;Rowan-Robinson et al 2017). In all cases we find the black holes to lie substantially below the local observed M bh -Mstar relation.…”

Comparison of cosmological simulations and deep submillimetre galaxy surveys

“…Chiang et al (submitted) have shown that the number fraction of galaxies hosting an AGN is roughly 60 per cent for L IR > 10 12 L ⊙ and that the fraction does not change along redshift up to 2. On the other hand, the database of Chang et al (2017) showed that approximately 10 per cent of galaxies have AGNs at the redshift range 2.0 ≤ z ≤ 2.5. Moreover, the contribution from AGNs is usually prominent at rest-frame mid-IR wavelengths according to the SED fitting by Chang et al (2017); thus, it is not obvious if the above percentage reflects the fraction of AGNs contributing to the STUDIES bands.…”

“…On the other hand, the database of Chang et al (2017) showed that approximately 10 per cent of galaxies have AGNs at the redshift range 2.0 ≤ z ≤ 2.5. Moreover, the contribution from AGNs is usually prominent at rest-frame mid-IR wavelengths according to the SED fitting by Chang et al (2017); thus, it is not obvious if the above percentage reflects the fraction of AGNs contributing to the STUDIES bands. Recently, Toba et al (2018) discovered an extremely IR-luminous galaxy (L IR ∼ 2 × 10 14 L ⊙ ) at z = 3.7, in which a significant fraction of far-IR emission is contributed by an AGN, although the star formation has a comparable contribution to the AGN at wavelengths relevant for the STUDIES observations.…”