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We reanalyze the mid-infrared (5-40 $\mu$m) Spitzer spectra of 86 low-redshift ($z < 0.5$) Palomar-Green quasars to investigate the nature of polycyclic aromatic hydrocarbon (PAH) emission and its utility as a star formation rate (SFR) indicator for the host galaxies of luminous active galactic nuclei (AGNs). We decompose the spectra with our recently developed template-fitting technique to measure PAH fluxes and upper limits, which we interpret using mock spectra that simulate the effects of AGN dilution. While luminous quasars can severely dilute and affect the detectability of emission lines, PAHs are intrinsically weak in some sources that are otherwise gas-rich and vigorously forming stars, conclusively demonstrating that powerful AGNs destroy PAH molecules. Comparing PAH-based SFRs with independent SFRs derived from the mid-infrared fine-structure neon lines and the total infrared luminosity reveals that PAHs can trace star formation activity in quasars with bolometric luminosities $\lesssim 10^{46}\, \rm erg\,s^{-1}$, but increasingly underestimate the SFR for more powerful quasars, typically by $\sim 0.5$ dex. Relative to star-forming galaxies and low-luminosity AGNs, quasars have a comparable PAH 11.3 $\mu$m/7.7 $\mu$m ratio but characteristically lower ratios of 6.2 $\mu$m/7.7 $\mu$m, 8.6 $\mu$m/7.7 $\mu$m, and 11.3 $\mu$m/17.0 $\mu$m. We suggest that these trends indicate that powerful AGNs preferentially destroy small grains and enhance the PAH ionization fraction., Comment: 20 pages, 11 figures and 1 Table. Accepted for publication in ApJ

The star formation activity of the host galaxies of active galactic nuclei (AGNs) provides valuable insights into the complex interconnections between black hole growth and galaxy evolution. A major obstacle arises from the difficulty of estimating accurate star formation rates in the presence of a strong AGN. Analyzing the $1-500\, \mu m$ spectral energy distributions and high-resolution mid-infrared spectra of low-redshift ($z < 0.5$) Palomar-Green quasars with bolometric luminosity $\sim 10^{44.5}-10^{47.5}\rm\,erg\,s^{-1}$, we find, from comparison with an independent star formation rate indicator based on [Ne II] 12.81$\, \mu m$ and [Ne III] 15.56$\, \mu m$, that the torus-subtracted, total infrared ($8-1000\, \mu m$) emission yields robust star formation rates in the range $\sim 1-250\,M_\odot\,{\rm yr^{-1}}$. Combined with available stellar mass estimates, the vast majority ($\sim 75\%-90\%$) of the quasars lie on or above the main sequence of local star-forming galaxies, including a significant fraction ($\sim 50\%-70\%$) that would qualify as starburst systems. This is further supported by the high star formation efficiencies derived from the gas content inferred from the dust masses. Inspection of high-resolution Hubble Space Telescope images reveals a wide diversity of morphological types, including a number of starbursting hosts that have not experienced significant recent dynamical perturbations. The origin of the high star formation efficiency is unknown., Comment: 17 pages, 5 figures, 2 tables. Accepted for publication in ApJ

The mid-infrared spectrum contains rich diagnostics to probe the physical properties of galaxies, among which the pervasive emission features from polycyclic aromatic hydrocarbons (PAHs) offer a promising means of estimating the star formation rate (SFR) relatively immune from dust extinction. This paper investigates the effectiveness of PAH emission as a SFR indicator on sub-kpc scales by studying the Spitzer/IRS mapping-mode observations of the nearby grand-design spiral galaxy M51. We present a new approach of analyzing the spatial elements of the spectral datacube that simultaneously maximizes spatial resolution and spatial coverage, while yielding reliable measurements of the total, integrated $5-20\,\mu$m PAH emission. We devise a strategy of extracting robust PAH emission using spectra with only partial spectral coverage, complementing missing spectral regions with properly combined mid-infrared photometry. We find that in M51 the PAH emission correlates tightly with the extinction-corrected far-ultraviolet, near-ultraviolet, and H$\alpha$ emission, from scales$\sim 0.4$ kpc close to the nucleus to 6 kpc out in the disk of the galaxy, indicating that PAH serves as an excellent tracer of SFR over a wide range of galactic environments. But regional differences exist. Close to the active nucleus of M51 the 6.2 $\mu$m feature is weaker, and the overall level of PAH emission is suppressed. The spiral arms and the central star-forming region of the galaxy emit stronger 7.7 and 8.6 $\mu$m PAH features than the inter-arm regions., Comment: 16 pages, 9 figures, accepted for publication in the Astronomical Journal

