Photometric redshifts from reconstructed QSO templates

From SDSS commissioning photometric and spectroscopic data, we investigate the utility of photometric redshift techniques to the task of estimating QSO redshifts. We consider empirical methods (e.g. nearest-neighbor searches and polynomial fitting), standard spectral template fitting and hybrid approaches (i.e. training spectral templates from spectroscopic and photometric observations of QSOs). We find that in all cases, due to the presence of strong emission-lines within the QSO spectra, the nearest-neighbor and template fitting methods are superior to the polynomial fitting approach. Applying a novel reconstruction technique, we can, from the SDSS multicolor photometry, reconstruct a statistical representation of the underlying SEDs of the SDSS QSOs. Although, the reconstructed templates are based on only broadband photometry the common emission lines present within the QSO spectra can be recovered in the resulting spectral energy distributions. The technique should be useful in searching for spectral differences among QSOs at a given redshift, in searching for spectral evolution of QSOs, in comparing photometric redshifts for objects beyond the SDSS spectroscopic sample with those in the well calibrated photometric redshifts for objects brighter than 20th magnitude and in searching for systematic and time variable effects in the SDSS broad band photometric and spectral photometric calibrations.Comment: 21 pages, 9 figures, LaTeX AASTeX, submitted to A

Properties of star forming galaxies in AKARI Deep Field-South

The main aim of this work is the characterization of physical properties of galaxies detected in the far infrared (FIR) in the AKARI Deep Field-South (ADF-S) survey. Starting from a catalog of the 1 000 brightest ADF-S sources in the WIDE-S (90$\mu$m) AKARI band, we constructed a subsample of galaxies with spectral coverage from the ultraviolet to the far infrared. We then analyzed the multiwavelength properties of this 90$\mu$m selected sample of galaxies. For galaxies without known spectroscopic redshifts we computed photometric redshifts using the codes Photometric Analysis for Redshift Estimate (Le PHARE) and Code Investigating GALaxy Emission (CIGALE), tested these photometric redshifts using spectroscopic redshifts, and compared the performances of both codes. To test the reliability of parameters obtained by fitting spectral energy distributions, a mock cataloge was generated. We built a large multiwavelength catalog of more than 500 ADF-S galaxies. We successfully fitted Spectral Energy Distributions of 186 galaxies with $\rm{\chi^2_{min}<4}$, and analyzed the output parameters of the fits. We conclude that our sample consists mostly of nearby actively star-forming galaxies, and all our galaxies have a relatively high metallicity. We estimated photometric redshifts for 113 galaxies from the whole ADF-S sample. Comparing the performance of Le PHARE and CIGALE, we found that CIGALE gives more reliable redshift estimates for our galaxies, which implies that including the IR photometry allows for substantial improvement of photometric redshift estimation.Comment: 16 pages, 20 figures, accepted for publication in A&

Stochastic model of optical variability of BL Lacertae

We use optical photometric and polarimetric data of BL Lacertae that cover a period of 22 years to study the variability of the source. The long-term observations are employed for establishing parameters of a stochastic model consisting of the radiation from a steady polarized source and a number of variable components with different polarization parameters, proposed by Hagen-Thorn et al. earlier. We infer parameters of the model from the observations using numerical simulations based on a Monte Carlo method, with values of each model parameter selected from a Gaussian distribution. We determine the best set of model parameters by comparing model distributions to the observational ones using the chi-square criterion. We show that the observed photometric and polarimetric variability can be explained within a model with a steady source of high polarization, ~40%, and with direction of polarization parallel to the parsec scale jet, along with 10+-5 sources of variable polarization.Comment: 4 pages, 10 figures, published by Astronomy and Astrophysics; v2: typos correcte

Photometric Redshifts of Galaxies in COSMOS

We measure photometric redshifts and spectral types for galaxies in the COSMOS survey. We use template fitting technique combined with luminosity function priors and with the option to simultaneously estimate dust extinction (i.e. E(B-V)) for each galaxy.Our estimated redshifts are accurate to i<25 and z~1.2. Using simulations with sampling and noise characteristics similar to those in COSMOS, the accuracy and reliability is estimated for the photometric redshifts as a function of the magnitude limits of the sample, S/N ratios and the number of bands used. From the simulations we find that the ratio of derived 95% confidence interval in the redshift probability distribution to the estimated photometric redshift (D95) can be used to identify and exclude the catastrophic failures in the photometric redshift estimates. We compare the derived redshifts with high-reliability spectroscopic redshifts for a sample of 868 normal galaxies with z < 1.2 from zCOSMOS. Considering different scenarios, depending on using prior, no prior and/or extinction, we compare the photometric and spectroscopic redshifts for this sample. This corresponds to an rms scatter of 0.031, with a small number of outliers (<2.5%). We also find good agreement (rms=0.10) between photometric and spectroscopic redshifts for Type II AGNs. We compare results from our photometric redshift procedure with three other independent codes and find them in excellent agreement. We show preliminary results, based on photometric redshifts for the entire COSMOS sample (to i < 25 mag.).Comment: 38 pages; 14 Figures; 7 Tables. Accepted for Publication in ApJS. COSMOS Special Issu

