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Register a new userThe AGN contribution to mid-infrared surveys
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Dario Fadda
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We provide constraints on the AGN contribution to the mid-IR extragalactic background light from a correlation analysis of deep X-ray and mid-IR observations in two regions centred on the Lockman Hole (LH) and Hubble Deep Field North (HDFN). Among the 76 galaxies detected by XMM in the LH area (200 square arcminutes), 24 show mid-IR emission, but the relative percentage of X-ray sources with mid-IR counterparts increases with the band energy: from 30% of the 0.5-2 keV sources up to 63% of the 5-10 keV sources. In contrast, only a small fraction of the mid-IR sources (around 10%) show X-ray emission. In the region centred on the HDFN (24 square arcminutes), 25% of the mid-IR sources are detected in the X-ray, while 30-40% of the X-ray sources show mid-IR emission. Under the conservative assumption that all XMM sources are AGN-dominated, AGNs contribute 15% of the total mid-IR flux in the LH. For the HDFN we have assumed that AGN-dominated sources are luminous X-ray sources with radio to X-ray SEDs typical of local AGNs, in which case we find that 18% of the mid-IR flux are due to AGN emission. If we put together all the existing information from the deepest HDFN data to the shallow large-area BeppoSAX observations in the ELAIS S1 region using the median mid-IR to X-ray spectral indices as a function of the X-ray flux, we find an AGN contribution to the 15um background of 17%. We conclude that the population of IR luminous galaxies detected in the ISOCAM deep surveys are mainly constituted by dust-obscured starbursts.
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We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z>0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 micron sources are selected from a Spitzer/MIPS survey of the NOAO Deep Wide-Field Survey Bootes field and have corresponding 8 micron data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 micron to 8 micron flux ratio. The X-ray detected sources fall within the peak corresponding to a flat spectrum in nufnu, implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 micron to 8 micron flux ratio is likely due to LIRGs whose infrared emission is powered by starbursts. The 24 micron emission is increasingly dominated by AGN at higher 24 micron flux densities (f_24): the AGN fraction of the z>0.6 sources increases from ~9% at f_24 ~ 0.35 mJy to 74+/-20% at f_24 ~ 3 mJy in good agreement with model predictions. Deep 24 micron, small area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for ~ 3-7% of the total 24 micron background.
We present the broad-band Spectral Energy Distributions (SEDs) of the largest available highly (72%) complete spectroscopic sample of mid-infrared (MIR) selected galaxies and AGN at intermediate redshift. The sample contains 203 extragalactic sources from the 15-micron survey in the ELAIS-SWIRE field S1, all with measured spectroscopic redshift. Most of these sources have full multi-wavelength coverage from the far-UV to the far-infrared and lie in the redshift range 0.1<z<1.3. Due to its size, this sample allows us for the first time to characterise the spectral properties of the sources responsible for the strong evolution observed in the MIR. Based on SED-fitting technique we have classified the MIR sources, identifying AGN signatures in about 50% of them. This fraction is significantly higher than that derived from optical spectroscopy (~29%) and is due in particular to the identification of AGN activity in objects spectroscopically classified as galaxies. It is likely that in most of our objects, the AGN is either obscured or of low-luminosity, and thus it does not dominate the energetic output at any wavelength, except in the MIR, showing up just in the range where the host galaxy SED has a minimum. The fraction of AGN strongly depends on the flux density, with that derived through the SED-fitting being about 20% at S(15)~0.5-1 mJy and gradually increasing up to 100% at S(15)>10 mJy, while that obtained from optical spectroscopy never being >30%, even at the higher flux densities. The results of this work will be very useful for updating all the models aimed at interpreting the deep infrared survey data and, in particular, for constraining the nature and the role of dust-obscured systems in the intermediate/high-redshift Universe.
