Disentangling accretion disk and dust emissions in the infrared spectrum of type 1 AGN

We use a semi-empirical model to reproduce the 0.1-10um spectral energy distribution (SED) of a sample of 85 luminous quasars. In the model, the continuum emission from the accretion disk as well as the nebular lines are represented by a single empirical template (disk), where differences in the optical spectral index are reproduced by varying the amount of extinction. The near- and mid-infrared emission of the AGN-heated dust is modelled as the combination of two black-bodies (dust). The model fitting shows that the disk and dust components are remarkably uniform among individual quasars, with differences in the observed SED largely accounted for by varying levels of obscuration in the disk as well as differences in the relative luminosity of the disk and dust components. By combining the disk-subtracted SEDs of the 85 quasars, we generate a template for the 1-10um emission of the AGN-heated dust. Additionally, we use a sample of local Seyfert 1 galaxies with full spectroscopic coverage in the 0.37um to 39um range to demonstrate a method for stitching together spectral segments obtained with different PSF and extraction apertures. We show that the disk and dust templates obtained from luminous quasars also reproduce the optical-to-mid-infrared spectra of local Seyfert 1s when the contribution from the host galaxy is properly subtracted.Comment: 8 pages, 4 figures; accepted for publication in Frontiers in Astronomy and Space Sciences, Research Topic "Quasars at all cosmic epochs"; proceedings of the conference "Quasars at all cosmic epochs", held in Padova, April 2-7, 201

Automated measurement of redshifts from mid-infrared low-resolution spectroscopy

Obtaining accurate redshifts from mid-infrared (MIR) low-resolution (R ~ 100) spectroscopy is challenging because the wavelength resolution is too low to detect narrow lines in most cases. Yet, the number of degrees of freedom and diversity of spectral features are too high for regular spectral energy distribution (SED) fitting techniques to be convenient. Here we present a new SED-fitting-based routine for redshift determination that is optimized for MIR low-resolution spectroscopy. Its flexible template scaling increases the sensitivity to slope changes and small-scale features in the spectrum, while a new selection algorithm called maximum combined pseudo-likelihood (MCPL) provides increased accuracy and a lower number of outliers compared to the standard maximum-likelihood (ML) approach. Unlike ML approach, the MCPL approach searches for local (instead of absolute) maxima of a 'pseudo-likelihood' (PL) function, and combines results obtained for all the templates in the library to weed out spurious redshift solutions. The capabilities of the MCPL approach are demonstrated by comparing its redshift estimates to those of the regular ML approach and to the optical spectroscopic redshifts of a sample of 491 Spitzer/Infrared Spectrograph spectra from extragalactic sources at 0 0.02] is 14 per cent for the MCPL approach and 22 per cent for the ML approach. χ 2 values for ML solutions are found to correlate with the signal-to-noise ratio of the spectra, but not with redshift accuracy. By contrast, the peak value of the normalized combined PL (γ) is found to provide a good indication on the reliability of the MCPL solution for individual sources. The accuracy and reliability of the redshifts depend strongly on the MIR SED. Sources with significant polycyclic aromatic hydrocarbon emission obtain much better results compared to sources dominated by active galactic nucleus continua. Nevertheless, for a given γ the frequency of accurate solutions and outliers is largely independent of their SED type. This reliability indicator for MCPL solutions allows to select subsamples with highly reliable redshifts. In particular, a γ > 0.15 threshold retains 79 per cent of the sources with Δ(z)/(1 + z) < 0.005 while reducing the outlier rate to 3.8 per cent. © 2012 The Author Monthly Notices of the Royal Astronomical Society. © 2012 RAS.AH-C is funded by the Universidad de Cantabria Augusto González Linares programme.Peer Reviewe

A complete census of silicate features in the mid-infrared spectra of active galaxies

We present a comprehensive study of the silicate features at 9.7 and 18 micron of a sample of almost 800 active galactic nuclei (AGN) with available spectra from the Spitzer InfraRed Spectrograph (IRS). We measure the strength of the silicate feature at 9.7 micron, S9.7, before and after subtracting the host galaxy emission from the IRS spectra. The numbers of type 1 and 2 AGN with the feature in emission increase by 20 and 50%, respectively, once the host galaxy is removed, while 35% of objects with the feature originally in absorption exhibit it in even deeper absorption. The peak of S9.7, lambda_peak, has a bimodal distribution when the feature is in emission, with about 65% of the cases showing lambda_peak > 10.2 micron. Silicates can appear in emission in objects with mid-infrared (MIR) luminosity spanning over six orders of magnitude. The derived distributions of the strength of the silicate features at 9.7 and 18 micron provide a solid test bed for modeling the dust distribution in AGN. Clumpiness is needed in order to produce absorption features in unobscured AGN and can also cause the silicates to be in absorption at 9.7 micron and in emission at 18 micron in type 1 sources. We find the `cosmic' silicates of Ossenkopf et al. to be more consistent with the observations than Draine's `astronomical' silicates. Finally, we discuss the possibility of a foreground absorber to explain the deep silicate absorption features in the MIR spectra of some type 2 AGN.Comment: 10 pages, 11 figures. Accepted for publication in Ap

