Color-magnitude relation and morphology of ultraluminous infrared galaxies from z ~ 0 to z ~ 1
Ultraluminous infrared galaxies (ULIRGs) are among the most powerful galaxies and play an important role in galaxy formation and evolution. In this dissertation, I present by far the most detailed and latest studies of ULIRGs both in the nearby universe and at epochs only half of today’s universe’s age, through their color-magnitude relation (CMR) and morphology.
I studied the most luminous local ULIRGs in the SDSS, and found that local ULIRGs are optically luminous, on average 1 magnitude brighter than field galaxies, and they are as blue as the blue cloud in the color-magnitude diagram of galaxies. Only few ULIRGs appear in the green valley, the low density region between the red sequence and blue cloud, and none of which harbors an AGN. Therefore, we are not witnessing the AGN quenching of star formation, and the blue colors of ULIRGs is consistent with a working hypothesis in which the dust geometry is patchy and blue lights originate from unshielded stellar components, many of which are at large distances. We also imaged nine most luminous ULIRGs at z ∼ 1 using HST in ACS/F814W and NICMOS2/F160W filters. The z ∼ 1 ULIRGs, unlike the z ∼ 0 ULIRGs, appear to be more concentrated in the green valley. The concentration of z ∼ 1 ULIRGs in the green valley is possible due to the color selection in studying the CMR.
Quantitative morphology measurements have been widely used in describing galaxy morphology and classifying galaxies, but their limitations have not been systematically studied. In this dissertation I present detailed studies of the limitations of non-parametric quantitative morphology measurements, the Gini coefficient (G) and M 20. Both z ∼ 0 and z ∼ 1 ULIRGs distribute heterogeneously in G – M20 space, which is unexpected from the proposed effectiveness of G –M 20 in selecting merging galaxies. I performed a series simulations, which shows that seen against higher background noise, G decreases and M20 increases systematically. Observationally G and M20 are also more sensitive to certain merging stages, consistent with the findings from other numerical simulations. As a conclusion, cautions must be made when applying the quantitative morphology measurements.
There are strong color gradients across almost all z ∼ 0 and z ∼ 1 ULIRGs. Benefited from high spatial resolutions of HST, I developed a new technique to investigate the pixel-based color distribution and produced pixel-based color-magnitude diagrams (pCMDs) for the z ∼ 1 ULIRGs. A unique feature in the pCMD, "horizontal blue shelf", has been discovered among all z ∼ 1 ULIRGs, which is consistent with typical colors of today's disk, and can be modeled by young stellar populations with an age between 10 Myr and 500 Myr with moderate extinction.