Mid-Infrared Spectroscopy of Optically Faint Extragalactic 70 micron Sources

We present mid-infrared spectra of sixteen optically faint sources with 70 micron fluxes in the range 19-38mJy. The sample spans a redshift range of 0.35<z<1.9, with most lying between 0.8<z<1.6, and has infrared luminosities of 10^{12} - 10^{13} solar luminosities. Ten of 16 objects show prominent polycyclic aromatic hydrocarbon (PAH) emission features; four of 16 show weak PAHs and strong silicate absorption, and two objects have no discernable spectral features. Compared to samples with 24 micron fluxes >10mJy, the 70\um sample has steeper IR continua and higher luminosities. The PAH dominated sources are among the brightest starbursts seen at any redshift, and reside in a redshift range where other selection methods turn up relatively few sources. The absorbed sources are at higher redshifts and have higher luminosities than the PAH dominated sources, and may show weaker luminosity evolution. We conclude that a 70 micron selection extending to ~20mJy, in combination with selections at mid-IR and far-IR wavelengths, is necessary to obtain a complete picture of the evolution of IR-luminous galaxies over 0<z<2.Comment: ApJ accepte

Redshifts from Spitzer Spectra for Optically Faint, Radio Selected Infrared Sources

Spectra have been obtained with the Infrared Spectrograph on the Spitzer Space Telescope for 18 optically faint sources (R > 23.9,mag) having f(nu) (24um) > 1.0,mJy and having radio detections at 20 cm to a limit of 115 microJy. Sources are within the Spitzer First Look Survey. Redshifts are determined for 14 sources from strong silicate absorption features (12 sources) or strong PAH emission features (2 sources), with median redshift of 2.1. Results confirm that optically faint sources of ~1 mJy at 24um are typically at redshifts z ~ 2, verifying the high efficiency in selecting high redshift sources based on extreme infrared to optical flux ratio, and indicate that 24um sources which also have radio counterparts are not systematically different than samples chosen only by their infrared to optical flux ratios. Using the parameter q = log[f(nu)(24um)/f(nu)(20 cm)] 17 of the 18 sources observed have values of 0<q<1, in the range expected for starburst-powered sources, but only a few of these show strong PAH emission as expected from starbursts, with the remainder showing absorbed or power-law spectra consistent with an AGN luminosity source. This confirms previous indications that optically faint Spitzer sources with f(nu)(24um) > 1.0mJy are predominately AGN and represent the upper end of the luminosity function of dusty sources at z ~ 2. Based on the characteristics of the sources observed so far, we predict that the nature of sources selected at 24um will change for f(nu)(24um) < 0.5 mJy to sources dominated primarily by starbursts.Comment: Accepted ApJ 20 February 2006, v638 2 issue, 10pages including 3 figure

[CII] 158 micron Luminosities and Star Formation Rate in Dusty Starbursts and AGN

Results are presented for [CII] 158 micron line fluxes observed with the Herschel PACS instrument in 112 sources with both starburst and AGN classifications, of which 102 sources have confident detections. Results are compared with mid-infrared spectra from the Spitzer Infrared Spectrometer and with L(IR) from IRAS fluxes; AGN/starburst classifications are determined from equivalent width of the 6.2 micron PAH feature. It is found that the [CII] line flux correlates closely with the flux of the 11.3 micron PAH feature independent of AGN/starburst classification, log [f([CII] 158 micron)/f(11.3 micron PAH)] = -0.22 +- 0.25. It is concluded that [CII] line flux measures the photodissociation region associated with starbursts in the same fashion as the PAH feature. A calibration of star formation rate for the starburst component in any source having [CII] is derived comparing [CII] luminosity L([CII]) to L(IR) with the result that log SFR = log L([CII)]) - 7.08 +- 0.3, for SFR in solar masses per year and L([CII]) in solar luminosities. The decreasing ratio of L([CII]) to L(IR) in more luminous sources (the "[CII] deficit") is shown to be a consequence of the dominant contribution to L(IR) arising from a luminous AGN component because the sources with largest L(IR) and smallest L([CII])/L(IR) are AGN.Comment: Accepted for publication in The Astrophysical Journa

Obscuration in extremely luminous quasars

The spectral energy distributions and infrared (IR) spectra of a sample of obscured AGNs selected in the mid-IR are modeled with recent clumpy torus models to investigate the nature of the sources, the properties of the obscuring matter, and dependencies on luminosity. The sample contains 21 obscured AGNs at z=1.3-3 discovered in the largest Spitzer surveys (SWIRE, NDWFS, & FLS) by means of their extremely red IR to optical colors. All sources show the 9.7micron silicate feature in absorption and have extreme mid-IR luminosities (L(6micron)~10^46 erg/s). The IR SEDs and spectra of 12 sources are well reproduced with a simple torus model, while the remaining 9 sources require foreground extinction from a cold dust component to reproduce both the depth of the silicate feature and the near-IR emission from hot dust. The best-fit torus models show a broad range of inclinations, with no preference for the edge-on torus expected in obscured AGNs. Based on the unobscured QSO mid-IR luminosity function, and on a color-selected sample of obscured and unobscured IR sources, we estimate the surface densities of obscured and unobscured QSOs at L(6micron)>10^12 Lsun, and z=1.3-3.0 to be about 17-22 deg^-2, and 11.7 deg^-2, respectively. Overall we find that ~35-41% of luminous QSOs are unobscured, 37-40% are obscured by the torus, and 23-25% are obscured by a cold absorber detached from the torus. These fractions constrain the torus half opening angle to be ~67 deg. This value is significantly larger than found for FIR selected samples of AGN at lower luminosity (~46 deg), supporting the receding torus scenario. A far-IR component is observed in 8 objects. The estimated far-IR luminosities associated with this component all exceed 3.3x10^12 Lsun, implying SFRs of 600-3000 Msun/yr. (Abridged)Comment: ApJ accepte

