Dust-obscured Active Galactic Nuclei Research Articles

Structures Of Dust and gAs (SODA): Constraining the innermost dust properties of II Zw96 with JWST observations of H2O and CO

We analyze JWST NIRSpec+MIRI/MRS observations of the infrared (IR) gas-phase molecular bands of the most enshrouded source (D1) within the interacting system and luminous IR galaxy II Zw 096. We report the detection of rovibrational lines of H2O ν2 = 1 − 0 (∼5.3–7.2 μm) and 12CO ν = 1 − 0 (∼4.45–4.95 μm) in D1. The CO band shows the R- and P-branches in emission and the spectrum of the H2O band shows the P-branch in emission and the R-branch in absorption. The H2O R-branch in absorption unveils an IR-bright embedded compact source in D1 and the CO broad component features a highly turbulent environment. From both bands, we also identified extended intense star-forming (SF) activity associated with circumnuclear photodissociation regions (PDRs), consistent with the strong emission of the ionised 7.7 μm polycyclic aromatic hydrocarbon band in this source. By including the 4.5–7.0 μm continuum information derived from the H2O and CO analysis, we modelled the IR emission of D1 with a dusty torus and SF component. The torus is very compact (diameter of ∼3 pc at 5 μm) and characterised by warm dust (∼370 K), giving an IR surface brightness of ∼3.6 × 108 L⊙ pc−2. This result suggests the presence of a dust-obscured active galactic nucleus (AGN) in D1, which has an exceptionally high covering factor that prevents the direct detection of AGN emission. Our results open a new way to investigate the physical conditions of inner dusty tori via modelling the observed IR molecular bands.

Central concentration of warm and dense molecular gas in a strongly lensed submillimeter galaxy at z = 6

Abstract We report the detection of the CO(12–11) line emission toward G09-83808 (or H-ATLAS J090045.4+004125), a strongly-lensed submillimeter galaxy at z = 6.02, with Atacama Large Millimeter/submillimeter Array observations. Combining previously detected [O iii] 88 μm, [N ii] 205 μm, and dust continuum at 0.6 mm and 1.5 mm, we investigate the physical properties of the multi-phase interstellar medium in G09-83808. A source-plane reconstruction reveals that the region of the CO(12–11) emission is compact ($R_\mathrm{{e, CO}}=0.49^{+0.29}_{-0.19}\:\mbox{kpc}$) and roughly coincides with that of the dust continuum. Non-local thermodynamic equilibrium radiative transfer modeling of CO spectral-line energy distribution reveals that most of the CO(12–11) emission comes from a warm (kinetic temperature of Tkin = 320 ± 170 K) and dense [log (nH2/cm−3) = 5.4 ± 0.6] gas, indicating that the warm and dense molecular gas is concentrated in the central 0.5 kpc region. The luminosity ratio in G09-83808 is estimated to be LCO(12-11)/LCO(6-5) = 1.1 ± 0.2. The high ratio is consistent with those in local active galactic nuclei (AGNs) and 6 < z < 7 quasars, the fact of which implies that G09-83808 would be a good target to explore dust-obscured AGNs in the epoch of reionization. In the reconstructed [O iii] 88 μm and [N ii] 205 μm cubes, we also find that a monotonic velocity gradient is extending over the central starburst region by a factor of 2 and that star-forming sub-components exist. High-resolution observations of bright [C ii] 158 μm line emissions will enable us to characterize the kinematics of a possible rotating disk and the nature of the sub-components.

Toward a More Complete Optical Census of Active Galactic Nuclei via Spatially Resolved Spectroscopy

Abstract While emission line flux ratio diagnostics are the most common technique for identifying active galactic nuclei (AGNs) in optical spectra, applying this approach to single-fiber spectra of galaxies can omit entire subpopulations of AGNs. Here, we use spatially resolved spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to construct a sample of 10 galaxies where Baldwin–Philips–Terlevich line flux ratio diagnostics classify each galaxy’s central 3″ spectrum as LINER or star-forming, while >10% of the spaxels in the galaxy’s MaNGA footprint are classified as Seyfert. We obtain Chandra observations of these 10 galaxies with off-nuclear Seyfert regions to determine whether AGNs are actually present in them. Our main result is that 7–10 (depending on strictness of criteria) of the galaxies host one or more X-ray AGNs, even though none of them were classified as AGNs based on their single-fiber optical spectra. We find that these AGNs were not identified in the single-fiber spectra because they are AGNs in the nuclei of companion galaxies, low-luminosity AGNs, dust-obscured AGNs, and/or flickering AGNs. In summary, we find that off-nuclear AGN signatures may increase the number of known AGNs by a factor of two over what conventional single nuclear fiber spectra identify. Our results show that spatially resolved spectroscopy can be leveraged to reveal a more complete census of AGNs that are traditionally missed by single-fiber spectra.

