Energy Diagnoses of Nine Infrared Luminous Galaxies Based on 3–4 Micron Spectra
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Abstract:
The energy sources of nine infrared luminous galaxies (IRLGs) are diagnosed based on their ground-based 3-4 μm spectra. Both the equivalent width of the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature and the 3.3 μm PAH to far-infrared luminosity ratio (L3.3/LFIR) are analyzed. Assuming that nuclear compact starburst activity in these sources produces the 3.3 μm PAH emission as strongly as that in starburst galaxies with lower far-infrared luminosities, the following results are found. For six IRLGs, both the observed equivalent widths and the L3.3/LFIR ratios are too small to explain the bulk of their far-infrared luminosities by compact starburst activity, indicating that active galactic nucleus (AGN) activity is a dominant energy source. For the other three IRLGs, while the 3.3 μm PAH equivalent widths are within the range of starburst galaxies, the L3.3/LFIR ratios after correction for screen dust extinction are a factor of ~3 smaller. The uncertainty in the dust extinction correction factor and in the scatter of the intrinsic L3.3/LFIR ratios for starburst galaxies does not allow a determination of the ultimate energy sources for these three IRLGs.Keywords:
Extinction (optical mineralogy)
In order to examine the relative importance of powerful starbursts and Compton-thick AGNs in NGC 6240, we have obtained mid-infrared images and low-resolution spectra of the galaxy with sub-arcsecond spatial resolution using the Keck Telescopes. Despite the high spatial resolution (~200 pc) of our data, no signature of the hidden AGNs has been detected in the mid-infrared. The southern nucleus, which we show provides 80-90% of the total 8-25 um luminosity of the system, has a mid-infrared spectrum and a mid-/far-infrared spectral energy distribution consistent with starbursts. At the same time, however, it is also possible to attribute up to 60% of the bolometric luminosity to an AGN, consistent with X-ray observations, if the AGN is heavily obscured and emits mostly in the far-infrared. This ambiguity arises because the intrinsic variation of properties among a given galaxy population (e.g., starbursts) introduces at least a factor of a few uncertainty even into the most robust AGN-starburst diagnostics. We conclude that with present observations it is not possible to determine the dominant power source in galaxies when AGN and starburst luminosities are within a factor of a few of each other.
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We investigate the fraction of starbursts, starburst-AGN composites, Seyferts, and LINERs as a function of infrared luminosity (L_IR) and merger progress for ~500 infrared-selected galaxies. Using the new optical classifications afforded by the extremely large data set of the Sloan Digital Sky Survey, we find that the fraction of LINERs in IR-selected samples is rare (< 5%) compared with other spectral types. The lack of strong infrared emission in LINERs is consistent with recent optical studies suggesting that LINERs contain AGN with lower accretion rates than in Seyfert galaxies. Most previously classified infrared-luminous LINERs are classified as starburst-AGN composite galaxies in the new scheme. Starburst-AGN composites appear to "bridge" the spectral evolution from starburst to AGN in ULIRGs. The relative strength of the AGN versus starburst activity shows a significant increase at high infrared luminosity. In ULIRGs (L_IR >10^12 L_odot), starburst-AGN composite galaxies dominate at early--intermediate stages of the merger, and AGN galaxies dominate during the final merger stages. Our results are consistent with models for IR-luminous galaxies where mergers of gas-rich spirals fuel both starburst and AGN, and where the AGN becomes increasingly dominant during the final merger stages of the most luminous infrared objects.
