arXiv subjects

B. T. Soifer

Publications and source records attributed to B. T. Soifer.

At least 55 records · Page 3Linked to original sources

arXiv2006

The Spitzer Space Telescope Extra-Galactic First Look Survey: 24 micron data reduction, catalog, and source identification

We present the reduction of the 24 micron data obtained during the first cosmological survey performed by the Spitzer Space Telescope (First Look Survey, FLS). The survey consists of a shallow observation of 2.5x2 sq deg centered at 17h18m +59d30m (main survey) and a deeper observation of 1x0.5 sq deg centered at 17h17m +59d45m(verification survey). Issues with the reduction of the 24 micron MIPS data are discussed and solutions to attenuate instrumental effects are proposed and applied to the data. Approximately 17000 sources are extracted with a SNR greater than five. The photometry of the point sources is evaluated through PSF fitting using an empirical PSF derived from the data. Aperture corrections and the absolute calibration have been checked using stars in the field. Astrometric and photometric errors depend on the SNR of the source varying between 0.35-1 arcsec and 5-15%, respectively, for sources detected at 20-5 sigma. The flux of the 123 extended sources have been estimated through aperture photometry. The extended sources cover less than 0.3% of the total area of the survey. Based on simulations, the main and verification surveys are 50% complete at 0.3 and 0.15 mJy, respectively. Counterparts have been searched for in optical and radio catalogs. More than 80% of the 24 micron sources have a reliable optical counterpart down to R=25.5. 16% of the sources have a 20 cm counterpart down to 0.1 mJy and ~ 80% of the radio-infrared associations have a reliable optical counterpart. A residual map is obtained by subtracting point sources detected at the 3-sigma level and interpolating the regions occupied by extended sources. Several galactic clouds with low and intermediate velocities are identified by comparison with neutral Hydrogen data from this field.

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arXiv2006

The AGN Contribution to the Mid-IR Emission of Luminous Infrared Galaxies

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 Boötes 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.

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arXiv2006

Silicate Emission in the Spitzer IRS spectrum of FSC 10214+4724

We present the first MIR spectrum of the z=2.2856 ultraluminous, infrared galaxy FSC 10214+4724, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope. The spectrum spans a rest wavelength range of 2.3-11.5 microns, covering a number of key diagnostic emission and absorption features. The most prominent feature in the IRS spectrum is the silicate emission at rest-frame 10 microns. We also detect an unresolved emission line at a rest wavelength of 7.65 microns which we identify with [NeVI], and a slightly resolved feature at 5.6 microns identified as a blend of [Mg VII] and [Mg V]. There are no strong PAH emission features in the FSC 10214+4724 spectrum. We place a limit of 0.1 micron on the equivalent width of 6.2 micron PAH emission but see no evidence of a corresponding 7.7 micron feature. Semi-empirical fits to the spectral energy distribution suggest about 45% of the bolometric luminosity arises from cold 50 K dust, half arises from warm (190 K) dust, and the remainder, 5%, originates from hot (640 K) dust. The hot dust is required to fit the blue end of the steep MIR spectrum. The combination of a red continuum, strong silicate emission, little or no PAH emission, and no silicate absorption, makes FSC 10214+4724 unlike most other ULIRGs or AGN observed thus far with IRS. These apparently contradictory properties may be explained by an AGN which is highly magnified by the lens, masking a (dominant) overlying starburst with unusually weak PAH emission.

