D. Fadda

Obscured and unobscured active galactic nuclei in the Spitzer Space Telescope First Look Survey

Selection of active galactic nuclei (AGNs) in the infrared facilitates the discovery of AGNs whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess the fraction of AGNs with mid-infrared (MIR) luminosities that are comparable to quasars and that are missed in optical quasar surveys because of dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg^2 extragalactic FLS. These quasars occupy a distinct region in MIR color space by virtue of their strong, red continua. This has allowed us to define an MIR color criterion for selecting AGN candidates. About 2000 FLS objects have colors that are consistent with them being AGNs, but most are much fainter in the MIR than the SDSS quasars, which typically have 8 μm flux densities S_(8.0) ~ 1 mJy. We have investigated the properties of 43 objects with S_(8.0) ≥ 1 mJy that satisfy our AGN color selection. This sample should contain both unobscured quasars as well as AGNs that are absent from the SDSS survey because of extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects that are likely to be obscured quasars or luminous Seyfert 2 galaxies. This suggests that the numbers of obscured and unobscured AGNs are similar in samples selected in the MIR at S_(8.0) ~ 1 mJy.

Star formation and extinction in redshift z~2 galaxies: inferences from spitzer mips observations

We use very deep Spitzer MIPS 24 ?m observations to examine the bolometric luminosities (Lbol) and UV extinction properties of more than 200 spectroscopically identified, optically selected (UnG) z ~ 2 galaxies, supplemented with near-IR-selected (BzK and DRG) and submillimeter galaxies at similar redshifts, in the GOODS-N field. Focusing on redshifts 1.5 1012 L?, with a mean LIR 2 ? 1011 L?. Using 24 ?m observations as an independent probe of dust extinction, we find that, as in the local universe, the obscuration LIR/L1600 is strongly dependent on Lbol and ranges in value from <1 to ~1000 within the sample considered. However, the obscuration is generally ~10 times smaller at a given Lbol at z ~ 2 than at z ~ 0. We show that the values of LIR and obscuration inferred from the UV spectral slope ? generally agree well with the values inferred from L5-8.5 ?m for Lbol < 1012 L?. Using the specific SFRs of galaxies as a proxy for cold gas fraction, we find a wide range in the evolutionary state of galaxies at z ~ 2, from galaxies that have just begun to form stars to those that have already accumulated most of their stellar mass and are about to become, or already are, passively evolving.

Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. I. The Stellar Calibrator Sample and the 24 μm Calibration

We present the stellar calibrator sample and the conversion from instrumental to physical units for the 24 μm channel of the Multiband Imaging Photometer for Spitzer (MIPS). The primary calibrators are A stars, and the calibration factor based on those stars is MJy sr^−1 (DN s^−1)^−1, with a nominal uncertainty of 2%. We discuss the data reduction procedures required to attain this accuracy; without these procedures, the calibration factor obtained using the automated pipeline at the Spitzer Science Center is lower. We extend this work to predict 24 μm flux densities for a sample of 238 stars that covers a larger range of flux densities and spectral types. We present a total of 348 measurements of 141 stars at 24 μm. This sample covers a factor of 460 in 24 μm flux density, from 8.6 mJy up to 4.0 Jy. We show that the calibration is linear over that range with respect to target flux and background level. The calibration is based on observations made using 3 s exposures; a preliminary analysis shows that the calibration factor may be 1% and 2% lower for 10 and 30 s exposures, respectively. We also demonstrate that the calibration is very stable: over the course of the mission, repeated measurements of our routine calibrator, HD 159330, show a rms scatter of only 0.4%. Finally, we show that the point-spread function (PSF) is well measured and allows us to calibrate extended sources accurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a sample of nearby galaxies are identical within the uncertainties.

Absolute calibration and characterization of the multiband imaging photometer for Spitzer. II. 70 μm imaging

The absolute calibration and characterization of the Multiband Imaging Photometer for Spitzer (MIPS) 70 μm coarse‐and fine‐scale imaging modes are presented based on over 2.5 yr of observations. Accurate photometry (especially for faint sources) requires two simple processing steps beyond the standard data reduction to remove long‐term detector transients. Point‐spread function (PSF) fitting photometry is found to give more accurate flux densities than aperture photometry. Based on the PSF fitting photometry, the calibration factor shows no strong trend with flux density, background, spectral type, exposure time, or time since anneals. The coarse‐scale calibration sample includes observations of stars with flux densities from 22 mJy to 17 Jy, on backgrounds from 4 to 26 MJy sr^(−1), and with spectral types from B to M. The coarse‐scale calibration is 702 ± 35 MJy sr^(−1) MIPS70^(−1) (5% uncertainty) and is based on measurements of 66 stars. The instrumental units of the MIPS 70 μm coarse‐ and fine‐scale imaging modes are called MIPS70 and MIPS70F, respectively. The photometric repeatability is calculated to be 4.5% from two stars measured during every MIPS campaign and includes variations on all timescales probed. The preliminary fine‐scale calibration factor is 2894 ± 294 MJy sr^(−1) MIPS70F^(−1) (10% uncertainty) based on 10 stars. The uncertainties in the coarse‐ and fine‐scale calibration factors are dominated by the 4.5% photometric repeatability and the small sample size, respectively. The 5 σ, 500 s sensitivity of the coarse‐scale observations is 6–8 mJy. This work shows that the MIPS 70 μm array produces accurate, well‐calibrated photometry and validates the MIPS 70 μm operating strategy, especially the use of frequent stimulator flashes to track the changing responsivities of the Ge:Ga detectors.