Infrared emission of z ∼ 6 galaxies: AGN imprints

ABSTRACT We investigate the infrared (IR) emission of high-redshift (z ∼ 6), highly star-forming (${{\rm SFR}\gt 100\,{\rm M}_{\odot }\, {\rm yr}^{-1}}$) galaxies, with/without active galactic nuclei (AGN), using a suite of cosmological simulations featuring dust radiative transfer. Synthetic spectral energy distributions (SEDs) are used to quantify the relative contribution of stars/AGN to dust heating. In dusty (Md ≳ 3 × 107 M⊙) galaxies, ≳50–90 per cent of the ultraviolet (UV) radiation is obscured by dust inhomogeneities on scales ≳100 pc. In runs with AGN, a clumpy, warm (≈250 K) dust component coexists with a colder (≈60 K) and more diffuse one, heated by stars. Warm dust provides up to ${50 {{\ \rm per\ cent}}}$ of the total infrared (IR) luminosity, but only ${\lesssim}0.1 {{\ \rm per\ cent}}$ of the total mass content. The AGN boosts the MIR flux by 10–100 times with respect to star-forming galaxies, without significantly affecting the far-IR. Our simulations successfully reproduce the observed SED of bright (MUV ∼ −26) z ∼ 6 quasars, and show that these objects are part of complex, dust-rich merging systems, containing multiple sources (accreting black holes and/or star-forming galaxies) in agreement with recent HST and ALMA observations. Our results show that the proposed ORIGINS missions will be able to investigate the mid-IR (MIR) properties of dusty star-forming galaxies and to obtain good-quality spectra of bright quasars at z ∼ 6. Finally, the MIR-to-FIR flux ratio of faint (MUV ∼ −24) AGN is >10 times higher than for normal star-forming galaxies. This implies that combined JWST/ORIGINS/ALMA observations will be crucial to identify faint and/or dust-obscured AGN in the distant Universe.

Evidence for a Buried AGN in an Extremely Bright Dusty Galaxy at z = 2

Abstract We present preliminary results of a submillimeter spectral line survey of a strongly lensed dusty star-forming galaxy (DSFG) at z ∼ 2, near the peak epoch of cosmic star formation. We use data from the Submillimeter Array, which recently completed a systems upgrade that substantially increased the instantaneous bandwidth. Focusing on the brightest spectral lines of CO and atomic carbon, we characterize the interstellar medium properties of this galaxy. We find highly excited CO and [C i] emission, and a large [C i] line ratio implies a higher excitation temperature than other extreme high-redshift sources. Based on this evidence, we infer that the source likely hosts a dust-obscured active galactic nucleus similar to other well-studied DSFGs.

Far-infrared dust properties of highly dust-obscured active galactic nuclei from the AKARI and WISE all-sky surveys

Abstract The combination of the AKARI and WISE infrared all-sky surveys provides a unique opportunity to identify and characterize the most highly dust-obscured active galactic nuclei (AGNs) in the universe. Dust-obscured AGNs are not easily detectable and are potentially underrepresented in extragalactic surveys due to their high optical extinction, but are readily found in the WISE catalog due to their extremely red mid-infrared (IR) colors. Combining these surveys with photometry from Pan-STARRS and Herschel, we use spectral energy distribution (SED) modeling to characterize the extinction and dust properties of these AGNs. From mid-IR WISE colors we are able to compute bolometric corrections to AGN luminosities. Using AKARI’s far-IR wavelength photometry and broadband AGN/galaxy spectral templates we estimate AGN dust mass and temperature using simple analytic models with three or four parameters. Even without spectroscopic data we can determine a number of AGN dust properties only using SED analysis. These methods, combined with the abundance of archival photometric data publicly available, will be valuable for large-scale studies of dusty, IR-luminous AGNs.