(abridged) Infrared selection is a potentially powerful way to identify heavily obscured AGN missed in even the deepest X-ray surveys. Using a 24 micron-selected sample in GOODS-S, we test the reliability and completeness of three infrared AGN selection methods: (1) IRAC color-color selection, (2) IRAC power-law selection, and (3) IR-excess selection; we also evaluate a number of infrared excess approaches. We find that the vast majority of non-power-law IRAC color-selected AGN candidates in GOODS-S have colors consistent with those of star-forming galaxies. Contamination by star-forming galaxies is most prevalent at low 24 micron flux densities (~100 uJy) and high redshifts (z~2), but the fraction of potential contaminants is still high (~50%) at 500 uJy, the highest flux density probed reliably by our survey. AGN candidates selected via a simple, physically-motivated power-law criterion (PLGs), however, appear to be reliable. We confirm that the infrared excess methods successfully identify a number of AGN, but we also find that such samples may be significantly contaminated by star-forming galaxies. Adding only the secure Spitzer-selected PLG, color-selected, IR-excess, and radio/IR-selected AGN candidates to the deepest X-ray-selected AGN samples directly increases the number of known X-ray AGN (84) by 54-77%, and implies an increase to the number of 24 micron-detected AGN of 71-94%. Finally, we show that the fraction of MIR sources dominated by an AGN decreases with decreasing MIR flux density, but only down to f_24 = 300 uJy. Below this limit, the AGN fraction levels out, indicating that a non-negligible fraction (~10%) of faint 24 micron sources (the majority of which are missed in the X-ray) are powered not by star formation, but by the central engine.
We present the results of a program of optical and near-infrared spectroscopic follow-up of candidate Active Galactic Nuclei (AGN) selected in the mid-infrared. This survey selects both normal and obscured AGN closely matched in luminosity across a wide range, from Seyfert galaxies with bolometric luminosities L_bol~10^10L_sun, to highly luminous quasars (L_bol~10^14L_sun), and with redshifts from 0-4.3. Samples of candidate AGN were selected through mid-infrared color cuts at several different 24 micron flux density limits to ensure a range of luminosities at a given redshift. The survey consists of 786 candidate AGN and quasars, of which 672 have spectroscopic redshifts and classifications. Of these, 137 (20%) are type-1 AGN with blue continua, 294 (44%) are type-2 objects with extinctions A_V>~5 towards their AGN, 96 (14%) are AGN with lower extinctions (A_V~1) and 145 (22%) have redshifts, but no clear signs of AGN activity in their spectra. 50% of the survey objects have L_bol >10^12L_sun, in the quasar regime. We present composite spectra for type-2 quasars and for objects with no signs of AGN activity in their spectra. We also discuss the mid-infrared - emission-line luminosity correlation and present the results of cross-correlations with serendipitous X-ray and radio sources. The results show that: (1) obscured objects dominate the overall AGN population, (2) there exist mid-infrared selected AGN candidates which lack AGN signatures in their optical spectra, but have AGN-like X-ray or radio counterparts, and (3) X-ray and optical classifications of obscured and unobscured AGN often differ.
We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 micron mid-infrared extraphotospheric excesses in the MidCourse Space Experiment (MSX) and Spitzer Space Telescope Galactic Legacy MidPlane Survey Extraordinaire (GLIMPSE) surveys. Of these, 183 are either OB stars earlier than B8 in which the excess plausibly arises from a thermal bremsstrahlung component or evolved stars in which the excess may be explained by an atmospheric dust component. The remaining 47 stars have spectral classifications B8 or later and appear to be main sequence or late pre-main-sequence objects harboring circumstellar disks. Six of the 47 stars exhibit multiple signatures characteristic of pre-main-sequence circumstellar disks, including emission lines, near-infrared K-band excesses, and X-ray emission. Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth. Of the 25 GLIMPSE stars with SST data at >24 microns, 20 also show an excess at 24 microns. Three additional objects have 24 micron upper limits consistent with possible excesses, and two objects have photospheric measurements at 24 microns. Six MSX sources had a measurement at wavelengths >8 microns. We modeled the excesses in 26 stars having two or more measurements in excess of the expected photospheres as single-component blackbodies. We determine probable disk temperatures and fractional infrared luminosities in the range 191 < T < 787 and 3.9x10^-4 < L_IR/L_* < 2.7x10^-1. We estimate a lower limit on the fraction of Tycho-2 Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses to be 1.0+-0.3%.
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