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Register a new userA systematic study of silicate absorption features in heavily obscured AGNs observed by Spitzer/IRS
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Heavily obscured active galactic nuclei (AGNs) are known to show deep silicate absorption features in the mid-infrared (IR) wavelength range of 10--20~$mu$m. The detailed profiles of the features reflect the properties of silicate dust, which are likely to include information on AGN activities obscured by large amounts of dust. In order to reveal AGN activities obscured by large amounts of dust, we select 115 mid-IR spectra of heavily obscured AGNs observed by Spitzer/IRS, and systematically analyze the composition of silicate dust by spectral fitting using the 10~$mu$m amorphous and 23~$mu$m crystalline bands. We find that the main component of the silicate dust obscuring AGNs is amorphous olivine, the median mass column density of which is one order of magnitude higher than those of the minor components of amorphous pyroxene and crystalline forsterite. The median mass fraction of the amorphous pyroxene, $sim$2%, is significantly lower than that of the diffuse interstellar medium (ISM) dust in our Galaxy, while the median mass fraction of the crystalline forsterite, $sim$6%, is higher than that of the diffuse ISM dust. We also find that the mass fractions of the amorphous pyroxene and the crystalline forsterite positively correlate with each other. The low mass fraction of the amorphous pyroxene suggests that the obscuring silicate dust is newly formed, originating from starburst activities. The relatively high mass fraction of crystalline forsterite implies that the silicate dust is processed in the high temperature environment close to the nucleus and transported to outer cooler regions by molecular outflows. The positive correlation between the mass fractions can be naturally explained considering that amorphous pyroxene is transformed from crystalline forsterite by ion bombardments.
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Spectroscopic studies play a key role in the identification and analysis of interstellar ices and their structure. Some molecules have been identified within the interstellar ices either as pure, mixed, or even as layered structures. Absorption band features of water ice can significantly change with the presence of different types of impurities (CO, CO2, CH3OH, H2CO, etc.). In this work, we carried out a theoretical investigation to understand the behavior of water band frequency, and strength in the presence of impurities. The computational study has been supported and complemented by some infrared spectroscopy experiments aimed at verifying the effect of HCOOH, NH3 , and CH3 OH on the band profiles of pure H2O ice. Specifically, we explored the effect on the band strength of libration, bending, bulk stretching, and free-OH stretching modes. Computed band strength profiles have been compared with our new and existing experimental results, thus pointing out that vibrational modes of H2O and their intensities can change considerably in the presence of impurities at different concentrations. In most cases, the bulk stretching mode is the most affected vibration, while the bending is the least affected mode. HCOOH was found to have a strong influence on the libration, bending, and bulk stretching band profiles. In the case of NH3, the free-OH stretching band disappears when the impurity concentration becomes 50%. This work will ultimately aid a correct interpretation of future detailed spaceborne observations of interstellar ices by means of the upcoming JWST mission.
We investigate the connection between X-ray and radio-loud AGNs and the physical properties of their evolved and massive host galaxies, focussing on the mass-related quenching channel followed by $mathcal{M}^star (simeq 10^{10.6} M_odot)$ galaxies in the rest-frame NUVrK colour diagram at $0.2 < z < 0.5$. While our results confirm that (1) radio-loud AGNs are predominantly hosted by already-quenched and very massive ($M_*>10^{11}M_odot$) galaxies, ruling out their feedback as a primary driver of $mathcal{M}^star$ galaxy quenching, we found that (2) X-ray AGNs affected by heavy obscuration of their soft X-ray emission are mostly hosted by $mathcal{M}^star$ galaxies that are in the process of quenching. This is consistent with a quenching scenario that involves mergers of (gas-poor) $mathcal{M}^star$ galaxies $after$ the onset of the quenching process, i.e., a scenario where $mathcal{M}^star$ galaxy mergers are not the cause but rather an aftermath of the quenching mechanism(s). In that respect, we discuss how our results may support a picture where the slow quenching of $mathcal{M}^star$ galaxies happens due to halo-halo mergers along cosmic filaments.
We measured the mid-infrared (MIR) extinction using Spitzer photometry and spectroscopy (3.6--37 micron) for a sample of Milky Way sightlines (mostly) having measured ultraviolet extinction curves. We used the pair method to determine the MIR extinction that we then fit with a power law for the continuum and modified Drude profiles for the silicate features. We derived 16 extinction curves having a range of A(V) (1.8-5.5) and R(V) values (2.4-4.3). Our sample includes two dense sightlines that have 3 micron ice feature detections and weak 2175 A bumps. The average A(lambda)/A(V) diffuse sightline extinction curve we calculate is lower than most previous literature measurements. This agrees better with literature diffuse dust grain models, though it is somewhat higher. The 10 micron silicate feature does not correlate with the 2175 A bump, for the first time providing direct observational confirmation that these two features arise from different grain populations. The strength of the 10 micron silicate feature varies by $sim$2.5 and is not correlated with A(V) or R(V). It is well fit by a modified Drude profile with strong correlations seen between the central wavelength, width, and asymmetry. We do not detect other features with limits in A(lambda)/A(V) units of 0.0026 (5--10 micron), 0.004 (10--20 micron), and 0.008 (20-40 micron). We find that the standard prescription of estimating R(V) from C times E(K_s-V)/E(B-V) has C = -1.14 and a scatter of $sim$7%. Using the IRAC 5.6 micron band instead of K_s gives C = -1.03 and the least scatter of $sim$3%.
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.
A primary aim of the Nuclear Spectroscopic Telescope Array (NuSTAR) mission is to find and characterize heavily obscured Active Galactic Nuclei (AGNs). Based on mid-infrared photometry from the Wide-Field Infrared Survey Explorer (WISE) and optical photometry from the Sloan Digital Sky Surveys, we have selected a large population of luminous obscured AGN (i.e., obscured quasars). Here we report NuSTAR observations of four WISE-selected heavily obscured quasars for which we have optical spectroscopy from the Southern African Large Telescope and W. M. Keck Observatory. Optical diagnostics confirm that all four targets are AGNs. With NuSTAR hard X-ray observations, three of the four objects are undetected, while the fourth has a marginal detection. We confirm that these objects have observed hard X-ray (10-40 keV) luminosities at or below ~ 10^43 erg s^-1. We compare X-ray and IR luminosities to obtain estimates of the hydrogen column densities (N_H) based on the suppression of the hard X-ray emission. We estimate N_H of these quasars to be at or larger than 10^25 cm^-2, confirming that WISE and optical selection can identify very heavily obscured quasars that may be missed in X-ray surveys, and do not contribute significantly to the cosmic X-ray background. From the optical Balmer decrements, we found that our three extreme obscured targets lie in highly reddened host environments. This galactic extinction is not adequate to explain the more obscured AGN, but it may imply a different scale of obscuration in the galaxy.
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