Reconstructing Galaxy Spectral Energy Distributions from Broadband Photometry

We present a novel approach to photometric redshifts, one that merges the advantages of both the template fitting and empirical fitting algorithms, without any of their disadvantages. This technique derives a set of templates, describing the spectral energy distributions of galaxies, from a catalog with both multicolor photometry and spectroscopic redshifts. The algorithm is essentially using the shapes of the templates as the fitting parameters. From simulated multicolor data we show that for a small training set of galaxies we can reconstruct robustly the underlying spectral energy distributions even in the presence of substantial errors in the photometric observations. We apply these techniques to the multicolor and spectroscopic observations of the Hubble Deep Field building a set of template spectra that reproduced the observed galaxy colors to better than 10%. Finally we demonstrate that these improved spectral energy distributions lead to a photometric-redshift relation for the Hubble Deep Field that is more accurate than standard template-based approaches.Comment: 23 pages, 8 figures, LaTeX AASTeX, accepted for publication in A

Evolution of the Clustering of Photometrically Selected SDSS Galaxies

We measure the angular auto-correlation functions (w) of SDSS galaxies selected to have photometric redshifts 0.1 < z < 0.4 and absolute r-band magnitudes Mr < -21.2. We split these galaxies into five overlapping redshift shells of width 0.1 and measure w in each subsample in order to investigate the evolution of SDSS galaxies. We find that the bias increases substantially with redshift - much more so than one would expect for a passively evolving sample. We use halo-model analysis to determine the best-fit halo-occupation-distribution (HOD) for each subsample, and the best-fit models allow us to interpret the change in bias physically. In order to properly interpret our best-fit HODs, we convert each halo mass to its z = 0 passively evolved bias (bo), enabling a direct comparison of the best-fit HODs at different redshifts. We find that the minimum halo bo required to host a galaxy decreases as the redshift decreases, suggesting that galaxies with Mr < -21.2 are forming in halos at the low-mass end of the HODs over our redshift range. We use the best-fit HODs to determine the change in occupation number divided by the change in mass of halos with constant bo and we find a sharp peak at bo ~ 0.9 - corresponding to an average halo mass of ~ 10^12Msol/h. We thus present the following scenario: the bias of galaxies with Mr < -21.2 decreases as the Universe evolves because these galaxies form in halos of mass ~ 10^12Msol/h (independent of redshift), and the bias of these halos naturally decreases as the Universe evolves.Comment: 17 pages, 14 figures, matches version accepted for publication in MNRA

Modelling and interpreting spectral energy distributions of galaxies with BEAGLE

We present a new-generation tool to model and interpret spectral energy distributions (SEDs) of galaxies, which incorporates in a consistent way the production of radiation and its transfer through the interstellar and intergalactic media. This flexible tool, named BEAGLE (for BayEsian Analysis of GaLaxy sEds), allows one to build mock galaxy catalogues as well as to interpret any combination of photometric and spectroscopic galaxy observations in terms of physical parameters. The current version of the tool includes versatile modeling of the emission from stars and photoionized gas, attenuation by dust and accounting for different instrumental effects, such as spectroscopic flux calibration and line spread function. We show a first application of the BEAGLE tool to the interpretation of broadband SEDs of a published sample of ${\sim}10^4$ galaxies at redshifts $0.1 \lesssim z\lesssim8$. We find that the constraints derived on photometric redshifts using this multi-purpose tool are comparable to those obtained using public, dedicated photometric-redshift codes and quantify this result in a rigorous statistical way. We also show how the post-processing of BEAGLE output data with the Python extension PYP-BEAGLE allows the characterization of systematic deviations between models and observations, in particular through posterior predictive checks. The modular design of the BEAGLE tool allows easy extensions to incorporate, for example, the absorption by neutral galactic and circumgalactic gas, and the emission from an active galactic nucleus, dust and shock-ionized gas. Information about public releases of the BEAGLE tool will be maintained on http://www.jacopochevallard.org/beagle.Comment: added missing term in equation 4.1 (Erratum submitted to MNRAS

UV star-formation rates of GRB host galaxies

We study a magnitude-limited sample of 10 gamma-ray burst (GRB) host galaxies with known spectroscopic redshifts (0.43 < z < 2.04). From an analysis of the spectral energy distributions (SEDs), based on published broad-band optical and near-infrared photometry, we derive photometric redshifts, galaxy types, ages of the dominant stellar populations, internal extinctions, and ultraviolet (UV) star-formation rates (SFRs) of the host galaxies. The photometric redshifts are quite accurate despite the heterogeneous nature of the sample: The r.m.s. errors are sigma(z) = 0.21 and sigma(Delta z/(1+z_spec)) = 0.16 with no significant systematic offsets. All the host galaxies have SEDs similar to young starburst galaxies with moderate to low extinction. A comparison of specific SFRs with those of high-redshift galaxies in the Hubble Deep Fields shows that GRB hosts are most likely similar to the field galaxies with the largest specific SFRs. On the other hand, GRB hosts are not significantly younger than starburst field galaxies at similar redshifts, but are found to be younger than a sample of all types of field galaxies.Comment: 15 pages, 7 figures, accepted for publication in A&