SPITZER MID-INFRARED SPECTROSCOPY OF INFRARED LUMINOUS GALAXIES AT z 2. I. THE SPECTRA

We present the mid-infrared spectra obtained with the Spitzer IRS for 52 sources, selected as infrared luminous, z ≳ 1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f_(24 μm) ≳ 0.9 mJy, νf_ν(24 μm)/νf_ν(8 μm) ≳ 3.16, and νf_ν(24 μm)/νf_ν(0.7 μm) ≳ 10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with 35/47 (74%) at 1.5 ≲ z ≲ 3.2. Keck spectroscopy of a subsample (17/47) agrees with the mid-IR redshift measurements. The observed spectra fall into three categories: (1) 33% (17/52) have strong PAH emission and are probably powered by star formation with total IR luminosity roughly a factor of 5 higher than the local starburst ULIRGs; (2) 33% (17/52) have only deep silicate absorption at 9.8 μm, indicatiing deeply embedded dusty systems—these data alone cannot determine the energetic nature of the heating sources in these systems; and (3) the remaining 34% are mid-IR continuum-dominated systems with weak PAH emission and/or silicate absorption. This subsample is probably AGNs. We derived monochromatic, rest-frame 5.8 μm, continuum luminosities (νL_ν), ranging from 10^(10.3) to 10^(12.6) L_☉. Our spectra have mid-IR slope α_(5-15 μm) ≳ 2.1, much redder than the median value of 1.3 for the optically selected PG quasars. From the silicate absorption feature, we estimate that roughly two-thirds of the sample have optical depth τ_(9.8 μm) > 1. Their L_(1600 A) and L_(IR) suggest that our sample is among the most luminous and most dust-enshrouded systems of its epoch. Our study has revealed a significant population of dust-enshrouded galaxies at z ~ 2, whose enormous energy output, comparable to that of quasars, is generated by AGNs, as well as starbursts. This IR-luminous population has very little overlap with submillimeter and UV-selected populations.

The obscuration by dust of most of the growth of supermassive black holes

Supermassive black holes underwent periods of exponential growth during which we see them as quasars in the distant Universe. The summed emission from these quasars generates the cosmic X-ray background, the spectrum of which has been used to argue that most black-hole growth is obscured. There are clear examples of obscured black-hole growth in the form of ‘type-2’ quasars, but their numbers are fewer than expected from modelling of the X-ray background. Here we report the direct detection of a population of distant type-2 quasars, which is at least comparable in size to the well-known unobscured type-1 population. We selected objects that have mid-infrared and radio emissions characteristic of quasars, but which are faint at near-infrared and optical wavelengths. We conclude that this population is responsible for most of the black-hole growth in the young Universe and that, throughout cosmic history, black-hole growth occurs in the dusty, gas-rich centres of active galaxies.

Spitzer detection of polycyclic aromatic hydrocarbon and silicate dust features in the mid-infrared spectra of z ∼ 2 ultraluminous infrared galaxies

We report the initial results from a Spitzer GO-1 program to obtain low-resolution, mid-infrared spectra of infrared luminous galaxies at z ~ 1-2. This paper presents the spectra of eight sources observed with the Spitzer Infrared Spectrograph (IRS). Of the eight spectra, six have mid-IR spectral features, either emission from polycyclic aromatic hydrocarbon (PAH) or silicate absorption. Based on these mid-IR features, the inferred six redshifts are in the range of 1.8-2.6. The remaining two spectra detect only strong continua and thus do not yield redshift information. Strong, multiple PAH emission features are detected in two sources, and weak PAH emission is detected in another two. These data provide direct evidence that PAH molecules are present and directly observable in ultraluminous infrared galaxies (ULIRGs) at z ~ 2. The six sources with measured redshifts are dusty, infrared luminous galaxies at z ~ 2 with estimated Lbol ~ 1013 L☉. Of the eight sources, two appear to be starburst dominated; two with only power-law continua are probably type 1 QSOs; and the remaining four are likely composite systems containing a buried active galactic nucleus (AGN) and a starburst component. Since half of our sample contains optically faint sources with R ≥ 25.5 mag (Vega), our results demonstrate the potential of using mid-infrared spectroscopy, especially the aromatic and silicate features produced by small dust grains, to directly probe optically faint and infrared luminous populations at high redshift.

Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2. II. Diagnostics

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z ~ 1-3 and likely L_(IR) ~ 10^(12)-10^(13) L_⊙. Our study aims at quantifying the star formation and active galactic nucleus (AGN) processes in these sources that recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum (i.e., AGN) dominated sources, but ~60% of these show PAH emission, suggesting the presence of star formation activity. These sources have redder mid-IR colors than typical optically selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5 μm hot dust continua. However, their steep 30-14 μm slopes suggest that star formation might dominate the total infrared luminosity. Six of our z ~ 2 sources have EW_(6.2) ≳ 0.3 μm and L_(14 μm) ≳ 10^(12) L_⊙ (implying L_(IR) ≳ 10^(13) L_⊙). At these luminosities, such high EW_(6.2) ULIRGs do not exist in the local universe. We find a median optical depth at 9.7 μm of = 1.4. This is consistent with local IRAS-selected ULIRGs but differs from early results on SCUBA-selected z ~ 2 ULIRGs. Similar to local ULIRGs, about 25% of our sample shows extreme obscuration (τ_(9.7 μm) ≳ 3), suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs but are an order of magnitude more luminous. It is not clear whether our z ~ 2 ULIRGs are simply scaled-up versions of local ULIRGs or subject to fundamentally different physical processes.

OPTICAL SPECTROSCOPY AND X-RAY DETECTIONS OF A SAMPLE OF QUASARS AND ACTIVE GALACTIC NUCLEI SELECTED IN THE MID-INFRARED FROM TWO SPITZER SPACE TELESCOPE WIDE-AREA SURVEYS

We present optical spectroscopy of a sample of 77 luminous active galactic nuclei (AGNs) and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and the SWIRE XMM-Newton Large-Scale Structure Survey (XMM-LSS) fields, with a typical 24 μm flux density of 5 mJy. The median redshift is 0.6, with a range of ~0.05-4. Only 33% (25 out of 77) of these objects are normal type 1 quasars, with no obscuration. Forty-four percent (34 out of 77) are type 2 objects, with high-ionization, narrow emission lines, and 14% (11 out of 77) are dust-reddened type 1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. Nine percent (7 out of 77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra that could be of either starburst or LINER type. These latter objects are analogous to the X-ray-detected population of AGNs with weak or nonexistent optical AGN emission (the X-ray-bright, optically normal galaxies). Of our objects from the SWIRE field, 21 fall within moderately deep XMM-Newton exposures. All the unobscured quasars and about half the obscured quasars are detected in these exposures. This sample, when taken together with other samples of Spitzer-selected AGNs and quasars and results from X-ray studies, confirms that obscured AGNs dominate the AGN and quasar number counts of all rapidly accreting supermassive black hole systems, at least for z ≾ 4. This implies a high radiative efficiency for the black hole accretion process.

The Role of Galaxy Interactions and Mergers in Star Formation at z ≤ 1.3: Mid-Infrared Properties in the Spitzer First Look Survey*

By combining the 0.12 deg2 F814W Hubble Space Telescope (HST) and Spitzer MIPS 24 μm imaging in the First Look Survey (FLS), we investigate the properties of interacting and merging mid-infrared bright and faint sources at 0.2 ≤ z ≤ 1.3. We find a marginally significant increase in the pair fraction for MIPS 24 μm detected, optically selected close pairs, with a pair fraction of 0.25 ± 0.10 at z ~ 1, in contrast to 0.11 ± 0.08 at z ~ 0.4, while galaxies below our 24 μm MIPS detection limit show a pair fraction consistent with zero at all redshifts. In addition, 24 μm detected galaxies with fluxes ≥ 0.1 mJy are on average 5 times more likely to be in a close galaxy pair in the range 0.2 ≤ z ≤ 1.3 than galaxies below this flux limit. Using the 24 μm flux to derive the total far-IR luminosity, we find that paired galaxies (early-stage mergers) are responsible for 27% ± 9% of the IR luminosity density resulting from star formation at z ~ 1, while morphologically classified (late stage) mergers make up 34% ± 11%. This implies that 61% ± 14% of the infrared luminosity density and, in turn, ~40% of the star formation rate density at z ~ 1 can be attributed to galaxies at some stage of a major merger or interaction. We argue that close pairs/mergers in a LIRG/ULIRG phase become increasingly important contributors to the IR luminosity and star formation rate density of the universe at z > 0.7.