Mid Infrared Spectra of Radio Galaxies and Quasars

Spitzer Infrared Spectrograph (IRS) observations of 3C radio galaxies and quasars shed new light on the nature of the central engines of AGN. Emission from silicate dust obscuring the central engine can be used to estimate the bolometric luminosity of an AGN. Emission lines from ions such as O IV and Ne V give another indication of the presence or lack of a hidden source of far-UV photons in the nucleus. Radio-loud AGN with relative-to-Eddington luminosity ratios of L/L_Edd < 3E-3 do not appear to have broad optical emission lines, though some do have strong silicate emission. Aromatic emission features from star formation activity are common in low-luminosity radio galaxies. Strong molecular hydrogen pure-rotational emission lines are also seen in some mid-IR weak radio galaxies, caused by either merger shocks or jet shocks in the interstellar medium.Comment: Conference proceedings to appear in "The Central Engine of Active Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP

Ultraluminous Star-forming Galaxies and Extremely Luminous Warm Molecular Hydrogen Emission at z = 2.16 in the PKS 1138–26 Radio Galaxy Protocluster

A deep Spitzer Infrared Spectrograph map of the PKS 1138–26 galaxy protocluster reveals ultraluminous polycyclic aromatic hydrocarbon (PAH) emission from obscured star formation in three protocluster galaxies, including Hα-emitter (HAE) 229, HAE 131, and the central Spiderweb Galaxy. Star formation rates of ~500-1100 M_☉ yr^(–1) are estimated from the 7.7 μm PAH feature. At such prodigious formation rates, the galaxy stellar masses will double in 0.6-1.1 Gyr. We are viewing the peak epoch of star formation for these protocluster galaxies. However, it appears that extinction of Hα is much greater (up to a factor of 40) in the two ULIRG HAEs compared to the Spiderweb. This may be attributed to different spatial distributions of star formation-nuclear star formation in the HAEs versus extended star formation in accreting satellite galaxies in the Spiderweb. We find extremely luminous mid-IR rotational line emission from warm molecular hydrogen in the Spiderweb Galaxy, with L(H_2 0-0 S(3)) = 1.4 × 10^(44) erg s^(–1) (3.7 × 10^(10) L_☉), ~20 times more luminous than any previously known H2 emission galaxy (MOHEG). Depending on the temperature, this corresponds to a very large mass of >9 × 10^(6)-2 × 10^9 M_☉ of T > 300 K molecular gas, which may be heated by the PKS 1138–26 radio jet, acting to quench nuclear star formation. There is >8 times more warm H_2 at these temperatures in the Spiderweb than what has been seen in low-redshift (z < 0.2) radio galaxies, indicating that the Spiderweb may have a larger reservoir of molecular gas than more evolved radio galaxies. This is the highest redshift galaxy yet in which warm molecular hydrogen has been directly detected

Quasar 3C 298: a test-case for meteoritic nanodiamond 3.5 µm emission

Aims. We calculate the dust emission expected at 3.43 and 3.53 µm if meteoritic (i.e. hydrogenated) nanodiamonds are responsible for most of the far-UV break observed in quasars. Methods. We integrate the UV flux that hydrogenated nanodiamonds must absorb to reproduce the far-UV break. Based on laboratory spectra of H-terminated diamond surfaces, we analyse the radiative energy budget and derive theoretically the IR emission profiles expected for possible C-H surface stretch modes of the diamonds. Results. Using as test case a spectrum of 3C 298 provided by the Spitzer Observatory, we do not find evidence of these emission bands. Conclusions. While diamonds without surface adsorbates remain a viable candidate for explaining the far-UV break observed in quasars, hydrogenated nanodiamonds appear to be ruled out, as they would give rise to IR emission bands, which have not been observed so far

Chandra Observations of the X-ray Narrow-Line Region in NGC 4151

We present the first high-resolution X-ray spectrum of the Seyfert 1.5 galaxy NGC 4151. Observations with the Chandra High Energy Transmission Grating Spectrometer reveal a spectrum dominated by narrow emission lines from a spatially resolved (1.6 kpc), highly ionized nebula. The X-ray narrow-line region is composite, consisting of both photoionized and collisionally ionized components. The X-ray emission lines have similar velocities, widths, and spatial extent to the optical emission lines, showing that they arise in the same region. The clouds in the narrow-line region must contain a large range of ionization states in order to explain both the optical and X-ray photoionized emission. Chandra data give the first direct evidence of X-ray line emission from a hot plasma (T~1e7 K) which may provide pressure confinement for the cooler (T=3e4 K) photoionized clouds.Comment: 13 pages, 3 figures, to be published in Astrophysical Journal Letter