The Infrared Database of Extragalactic Observables from Spitzer I: the redshift catalog

This is the first of a series of papers on the Infrared Database of Extragalactic Observables from Spitzer (IDEOS). In this work we describe the identification of optical counterparts of the infrared sources detected in Spitzer Infrared Spectrograph (IRS) observations, and the acquisition and validation of redshifts. The IDEOS sample includes all the spectra from the Cornell Atlas of Spitzer/IRS Sources (CASSIS) of galaxies beyond the Local Group. Optical counterparts were identified from correlation of the extraction coordinates with the NASA Extragalactic Database (NED). To confirm the optical association and validate NED redshifts, we measure redshifts with unprecedented accuracy on the IRS spectra ({\sigma}(dz/(1+z))=0.0011) by using an improved version of the maximum combined pseudo-likelihood method (MCPL). We perform a multi-stage verification of redshifts that considers alternate NED redshifts, the MCPL redshift, and visual inspection of the IRS spectrum. The statistics is as follows: the IDEOS sample contains 3361 galaxies at redshift 0<z<6.42 (mean: 0.48, median: 0.14). We confirm the default NED redshift for 2429 sources and identify 124 with incorrect NED redshifts. We obtain IRS-based redshifts for 568 IDEOS sources without optical spectroscopic redshifts, including 228 with no previous redshift measurements. We provide the entire IDEOS redshift catalog in machine-readable formats. The catalog condenses our compilation and verification effort, and includes our final evaluation on the most likely redshift for each source, its origin, and reliability estimates.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in MNRAS. Full redshift table in machine-readable format available at http://ideos.astro.cornell.edu/redshifts.htm

Selection of AGN candidates in the GOODS-South Field through SPITZER/MIPS 24 μ\mum variability

We present a study of galaxies showing mid-infrared variability in data taken in the deepest Spitzer/MIPS 24 $\mu$m surveys in the GOODS-South field. We divide the dataset in epochs and subepochs to study the long-term (months-years) and the short-term (days) variability. We use a $\chi^2$-statistics method to select AGN candidates with a probability $\leq$ 1% that the observed variability is due to statistical errors alone. We find 39 (1.7% of the parent sample) sources that show long-term variability and 55 (2.2% of the parent sample) showing short-term variability. That is, 0.03 sources $\times$ arcmin$^{-2}$ for both, long-term and short-term variable sources. After removing the expected number of false positives inherent to the method, the estimated percentages are 1.0% and 1.4% of the parent sample for the long-term and short-term respectively. We compare our candidates with AGN selected in the X-ray and radio bands, and AGN candidates selected by their IR emission. Approximately, 50% of the MIPS 24 $\mu$m variable sources would be identified as AGN with these other methods. Therefore, MIPS 24 $\mu$m variability is a new method to identify AGN candidates, possibly dust obscured and low luminosity AGN, that might be missed by other methods. However, the contribution of the MIPS 24 $\mu$m variable identified AGN to the general AGN population is small ($\leq$ 13%) in GOODS-South.Comment: Accepted for publication in MNRA

The miniJPAS survey : photometric redshift catalogue

MiniJPAS is a ∼1 deg2 imaging survey of the AEGIS field in 60 bands, performed to demonstrate the scientific potential of the upcoming Javalambre-Physics of the Accelerating Universe Astrophysical Survey (J-PAS). Full coverage of the 3800–9100 Å range with 54 narrow-band filters, in combination with 6 optical broad-band filters, allows for extremely accurate photometric redshifts (photo-z), which, applied over areas of thousands of square degrees, will enable new applications of the photo-z technique, such as measurement of baryonic acoustic oscillations. In this paper we describe the method we used to obtain the photo-z that is included in the publicly available miniJPAS catalogue, and characterise the photo-z performance. We built photo-spectra with 100 Å resolution based on forced-aperture photometry corrected for point spread function. Systematic offsets in the photometry were corrected by applying magnitude shifts obtained through iterative fitting with stellar population synthesis models. We computed photo-z with a customised version of LePhare, using a set of templates that is optimised for the J-PAS filter-set. We analysed the accuracy of miniJPAS photo-z and their dependence on multiple quantities using a subsample of 5266 galaxies with spectroscopic redshifts from SDSS and DEEP, which we find to be representative of the whole r 0.03), regardless of the magnitude, redshift, or spectral type of the sources. We show that the two main summary statistics characterising the photo-z accuracy for a population of galaxies (σNMAD and η) can be predicted by the distribution of odds in this population, and we use this to estimate the statistics for the whole miniJPAS sample. At r 0.82 with η = 0.05, at the cost of decreasing the density of selected galaxies to n ∼ 5200 deg−2 (∼2600 of which have |∆z| < 0.003)

