Mid‐Infrared Spectra of Classical AGNs Observed with the<i>Spitzer Space Telescope</i>

Weedman, D. W.; Hao, Lei; Higdon, Sarah J.U.; Devost, D.; Wu, Yanling; Charmandaris, V.; Brandl, Bernhard R.; Bass, Ethan; Houck, J. R.

doi:10.1086/466520

Cited by 210 publications

(313 citation statements)

References 40 publications

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“…It has proven to be a good indicator of the SB content given its correlation with EW(PAH 11.3 µm) (e.g. Wu et al 2009;Weedman et al 2005;Brandl et al 2006;LaMassa et al 2012). This steepness has been defined in several bands by different authors; e.g.…”

Section: Steepness Of the Mid-ir Spectramentioning

confidence: 99%

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Nuclear obscuration in LINERs

González‐Martín

Masegosa

Márquez

et al. 2015

A&A

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Context. Most of the optically classified low-ionisation, narrow emission-line regions (LINERs) nuclei host an active galactic nucleus (AGN). However, how they fit into the unified model (UM) of AGN is still an open question.Aims. The aims of this work are to study at mid-infrared (mid-IR) (1) the Compton-thick nature of LINERs (i.e. hydrogen column densities of N H > 1.5 × 10 24 cm −2 ) and (2) the disappearance of the dusty torus in LINERs predicted from theoretical arguments. Methods. We have compiled all the available low spectral-resolution, mid-IR spectra of LINERs from the InfraRed Spectrograph (IRS) onboard Spitzer. The sample contains 40 LINERs. We have complemented the LINER sample with Spitzer/IRS spectra of PG QSOs, Type-1 Seyferts (S1s), Type-2 Seyferts (S2s), and StarBurst (SB) nuclei. We studied the AGN compared to the starburst content in our sample using different indicators: the equivalent width of the polycyclic aromatic hydrocarbon at 6.2 µm, the strength of the silicate feature at 9.7 µm, and the steepness of the mid-IR spectra. We classified the spectra as SB-dominated and AGN-dominated, according to these diagnostics and compared the average mid-IR spectra of the various classes. Moreover, we studied the correlation between the 12 µm luminosity, νL ν (12 µm), and the 2−10 keV energy band X-ray luminosity, L X (2−10 keV). Results. In 25 out of the 40 LINERs (i.e. 62.5%), the mid-IR spectra are not SB-dominated, similar to the comparison S2 sample (67.7%). The average spectra of both SB-dominated LINERs and S2s are very similar to the average spectrum of the SB class. The average spectrum of AGN-dominated LINERs is different from the average spectra of the other optical classes, showing a rather flat spectrum at 6−28 µm. We find that the average spectrum of AGN-dominated LINERs with X-ray luminosities L X (2−10 keV) > 10 41 erg/s is similar to the average mid-IR spectrum of AGN-dominated S2s. However, faint LINERs (i.e. L X (2−10 keV) < 10 41 erg/s) show flat spectra different from any of the other optical classes. The correlation between νL ν (12 µm) and L X (2−10 keV) for AGN nicely extends towards low luminosities only if SB-dominated LINERs are excluded and if the 2−10 keV band X-ray luminosity is corrected in Compton-thick LINER candidates. Conclusions. We find that LINERs proposed as Compton-thick candidates at X-ray wavelengths may be confirmed according to the X-ray to mid-IR luminosity relation. We show evidence that the dusty-torus disappear when their bolometric luminosity is below L bol 10 42 erg/s. We suggest that the dominant emission at mid-IR of faint LINERs might be a combination of an elliptical galaxy host (characterised by the lack of gas), a starburst, a jet, and/or ADAF emission. Alternatively, the mid-IR emission of some of these faint LINERs could be a combination of elliptical galaxy plus carbon-rich planetary nebulae. To reconcile the Compton-thick nature of a large number of LINERs without dusty-torus signatures, we suggest that the material producing the ...