Polycyclic aromatic hydrocarbon (PAH) emission has long been proposed to be a potential star formation rate indicator, as it arises from the photodissociation region bordering the Str\"omgren sphere of young, massive stars. We apply a recently developed technique of mid-infrared spectral decomposition to obtain a uniform set of PAH measurements from Spitzer low-resolution spectra of a large sample of star-forming galaxies spanning a wide range in stellar mass ($M_\star \approx 10^6-10^{11.4}\,M_{\odot}$) and star formation rate ($\sim 0.1- 2000\,M_{\odot}\rm\,yr^{-1}$). High-resolution spectra are also analyzed to measure [Ne II] 12.8 $\mu$m and [Ne III] 15.6 $\mu$m, which effectively trace the Lyman continuum. We present a new relation between PAH luminosity and star formation rate based on the [Ne II] and [Ne III] lines. Calibrations are given for the integrated $5-15$ $\mu$m PAH emission, the individual features at 6.2, 7.7, 8.6, and 11.3 $\mu$m, as well as several mid-infrared bandpasses sensitive to PAH. We confirm that PAH emission is suppressed in low-mass, dwarf galaxies, and we discuss the possible physical origin of this effect., Comment: 16 pages, 5 figures, 2 tables, accepted for publication in ApJ

The interstellar medium is a key ingredient that governs star formation in galaxies. We present a detailed study of the infrared (~ 1-500 micron) spectral energy distributions of a large sample of 193 nearby (z ~ 0.088) luminous infrared galaxies (LIRGs) covering a wide range of evolutionary stages along the merger sequence. The entire sample has been observed uniformly by 2MASS, WISE, Spitzer, and Herschel. We perform multi-component decomposition of the spectra to derive physical parameters of the interstellar medium, including the intensity of the interstellar radiation field and the mass and luminosity of the dust. We also constrain the presence and strength of nuclear dust heated by active galactic nuclei. The radiation field of LIRGs tends to have much higher intensity than in quiescent galaxies, and it increases toward advanced merger stages as a result of central concentration of the interstellar medium and star formation. The total gas mass is derived from the dust mass and the galaxy stellar mass. We find that the gas fraction of LIRGs is on average ~ 0.3 dex higher than that of main-sequence star-forming galaxies, rising moderately toward advanced merger stages. All LIRGs have star formation rates that place them above the galaxy star formation main sequence. Consistent with recent observations and numerical simulations, the global star formation efficiency of the sample spans a wide range, filling the gap between normal star-forming galaxies and extreme starburst systems., Comment: 23 pages, 14 figure, 3 tables; ApJ accepted

Interstellar dust spans a wide range in size distribution, ranging from ultrasmall grains of a few Angstroms to micrometer-size grains. While the presence of nanometer-size dust grains in the Galactic interstellar medium was speculated six decades ago and was previously suggested based on early infrared observations, systematic and direct analysis of their properties over a wide range of environments has been lacking. Here we report the detection of nanometer-size dust grains that appear to be universally present in a wide variety of astronomical environments, from Galactic high-latitude clouds to nearby star-forming galaxies and galaxies with low levels of nuclear activity. The prevalence of such a grain population is revealed conclusively as prominent mid-infrared continuum emission at wavelength short than 10\mu m seen in the Spitzer/IRS data, characterized by temperatures of ~300-400K that are significantly higher than the equilibrium temperatures of common, submicron-size grains in typical galactic environments. We propose that the optimal carriers of this pervasive, featureless hot dust component are very small carbonaceous (e.g., graphite) grains of nanometer size that are transiently heated by single-photon absorption. This grain population accounts for ~1.4% of the total infrared emission at ~5-3000\mu m and ~0.4% of the total interstellar dust mass., Comment: 25 pages, 5 figures, 2 tables; accepted for publication in The Astrophysical Journal