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arXiv2005

IRS spectra of two ultraluminous infrared galaxies at z=1.3

We present low-resolution (64 < R < 124) mid-infrared (8--38 micron) Spitzer/IRS spectra of two z~1.3 ultraluminous infrared galaxies (LFIR~10^13) discovered in a Spitzer/MIPS survey of the Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). MIPS J142824.0+352619 is a bright 160 micron source with a large infrared-to-optical flux density ratio and a possible lensing amplification of <~10. The 6.2, 7.7, 11.3, and 12.8 micron PAH emission bands in its IRS spectrum indicate a redshift of z~1.3. The large equivalent width of the 6.2 micron PAH feature indicates that at least 50% of the mid-infrared energy is generated in a starburst, an interpretation that is supported by a large [NeII]/[NeIII] ratio and a low upper limit on the X-ray luminosity. SST24 J142827.19+354127.71 has the brightest 24 micron flux (10.55 mJy) among optically faint (R > 20) galaxies in the NDWFS. Its mid-infrared spectrum lacks emission features, but the broad 9.7 micron silicate absorption band places this source at z~1.3. Given this redshift, SST24 J142827.19+354127.71 has among the largest rest-frame 5 micron luminosities known. The similarity of its SED to those of known AGN-dominated ULIRGs and its lack of either PAH features or large amounts of cool dust indicate that the powerful mid-infrared emission is dominated by an active nucleus rather than a starburst. Our results illustrate the power of the IRS in identifying massive galaxies in the ``redshift desert'' and in discerning their power sources. Because they are bright, MIPS J142824.0+352619 (pending future observations to constrain its lensing amplification) and SST24 J142827.19+354127.71 are useful z>1 templates of a high luminosity starburst and AGN, respectively.

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arXiv2005

Detection of the Buried AGN in NGC 6240 with the Infrared Spectrograph on The Spitzer Space Telescope

We present mid-infrared spectra of the nearby, Ultraluminous Infrared Galaxy NGC 6240 taken with the IRS on Spitzer. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and PAH emission features. The H2 line fluxes suggest molecular gas at a variety of temperatures. A simple two-temperature fit to the S(0) through S(7) lines implies a mass of 6.7E6 solar masses at T=957K and 1.6E9 solar masses at T=164K, or about 15% of the total molecular gas mass in this system. Noteably, we have detected the [NeV] 14.3 micron emission line, with a flux of 5E-14 erg/cm^2/s, providing the first direct detection of the buried AGN in the mid-infrared. Modelling of the total SED from near to far-infrared wavelengths requires the presence of a hot dust (T=700K) component, which we also associate with the buried AGN. The small [NeV]/[NeII] and [NeV]/IR flux ratios, the relative fraction of hot dust emission, as well as the large 6.2 micron PAH EQW, are all consistent with an apparent AGN contribution of only 3-5% to the bolometric luminosity. However, correcting the measured [NeV] flux by the extinction implied by the silicate optical depth and our SED fitting, suggests an intrinsic fractional AGN contribution to the bolometric luminosity of about 20-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum.

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arXiv2005

Sub-Arcsecond Mid-Infrared Observations of NGC 6240: Limitations of AGN-Starburst Power Diagnostics

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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arXiv2005

Star Formation Rates and Extinction Properties of IR-Luminous Galaxies in the Spitzer First Look Survey

We investigate the instantaneous star formation rates (SFR) and extinction properties for a large (N=274), near-infrared (2.2 micron) + mid-infrared (24 micron) selected sample of normal to ultra-luminous infrared galaxies (ULIRGs) [10^9 ~0.8 in the Spitzer Extragalactic First Look Survey. We combine Spitzer MIPS 24-micron observations with high-resolution, optical Keck Deimos spectroscopy to derive optical emission-line and infrared star formation rates (SFR_{opt} & SFR_{IR}, respectively). Direct comparison of these SFR diagnostics reveals that our sample exhibits a wide range of extinction (1.0 1, which make up a large fraction of our sample, have SFR~100 M_{sun}/yr and ~2.5 mag. This translates to a 97% mean attenuation of the [OII] forbidden line doublet, with the most extreme sources having as much as 99.7% of their [OII] line flux extinguished by dust. Based on a SFR_{IR}/SFR_{opt} diagnostic, we derive an IR-luminosity-dependent Av^{IR} function [Av^{IR}=0.75*log(L_{IR}/L_{sun})-6.35 mag] that we use to extinction correct our emission line luminosities. Application of this correction results in a correlation between SFR_{IR} and SFR_{opt} that has a dispersion of 0.2 dex. Investigation of the Av dependence on redshift reveals that for a fixed L_{IR}, there is no significant Av evolution. The mean attenuation of our sample is intermediate between that of local optical/UV- and radio-selected samples and has a marginally stronger L_{IR} dependence.