The AGN–Star Formation Connection: Future Prospects with JWST

Abstract The bulk of the stellar growth over cosmic time is dominated by IR-luminous galaxies at cosmic noon ( z = 1 – 2 ), many of which harbor a hidden active galactic nucleus (AGN). We use state-of-the-art infrared color diagnostics, combining Spitzer and Herschel observations, to separate dust-obscured AGNs from dusty star-forming galaxies (SFGs) in the CANDELS and COSMOS surveys. We calculate 24 μm counts of SFGs, AGN/star-forming “Composites,” and AGNs. AGNs and Composites dominate the counts above 0.8 mJy at 24 μm, and Composites form at least 25% of an IR sample even to faint detection limits. We develop methods to use the Mid-Infrared Instrument (MIRI) on JWST to identify dust-obscured AGNs and Composite galaxies from z ∼ 1 – 2 . With the sensitivity and spacing of MIRI filters, we will detect >4 times as many AGN hosts as with Spitzer/IRAC criteria. Any star formation rates based on the 7.7 μm PAH feature (likely to be applied to MIRI photometry) must be corrected for the contribution of the AGN, or the star formation rate will be overestimated by ∼35% for cases where the AGN provides half the IR luminosity and ∼50% when the AGN accounts for 90% of the luminosity. Finally, we demonstrate that our MIRI color technique can select AGNs with an Eddington ratio of λ Edd ∼ 0.01 and will identify AGN hosts with a higher specific star formation rate than X-ray techniques alone. JWST/MIRI will enable critical steps forward in identifying and understanding dust-obscured AGNs and the link to their host galaxies.

Incidence ofWISE-selected obscured AGNs in major mergers and interactions from the SDSS

We use the Wide-field Infrared Survey Explorer (WISE) and the Sloan Digital Sky Survey (SDSS) to confirm a connection between dust-obscured active galactic nuclei (AGNs) and galaxy merging. Using a new, volume-limited (z <= 0.08) catalog of visually-selected major mergers and galaxy-galaxy interactions from the SDSS, with stellar masses above 2x10^10 Msun, we find that major mergers (interactions) are 5-17 (3-5) times more likely to have red [3.4]-[4.6] colors associated with dust-obscured or `dusty' AGNs, compared to non-merging galaxies with similar masses. Using published fiber spectral diagnostics, we map the [3.4]-[4.6] versus [4.6]-[12] colors of different emission-line galaxies and find one-quarter of Seyferts have colors indicative of a dusty AGN. We find that AGNs are five times more likely to be obscured when hosted by a merging galaxy, half of AGNs hosted by a merger are dusty, and we find no enhanced frequency of optical AGNs in merging over non-merging galaxies. We conclude that undetected AGNs missed at shorter wavelengths are at the heart of the ongoing AGN-merger connection debate. The vast majority of mergers hosting dusty AGNs are star-forming and located at the centers of Mhalo<10^13 Msun groups. Assuming plausibly short duration dusty-AGN phases, we speculate that a large fraction of gas-rich mergers experience a brief obscured AGN phase, in agreement with the strong connection between central star formation and black hole growth seen in merger simulations.

HOT DUST OBSCURED GALAXIES WITH EXCESS BLUE LIGHT: DUAL AGN OR SINGLE AGN UNDER EXTREME CONDITIONS?

ABSTRACT Hot dust-obscured galaxies (Hot DOGs) are a population of hyper-luminous infrared galaxies identified by the Wide-field Infrared Survey Explorer (WISE) mission from their very red mid-IR colors, and characterized by hot dust temperatures (T &gt; 60 K). Several studies have shown clear evidence that the IR emission in these objects is powered by a highly dust-obscured active galactic nucleus (AGN) that shows close to Compton-thick absorption at X-ray wavelengths. Thanks to the high AGN obscuration, the host galaxy is easily observable, and has UV/optical colors usually consistent with those of a normal galaxy. Here we discuss a sub-population of eight Hot DOGs that show enhanced rest-frame UV/optical emission. We discuss three scenarios that might explain the excess UV emission: (i) unobscured light leaked from the AGN by reflection over the dust or by partial coverage of the accretion disk; (ii) a second unobscured AGN in the system; or (iii) a luminous young starburst. X-ray observations can help discriminate between these scenarios. We study in detail the blue excess Hot DOG WISE J020446.13–050640.8, which was serendipitously observed by Chandra/ACIS-I for 174.5 ks. The X-ray spectrum is consistent with a single, hyper-luminous, highly absorbed AGN, and is strongly inconsistent with the presence of a secondary unobscured AGN. Based on this, we argue that the excess blue emission in this object is most likely either due to reflection or a co-eval starburst. We favor the reflection scenario as the unobscured star formation rate needed to power the UV/optical emission would be ≳1000 M ⊙ yr−1. Deep polarimetry observations could confirm the reflection hypothesis.

