The AGN, star-forming, and morphological properties of luminous IR-bright/optically-faint galaxies

J. L. Donley, George H. Rieke, D. M. Alexander, E. Egami, P. G. Pérez-González

Research output: Contribution to journalArticle

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Abstract

We present the active galactic nucleus (AGN), star-forming, and morphological properties of a sample of 13 MIR-luminous (f24 ≳ 700 μJy) IR-bright/optically-faint galaxies (IRBGs, f24/fR ≳ 1000). While these z ∼ 2 sources were drawn from deep Chandra fields with >200 ks X-ray coverage, only seven are formally detected in the X-ray and four lack X-ray emission at even the 2σ level. Spitzer InfraRed Spectrograph (IRS) spectra, however, confirm that all of the sources are AGN-dominated in the mid-IR, although half have detectable polycyclic aromatic hydrocarbon (PAH) emission responsible for ∼25% of their mid-infrared flux density. When combined with other samples, this indicates that at least 30%-40% of luminous IRBGs have star formation rates in the ultraluminous infrared galaxy (ULIRG) range (∼100-2000 Mȯ yr-1). X-ray hardness ratios and MIR to X-ray luminosity ratios indicate that all members of the sample contain heavily X-ray obscured AGNs, 80% of which are candidates to be Compton thick. Furthermore, the mean X-ray luminosity of the sample, log L2-10keV(erg s-1) ∼44.6, indicates that these IRBGs are Type 2 QSOs, at least from the X-ray perspective. While those sources most heavily obscured in the X-ray are also those most likely to display strong silicate absorption in the mid-IR, silicate absorption does not always accompany X-ray obscuration. Finally, ∼70% of the IRBGs are merger candidates, a rate consistent with that of sub-mm galaxies (SMGs), although SMGs appear to be physically larger than IRBGs. These characteristics are consistent with the proposal that these objects represent a later, AGN-dominated, and more relaxed evolutionary stage following soon after the star-formation-dominated one represented by the SMGs.

Original languageEnglish (US)
Pages (from-to)1393-1407
Number of pages15
JournalAstrophysical Journal
Volume719
Issue number2
DOIs
StatePublished - Aug 20 2010

Fingerprint

active galactic nuclei
galaxies
stars
x rays
silicates
silicate
luminosity
occultation
polycyclic aromatic hydrocarbons
star formation rate
erg
merger
quasars
hardness
spectrographs
proposals
star formation
PAH
flux density

Keywords

  • Galaxies: active
  • Infrared: galaxies
  • X-rays: galaxies

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

The AGN, star-forming, and morphological properties of luminous IR-bright/optically-faint galaxies. / Donley, J. L.; Rieke, George H.; Alexander, D. M.; Egami, E.; Pérez-González, P. G.

In: Astrophysical Journal, Vol. 719, No. 2, 20.08.2010, p. 1393-1407.

Research output: Contribution to journalArticle

Donley, J. L. ; Rieke, George H. ; Alexander, D. M. ; Egami, E. ; Pérez-González, P. G. / The AGN, star-forming, and morphological properties of luminous IR-bright/optically-faint galaxies. In: Astrophysical Journal. 2010 ; Vol. 719, No. 2. pp. 1393-1407.
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T1 - The AGN, star-forming, and morphological properties of luminous IR-bright/optically-faint galaxies

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AU - Egami, E.

AU - Pérez-González, P. G.

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N2 - We present the active galactic nucleus (AGN), star-forming, and morphological properties of a sample of 13 MIR-luminous (f24 ≳ 700 μJy) IR-bright/optically-faint galaxies (IRBGs, f24/fR ≳ 1000). While these z ∼ 2 sources were drawn from deep Chandra fields with >200 ks X-ray coverage, only seven are formally detected in the X-ray and four lack X-ray emission at even the 2σ level. Spitzer InfraRed Spectrograph (IRS) spectra, however, confirm that all of the sources are AGN-dominated in the mid-IR, although half have detectable polycyclic aromatic hydrocarbon (PAH) emission responsible for ∼25% of their mid-infrared flux density. When combined with other samples, this indicates that at least 30%-40% of luminous IRBGs have star formation rates in the ultraluminous infrared galaxy (ULIRG) range (∼100-2000 Mȯ yr-1). X-ray hardness ratios and MIR to X-ray luminosity ratios indicate that all members of the sample contain heavily X-ray obscured AGNs, 80% of which are candidates to be Compton thick. Furthermore, the mean X-ray luminosity of the sample, log L2-10keV(erg s-1) ∼44.6, indicates that these IRBGs are Type 2 QSOs, at least from the X-ray perspective. While those sources most heavily obscured in the X-ray are also those most likely to display strong silicate absorption in the mid-IR, silicate absorption does not always accompany X-ray obscuration. Finally, ∼70% of the IRBGs are merger candidates, a rate consistent with that of sub-mm galaxies (SMGs), although SMGs appear to be physically larger than IRBGs. These characteristics are consistent with the proposal that these objects represent a later, AGN-dominated, and more relaxed evolutionary stage following soon after the star-formation-dominated one represented by the SMGs.

AB - We present the active galactic nucleus (AGN), star-forming, and morphological properties of a sample of 13 MIR-luminous (f24 ≳ 700 μJy) IR-bright/optically-faint galaxies (IRBGs, f24/fR ≳ 1000). While these z ∼ 2 sources were drawn from deep Chandra fields with >200 ks X-ray coverage, only seven are formally detected in the X-ray and four lack X-ray emission at even the 2σ level. Spitzer InfraRed Spectrograph (IRS) spectra, however, confirm that all of the sources are AGN-dominated in the mid-IR, although half have detectable polycyclic aromatic hydrocarbon (PAH) emission responsible for ∼25% of their mid-infrared flux density. When combined with other samples, this indicates that at least 30%-40% of luminous IRBGs have star formation rates in the ultraluminous infrared galaxy (ULIRG) range (∼100-2000 Mȯ yr-1). X-ray hardness ratios and MIR to X-ray luminosity ratios indicate that all members of the sample contain heavily X-ray obscured AGNs, 80% of which are candidates to be Compton thick. Furthermore, the mean X-ray luminosity of the sample, log L2-10keV(erg s-1) ∼44.6, indicates that these IRBGs are Type 2 QSOs, at least from the X-ray perspective. While those sources most heavily obscured in the X-ray are also those most likely to display strong silicate absorption in the mid-IR, silicate absorption does not always accompany X-ray obscuration. Finally, ∼70% of the IRBGs are merger candidates, a rate consistent with that of sub-mm galaxies (SMGs), although SMGs appear to be physically larger than IRBGs. These characteristics are consistent with the proposal that these objects represent a later, AGN-dominated, and more relaxed evolutionary stage following soon after the star-formation-dominated one represented by the SMGs.

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