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We present the results of Spitzer Infrared Spectrograph low-resolution infrared 5–35 μm spectroscopy of 17 nearby ultraluminous infrared galaxies (ULIRGs) at z < 0.2, optically classified as non-Seyferts. The presence of optically elusive, but intrinsically luminous, buried active galactic nuclei (AGNs) is investigated, based on the strengths of polycyclic aromatic hydrocarbon emission and silicate dust absorption features detected in the spectra. The signatures of luminous buried AGNs, whose intrinsic luminosities range up to ∼1012 L☉, are found in eight sources. We combine these results with those of our previous research to investigate the energy function of buried AGNs in a complete sample of optically non-Seyfert ULIRGs in the local universe at z < 0.3 (85 sources). We confirm a trend that we previously discovered: that buried AGNs are more common in galaxies with higher infrared luminosities. Because optical Seyferts also show a similar trend, we argue more generally that the energetic importance of AGNs is intrinsically higher in more luminous galaxies, suggesting that the AGN–starburst connections are luminosity dependent. This may be related to the stronger AGN feedback scenario in currently more massive galaxy systems, as a possible origin of the galaxy downsizing phenomenon.
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We present observations of a sample of 28 luminous infrared galaxies (LIRGs) from the IRAS Bright Galaxy Catalog. These galaxies were previously observed by Goldader et al. using 2 μm spectroscopy on UKIRT. They found that stellar population synthesis models constrained by the spectroscopic diagnostics implied star formation rates that accounted for their infrared luminosities, and there was also no spectroscopic evidence for "buried" active galactic nuclei (AGNs) at 2 μm. To search for evidence of AGNs buried deeply in extinction we have supplemented the UKIRT spectra with Spitzer mid-infrared (5.2–38.0 μm) spectra. Using a variety of spectroscopic diagnostics we find that ∼50% of the sample shows some evidence for an AGN. We also find that the luminosity of about ∼(17 ± 4)% of our sample is probably dominated by emission from AGNs, and the remaining ∼(80 ± 4)% have luminosities dominated by starbursts. Since ∼50% of the sample shows some evidence of concurrent AGN and starburst activities this suggests that both AGNs and starbursts commonly coexist in the LIRGs' phase of evolution. The sample consists of galaxies that show no AGN signatures at wavelengths less than 2 μm, so it appears that at wavelengths below 2 μm extinction in these galaxies masks the detection of AGNs in these and other LIRGs.
Extinction (optical mineralogy)
Extragalactic astronomy
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We study the X-ray emission of a representative sample of 27 local luminous infrared galaxies (LIRGs). The median IR luminosity of our sample is log L_IR/L_sun = 11.2, thus the low-luminosity end of the LIRG class is well represented. We used new XMM-Newton data as well as Chandra and XMM-Newton archive data. The soft X-ray (0.5-2 keV) emission of most of the galaxies (>80%), including LIRGs hosting a Seyfert 2 nucleus, is dominated by star-formation related processes. These LIRGs follow the star-formation rate (SFR) versus soft X-ray luminosity correlation observed in local starbursts. We find that ~15% of the non-Seyfert LIRGs (3 out of 20) have an excess hard X-ray emission relative to that expected from star-formation that might indicate the presence of an obscured AGN. The rest of the non-Seyfert LIRGs follow the SFR versus hard X-ray (2-10 keV) luminosity correlation of local starbursts. The non-detection of the 6.4 keV Fe K alpha emission line in the non-Seyfert LIRGs allows us to put an upper limit to the bolometric luminosity of an obscured AGN, L_bol <1043 erg s-1 . That is, in these galaxies, if they hosted a low luminosity AGN, its contribution to total luminosity would be less than 10%. Finally we estimate that the AGN contribution to the total luminosity for our sample of local LIRGs is between 7% and 10%.
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We present 71 Emission Line objects selected from the REX survey. Except for 3 of them, for which the presence of an active nucleus is dubious, all these sources are Active Galactic Nuclei (QSOs, Seyfert galaxies, emission line radiogalaxies). In addition, we present the spectra of other 19 AGNs included in a preliminary version of the REX catalog but not in the final one. The majority (80) of the 90 sources presented in this paper is newly discovered. Finally, we present the general properties in the radio and in the X-ray band of all the AGNs discovered so far in the REX survey.
