Color Magnitude Relation and Morphology of Low-Redshift ULIRGs in SDSS

TL;DR

This study analyzes the color-magnitude and morphological properties of 54 low-redshift ULIRGs in SDSS, revealing their typical blue colors, diverse morphologies, and ongoing merger activity, providing a baseline for high-redshift ULIRG studies.

Contribution

It offers a comprehensive morphological and color analysis of low-redshift ULIRGs, including their merger status and star formation activity, using SDSS data and Gini-M20 coefficients.

Findings

Most ULIRGs are in the blue cloud with high luminosity.

Few ULIRGs are in the green valley or host AGNs.

Approximately 42% of ULIRGs are in the merger region of G-M20 space.

Abstract

We present color-magnitude and morphological analysis of 54 low-redshift ULIRGs, a subset of the IRAS 1Jy sample (Kim & Sanders, 1998), in the SDSS. The ULIRGs are on average 1 magnitude brighter in M0.1r than the SDSS galaxies within the same redshift range. The majority of the ULIRGs (~87%) have the colors typical of the blue cloud, and only 4 sources (~7%) are located in the red sequence. While ULIRGs are popularly thought to be precursors to a QSO phase, we find few (~6%) in the "green valley" where the majority of the X-ray and IR selected AGNs are found, and none of which harbors an AGN. For the 14 previously spectroscopic identified AGNs (~28%), we perform PSF subtractions and find that on average the central point sources contribute less than one third to the total luminosity, and that their high optical luminosities and overall blue colors are apparently the result of star formation activity of the host galaxies. Visual inspection of the SDSS images reveals a wide range of disturbed morphologies. A detailed morphology analysis using Gini and M20 coefficients shows that slightly less than one half (~42% in g band) of the ULIRGs are located in the region where most local mergers are found. The heterogeneous distribution of ULIRGs in the G-M20 space is qualitatively consistent with the results found by numerical simulations of disk-disk mergers. Our study also shows that the measured morphological parameters are systematically affected by the SNR and thus the merging galaxies can appear at various regions in the G-M20 space. In general, our results reinforce the view that ULIRGs contain young stellar populations and are mergers in progress. Our study provides a uniform comparison sample for studying ULIRGs at higher redshifts such as Spitzer mid-IR selected ULIRGs at z=1~2 and submm galaxies.