Automated measurement of redshift from mid-infrared low resolution spectroscopy

We present a new SED-fitting based routine for redshift determination that is optimised for mid-infrared (MIR) low-resolution spectroscopy. Its flexible template scaling increases the sensitivity to slope changes and small scale features in the spectrum, while a new selection algorithm called Maximum Combined Pseudo-Likelihood (MCPL) provides increased accuracy and a lower number of outliers compared to the standard maximum-likelihood (ML) approach. Unlike ML, MCPL searches for local (instead of absolute) maxima of a 'pseudo-likelihood' (PL) function, and combines results obtained for all the templates in the library to weed out spurious redshift solutions. The capabilities of MCPL are demonstrated by comparing its results to those of regular ML and to the optical spectroscopic redshifts of a sample of 491 Spitzer/IRS spectra from sources at 0<z<3.7. MCPL achieves a redshift accuracy dz/(1+z)<0.005 for 78% of the galaxies in the sample compared to 68% for ML. The rate of outliers (dz/(1+z)>0.02) is 14% for MCPL and 22% for ML. chi^2 values for ML solutions are found to correlate with the SNR of the spectra, but not with redshift accuracy. By contrast, the peak value of the normalised combined PL (gamma) is found to provide a good indication on the reliability of the MCPL solution for individual sources. The accuracy and reliability of the redshifts depends strongly on the MIR SED. Sources with significant polycyclic aromatic hydrocarbon emission obtain much better results compared to sources dominated by AGN continuum. Nevertheless, for a given gamma the frequency of accurate solutions and outliers is largely independent on their SED type. This reliability indicator for MCPL solutions allows to select subsamples with highly reliable redshifts. In particular, a gamma>0.15 threshold retains 79% of the sources with dz/(1+z)<0.005 while reducing the outlier rate to 3.8% (abridged).Comment: 23 pages, 12 figures, 5 tables. Accepted for publication in MNRA

Automated measurement of redshift from mid-infrared low resolution spectroscopy

We present a new SED-fitting based routine for redshift determination that is optimised for mid-infrared (MIR) low-resolution spectroscopy. Its flexible template scaling increases the sensitivity to slope changes and small scale features in the spectrum, while a new selection algorithm called Maximum Combined Pseudo-Likelihood (MCPL) provides increased accuracy and a lower number of outliers compared to the standard maximum-likelihood (ML) approach. Unlike ML, MCPL searches for local (instead of absolute) maxima of a 'pseudo-likelihood' (PL) function, and combines results obtained for all the templates in the library to weed out spurious redshift solutions. The capabilities of MCPL are demonstrated by comparing its results to those of regular ML and to the optical spectroscopic redshifts of a sample of 491 Spitzer/IRS spectra from sources at 0<z<3.7. MCPL achieves a redshift accuracy dz/(1+z)<0.005 for 78% of the galaxies in the sample compared to 68% for ML. The rate of outliers (dz/(1+z)>0.02) is 14% for MCPL and 22% for ML. chi^2 values for ML solutions are found to correlate with the SNR of the spectra, but not with redshift accuracy. By contrast, the peak value of the normalised combined PL (gamma) is found to provide a good indication on the reliability of the MCPL solution for individual sources. The accuracy and reliability of the redshifts depends strongly on the MIR SED. Sources with significant polycyclic aromatic hydrocarbon emission obtain much better results compared to sources dominated by AGN continuum. Nevertheless, for a given gamma the frequency of accurate solutions and outliers is largely independent on their SED type. This reliability indicator for MCPL solutions allows to select subsamples with highly reliable redshifts. In particular, a gamma>0.15 threshold retains 79% of the sources with dz/(1+z)<0.005 while reducing the outlier rate to 3.8% (abridged).Comment: 23 pages, 12 figures, 5 tables. Accepted for publication in MNRA