POLYCYCLIC AROMATIC HYDROCARBON AND MID-INFRARED CONTINUUM EMISSION IN Az&gt; 4 SUBMILLIMETER GALAXY

We report the detection of 6.2um polycyclic aromatic hydrocarbon (PAH) and rest-frame 4-7um continuum emission in the z=4.055 submillimeter galaxy GN20, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. This represents the first detection of PAH emission at z>4. The strength of the PAH emission feature is consistent with a very high star formation rate of ~1600Msun/yr. We find that this intense starburst powers at least ~1/3 of the faint underlying 6um continuum emission, with an additional, significant (and perhaps dominant) contribution due to a power-law-like hot dust source, which we interpret to likely be a faint, dust-obscured active galactic nucleus (AGN). The inferred 6um AGN continuum luminosity is consistent with a sensitive upper limit on the hard X-ray emission as measured by the Chandra X-Ray Observatory if the previously undetected AGN is Compton-thick. This is in agreement with the finding at optical/infrared wavelengths that the galaxy and its nucleus are heavily dust-obscured. Despite the strong power-law component enhancing the mid-infrared continuum emission, the intense starburst associated with the photon-dominated regions that give rise to the PAH emission appears to dominate the total energy output in the infrared. GN20 is one of the most luminous starburst galaxies known at any redshift, embedded in a rich protocluster of star-forming galaxies. This investigation provides an improved understanding of the energy sources that power such exceptional systems, which represent the extreme end of massive galaxy formation at early cosmic times.

THE NATURE OF EXTREMELY RED H – [4.5] &gt; 4 GALAXIES REVEALED WITH SEDS AND CANDELS

We have analysed a sample of 25 extremely red H-[4.5]>4 galaxies, selected using 4.5 micron data from the Spitzer SEDS survey and deep H-band data from the Hubble Space Telescope (HST) CANDELS survey, over ~180 square arcmin of the UKIDSS Ultra Deep Survey (UDS) field. Our aim is to investigate the nature of this rare population of mid-infrared (mid-IR) sources that display such extreme near-to-mid-IR colours. Using up to 17-band photometry (U through 8.0 microns), we have studied in detail their spectral energy distributions, including possible degeneracies in the photometric redshift/internal extinction (zphot-Av) plane. Our sample appears to include sources of very different nature. Between 45% and 75% of them are dust-obscured, massive galaxies at 3<zphot<5. All of the 24 micron-detected sources in our sample are in this category. Two of these have S(24 micron)>300 microJy, which at 3<zphot<5 suggests that they probably host a dust-obscured active galactic nucleus (AGN). Our sample also contains four highly obscured (Av>5) sources at zphot<1. Finally, we analyse in detail two zphot~6 galaxy candidates, and discuss their plausibility and implications. Overall, our red galaxy sample contains the tip of the iceberg of a larger population of z>3 galaxies to be discovered with the future James Webb Space Telescope.

SpitzerIRAC infrared colours of submillimetre-bright galaxies

High-redshift submillimetre-bright galaxies identified by blank field surveys at millimetre and submillimetre wavelengths appear in the region of the IRAC colour-colour diagrams previously identified as the domain of luminous active galactic nuclei (AGNs). Our analysis using a set of empirical and theoretical dusty starburst spectral energy distribution (SED) models shows that power-law continuum sources associated with hot dust heated by young ( 2 also occupy the same general area as AGNs in the IRAC colour-colour plots. A detailed comparison of the IRAC colours and SEDs demonstrates that the two populations are distinct from each other, with submillimetre-bright galaxies having a systematically flatter IRAC spectrum (>1 mag bluer in the observed [4.5]-[8.0] colour). Only about 20% of the objects overlap in the colour-colour plots, and this low fraction suggests that submillimetre galaxies powered by a dust-obscured AGN are not common. The red IR colours of the submillimetre galaxies are distinct from those of the ubiquitous foreground IRAC sources, and we propose a set of IR colour selection criteria for identifying SMG counterparts that can be used even in the absence of radio or Spitzer MIPS 24 micron data.