A Spitzer Unbiased Ultradeep Spectroscopic Survey

We carried out an unbiased, spectroscopic survey using the low-resolution module of the infrared spectrograph (IRS) on board Spitzer targeting two 2.6 square arcminute regions in the GOODS-North field. IRS was used in spectral mapping mode with 5 hours of effective integration time per pixel. One region was covered between 14 and 21 microns and the other between 20 and 35 microns. We extracted spectra for 45 sources. About 84% of the sources have reported detections by GOODS at 24 microns, with a median F_nu(24um) ~ 100 uJy. All but one source are detected in all four IRAC bands, 3.6 to 8 microns. We use a new cross-correlation technique to measure redshifts and estimate IRS spectral types; this was successful for ~60% of the spectra. Fourteen sources show significant PAH emission, four mostly SiO absorption, eight present mixed spectral signatures (low PAH and/or SiO) and two show a single line in emission. For the remaining 17, no spectral features were detected. Redshifts range from z ~ 0.2 to z ~ 2.2, with a median of 1. IR Luminosities are roughly estimated from 24 microns flux densities, and have median values of 2.2 x 10^{11} L_{\odot} and 7.5 x 10^{11} L_{\odot} at z ~ 1 and z ~ 2 respectively. This sample has fewer AGN than previous faint samples observed with IRS, which we attribute to the fainter luminosities reached here.Comment: Published in Ap

Powerful H2_2 Line-cooling in Stephan's Quintet : I - Mapping the Significant Cooling Pathways in Group-wide Shocks

We present results from the mid-infrared spectral mapping of Stephan's Quintet using the Spitzer Space Telescope. A 1000 km/s collision has produced a group-wide shock and for the first time the large-scale distribution of warm molecular hydrogen emission is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 $\times10^{8}$ M$_{\odot}$ of warm H$_2$ spread over $\sim$ 480 kpc$^2$ and additionally report the discovery of a second major shock-excited H$_2$ feature. This brings the total H$_2$ line luminosity of the group in excess of 10$^42$ erg/s. In the main shock, the H$_2$ line luminosity exceeds, by a factor of three, the X-ray luminosity from the hot shocked gas, confirming that the H$_2$-cooling pathway dominates over the X-ray. [Si II]34.82$\mu$m emission, detected at a luminosity of 1/10th of that of the H$_2$, appears to trace the group-wide shock closely and in addition, we detect weak [FeII]25.99$\mu$m emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 < $V_{s}$ < 300 km/s) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, PAH and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H$_{2}$ power is driven by turbulent energy transfer from motions in a post-shocked layer. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.Comment: 23 pages, 15 figures, Accepted to Ap

Polarization of Broad Absorption Line QSOs I. A Spectropolarimetric Atlas

We present a spectropolarimetric survey of 36 broad absorption line quasi-stellar objects (BAL QSOs). The continuum, absorption trough, and emission line polarization of BAL QSOs yield clues about their structure. We confirm that BAL QSOs are in general more highly polarized than non-BAL QSOs, consistent with a more equatorial viewing direction for the former than the latter. We have identified two new highly-polarized QSOs in our sample (1232+1325 and 1333+2840). The polarization rises weakly to the blue in most objects, perhaps due to scattering and absorption by dust particles. We find that a polarization increase in the BAL troughs is a general property of polarized BAL QSOs, indicating an excess of scattered light relative to direct light, and consistent with the unification of BAL QSOs and non-BAL QSOs. We have also discovered evidence of resonantly scattered photons in the red wing of the C IV broad emission lines of a few objects. In most cases, the broad emission lines have lower polarization and a different position angle than the continuum. The polarization characteristics of low-ionization BAL QSOs are similar to those of high-ionization BAL QSOs, suggesting a similar BAL wind geometry.Comment: 39 pages, 6 figures (20 .gif files), accepted for publication in The Astrophysical Journal Supplement

Strong Far-IR Cooling Lines, Peculiar CO Kinematics and Possible Star Formation Suppression in Hickson Compact Group 57

We present [C II] and [O I] observations from Herschel and CO(1-0) maps from the Combined Array for{\dag} Research in Millimeter Astronomy (CARMA) of the Hickson Compact Group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG 57a has been previously shown to contain enhanced quantities of warm molecular hydrogen consistent with shock and/or turbulent heating. Our observations show that HCG 57d has strong [C II] emission compared to L$_{\rm FIR}$ and weak CO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a lies at the upper end of the normal distribution of [C II]/CO and [C II]/FIR ratios, and its far-IR cooling supports a low density warm diffuse gas that falls close to the boundary of acceptable PDR models. However, the power radiated in the [C II] and warm H$_2$ emission have similar magnitudes, as seen in other shock-dominated systems and predicted by recent models. We suggest that shock-heating of the [C II] is a viable alternative to photoelectric heating in violently disturbed diffuse gas. The existence of shocks is also consistent with peculiar CO kinematics in the galaxy, indicating highly non-circular motions are present. These kinematically disturbed CO regions also show evidence of suppressed star formation, falling a factor of 10-30 below normal galaxies on the Kennicutt-Schmidt relation. We suggest that the peculiar properties of both galaxies are consistent with a highly dissipative off-center collisional encounter between HCG 57d and 57a, creating ring-like morphologies in both systems. Highly dissipative gas-on-gas collisions may be more common in dense groups because of the likelihood of repeated multiple encounters. The possibility of shock-induced SF suppression may explain why a subset of these HCG galaxies have been found previously to fall in the mid-infrared green valley.Comment: ApJ accepted, 16 pages, 12 figures, 3 table