We present a new method to decompose the emission features of polycyclic aromatic hydrocarbons (PAHs) from mid-infrared spectra using theoretical PAH templates in conjunction with modified blackbody components for the dust continuum and an extinction term. The primary goal is to obtain robust measurements of the PAH features, which are sensitive to the star formation rate, in a variety of extragalactic environments. We demonstrate the effectiveness of our technique, starting with the simplest Galactic high-latitude clouds to extragalactic systems of ever-increasing complexity, from normal star-forming galaxies to low-luminosity active galaxies, quasars, and heavily obscured infrared-luminous galaxies. In addition to providing accurate measurements of the PAH emission, including the upper limits thereof, our fits can reproduce reasonably well the overall continuum shape and constrain the line-of-sight extinction. Our new PAH line flux measurements differ systematically and significantly from those of previous methods by ~15% to as much as a factor of ~6. The decomposed PAH spectra show remarkable similarity among different systems, suggesting a uniform set of conditions responsible for their excitation., Comment: 61 pages, 23 figures, accepted for publication in ApJ

The interstellar medium is crucial to understanding the physics of active galaxies and the coevolution between supermassive black holes and their host galaxies. However, direct gas measurements are limited by sensitivity and other uncertainties. Dust provides an efficient indirect probe of the total gas. We apply this technique to a large sample of quasars, whose total gas content would be prohibitively expensive to measure. We present a comprehensive study of the full (1 to 500 micron) infrared spectral energy distributions of 87 redshift <0.5 quasars selected from the Palomar-Green sample, using photometric measurements from 2MASS, WISE, and Herschel, combined with Spitzer mid-infrared (5 to 40 micron) spectra. With a newly developed Bayesian Markov Chain Monte Carlo fitting method, we decompose various overlapping contributions to the integrated spectral energy distribution, including starlight, warm dust from the torus, and cooler dust on galaxy scales. This procedure yields a robust dust mass, which we use to infer the gas mass, using a gas-to-dust ratio constrained by the host galaxy stellar mass. Most (90%) quasar hosts have gas fractions similar to those of massive, star-forming galaxies, although a minority (10%) seem genuinely gas-deficient, resembling present-day massive early-type galaxies. This result indicates that "quasar mode" feedback does not occur or is ineffective in the host galaxies of low-redshift quasars. We also find that quasars can boost the interstellar radiation field and heat dust on galactic scales. This cautions against the common practice of using the far-infrared luminosity to estimate the host galaxy star formation rate., Comment: 49 pages, 26 figures, 7 table. ApJ, in press

The large majority of the accreting supermassive black holes in the Universe are obscured by large columns of gas and dust. The location and evolution of this obscuring material have been the subject of intense research in the past decades, and are still highly debated. A decrease in the covering factor of the circumnuclear material with increasing accretion rates has been found by studies carried out across the electromagnetic spectrum. The origin of this trend has been suggested to be driven either by the increase in the inner radius of the obscuring material with incident luminosity due to the sublimation of dust; by the gravitational potential of the black hole; by radiative feedback; or by the interplay between outflows and inflows. However, the lack of a large, unbiased and complete sample of accreting black holes, with reliable information on gas column density, luminosity and mass, has left the main physical mechanism regulating obscuration unclear. Using a systematic multi-wavelength survey of hard X-ray-selected black holes, here we show that radiation pressure on dusty gas is indeed the main physical mechanism regulating the distribution of the circumnuclear material. Our results imply that the bulk of the obscuring dust and gas in these objects is located within the sphere of influence of the black hole (i.e., a few to tens of parsecs), and that it can be swept away even at low radiative output rates. The main physical driver of the differences between obscured and unobscured accreting black holes is therefore their mass-normalized accretion rate., Comment: To appear in the 28 September 2017 issue of Nature. This is the authors' version of the work

