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Register a new userSpatial resolution bias in the mid-infrared Starburst/AGN classification
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We present the effects of limited spatial resolution to the observed mid-infrared (MIR) spectrum of an active galactic nucleus (AGN) surrounded by a disk with massive star forming regions. Using MIR observations of the face-on nearby Seyfert 1 galaxy NGC 6814, we vary the observing aperture and examine the evolution of the observed AGN/starburst fraction with our MIR diagnostic. We show that the spatial resolution of ISOCAM is sufficient to disentangle AGN from starburst features in nuclear regions of nearby galaxies (D<50Mpc). However, with the exception of a few ultra-luminous galaxies, dilution effects hide completely the AGN contribution in more distant galaxies.
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Spectra have been obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope for 20 sources in the Lockman Hole field of the SWIRE survey. The sample is divided between sources with indicators of an obscured AGN, based primarily on X-ray detections of optically-faint sources, and sources with indicators of a starburst, based on optical and near-infrared spectral energy distributions (SEDs) which show a luminosity peak from stellar photospheric emission. Ten of the 11 AGN sources have IRS spectra which show silicate absorption or are power laws; only one AGN source shows PAH emission features. All 9 of the sources showing starburst SEDs in the near-infrared show PAH emission features in the IRS spectra. Redshifts are determined from the IRS spectra for all 9 starbursts (1.0 < z < 1.9) and 8 of the 11 AGN (0.6 < z < 2.5). Classification as AGN because of an X-ray detection, the classification as AGN or starburst derived from the photometric SED, and the IRS spectroscopic classification as AGN (silicate absorption) or starburst (PAH emission) are all consistent in 18 of 20 sources. The surface density for starbursts which are most luminous in the mid-infrared is less than that for the most luminous AGN within the redshift interval 1.7 < z < 1.9. This result implies that mid-infrared source counts at high redshift are dominated by AGN for f(24micron) > 1.0 mJy.
We present an atlas of Spitzer/IRS high resolution (R~600) 10-37um spectra for 24 well known starburst galaxies. The spectra are dominated by fine-structure lines, molecular hydrogen lines, and emission bands of polycyclic aromatic hydrocarbons. Six out of the eight objects with a known AGN component show emission of the high excitation [NeV] line. This line is also seen in one other object (NGC4194) with, a priori, no known AGN component. In addition to strong polycyclic aromatic hydrocarbon emission features in this wavelength range (11.3, 12.7, 16.4um), the spectra reveal other weak hydrocarbon features at 10.6, 13.5, 14.2um, and a previously unreported emission feature at 10.75um. An unidentified absorption feature at 13.7um is detected in many of the starbursts. We use the fine-structure lines to derive the abundance of neon and sulfur for 14 objects where the HI 7-6 line is detected. We further use the molecular hydrogen lines to sample the properties of the warm molecular gas. Several basic diagrams characterizing the properties of the sample are also shown. We have combined the spectra of all the pure starburst objects to create a high S/N template, which is available to the community.
We present diffraction-limited (FWHM ~ 0.3arcsec) Gemini/T-ReCS mid-infrared (MIR: N-band or narrow-band at 8.7micron) imaging of four Luminous Infrared Galaxies (LIRGs) drawn from a representative local sample. The MIR emission in the central few kpc is strikingly similar to that traced by Pa-alpha, and generally consists of bright nuclear emission and several compact circumnuclear and/or extranuclear HII regions. The central MIR emission is dominated by these powerful HII regions, consistent with the majority of AGN in this local sample of LIRGs contributing a minor part of the MIR emission. The luminous circumnuclear HII regions detected in LIRGs follow the extrapolation of the 8micron vs. Pa-alpha relation found for M51 HII knots. The integrated central 3-7kpc of galaxies, however, present elevated 8micron/Pa-alpha ratios with respect to individual HII regions, similar to the integrated values for star-forming galaxies. Our results show that the diffuse 8micron emission, not directly related to the ionizing stellar population, can be as luminous as that from the resolved HII regions. Therefore, calibrations of the star formation rate for distant galaxies should be based on the integrated 8micron emission of nearby galaxies, not that of the HII regions alone.
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.
High spatial resolution mid-infrared (MIR) 12 mum continuum imaging of low-luminosity active galactic nuclei (LLAGN) obtained by VLT/VISIR is presented. The goal of this investigation is to determine if the nuclear MIR emission of LLAGN is consistent with the existence of a dusty obscuring torus. A sample of 17 nearby LLAGN was selected and combined with archival VISIR data of 9 additional LLAGN with available X-ray measurements. Of the 17 observed LLAGN, 7 are detected, while upper limits are derived for the 10 non-detections. All detections except NGC 3125 appear point-like on a spatial scale of sim 0.35. The detections do not significantly deviate from the known MIR-X-ray correlation but extend it by a factor of sim 10 down to luminosities < 10^41 erg/s with a narrow scatter. The latter is dominated by the uncertainties in the X-ray luminosity. Interestingly, a similar correlation with comparable slope but with a normalization differing by sim 2.6 orders of magnitude has been found for local starburst galaxies. In addition, the VISIR data are compared with lower spatial resolution data from Spitzer/IRS and IRAS. By using a scaled starburst template SED and the PAH 11.3 mum emission line the maximum nuclear star formation contamination to the VISIR photometry is restricted to < 30% for 75% of the LLAGN. Exceptions are NGC 1097 and NGC 1566, which may possess unresolved strong PAH emission. Furthermore, within the uncertainties the MIR-X-ray luminosity ratio is unchanged over more than 4 orders of magnitude in accretion rate. These results are consistent with the existence of the dusty torus in all observed LLAGN, although the jet or accretion disk as origin of the MIR emission cannot be excluded. Finally, the fact that the MIR-X-ray correlation holds for all LLAGN and Seyferts makes it a very useful empirical tool for converting between the MIR and X-ray powers of these nuclei.
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