The DEEP2 Galaxy Redshift Survey: AEGIS Observations of a Dual AGN at z = 0.7

We present evidence of a dual active galactic nucleus (AGN) within an early-type galaxy at z = 0.709 in the extended Groth strip. The galaxy lies on the red sequence, with absolute magnitude MB = -21.0 (AB, with h = 0.7) and rest-frame color U - B = 1.38. Its optical spectrum shows strong, double-peaked [O III] emission lines and weak Hβ emission, with Seyfert-like line ratios. The two narrow peaks are separated by 630 km s-1 in velocity and arise from two distinct regions, spatially resolved in the DEIMOS spectrum, with a projected physical separation of 1.2 kpc. HST ACS imaging shows an early-type (E/S0) galaxy with hints of disturbed structure, consistent with the remnant of a dissipationless merger. Multiwavelength photometric information from the AEGIS consortium confirms the identification of a dust-obscured AGN in an early-type galaxy, with detections in X-ray, optical, infrared, and radio wave bands. These data are most readily explained as a single galaxy harboring two AGNs—the first such system to be observed in an otherwise typical early-type galaxy.

Infrared 3-4 μm Spectroscopy of Infrared Luminous Galaxies with Possible Signatures of Obscured Active Galactic Nuclei

We present the results of infrared 2.8-4.1 μm (L-band) spectroscopy of nearby infrared luminous galaxies with possible signatures of dust-obscured active galactic nuclei (AGNs) in data at other wavelengths. The samples are chosen to include sources with a radio excess relative to far-infrared emission, strong absorption features in mid-infrared 5-11.5 μm spectra, unusually weak [C II] 158 μm emission relative to the far-infrared continuum, and radio galaxies classified optically as narrow-line objects. Our aim is to investigate whether the signatures of possible obscured AGNs can be detected in our L-band spectra based on the strengths of emission and absorption features. Six of nine observed sources clearly show 3.3 μm polycyclic aromatic hydrocarbon emission features, a good starburst indicator. An absorption feature at 3.1 μm due to ice-covered dust is detected in IRAS 04154+1755 and IRAS 17208-0014. The signature of a bare carbonaceous dust absorption feature at 3.4 μm is seen in NGC 1377. Our L-band spectra reveal strong signatures of obscured AGNs in all three optical Seyfert 2 galaxies (IRAS 04154+1755, Cygnus A, and 3C 234) and two galaxies classified optically as non-Seyfert galaxies (NGC 828 and NGC 1377). Among the remaining optical non-Seyfert galaxies, IRAS 17208-0014 might also show a buried AGN signature, whereas no explicit AGN evidence is seen in the L-band spectra of the mid-infrared absorption feature source IRAS 15250+3609 and two weak [C II] emitters, IC 860 and CGCG 1510.8+0725.

Further Multiwavelength Observations of the SSA 22 Lyα‐Emitting Blob

We present new follow-up observations of the submillimeter luminous Lyα-emitting object in the SSA 22 z = 3.09 galaxy overdensity, referred to as blob 1 by Steidel and coworkers. In particular, we discuss high-resolution Hubble Space Telescope (HST) optical imaging, Owens Valley Radio Observatory (OVRO) spectral imaging, Keck spectroscopy, Very Large Array (VLA) 20 cm radio continuum imaging, and Chandra X-ray observations. We also present a more complete analysis of the existing James Clerk Maxwell Telescope (JCMT) submillimeter data. We detect several optical continuum components that may be associated with the core of the submillimeter-emitting region. A radio source at the position of one of the HST components [α = 22h17m2594, δ = +00°12'389] identifies it as the likely counterpart to the submillimeter source. We also tentatively detect the CO(4-3) molecular line centered on the radio position. We use the CO(4-3) intensity to estimate a limit on the gas mass for the system. The optical morphology of sources within the Lyα cloud appears to be filamentary, while the optical source identified with the radio source has a dense knot that may be an active galactic nucleus (AGN) or compact starburst. We obtain a Keck Low-Resolution Imaging Spectrograph (LRIS) spectrum of this object, despite its faintness (R = 26.8). The spectrum reveals weak Lyα emission but no other obvious features, suggesting that the source is not an energetic AGN (or that it is extremely obscured). We use nondetections in deep Chandra X-ray images to constrain the nature of the blob. Although conclusive evidence regarding the nature of the object remains hard to obtain at this redshift, the evidence presented here is at least consistent with a dust-obscured AGN surrounded by a starburst situated at the heart of this giant Lyα cloud.