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Section: Steepness Of the Mid-ir Spectramentioning

confidence: 99%

“…This steepness has been defined in several bands by different authors; e.g. 20−30 µm (Baum et al 2010;Weedman et al 2005) or 15−30 µm (Brandl et al 2006;Wu et al 2009;Nardini et al 2008). We present the 20 and 30 µm luminosities to produce an estimate of the steepness of the mid-IR spectra in our sample.…”

Section: Steepness Of the Mid-ir Spectramentioning

confidence: 99%

Nuclear obscuration in LINERs

González‐Martín

Masegosa

Márquez

et al. 2015

A&A

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“…9, bottom), considering both X-ray 8 For sources like Mkn 3 and Mkn 231, this is a good estimate of the AGN 6 µm luminosity since their MIR spectra are dominated by the AGN (e.g. Weedman et al 2005;Goulding et al 2012); however, for other sources, e.g. NGC 4945, where the MIR emission is dominated by star formation (e.g.…”

Section: Infrared Agnmentioning

confidence: 99%

GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies

Moro

Alexander

Mullaney

et al. 2012

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Context. A tight correlation exists between far-infrared and radio emission for star-forming galaxies (SFGs), which seems to hold out to high redshifts (z ≈ 2). Any excess of radio emission over that expected from star formation processes is most likely produced by an active galactic nucleus (AGN), often hidden by large amounts of dust and gas. Identifying these radio-excess sources will allow us to study a population of AGN unbiased by obscuration and thus find some of the most obscured, Compton-thick AGN, which are in large part unidentified even in the deepest X-ray and infrared (IR) surveys. Aims. We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant star-forming galaxies in the GOODS-Herschel (North) field and to reveal the presence of hidden, highly obscured AGN. Methods. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 µm detections using some of the deepest Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN (∼1300 deg −2 ) out to redshift z ≈ 3. These radio-excess AGN have a significantly lower far-IR/radio ratio (q < 1.68, 3σ) than the typical relation observed for star-forming galaxies (q ≈ 2.2). Results. We find that ≈45% of these radio-excess sources have a dominant AGN component in the mid-IR band, while for the remainders the excess radio emission is the only indicator of AGN activity. The presence of an AGN is also confirmed by the detection of a compact radio core in deep VLBI 1.4 GHz observations for eight of our radio-excess sources (≈16%; ≈66% of the VLBI detected sources in this field), with the excess radio flux measured from our SED analysis agreeing, to within a factor of two, with the radio core emission measured by VLBI. We find that the fraction of radio-excess AGN increases with X-ray luminosity reaching ∼60% at L X ≈ 10 44 −10 45 erg s −1 , making these sources an important part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data, suggesting that some of these sources might be heavily obscured. Amongst the radio-excess AGN we can distinguish three groups of objects: i) AGN clearly identified in infrared (and often in X-rays), a fraction of which are likely to be distant Compton-thick AGN; ii) moderate luminosity AGN (L X 10 43 erg s −1 ) hosted in strong star-forming galaxies; and iii) a small fraction of low accretion-rate AGN hosted in passive (i.e. weak or no star-forming) galaxies. We also find that the specific star formation rates (sSFRs) of the radio-excess AGN are on average lower that those observed for X-ray selected AGN hosts, indicating that our sources are forming stars more slowly than typical AGN hosts, and possibly their star formation is progressively quenching.

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“…In addition, there is a weak PAH emission in their mid-IR spectrum, which is attributed to the destruction of their carriers by the strong radiation field surrounding the AGN (i.e. see Weedman et al 2005;Brandl et al 2006;Smith et al 2007b). We can use our Spitzer infrared photometric measurements to probe for an AGN among the galaxies in our sample.…”

Section: Irac Colors and Agn Diagnosticsmentioning

confidence: 99%

A mid-IR study of Hickson compact groups

Bitsakis¹,

Charmandaris

Floc’h

et al. 2010

A&A

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Hickson compact groups (HCGs) are among the densest galaxy environments of the local universe. To examine the effects of the environment on the infrared properties of these systems, we present an analysis of Spitzer and ISO mid-infrared imaging, as well as of deep ground-based near-infrared imaging of 14 HCGs containing a total of 69 galaxies. Based on mid-infrared color diagnostics we identify the galaxies that appear to host an active nucleus, while using a suite of templates, and fit the complete infrared spectral energy distribution for each group member. We compare our estimates of galaxy mass, star formation rate, total infrared luminosities, and specific star formation rates (sSFR) for our HCG sample to samples of isolated galaxies and interacting pairs and find that overall there is no discernible difference among them. However, HCGs that can be considered as dynamically "old" host late-type galaxies with a slightly lower sSFR than the one found in dynamically "young" groups. This could be attributed to multiple past interactions among the galaxies in old groups, that have led to the build up of their stellar mass. It is also consistent with our prediction of the presence of diffuse cold dust in the intergalactic medium of 9 of the dynamically "old" groups.

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Mid‐Infrared Spectra of Classical AGNs Observed with theSpitzer Space Telescope

Abstract: Full low resolution (65 Show more

Cited by 210 publications

References 40 publications

Nuclear obscuration in LINERs

Nuclear obscuration in LINERs

GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies

A mid-IR study of Hickson compact groups

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