Robust automatic photometry of local galaxies from SDSS Dissecting the color magnitude relation with color profiles

TL;DR

This paper introduces an automatic photometry method for SDSS galaxy images, effectively addressing the shredding problem and enabling analysis of a large galaxy sample, revealing detailed color-magnitude relations and profiles.

Contribution

The authors develop a new automated procedure for galaxy photometry that overcomes SDSS pipeline limitations, allowing comprehensive analysis of local galaxies including previously missed ones.

Findings

The method accurately derives Petrosian magnitudes for all galaxies, including 25% missed by SDSS.

Color-magnitude diagrams show well-separated red and blue sequences after extinction correction.

Elliptical galaxies exhibit no color gradients, while spirals show increasing gradients with mass.

Abstract

We present an automatic procedure to perform reliable photometry of galaxies on SDSS images. We selected a sample of 5853 galaxies in the Coma and Virgo superclusters. For each galaxy, we derive Petrosian g and i magnitudes, surface brightness profiles and color profiles. Unlike the SDSS pipeline, our procedure is not affected by the well known shredding problem and efficiently extracts Petrosian magnitudes for all galaxies. Hence we derived magnitudes even from the population of galaxies missed by the SDSS that represents 25% of all Local supercluster galaxies and ~95% of galaxies with g < 11 mag. After correcting the g and i magnitudes for Galactic and internal extinction, the blue and red sequences in the color magnitude diagram are well separated, with similar slopes. In addition, we study (i) the color-magnitude diagrams in different galaxy regions, the inner (r <= 1 kpc), intermediate (0.2R{Pet} <=r<= 0.3R{Pet}) and outer, disk-dominated (r>=0.35R{Pet})) zone, and (ii), we compute template color profiles, discussing the dependences of the templates on the galaxy masses and on their morphological type. The two analyses consistently lead to a picture where elliptical galaxies show no color gradients, irrespective of their masses. Spirals, instead, display a steeper gradient in their color profiles with increasing mass, consistently with the growing relevance of a bulge and/or a bar component above 10^{10} Modot.