Engineering topics

Capak, Peter

Publications and source records attributed to Capak, Peter.

21 records · Page 2

DOE OSTI2015

MAPPING THE GALAXY COLOR–REDSHIFT RELATION: OPTIMAL PHOTOMETRIC REDSHIFT CALIBRATION STRATEGIES FOR COSMOLOGY SURVEYS

Calibrating the photometric redshifts of ≳10{sup 9} galaxies for upcoming weak lensing cosmology experiments is a major challenge for the astrophysics community. The path to obtaining the required spectroscopic redshifts for training and calibration is daunting, given the anticipated depths of the surveys and the difficulty in obtaining secure redshifts for some faint galaxy populations. Here we present an analysis of the problem based on the self-organizing map, a method of mapping the distribution of data in a high-dimensional space and projecting it onto a lower-dimensional representation. We apply this method to existing photometric data from the COSMOS survey selected to approximate the anticipated Euclid weak lensing sample, enabling us to robustly map the empirical distribution of galaxies in the multidimensional color space defined by the expected Euclid filters. Mapping this multicolor distribution lets us determine where—in galaxy color space—redshifts from current spectroscopic surveys exist and where they are systematically missing. Crucially, the method lets us determine whether a spectroscopic training sample is representative of the full photometric space occupied by the galaxies in a survey. We explore optimal sampling techniques and estimate the additional spectroscopy needed to map out the color–redshift relation, finding that sampling the galaxy distribution in color space in a systematic way can efficiently meet the calibration requirements. While the analysis presented here focuses on the Euclid survey, similar analysis can be applied to other surveys facing the same calibration challenge, such as DES, LSST, and WFIRST.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

A turnover in the galaxy main sequence of star formation at M{sub *} ∼ 10{sup 10}M{sub ☉} for Redshifts z < 1.3

The relationship between galaxy star formation rates (SFRs) and stellar masses (M {sub *}) is reexamined using a mass-selected sample of ∼62,000 star-forming galaxies at z ≤ 1.3 in the COSMOS 2 deg{sup 2} field. Using new far-infrared photometry from Herschel-PACS and SPIRE and Spitzer-MIPS 24 μm, along with derived infrared luminosities from the NRK method based on galaxies' locations in the restframe color-color diagram (NUV – r) versus (r – K), we are able to more accurately determine total SFRs for our complete sample. At all redshifts, the relationship between median SFR and M {sub *} follows a power law at low stellar masses, and flattens to nearly constant SFR at high stellar masses. We describe a new parameterization that provides the best fit to the main sequence and characterizes the low mass power-law slope, turnover mass, and overall scaling. The turnover in the main sequence occurs at a characteristic mass of about M {sub 0} ∼ 10{sup 10} M {sub ☉} at all redshifts. The low mass power-law slope ranges from 0.9-1.3 and the overall scaling rises in SFR as a function of (1 + z){sup 4.12 ± 0.10}. A broken power-law fit below and above the turnover mass gives relationships of SFR∝M{sub ∗}{sup 0.88±0.06} below the turnover mass and SFR∝M{sub ∗}{sup 0.27±0.04} above the turnover mass. Galaxies more massive than M {sub *} ≳ 10{sup 10} M {sub ☉} have a much lower average specific star formation rate (sSFR) than would be expected by simply extrapolating the traditional linear fit to the main sequence found for less massive galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

DUST ATTENUATION IN HIGH REDSHIFT GALAXIES: 'DIAMONDS IN THE SKY'

We use observed optical to near-infrared spectral energy distributions (SEDs) of 266 galaxies in the COSMOS survey to derive the wavelength dependence of the dust attenuation at high redshift. All of the galaxies have spectroscopic redshifts in the range z = 2-6.5. The presence of the C IV absorption feature, indicating that the rest-frame UV-optical SED is dominated by OB stars, is used to select objects for which the intrinsic, unattenuated spectrum has a well-established shape. Comparison of this intrinsic spectrum with the observed broadband photometric SED then permits derivation of the wavelength dependence of the dust attenuation. The derived dust attenuation curve is similar in overall shape to the Calzetti curve for local starburst galaxies. We also see the 2175 Å bump feature which is present in the Milky Way and Large Magellanic Cloud extinction curves but not seen in the Calzetti curve. The bump feature is commonly attributed to graphite or polycyclic aromatic hydrocarbons. No significant dependence is seen with redshift between sub-samples at z = 2-4 and z = 4-6.5. The 'extinction' curve obtained here provides a firm basis for color and extinction corrections of high redshift galaxy photometry.

79 ASTRONOMY AND ASTROPHYSICS↗