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arXiv2005

The Detection of Crystalline Silicates in Ultra-Luminous Infrared Galaxies

Silicates are an important component of interstellar dust and the structure of these grains -- amorphous versus crystalline -- is sensitive to the local physical conditions. We have studied the infrared spectra of a sample of ultra-luminous infrared galaxies. Here, we report the discovery of weak, narrow absorption features at 11, 16, 19, 23, and 28 microns, characteristic of crystalline silicates, superimposed on the broad absorption bands at 10 and 18 microns due to amorphous silicates in a subset of this sample. These features betray the presence of forsterite (Mg_2SiO_4), the magnesium-rich end member of the olivines. Previously, crystalline silicates have only been observed in circumstellar environments. The derived fraction of forsterite to amorphous silicates is typically 0.1 in these ULIRGs. This is much larger than the upper limit for this ratio in the interstellar medium of the Milky Way, 0.01. These results suggest that the timescale for injection of crystalline silicates into the ISM is short in a merger-driven starburst environment (e.g., as compared to the total time to dissipate the gas), pointing towards massive stars as a prominent source of crystalline silicates. Furthermore, amorphization due to cosmic rays, which is thought to be of prime importance for the local ISM, lags in vigorous starburst environments.

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arXiv2005

Spitzer 70 and 160-micron Observations of the Extragalactic First Look Survey

We present Spitzer 70um and 160um observations of the Spitzer extragalactic First Look Survey (xFLS). The data reduction techniques and the methods for producing co-added mosaics and source catalogs are discussed. Currently, 26% of the 70um sample and 49% of the 160um-selected sources have redshifts. The majority of sources with redshifts are star-forming galaxies at z<0.5, while about 5% have infrared colors consistent with AGN. The observed infrared colors agree with the spectral energy distribution (SEDs) of local galaxies previously determined from IRAS and ISO data. The average 160um/70um color temperature for the dust is Td~= 30+/-5 K, and the average 70um/24um spectral index is alpha~= 2.4+/-0.4. The observed infrared to radio correlation varies with redshift as expected out to z~1 based on the SEDs of local galaxies. The xFLS number counts at 70um and 160um are consistent within uncertainties with the models of galaxy evolution, but there are indications that the current models may require slight modifications. Deeper 70um observations are needed to constrain the models, and redshifts for the faint sources are required to measure the evolution of the infrared luminosity function.

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arXiv2005

MIPS J142824.0+352619: A Hyperluminous Starburst Galaxy at z=1.325

Using the SHARC-II camera at the Caltech Submillimeter Observatory to obtain 350 micron images of sources detected with the MIPS instrument on Spitzer, we have discovered a remarkable object at z=1.325+/-0.002 with an apparent Far-Infrared luminosity of 3.2(+/-0.7) x 10^13 Lsun. Unlike other z>1 sources of comparable luminosity selected from mid-IR surveys, MIPS J142824.0+352619 lacks any trace of AGN activity, and is likely a luminous analog of galaxies selected locally by IRAS, or at high redshift in the submillimeter. This source appears to be lensed by a foreground elliptical galaxy at z=1.034, although the amplification is likely modest (~10). We argue that the contribution to the observed optical/Near-IR emission from the foreground galaxy is small, and hence are able to present the rest-frame UV through radio Spectral Energy Distribution of this galaxy. Due to its unusually high luminosity, MIPS J142824.0+352619 presents a unique chance to study a high redshift dusty starburst galaxy in great detail.

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arXiv2005

The Infrared Array Camera component of the Spitzer Space Telescope Extragalactic First Look Survey

We present Infrared Array Camera (IRAC) data and source catalogs from the Spitzer Space Telescope Extragalactic First Look Survey. The data were taken in four broad bands centered at nominal wavelengths of 3.6, 4.5, 5.8 and 8.0 microns. A set of mosaics and catalogs have been produced which are ~80% complete and ~99% reliable to their chosen flux density limits. The main field survey covers 3.8 deg^2, and has flux density limits of 20muJy, 25muJy, 100muJy and 100muJy at wavelengths of 3.6, 4.5, 5.8 and 8.0 microns,respectively. The deeper ``verification'' survey covers 0.25 deg^2 with limits of 10muJy, 10muJy, 30muJy and 30muJy, respectively. We also include deep data in the ELAIS-N1 field which covers 0.041deg^2 with limits of 4muJy, 3muJy, 10muJy and 10muJy, respectively, but with only two wavelength coverage at a given sky position. The final bandmerged catalogs contain 103193 objects in the main field, 12224 in the verification field and 5239 in ELAIS-N1. Flux densities of high signal-to-noise objects are accurate to about 10%, and the residual systematic error in the absolute flux density scale is ~2-3%. We have successfully extracted sources at source densities as high as 100000 deg^-2 in our deepest 3.6 and 4.5 micron data. The mosaics and source catalogs will be made available through the Spitzer Science Center archive and the Infrared Science Archive.