The tori around active galactic nuclei (AGN) are potential formation sites for large amounts of dust, and they may help resolve the so-called dust budget crisis at high redshift. We investigate the dust composition in 53 of the 87 Palomar Green (PG) quasars showing the 9.7 micron silicate feature in emission. By simultaneously fitting the mid-infrared spectroscopic features and the underlying continuum, we estimate the mass fraction in various amorphous and crystalline dust species. We find that the dust consists predominantly of alumina and amorphous silicates, with a small fraction in crystalline form. The mean crystallinity is 8 +/- 6%, with more than half of the crystallinities greater than 5%, well above the upper limit determined for the Galaxy. Higher values of crystallinity are found for higher oxide fractions and for more luminous sources., Comment: 24 pages, 9 figures, accepted for publication in Planetary and Space Science

The unification theory of active galactic nuclei (AGNs) hypothesizes that all AGNs are surrounded by an anisotropic dust torus and are essentially the same objects but viewed from different angles. However, little is known about the dust which plays a central role in the unification theory. There are suggestions that the AGN dust extinction law appreciably differs from that of the Galaxy. Also, the silicate emission features observed in type 1 AGNs appear anomalous (i.e., their peak wavelengths and widths differ considerably from that of the Galaxy). In this work, we explore the dust properties of 147 AGNs of various types at redshifts z<0.5, with special attention paid to 93 AGNs which exhibit the 9.7 and 18 $\mu$m silicate emission features. We model their silicate emission spectra obtained with the Infrared Spectrograph aboard the Spitzer Space Telescope. We find that 60/93 of the observed spectra can be well explained with "astronomical silicate", while the remaining sources favor amorphous olivine or pyroxene. Most notably, all sources require the dust to be $\mu$m-sized (with a typical size of ~1.5$\pm$0.1 $\mu$m), much larger than sub-$\mu$m-sized Galactic interstellar grains, implying a flat or "gray" extinction law for AGNs. We also find that, while the 9.7 $\mu$m emission feature arises predominantly from warm silicate dust of temperature T~270 K, the ~5--8 $\mu$m continuum emission is mostly from carbon dust of T~640 K. Finally, the correlations between the dust properties (e.g., mass, temperature) and the AGN properties (e.g., luminosity, black hole mass) have also been investigated., Comment: 68 pages, 24 figures, 6 tables; accepted for publication in The Astrophysical Journal Supplement Series

We investigate the dust properties in three spectroscopically anomalous galaxies (IRAS F10398+1455, IRAS F21013-0739 and SDSS J0808+3948). Their Spitzer/IRS spectra are characterized by a steep ~5-8 micron emission continuum, strong emission bands from polycyclic aromatic hydrocarbon (PAH) molecules, and prominent 10 micron silicate emission. The steep ~5-8 micron continuum and strong PAH emission features suggest the presence of starbursts, while the silicate emission is indicative of significant heating from AGNs. The simultaneous detection of these two observational properties has rarely been reported on galactic scale. We employ the PAHFIT software to estimate their starlight contributions, and the CLUMPY model for the components contributed by the AGN tori. We find that the CLUMPY model is generally successful in explaining the overall dust infrared emission, although it appears to emit too flat at the ~5-8 micron continuum to be consistent with that observed in IRAS F10398+1455 and IRAS F21013-0739. The flat ~5-8 micron continuum calculated from the CLUMPY model could arise from the adopted specific silicate opacity of Ossenkopf et al. (1992) which exceeds that of the Draine & Lee (1984) "astronomical silicate" by a factor up to 2 in the ~5-8 micron wavelength range. Future models with a variety of dust species incorporated in the CLUMPY radiation transfer regime are needed for a thorough understanding of the dust properties of these spectroscopically anomalous galaxies., Comment: 23 pages, 4 figures, 2 tables; accepted for publication in Planetary and Space Science, special issue on "Cosmic Dust"

