Photometric redshifts and properties of galaxies from the sloan digital sky survey

The determination of photometric redshifts is essential for many subjects in cosmology and extragalactic astronomy, like the large scale structure of the Universe, gravitational lensing, or galaxy evolution. If the spectral energy distribution (SED) of a galaxy is measured with high enough spectral resolution, the redshift can be easily derived through the absorption and emission lines which are created by the elements in the galaxy. However, currently more telescopes are equipped with large cameras with charged coupled devices (CCDs) that observe the sky through optical filters. With these photometric observations it is possible to detect much fainter astronomical objects than with spectroscopy. Furthermore, photometric observations are less time consuming and cheaper in comparison, wherefore they are preferentially used for observations of statistical meaningful cosmological volumes. Nonetheless, photometric data, which are often gained by observations through broadband filters, are not as precisely resolved as spectra. Therefore one does not have information about the accurate position in wavelength of spectral lines, but only about the overall shape of the SED. This is the reason why so-called photometric redshifts have to be derived by statistical means. One approach to estimate the redshift through photometry alone are template fitting methods which compare the fluxes predicted by model spectra with the observations. After that, a likelihood analysis is performed with which a probability density function P(z) and the most probable value of z can be derived. To achieve high accuracies with photometric redshift template fitting techniques, the model spectra as well as their corresponding prior probabilities have to be chosen carefully. In this work I use photometric and spectroscopic data of luminous red galaxies from the Sloan Digital Sky Survey (SDSS). I analyze the precision of photometric redshifts estimated with model SEDs specifically designed to match the set of luminous red galaxies of SDSS-II at redshifts z ≤ 0.5 in color and I compare them with published results. These models were created without information on their properties at wavelengths shorter than the SDSS u band. However, the galaxy UV characteristics derived from the model SEDs match those of other observations. Furthermore, I investigate the SED properties derived from the best fitting models with respect to spectroscopic data as functions of redshift and luminosity. At lower redshifts less luminous galaxies from our sample on average show increased signs of star formation in comparison to galaxies with higher luminosities. This is supported by analyses of the line strengths in the spectra. Moreover, star formation activity increases with increasing redshift which is caused by the aging of the galaxy population from higher to lower redshifts. I also generate model spectra for red galaxies from the SDSS-III located at even higher redshifts 0.45 ≤ z ≤ 0.9. For this I modify the shape of theoretical spectra to match the data of the analyzed galaxies to a better extent. The multidimensional space defined by the colors and the absolute magnitude of the galaxies is reduced to two dimensions through a self-organizing map. The map is then partitioned by a k-means algorithm which identifies clusters in the data. From the cluster cells I select model spectra which represent the galaxies from within the same cell. A selection of the models is then used as a template set for photometric redshift estimation. I find that our models improve the redshift accuracy in comparison to the results published by SDSS.Die Bestimmung kosmologischer Rotverschiebungen ist für viele Untersuchungen in der Kosmologie und extragalaktischen Astronomie essentiell. Dies sind z.B. Analysen der großskaligen Struktur des Universums, des Gravitationslinseneffekts oder der Galaxienentwicklung. In der Kosmologie statistisch aussagekräftige Volumina werden heutzutage meist durch Breitbandfilter beobachtet. Mit diesen photometrischen Beobachtungen kann man nur Aussagen über die grobe Form des Spektrums machen, weshalb man sich statistischer Mittel bedienen muss um die Rotverschiebung zu schätzen. Eine Methode zur Bestimmung photometrischer Rotverschiebungen ist es, Flüsse von Modellgalaxien in den Filtern bei verschiedenen Rotverschiebungen z vorherzusagen und mit den beobachteten Flüssen zu vergleichen (template fitting). Danach wird eine Likelihood-Analyse durchgeführt, in der die Vorhersagen mit den Beobachtungen verglichen werden, um die Wahrscheinlichkeitsdichte P(z) und die wahrscheinlichste Rotverschiebung zu bestimmen. Um mit template fitting Methoden möglichst genaue Ergebnisse zu erzielen, müssen die Modellspektren, sowie die zugehörigen a priori Wahrscheinlichkeiten sorgfältig ausgewählt werden. In dieser Arbeit nutze ich photometrische und spektroskopische Daten leuchtkräftiger roter Galaxien (LRGs) aus dem Sloan Digital Sky Survey (SDSS). Ich untersuche die Genauigkeit photometrischer Rotverschiebungen, die mit Modellspektren erreicht werden, welche von mir speziell für den Satz LRGs bei z ≤ 0.5 (SDSS-II) entwickelt wurden, und vergleiche sie mit publizierten Ergebnissen. Diese Modelle wurden ohne Informationen aus Wellenlängenbereichen, die kurzwelliger als das SDSS u Band sind, erstellt. Die Galaxieneigenschaften, die wir aus den Modellen für den UV Bereich vorhersagen können decken sich allerdings mit denen aus anderen Beobachtungen. Darüber hinaus analysiere ich die sich daraus ergebenden Eigenschaften der am besten fittenden Modellspektren und vergleiche sie mit den spektroskopischen Daten. Aus den Ergebnissen ist abzuleiten, dass leuchtschwächere rote Galaxien bei niedriger Rotverschiebung im Mittel größere Anzeichen von Sternentstehung zeigen als leuchtkräftige, was durch Analysen der Spektren bestätigt wird. Überdies können wir einen Abfall im UV Fluss von höheren zu niedrigeren Rotverschiebungen hin beobachten, welcher durch die Alterung der Galaxienpopulation erzeugt wird. Desweiteren generiere ich Modellspektren für leuchtkräftige rote Galaxien aus SDSS-III bei höheren Rotverschiebungen 0.45 ≤ z ≤ 0.9. Ich modifiziere hierzu die Form theoretischer spektraler Energieverteilungen um die Farben der untersuchten Galaxien mit den Modellen bestmöglich wiedergeben zu können. Ich reduziere die Dimension des Raums, der durch die Farben und absoluten Helligkeiten aufgespannt wird, auf zwei Dimensionen mit einer selbstorganisierenden Karte. Diese wird mit einem k-means Algorithmus partitioniert indem wir Häufungspunkte der Daten identifizieren. Aus den sich ergebenden Partitionen selektiere ich einzelne Modellspektren, die die zugrundeliegenden Galaxien repräsentieren. Eine Auswahl aus den erstellten Modellen wird danach für die Schätzung photometrischer Rotverschiebungen verwendet, deren Genauigkeit über die von SDSS publizierten Ergebnisse hinausgeht

