Francine Roxanne Marleau

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.

MIPSGAL: A Survey of the Inner Galactic Plane at 24 and 70 μm

MIPSGAL is a 278 deg^2 survey of the inner Galactic plane using the Multiband Infrared Photometer for Spitzer aboard the Spitzer Space Telescope. The survey field was imaged in two passbands, 24 and 70 μm with resolutions of 6″ and 18″, respectively. The survey was designed to provide a uniform, well-calibrated and well-characterized data set for general inquiry of the inner Galactic plane and as a longer-wavelength complement to the shorter-wavelength Spitzer survey of the Galactic plane: Galactic Plane Infrared Mapping Survey Extraordinaire. The primary science drivers of the current survey are to identify all high-mass (M > 5 M⊙) protostars in the inner Galactic disk and to probe the distribution, energetics, and properties of interstellar dust in the Galactic disk. The observations were planned to minimize data artifacts due to image latents at 24 μm and to provide full coverage at 70 μm. Observations at ecliptic latitudes within 15° of the ecliptic plane were taken at multiple epochs to help reject asteroids. The data for the survey were collected in three epochs, 2005 September–October, 2006 April, and 2006 October with all of the data available to the public. The estimated point-source sensitivities of the survey are 2 and 75 mJy (3 σ) at 24 and 70 μm, respectively. Additional data processing was needed to mitigate image artifacts due to bright sources at 24 μm and detector responsivity variations at 70 μm due to the large dynamic range of the Galactic plane. Enhanced data products including artifact-mitigated mosaics and point-source catalogs are being produced with the 24 μm mosaics already publicly available from the NASA/IPAC Infrared Science Archive. Some preliminary results using the enhanced data products are described.

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.

Point-Source Extraction with MOPEX

ABSTRACT MOPEX (Mosaicking and Point‐source Extraction) is a package developed at the Spitzer Science Center for astronomical image processing. We report on the point‐source extraction capabilities of MOPEX. Point‐source extraction is implemented as a two‐step process: point‐source detection and profile fitting. Nonlinear matched filtering of input images can be performed optionally to increase the signal‐to‐noise ratio and improve detection of faint point sources. Point response function (PRF) fitting of point sources produces the final point‐source list, which includes the fluxes and improved positions of the point sources, along with other parameters characterizing the fit. Passive and active deblending allows for successful fitting of confused point sources. Aperture photometry can also be computed for every extracted point source for an unlimited number of aperture sizes. The PRF is estimated directly from the input images. The implementation of efficient methods of background and noise estimation an...

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.

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.

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.

Spitzer/mips infrared imaging of m31: further evidence for a spiral/ring composite structure

New images of M31 at 24, 70, and 160 μm taken with the Multiband Imaging Photometer for Spitzer (MIPS) reveal the morphology of the dust in this galaxy. This morphology is well represented by a composite of two logarithmic spiral arms and a circular ring (radius ~10 kpc) of star formation offset from the nucleus. The two spiral arms appear to start at the ends of a bar in the nuclear region and extend beyond the star-forming ring. As has been found in previous work, the spiral arms are not continuous, but composed of spiral segments. The star-forming ring is very circular except for a region near M32 where it splits. The lack of well-defined spiral arms and the prominence of the nearly circular ring suggest that M31 has been distorted by interactions with its satellite galaxies. Using new dynamical simulations of M31 interacting with M32 and NGC 205, we find that, qualitatively, such interactions can produce an offset, split ring like that seen in the MIPS images.

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.

A population of high-redshift type 2 quasars – I. Selection criteria and optical spectra

We discuss the relative merits of mid-infrared and X-ray selection of type 2 quasars. We describe the mid-infrared, near-infrared and radio selection criteria used to find a population of redshift z∼ 2 type 2 quasars which we previously argued suggests that most supermassive black hole growth in the Universe is obscured. We present the optical spectra obtained from the William Herschel Telescope, and we compare the narrow emission-line luminosity, radio luminosity and maximum size of jets to those of objects from radio-selected samples. This analysis suggests that these are genuine radio-quiet type 2 quasars, albeit the radio-bright end of this population. We also discuss the possibility of two different types of quasar obscuration, which could explain how the ∼2–3:1 ratio of type 2 to type 1 quasars preferred by modelling our population can be reconciled with the ∼1:1 ratio predicted by unified schemes.