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arXiv2005

Discovery of a Large ~200 kpc Gaseous Nebula at z=2.7 with the Spitzer Space Telescope

We report the discovery of a very large, spatially extended Ly alpha -emitting nebula at z=2.656 associated with a luminous mid-infrared source. The bright mid-infrared source (F(24um)=0.86 mJy) was first detected in observations made using the Spitzer Space Telescope. Existing broad-band imaging data from the NOAO Deep Wide-Field Survey revealed the mid-infrared source to be associated with a diffuse, spatially extended, optical counterpart in the Bw band. Spectroscopy and further imaging of this target reveals that the optical source is an almost purely line-emitting nebula with little, if any, detectable diffuse continuum emission. The Lya nebula has a luminosity of L[Lya] ~ 1.7e44 erg/s and an extent of at least 20 arcsec (160 kpc). Its central ~8 arcsec shows an ordered, monotonic velocity profile; interpreted as rotation, this region encloses a mass M = 6e12 Msun. Several sources lie within the nebula. The central region of the nebula shows narrow (~365 km/s) emission lines of CIV and HeII. The mid-infrared source is a compact object lying within the nebula, but offset from the center by a projected distance of ~2.5 arcsec (20 kpc), and likely to be an enshrouded AGN. A young star-forming galaxy lies near the northern end of the nebula. We suggest that the nebula is a site of recent multiple galaxy and AGN formation, with the spatial distribution of galaxies within the nebula perhaps tracking the formation history of the system.

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arXiv2005

Spectroscopic Redshifts to z > 2 for Optically Obscured Sources Discovered with the Spitzer Space Telescope

We have surveyed a field covering 9.0 degrees^2 within the NOAO Deep Wide-Field Survey region in Bootes with the Multiband Imaging Photometer on the Spitzer Space Telescope (SST) to a limiting 24 um flux density of 0.3 mJy. Thirty one sources from this survey with F(24um) > 0.75 mJy which are optically very faint (R > 24.5 mag) have been observed with the low-resolution modules of the Infrared Spectrograph on SST. Redshifts derived primarily from strong silicate absorption features are reported here for 17 of these sources; 10 of these are optically invisible (R > 26 mag), with no counterpart in B_W, R, or I. The observed redshifts for 16 sources are 1.7 < z < 2.8. These represent a newly discovered population of highly obscured sources at high redshift with extreme infrared to optical ratios. Using IRS spectra of local galaxies as templates, we find that a majority of the sources have mid-infrared spectral shapes most similar to ultraluminous infrared galaxies powered primarily by AGN. Assuming the same templates also apply at longer wavelengths, bolometric luminosities exceed 10^13 L(solar).

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arXiv2005

Spitzer Observations of Optically "Invisible" Radio and X-Ray Sources: High Redshift AGN

We have combined a survey at 24 microns to 0.3 mJy with the Multiband Imaging Photometer (MIPS) on the Spitzer Space Telescope, a 20 cm A-array VLA survey covering 0.5 deg^2, and an existing 172 ks Chandra X-Ray Observatory exposure to investigate the nature of optically faint radio and X-ray sources in the NOAO Deep Wide-Field Survey (NDWFS) in Bootes. We find little overlap between the radio and infrared selected populations: only 9% of the infrared sources are detected in the radio and only 33% of the radio sources are detected in the infrared. Thirty-six (10%) of the 377 compact radio sources lack optical counterparts in the NDWFS Bw, R, & I images. We refer to these objects as optically invisible radio sources (OIRS). Only four of the thirty-one OIRSs observed with MIPS are detected at 24 microns. Comparisons of the radio and infrared properties of the OIRSs with various galaxy spectral energy distributions demonstrate that most of these sources are powered by AGN rather than starbursts. Similarly, eleven X-ray sources observed by both MIPS and the VLA are classified as optically invisible X-ray sources (OIXS). None are detected at 24 microns or 20 cm. All seven OIXSs detected in Chandra's 0.5-2 keV band have infrared to X-ray flux ratios consistent with their luminosity being dominated by an unobscured AGN. From these results we conclude that both the optically invisible radio and X-ray source populations are primarily AGN, relatively unaffected by dust and most likely at z > 1. No OIRSs are detected in X-ray emission and no OIXSs are detected at 20 cm. However, given the wide range in radio and X-ray properties of known AGN and the size of our samples, this lack of overlap does not necessarily imply AGN source populations.