The \textit{Spitzer}/Infrared Spectrograph spectra of three spectroscopically anomalous galaxies (IRAS~F10398+1455, IRAS~F21013-0739 and SDSS~J0808+3948) are modeled in terms of a mixture of warm and cold silicate dust, and warm and cold carbon dust. Their unique infrared (IR) emission spectra are characterized by a steep $\simali$5--8$\mum$ emission continuum, strong emission bands from polycyclic aromatic hydrocarbon (PAH) molecules, and prominent silicate emission. The steep $\simali$5--8$\mum$ emission continuum and strong PAH emission features suggest the dominance of starbursts, while the silicate emission is indicative of significant heating from active galactic nuclei (AGNs). With warm and cold silicate dust of various compositions ("astronomical silicate," amorphous olivine, or amorphous pyroxene) combined with warm and cold carbon dust (amorphous carbon, or graphite), we are able to closely reproduce the observed IR emission of these %spectroscopically anomalous galaxies. We find that the dust temperature is the primary cause in regulating the steep $\sim$5--8$\mum$ continuum and silicate emission, insensitive to the exact silicate or carbon dust mineralogy and grain size $a$ as long as $a\simlt1\mum$. More specifically, the temperature of the $\simali$5--8$\mum$ continuum emitter (which is essentially carbon dust) of these galaxies is $\sim$250--400$\K$, much lower than that of typical quasars which is $\sim$640$\K$. Moreover, it appears that larger dust grains are preferred in quasars. The lower dust temperature and smaller grain sizes inferred for these three galaxies compared with that of quasars could be due to the fact that they may harbor a young/weak AGN which is not maturely developed yet., Comment: 31 pages, 14 figures, accepted for publication in ApJ

On a galactic scale the 9.7um silicate emission is usually only seen in type 1 active galactic nuclei (AGNs). They usually also display a flat emission continuum at ~5--8um and the absence of polycyclic aromatic hydrocarbon (PAH) emission bands. In contrast, starburst galaxies, luminous infrared (IR) galaxies (LIRGs), and ultraluminous IR galaxies (ULIRGs) exhibit a red 5--8um emission continuum, strong 10um and 18um silicate absorption features, and strong PAH emission bands. Here we report the detection of anomalous dust properties by Spitzer/Infrared Spectrograph in three galaxies (IRASF10398+1455, IRASF21013-0739 and SDSSJ0808+3948) which are characterized by the simultaneous detection of a red 5--8um emission continuum, the 9.7 and 18um silicate emission features as well as strong PAH emission bands. These apparently contradictory dust IR emission properties are discussed in terms of iron-poor silicate composition, carbon dust deficit, small grain size and low dust temperature in the young AGN phase of these three galaxies., Comment: accepted for publication in ApJL

We have produced the next generation of quasar spectral energy distributions (SEDs), essentially updating the work of Elvis et al. (1994) by using high-quality data obtained with several space and ground-based telescopes, including NASA's Great Observatories. We present an atlas of SEDs of 85 optically bright, non-blazar quasars over the electromagnetic spectrum from radio to X-rays. The heterogeneous sample includes 27 radio-quiet and 58 radio-loud quasars. Most objects have quasi-simultaneous ultraviolet-optical spectroscopic data, supplemented with some far-ultraviolet spectra, and more than half also have Spitzer mid-infrared IRS spectra. The X-ray spectral parameters are collected from the literature where available. The radio, far-infrared, and near-infrared photometric data are also obtained from either the literature or new observations. We construct composite spectral energy distributions for radio-loud and radio-quiet objects and compare these to those of Elvis et al., finding that ours have similar overall shapes, but our improved spectral resolution reveals more detailed features, especially in the mid and near-infrared., Comment: 46 pages, 10 figures, 10 tables, Accepted by ApJS. Composite SED data files for radio-loud and radio-quiet quasars (rlmsedMR.txt, rqmsedMR.txt) are included in the source (Other formats -> Source). Supplemental figures are not included