Comparing Chandra and SIRTF Observations for Obscured Starbursts and AGN at High Redshift

Tracking 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 IR/X is defined as IR/X = (flux density observed in SIRTF MIPS 24 $\mu$m filter in mJy)/(total flux observed within 0.5-2.0 keV in units of 10^-16 ergs\s\cm^2). Based on observations of NGC 4038/39 (``The Antennae''), NGC 3690+IC 694 (Arp 299 or Mkn 171), M 82, and Arp 220, nine starburst regions are compared using mid-infrared spectra taken by the Infrared Space Observatory (ISO) and X-ray spectra obtained with Chandra . The IR/X are determined as they would appear for 1<z<3. The mean IR/X over this redshift range is 1.3 and is not a significant function of redshift or luminosity, indicating that SIRTF surveys reaching 0.4 mJy at 24 $\mu$m should detect the same starbursts as deep CXO surveys detect at a flux of 0.3x10^-16 ergs/s/cm^2. The lower bound of IR/X for starbursts is about 0.2, suggesting that objects with IR/X smaller than this have an AGN X-ray component in addition to the starburst. Values of IR/X for the obscured AGN within NGC 1068, the Circinus galaxy, and NGC 6240 are also determined for comparison although interpretation is complicated by the circumnuclear starbursts in these galaxies. Any sources found in surveys having IR/X>4 would not match any of the objects considered.Comment: accepted for publication in Ap

The distribution of silicate strength in Spitzer spectra of AGNs and ULIRGs

A sample of 196 AGNs and ULIRGs observed by the Infrared Spectrograph (IRS) on Spitzer is analyzed to study the distribution of the strength of the 9.7 micron silicate feature. Average spectra are derived for quasars, Seyfert 1 and Seyfert 2 AGNs, and ULIRGs. We find that quasars are characterized by silicate features in emission and Seyfert 1s equally by emission or weak absorption. Seyfert 2s are dominated by weak silicate absorption, and ULIRGs are characterized by strong silicate absorption (mean apparent optical depth about 1.5). Luminosity distributions show that luminosities at rest frame 5.5 micron are similar for the most luminous quasars and ULIRGs and are almost 10^5 times more luminous than the least luminous AGN in the sample. The distributions of spectral characteristics and luminosities are compared to those of optically faint infrared sources at z~2 being discovered by the IRS, which are also characterized by strong silicate absorption. It is found that local ULIRGs are a similar population, although they have lower luminosities and somewhat stronger absorption compared to the high redshift sources.Comment: Accepted for publication on ApJ

Mid-Infrared Spectra of Classical AGN Observed with the Spitzer Space Telescope

Full low resolution (65<R<130) and high resolution (R~600) spectra between 5 microns and 37 microns obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope are presented for eight classical active galactic nuclei (AGN) which have been extensively studied previously. Spectra of these AGN are presented as comparison standards for the many objects, including sources at high redshift, which are being observed spectroscopically in the mid-infrared for the first time using the IRS. The AGN are NGC4151, Markarian 3, I Zwicky 1, NGC 1275, Centaurus A, NGC 7469, Markarian 231, and NGC 3079. These sources are used to demonstrate the range of infrared spectra encountered in objects which have widely different classification criteria at other wavelengths but which unquestionably contain AGN. Overall spectral characteristics - including continuum shape, nebular emission lines, silicate absorption and emission features, and PAH emission features - are considered to understand how spectral classifications based on mid-infrared spectra relate to those previously derived from optical spectra. The AGN are also compared to the same parameters for starburst galaxies such as NGC 7714 and the compact, low metallicity starburst SBS 0335-052 previously observed with the IRS. Results confirm the much lower strengths of PAH emission features in AGN, but there are no spectral parameters in this sample which unambiguously distinguish AGN and starbursts based only on the slopes of the continuous spectra.Comment: Accepted by Ap

HST Morphologies of z~2 Dust Obscured Galaxies I: Power-law Sources

We present high spatial resolution optical and near-infrared imaging obtained using the ACS, WFPC2 and NICMOS cameras aboard the Hubble Space Telescope of 31 24um--bright z~2 Dust Obscured Galaxies (DOGs) identified in the Bootes Field of the NOAO Deep Wide-Field Survey. Although this subset of DOGs have mid-IR spectral energy distributions dominated by a power-law component suggestive of an AGN, all but one of the galaxies are spatially extended and not dominated by an unresolved component at rest-frame UV or optical wavelengths. The observed V-H and I-H colors of the extended components are 0.2-3 magnitudes redder than normal star-forming galaxies. All but 1 have axial ratios >0.3, making it unlikely that DOGs are composed of an edge-on star-forming disk. We model the spatially extended component of the surface brightness distributions of the DOGs with a Sersic profile and find effective radii of 1-6 kpc. This sample of DOGs is smaller than most sub-millimeter galaxies (SMGs), but larger than quiescent high-redshift galaxies. Non-parametric measures (Gini and M20) of DOG morphologies suggest that these galaxies are more dynamically relaxed than local ULIRGs. We estimate lower limits to the stellar masses of DOGs based on the rest-frame optical photometry and find that these range from ~10^(9-11) M_sun. If major mergers are the progenitors of DOGs, then these observations suggest that DOGs may represent a post-merger evolutionary stage.Comment: 23 pages, 9 figures, 6 tables, accepted to ApJ; lower limits on stellar mass revised upwards by factor of (1+z

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.Comment: Accepted in ApJ Sup. Spitzer Special Issue, 4 pages, 5 figure