J. A. Marshall

Observations of Ultraluminous Infrared Galaxies with the Infrared Spectrograph on the Spitzer Space Telescope. II. The IRAS Bright Galaxy Sample

We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5-38 μm region of the 10 ultraluminous infrared galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factor of 50 spread in the rest-frame 5.5-60 μm spectral slopes, and the 9.7 μm silicate optical depths range from at least τ_(9.7) ≤ 0.4 (A_V ~ 8) to τ_(9.7) ≥ 4.2 (A_V ≥ 78). There is evidence for water ice and hydrocarbon absorption and C_2H_2 and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [Ne V] emission in 3 of the 10 BGS ULIRGs, at flux levels of 5-18 × 10^(-14) ergs cm^(-2) s^(-1) and [Ne V] 14.3/[Ne II] 12.8 line flux ratios of 0.12-0.85. The remaining BGS ULIRGs have limits on their [Ne V]/[Ne II]line flux ratios, which range from ≤0.15 to ≤0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [Ne V]/[Ne II] or [O IV]/[Ne II] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2 μm PAH EQW feature. There is evidence for hot (T > 300 K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from ~1% to 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z ~ 1.3. The large range in diagnostic parameters among the nearest ULIRGs suggests that matching survey results to a small number of templates may lead to biased results about the fraction of luminous dusty starbursts and AGNs at high z.

Mid-Infrared Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width

We present a new diagnostic diagram for mid-infrared spectra of infrared galaxies based on the equivalent width of the 6.2 μm PAH emission feature and the strength of the 9.7 μm silicate feature. Based on the positions in this diagram, we classify galaxies into nine classes ranging from continuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra to absorption-dominated spectra of deeply obscured galactic nuclei. We find that galaxies are systematically distributed along two distinct branches: one of AGN and starburst-dominated spectra and one of deeply obscured nuclei and starburst-dominated spectra. The separation into two branches likely reflects a fundamental difference in the dust geometry in the two sets of sources: clumpy versus nonclumpy obscuration. Spectra of ULIRGs are found along the full length of both branches, reflecting the diverse nature of the ULIRG family.

The Detection of Silicate Emission from Quasars at 10 and 18 Microns

We report the spectroscopic detection of silicate emission at 10 and 18 mm in five PG quasars, the first detection of these two features in galaxies outside the Local Group. This finding is consistent with the unification model for active galactic nuclei (AGNs), which predicts that an AGN torus seen pole-on should show a silicate emission feature in the mid-infrared. The strengths of the detected silicate emission features range from 0.12 to 1.25 times the continuum at 10 mu m and from 0.20 to 0.79 times the continuum at 18 mu m. The silicate grain temperatures inferred from the ratio of 18 mu m to 10 mm silicate features under the assumption of optically thin emission range from 140 to 220 K.

Spitzer Observations of 3C Quasars and Radio Galaxies: Mid-Infrared Properties of Powerful Radio Sources

We have measured mid-infrared radiation from an orientation-unbiased sample of 3CRR galaxies and quasars at redshifts 0.4 ≤ z ≤ 1.2 with the IRS and MIPS instruments on Spitzer. Powerful emission (L24 μm > 1022.4 W Hz-1 sr-1) was detected from all but one of the sources. We fit the Spitzer data and other measurements from the literature with synchrotron and dust components. The IRS data provide powerful constraints on the fits. At 15 μm, quasars are typically 4 times brighter than radio galaxies with the same isotropic radio power. Based on our fits, half of this difference can be attributed to the presence of nonthermal emission in the quasars but not the radio galaxies. The other half is consistent with dust absorption in the radio galaxies but not the quasars. Fitted optical depths are anticorrelated with core dominance, from which we infer an equatorial distribution of dust around the central engine. The median optical depth at 9.7 μm for objects with core dominance factor R > 10-2 is ≈0.4; for objects with R ≤ 10-2, it is ≈1.1. We have thus addressed a long-standing question in the unification of FR II quasars and galaxies: quasars are more luminous in the mid-infrared than galaxies because of a combination of Doppler-boosted synchrotron emission in quasars and extinction in galaxies, both orientation-dependent effects.

PAH Emission from Ultraluminous Infrared Galaxies

We explore the relationships between the polycyclic aromatic hydrocarbon (PAH) feature strengths, mid-infrared continuum luminosities, far-infrared spectral slopes, optical spectroscopic classifications, and silicate optical depths in a sample of 107 ULIRGs observed with the Infrared Spectrograph on the Spitzer Space Telescope. The detected 6.2 μm PAH equivalent widths (EWs) in the sample span more than 2 orders of magnitude (~0.006-0.8 μm), and ULIRGs with H II-like optical spectra or steep far-infrared spectral slopes (S_(25)/S_(60) 2.3) silicate optical depths. The far-infrared spectral slope is strongly correlated with PAH EW, but not with silicate optical depth. In addition, the PAH EW decreases with increasing rest-frame 24 μm luminosity. We argue that this trend results primarily from dilution of the PAH EW by continuum emission from dust heated by a compact central source, probably an AGN. High-luminosity, high-redshift sources studied with Spitzer appear to have a much larger range in PAH EW than seen in local ULIRGs, which is consistent with extremely luminous starburst systems being absent at low redshift, but present at early epochs.