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arXiv2004

The First Measurements of Galaxy Clustering from IRAC Data of the Spitzer First Look Survey

We present the first results of the angular auto-correlation function of the galaxies detected by the Infrared Array Camera (IRAC) instrument in the First Look Survey (FLS) of the Spitzer Space Telescope. We detect significant signals of galaxy clustering within the survey area. The angular auto-correlation function of the galaxies detected in each of the four IRAC instrument channels is consistent with a power-law form $w(θ)=Aθ^{1-γ}$ out to $θ= 0.2\arcdeg$, with the slope ranging from $γ= 1.5$ to 1.8. We estimate the correlation amplitudes $A$ to be $2.95 \times 10^{-3}$, $2.03 \times 10^{-3}$, $4.53 \times 10^{-3}$, and $2.34 \times 10^{-3}$ at $θ=1\arcdeg$ for galaxies detected in the IRAC 3.6$μ$m, 4.5$μ$m, 5.8$μ$m, and 8.0$μ$m instrument channels, respectively. We compare our measurements at 3.6$μ$m with the previous K-band measurements, and discuss the implications of these results.

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arXiv2004

Infrared Properties of Radio-Selected Submillimeter Galaxies in the Spitzer First Look Survey Verification Field

We report on submillimeter and infrared observations of 28 radio-selected galaxies in the Spitzer First Look Survey Verification field (FLSV). All of the radio-selected galaxies that show evidence for emission at 850um with SCUBA have Spitzer counterparts at 24um, while only half of the radio-selected galaxies without 850um emission have detectable counterparts at 24um. The data show a wide range of infrared colors (S70/S24 < 5--30, S8/S3.6 < 0.3--4), indicative of a mixture of infrared-warm AGN and cooler starburst dominated sources. The galaxies showing 850um emission have Spitzer flux densities and flux density ratios consistent with the range of values expected for high-redshift (z=1--4) ultraluminous infrared galaxies.

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arXiv2004

Spitzer-IRS Spectroscopy of the Prototypical Starburst Galaxy NGC7714

We present observations of the starburst galaxy NGC 7714 with the Infrared Spectrograph IRS on board the Spitzer Space Telescope. The spectra yield a wealth of ionic and molecular features that allow a detailed characterization of its properties. NGC 7714 has an HII region-like spectrum with strong PAH emission features. We find no evidence for an obscured active galactic nucleus, and with [NeIII]/[NeII]~0.73, NGC7714 lies near the upper end of normal-metallicity starburst galaxies. With very little slicate absorption and a temperature of the hottest dust component of 340K, NGC 7714 is the perfect template for a young, unobscured starburst

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arXiv2004

First mid-infrared spectrum of a faint high-z galaxy: Observations of CFRS 14.1157 with the Infrared Spectrograph on the Spitzer Space Telescope

The unprecedented sensitivity of the Infrared Spectrograph on the Spitzer Space Telescope allows for the first time the measurement of mid-infrared spectra from 14 to 38 microns of faint high-z galaxies. This unique capability is demonstrated with observations of sources having 16 micron fluxes of 3.6 mJy (CFRS 14.1157) and 0.35 mJy (CFRS 14.9025). A spectral-fitting technique is illustrated which determines the redshift by fitting emission and absorption features characteristic of nearby galaxies to the spectrum of an unknown source. For CFRS 14.1157, the measured redshift is z = 1.00+/-0.20 in agreement with the published result of z = 1.15. The spectrum is dominated by emission from an AGN, similar to the nucleus of NGC 1068, rather than a typical starburst with strong PAH emission like M82. Such spectra will be crucial in characterizing the nature of newly discovered distant galaxies, which are too faint for optical follow-up.

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