205 research outputs found
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 m variability
We present a study of galaxies showing mid-infrared variability in data taken
in the deepest Spitzer/MIPS 24 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
-statistics method to select AGN candidates with a probability
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 arcmin 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 m variable sources would be
identified as AGN with these other methods. Therefore, MIPS 24 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 m variable identified AGN to the general AGN
population is small ( 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
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