The correlation between galaxy morphology and star formation activity at z~2

Press release of Bomee Lee et al. 2013 [image credit: ESA heic1315 Science Release]

Galaxy morphology in the local universe provides significant information about the formation and evolution of galaxies. Massive galaxies in the nearby universe are well described by the Hubble sequence, which correlates with the dominance of galaxy's central bulge, surface brightness and colors. In the classical picture, the late-type spiral galaxies are active star-forming structures with flattened, gas-rich, rotationally supported exponential disks, while early-type galaxies are more luminous, massive and quiescent systems with a so-called "de Vaucouleurs" (or similar) light profile. Until now, the key question of how and over what time-scale the Hubble Sequence has formed remain unanswered. We discuss the state of the assembly of the Hubble sequence in the mix of bright galaxies at redshift 1.4<z<2.5 with a large sample galaxies, selected from the HST/ACS and WFC3 images of the GOODS-South field obtained as part of the GOODS and CANDELS observations. We investigate the relationship between the star formation properties and morphology using various parametric diagnostics, such as the Sérsic light profile, Gini (G), M 20, concentration (C), asymmetry (A), and multiplicity (Ψ) parameters. Our sample clearly separates into massive, red, and passive galaxies versus less massive, blue, and star-forming ones, and this dichotomy correlates very well with the galaxies' morphological properties. Star-forming galaxies show a broad variety of morphological features, including clumpy structures and bulges mixed with faint low surface brightness features, generally characterized by disky-type light profiles. Passively evolving galaxies, on the other hand, very often have compact light distribution and morphology typical of today's spheroidal systems. Similar general trends are observed in the local universe among massive galaxies, suggesting that the backbone of the Hubble sequence was already in place at z ~ 2.

Robust measures of physical properties of galaxies at z~2 using investigation of star formation histories

Main sequence of star formation (MS) is a very tight correlation between stellar mass and star formation rate at z<2 which are commonly explained by a single power law. This implies that galaxies on the MS follow increasing star formation histories, not commonly used exponentially decreasing star formation histories. Renzini (2009) suggested that galaxies on, above and below the MS follow very different time evolution of SFR. We test this idea with a large sample of galaxies at 1<z<4 selected from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) in the GOODS-North and South fields. For the first time, we study their star formation histories (SFH) with an improved SED fitting technique, SpeedyMC [Acquaviva et al. 2011], by applying 5 different SFH for each galaxy. We find that stellar mass is insensitive to stellar mass, but star formation rate strongly depends on SFH. We analyze best-fitted SFHs for each galaxy and find that star-forming galaxies are mostly best fitted to increasing or constant SFH, but most passive galaxies are best fitted to exponentially decreasing SFH. Our studies are very important in aspect of that we constrain a SFH better for each galaxy and yield accurate stellar masse and star formation rates (SFR). Our robust measures of stellar masses and SFRs enable us to explore galaxies on the stellar mass vs. SFR plane, especially starbursts (exceeding three times the MS at a given stellar mass) and galaxies below the MS with a little star formation activities, which might be in a process of quenching.

Characteristics of galaxies on the SFR-Stellar mass diagram at z~2: How galaxies are evolved on the Main sequence?

Using CANDELS combined with GOODS-Herschel in the GOODS-North and South field, we investigate how galaxies quench their star formation and evolve on the stellar mass vs. SFR plane at 1<z<4. With an improved SED fitting technique, we are able to obtain more accurate stellar mass and SFR by testing various star formation histories (SFH) for each galaxy. We show that galaxies are apparently separated in four different populations: starbursts which lie above the main sequence of star formation (MS), normal star-forming galaxies on the tight MS, galaxies below the MS with a little star-forming activity and quiescent galaxies with different time evolution of SFR. We constrain the slope and the scatter better on the MS at 1<z<4. The MS slope cannot be explained by a single power-law and becomes flatter at high mass (log(M*)<10.5), indicating that star formation efficiency decrease at high masses. We study morphologies of galaxies using non-parametric (Sersic Index) and parametric measures as well as projected mass surface density. Using a projected mass surface density, more distinct morphological differences are shown among different galaxy populations. As star formation activities decrease, galaxies are more compact at all explored redshifts. In particular, the galaxies below the MS show very similar structures of the massive compact quiescent galaxies (QGs). Their morphologies and intermediate rest-frame colors between red QGs and blue SFGs indicate that they may be undergoing a quenching of star formation. We also find that the average morphologies of SB galaxies are disky and generally have much more diffuse optical light profile than massive compact early-type galaxies (ETGs). The sizes of the SB galaxies are clearly larger than those of the MS galaxies on average. Very compact SB galaxies are rather rare, and hence even from a statistical standpoint, our morphological analysis does not support the popular dissipative mechanism that powerful starbursts in wet mergers of gas-rich disks are the key driver to assemble very compact, massive early-type galaxies observed at z~2.