Spitzer Mid-Infrared Spectroscopy of 70um-Selected Distant Luminous Infrared Galaxies

We present mid-infrared spectroscopy obtained with the Spitzer Space Telescope of a sample of 11 optically faint, infrared luminous galaxies selected from a Spitzer MIPS 70um imaging survey of the NDWFS Bootes field. These are the first Spitzer IRS spectra presented of distant 70um-selected sources. All the galaxies lie at redshifts 0.3<z<1.3 and have very large infrared luminosities of L_IR~ 0.1-17 x 10^12 solar luminosities. Seven of the galaxies exhibit strong emission features attributed to polycyclic aromatic hydrocarbons (PAHs). The average IRS spectrum of these sources is characteristic of classical starburst galaxies, but with much larger infrared luminosities. The PAH luminosities of L(7.7) ~ 0.4 - 7 x 10^11 solar luminosities imply star formation rates of ~ 40 - 720 solar masses per year. Four of the galaxies show deep 9.7um silicate absorption features and no significant PAH emission features (6.2um equivalent widths < 0.03um). The large infrared luminosities and low f70/f24 flux density ratios suggests that these sources have AGN as the dominant origin of their large mid-infrared luminosities, although deeply embedded but luminous starbursts cannot be ruled out. If the absorbed sources are AGN-dominated, a significant fraction of all far-infrared bright, optically faint sources may be dominated by AGN.Comment: 8 Pages, ApJ accepte

Redshift Distribution of Extragalactic 24 micron Sources

We present the redshift distribution of a complete, unbiased sample of 24 micron sources down to fnu(24 micron) = 300 uJy (5-sigma). The sample consists of 591 sources detected in the Bootes field of the NOAO Deep Wide-Field Survey. We have obtained optical spectroscopic redshifts for 421 sources (71%). These have a redshift distribution peaking at z~0.3, with a possible additional peak at z~0.9, and objects detected out to z=4.5. The spectra of the remaining 170 (29%) exhibit no strong emission lines from which to determine a redshift. We develop an algorithm to estimate the redshift distribution of these sources, based on the assumption that they have emission lines but that these lines are not observable due to the limited wavelength coverage of our spectroscopic observations. The redshift distribution derived from all 591 sources exhibits an additional peak of extremely luminous (L(8-1000 micron) > 3 x 10^{12} Lsun) objects at z~2, consisting primarily of sources without observable emission lines. We use optical line diagnostics and IRAC colors to estimate that 55% of the sources within this peak are AGN-dominated. We compare our results to published models of the evolution of infrared luminous galaxies. The models which best reproduce our observations predict a large population of star-formation dominated ULIRGs at z > 1.5 rather than the AGN-dominated sources we observe.Comment: 21 pages, 11 figures; accepted for publication in Ap

Comparing Chandra and SIRTF Observations for Obscured Starbursts and Active Galactic Nuclei at High Redshift

International audienceTracking the star formation rate to high redshifts requires knowledge of the contribution from both optically visible and obscured sources. The dusty, optically obscured galaxies can be located by X-ray and infrared surveys. To establish criteria for selecting such sources based only on X-ray and infrared surveys, we determine the ratio of infrared to X-ray brightness that would be observed by SIRTF and Chandra for objects with the same spectral shapes as nearby starbursts if seen at high redshift. The parameter infrared/X-ray (IR/X) is defined as IR/X=(flux density observed in the SIRTF Multiband Imaging Photometer 24 mum filter in mJy)/(total flux observed within 0.5-2.0 keV in units of 10-16 ergs s-1 cm-2). Based on observations of NGC 4038/39 (``The Antennae''), NGC 3690+IC 694 (Arp 299 or Mrk 171), M82, and Arp 220, nine starburst regions are compared using mid-infrared spectra taken by the Infrared Space Observatory and X-ray spectra obtained with Chandra. The IR/X are determined as they would appear for

16 mum Imaging around the Hubble Deep Field-North with the Spitzer IRS

International audienceWe present a pilot study of 16 mum imaging within the GOODS northern field. Observations were obtained using the ``peak-up'' imaging capability of the Spitzer IRS. We survey 35 arcmin2 to an average 3 sigma depth of 0.075 mJy and detect 149 sources. The survey partially overlaps the area imaged at 15 mum by the ISO, and we demonstrate that our photometry and galaxy number counts are consistent with its measurements. We infer the total infrared luminosity of 16 mum detections using a comparison to local templates and find a wide range of LIR, from ~109 to 1012 Lsolar. Approximately 1/5 of the detected sources have counterparts in the Chandra 2 Ms catalog, and we show that the hard-band (2-8 keV) detected sources are likely to have strong active galactic nucleus (AGN) contributions to the X-ray flux. The ultradeep sensitivity of Chandra implies that some X-ray detections may be purely starbursting objects. We examine the 16 to 24 mum flux ratio and conclude that it shows evidence for the detection of redshifted polycyclic aromatic hydrocarbon (PAH) emission at z~0.5 and z>0.8

Identifying Silicate-absorbed ULIRGs at z ~ 1-2 in the Bootes Field Using the Spitzer IRS

International audienceUsing the 16 mum peak-up imager on the Infrared Spectrograph (IRS) on Spitzer, we present a serendipitous survey of 0.0392 deg2 within the area of the NOAO Deep Wide-Field Survey in Bootes. Combining our results with the available Multiband Imaging Photometer for Spitzer (MIPS) 24 mum survey of this area, we produce a catalog of 150 16 mum sources brighter than 0.18 mJy (3 sigma) for which we derive measures or limits on the 16 mum/24 mum colors. Such colors are especially useful in determining redshifts for sources whose mid-infrared spectra contain strong emission or absorption features that characterize these colors as a function of redshift. We find that the 9.7 mum silicate absorption feature in ultraluminous infrared galaxies (ULIRGs) results in sources brighter at 16 mum than at 24 mum at z~1-1.8 by at least 20%. With a threshold flux ratio of 1.2, restricting our analysis to >5 sigma detections at 16 mum, and using a 3 sigma limit on 24 mum nondetections, the number of silicate-absorbed ULIRG candidates is 36. This defines a strong upper limit of ~920 sources deg-2 on the population of silicate-absorbed ULIRGs at z~1-1.8. This source count is about half of the total number of sources predicted at z~1-2 by various phenomenological models. We note that the high 16 mum/24 mum colors measured cannot be reproduced by any of the mid-IR spectral energy distributions assumed by these models, which points to the strong limitations currently affecting our phenomenological and theoretical understanding of infrared galaxy evolution

First Mid-Infrared Spectrum of a Faint High-z Galaxy: Observations of CFRS 14.1157 with the Infrared Spectrograph on the Spitzer Space Telescope

International audienceThe 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 mum of faint high-z galaxies. This unique capability is demonstrated with observations of sources having 16 mum fluxes of 3.6 mJy (CFRS 14.1157) and 0.35 mJy (CFRS 14.9025). A spectral-fitting technique is illustrated that 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 active galactic nucleus, similar to the nucleus of NGC 1068, rather than a typical starburst with strong polycyclic aromatic hydrocarbon 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