Photometric Redshifts and Systematic Variations in the SEDs of Luminous Red Galaxies from the SDSS DR7

We describe the construction of a template set of spectral energy distributions (SEDs) for the estimation of photometric redshifts of luminous red galaxies (LRGs) with a Bayesian template fitting method. By examining the color properties of several publicly available SED sets within a redshift range of 0<z<0.5 and comparing them to SDSS DR7 data, we show that only some of the investigated SEDs approximately match the colors of the LRG data throughout the redshift range, however not at the quantitative level required for precise photometric redshifts. We generate new SEDs by superposing model SEDs of composite stellar populations with a burst model, allowing both components to be reddened by dust, in order to match the data in five different redshift bins. We select a set of SEDs which represents the LRG data in color space within five redshift bins, thus defining our new SED template set for photometric redshift estimates. The results we get with the new template set and our Bayesian template fitting photometric redshift code (PhotoZ) are nearly unbiased, with a scatter of \sigma(\Delta z)=0.027 (including outliers), and a fraction of catastrophic outliers (|z_phot-z_spec|/(1+z_spec)>0.15) of 0.12%. We show that templates that optimally describe the brightest galaxies (-24.5<M_R<-22.7) indeed vary from z=0.1 to z=0.5, consistent with aging of the stellar population. Furthermore, we find that templates that optimally describe galaxies at z<0.1 strongly differ as a function of the absolute magnitude of the galaxies, indicating an increase in star formation activity for less luminous galaxies. Our findings based on the photometry of the SDSS LRGs and our SED template fitting are supported by comparison to the average SDSS LRG spectra in different luminosity and redshift bins.Comment: 21 pages, 25 figures, accepted for publication in the Astrophysical Journal (ApJ

ELDAR, a new method to identify AGN in multi-filter surveys: the ALHAMBRA test-case

International audienceWe present eldar, a new method that exploits the potential of medium- and narrow-band filter surveys to securely identify active galactic nuclei (AGN) and determine their redshifts. Our methodology improves on traditional approaches by looking for AGN emission lines expected to be identified against the continuum, thanks to the width of the filters. To assess its performance, we apply eldar to the data of the ALHAMBRA (Advance Large Homogeneous Area Medium Band Redshift Astronomical) survey, which covered an effective area of 2.38 deg^2 with 20 contiguous medium-band optical filters down to F814W ≃ 24.5. Using two different configurations of  eldar in which we require the detection of at least two and three emission lines, respectively, we extract two catalogues of type-I AGN. The first is composed of 585 sources (79  per cent of them spectroscopically unknown) down to  F814W = 22.5 at z_phot > 1, which corresponds to a surface density of 209 deg^−2. In the second, the 494 selected sources (83  per cent of them spectroscopically unknown) reach F814W = 23 at z_phot > 1.5, for a corresponding number density of 176 deg^−2. Then, using samples of spectroscopically known AGN in the ALHAMBRA fields, for the two catalogues we estimate a completeness of 73  per cent and 67  per cent, and a redshift precision of 1.01  per cent and 0.86  per cent (with outliers fractions of 8.1  per cent and 5.8  per cent). At z > 2, where our selection performs best, we reach 85  per cent and 77  per cent completeness and we find no contamination from galaxies