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Doubly ionized oxygen
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We quantify the active galactic nucleus (AGN) contribution to the mid-infrared (mid-IR) and the total infrared (IR, 8-1000micron) emission in a complete volume-limited sample of 53 local luminous infrared galaxies (LIRGs). We decompose the Spitzer Infrared Spectrograph (IRS) low-resolution 5-38micron spectra of the LIRGs into AGN and starburst components using clumpy torus models and star-forming galaxy templates, respectively. We find that 50% (25/50) of local LIRGs have an AGN component detected with this method. There is good agreement between these AGN detections through mid-IR spectral decomposition and other AGN indicators, such as the optical spectral class, mid-IR spectral features and X-ray properties. Taking all the AGN indicators together, the AGN detection rate in the individual nuclei of LIRGs is ~62%. The derived AGN bolometric luminosities are in the range L_bol(AGN)=0.4 -50x10^{43} erg/s. The AGN bolometric contribution to the IR luminosities of the galaxies is generally small, with 70% of LIRGs having L_bol(AGN)/L_IR<0.05. Only ~8% of local LIRGs have a significant AGN bolometric contribution L_bol(AGN)/L_IR > 0.25. From the comparison of our results with literature results of ultraluminous infrared galaxies, we confirm that in the local universe the AGN bolometric contribution to the IR luminosity increases with the IR luminosity of the galaxy/system. If we add up the AGN bolometric luminosities we find that AGNs only account for 5%^{+8%}_{-3%} of the total IR luminosity produced by local LIRGs (with and without AGN detections). This proves that the bulk of the IR luminosity of local LIRGs is due to star formation activity. Taking the newly determined IR luminosity density of LIRGs in the local universe, we then estimate an AGN IR luminosity density of Omega_IR(AGN) = 3x10^5 L_sun Mpc^{-3}$ in LIRGs.
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Abstract Arp 187 is one of the fading active galactic nuclei (AGNs), whose AGN activity is currently decreasing in luminosity. We investigate the observational signatures of AGN in Arp 187, which trace various physical scales from less than 0.1 pc to the nearly 10 kpc, to estimate the long-term luminosity change over 10 4 yr. The Very Large Array 5 GHz and 8 GHz images and the ALMA 133 GHz images reveal bimodal jet lobes with ∼5 kpc size and the absence of the central radio core. The 6dF optical spectrum shows that Arp 187 hosts a narrow line region with the estimated size of ∼1 kpc, and the line strengths give the AGN luminosity of L bol = 1.5 × 10 46 erg s −1 . On the other hand, the current AGN activity estimated from the AGN torus emission gives the upper bound of L bol < 2.2 × 10 43 erg s −1 . The absence of the radio core gives the more strict upper bound of the current AGN luminosity of L bol < 8.0 × 10 40 erg s −1 , suggesting that the central engine is already quenched. These multiwavelength signatures indicate that Arp 187 hosts a “dying” AGN: the central engine is already dead, but the large-scale AGN indicators are still observable as the remnant of the past AGN activity. The central engine has experienced the drastic luminosity decline by a factor of ∼10 3–5 fainter within ∼10 4 yr, which is roughly consistent with the viscous timescale of the inner part of the accretion disk within ∼500 yr.
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We present a new diagnostic tool based on mid-infrared spectra (5–16 μ m) for distinguishing the emission triggered by the active galactic nucleus (AGN) and by the star formation activity. We show that the AGN spectra, contrary to the starburst spectra, present an important continuum below 9 μ m and an absence of Unidentified Infrared Bands (UIBs).
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We present 71 Emission Line objects selected from the REX survey. Except for 3 of them, for which the presence of an active nucleus is dubious, all these sources are Active Galactic Nuclei (QSOs, Seyfert galaxies, emission line radiogalaxies). In addition, we present the spectra of other 19 AGNs included in a preliminary version of the REX catalog but not in the final one. The majority (80) of the 90 sources presented in this paper is newly discovered. Finally, we present the general properties in the radio and in the X-ray band of all the AGNs discovered so far in the REX survey.
QSOS
Line (geometry)
Doubly ionized oxygen
Extragalactic astronomy
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