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 μm 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 μm are similar for the most luminous quasars and ULIRGs and are almost 105 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.

Detection of the Buried Active Galactic Nucleus 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 Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum of NGC 6240 is dominated by strong fine-structure lines, rotational H2 lines, and polycyclic aromatic hydrocarbon (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 ~6.7 × 106 M☉ at T ~ 957 K and ~1.6 × 109 M☉ at T ~ 164 K, or about 15% of the total molecular gas mass in this system. Notably, we have detected the [Ne V] 14.3 μm emission line, with a flux of 5 × 10-14 ergs cm-2 s-1, providing the first direct detection of the buried active galactic nucleus (AGN) in the mid-infrared. Modeling of the total spectral energy distribution (SED) from near- to far-infrared wavelengths requires the presence of a hot dust (T ~ 700 K) component, which we also associate with the buried AGN. The small [Ne V]/[Ne II] and [Ne V]/IR flux ratios, the relative fraction of hot dust emission, and the large 6.2 μm PAH equivalent width (EQW), are all consistent with an apparent AGN contribution of only 3%-5% to the bolometric luminosity. However, correcting the measured [Ne V] 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 ~20%-24% in NGC 6240, which lies within the range implied by fits to the hard X-ray spectrum.

Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry Toward Galactic Nuclei

The silicate cross section peak near 10 μm produces emission and absorption features in the spectra of dusty galactic nuclei observed with the Spitzer Space Telescope. Especially in ultraluminous infrared galaxies, the observed absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A foreground screen of obscuration cannot reproduce this observed feature, even at a large optical depth. Instead, the deep absorption requires a nuclear source to be deeply embedded in a smooth distribution of material that is both geometrically and optically thick. In contrast, a clumpy medium can produce only shallow absorption or emission, which are characteristic of optically identified active galactic nuclei. In general, the geometry of the dusty region and the total optical depth, rather than the grain composition or heating spectrum, determine the silicate features observable properties. The apparent optical depth calculated from the ratio of line to continuum emission generally fails to accurately measure the true optical depth. The obscuring geometry, not the nature of the embedded source, also determines the far-IR spectral shape.

Measuring PAH Emission in Ultradeep Spitzer IRS Spectroscopy of High Redshift IR Luminous Galaxies

The study of the dominant population of high-redshift IR-luminous galaxies (1011-1012 L☉ at 1 < z < 3), requires observation of sources at the ~0.1 mJy level in the mid-IR. We present the deepest spectra taken to date with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. We targeted two faint (f24 ~ 0.15 mJy) sources in the Southern GOODS field at z = 1.09 and z = 2.69. Spectra of the lower redshift target were taken in the observed-frame 8-21 μm range, while the spectrum of the higher redshift target covered 21-37 μm. We also present the spectra of two secondary sources within the slit. We detect strong PAH emission in all four targets, and compare the spectra to those of local galaxies observed by the IRS. The z = 1.09 source appears to be a typical, star-formation-dominated IR-luminous galaxy, while the z = 2.69 source is a composite source with strong star formation and a prominent AGN. The IRAC colors of this source show no evidence of rest-frame near-IR stellar photospheric emission. We demonstrate that an AGN that contributes only a small (~10%) fraction of the bolometric luminosity can produce enough hot dust emission to overwhelm the near-IR photospheric emission from stars. Such sources would be excluded from photometric surveys that rely on the near-IR bump to identify starbursts, leading to an underestimate of the star formation rate density.

Decomposing Dusty Galaxies. I. Multicomponent Spectral Energy Distribution Fitting

We present a new multicomponent SED decomposition method and use it to analyze the UV to millimeter wavelength SEDs of a sample of dusty infrared-luminous galaxies. Each SED is decomposed into emission from populations of stars, an AGN accretion disk, PAHs, atomic and molecular lines, and distributions of graphite and silicate grains. Decompositions of the SEDs of template starburst galaxies and AGNs provide baseline properties to aid in quantifying the strength of star formation and accretion in the composite systems NGC 6240 and Mrk 1014. We find that obscured radiation from stars is capable of powering the total dust emission from NGC 6240. The presence of a small quantity of 1260 K dust in this source suggests a ~2% AGN contribution, although we cannot rule out a larger contribution from a deeply embedded AGN visible only in X-rays. The decomposition of Mrk 1014 is consistent with ~65% of its power emerging from an AGN and ~35% from star formation. We suggest that many of the variations in our template starburst SEDs may be explained in terms of the different mean optical depths through the clouds of dust surrounding the young stars within each galaxy. Prompted by the divergent far-IR properties of our template AGNs, we suggest that variations in the relative orientation of their AGN accretion disks with respect to the disks of the galaxies hosting them may result in different amounts of AGN-heated cold dust emission emerging from their host galaxies. We estimate that 30%-50% of the far-IR and PAH emission from Mrk 1014 may originate from such AGN-heated material in its host galaxy disk.