Forming Compact Massive Galaxies

Abstract

In this paper we study a key phase in the formation of massive galaxies: the transition of star forming galaxies into massive (M stars ∼ 10 11 M ⊙ ), compact (r e ∼ 1 kpc) quiescent galaxies, which takes place from z ∼ 3 to z ∼ 1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star forming galaxies exists at z 2, using K-band spectroscopy of 25 of these objects at 2.0 < z < 2.5. They have a median [NII]/Hα ratio of 0.6, are highly obscured with SFR(tot)/SFR(Hα) ∼ 10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of Hα that the galaxies have rotating disks of ionized gas that are a factor of ∼ 2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from V rot ∼ 500 km s −1 at 1 kpc to V rot ∼ 250 km s −1 at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of ∆ log r e ∼ 0.3∆ log M stars . This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane.

Structural and Star-forming Relations since z ∼ 3: Connecting Compact Star-forming and Quiescent Galaxies

“…Thus, on average, galaxies would evolve along this relation as well. A similar statement can also be made about the r M e  -relation, as shown in van Dokkum et al (2015), which is equivalent in information to M e  S -. In that paper, the authors advocate for a slightly steeper slope (a=0.3) compared to that of the observed size-mass relation (a=0.22; van der ).…”

“…Previous works advanced a slightly different view in which quenching occurs when galaxies pass over a fixed threshold in effective (i.e., global) velocity dispersion that varies with redshift but not with mass (e.g., Franx et al 2008;Wake et al 2012;van Dokkum et al 2015). Figure 2 indicates that the accuracy of a threshold in e S or 1 S depends on the slope of the quiescent correlation with stellar mass, being more effective for weak correlations-when the slope is close to zero.…”

“…However, confining quenching to that track would likely overproduce the number of massive quiescent galaxies (e.g., Figure 27 of van Dokkum et al 2015). An alternative scenario to explain the emergence of denser quiescent galaxies is that at least some SFGs follow a steeper path upwards from the M SF  Srelation as a result of some period(s) of fast core growth relative to the smooth growth along the structural M  Srelations.…”

MOSFIRE SPECTROSCOPY OF QUIESCENT GALAXIES AT 1.5 < z < 2.5. I. EVOLUTION OF STRUCTURAL AND DYNAMICAL PROPERTIES

“…If rotation is indeed significant in high-redshift quiescent galaxies, we expect to see a variation of the measured velocity dispersion as a function of the inclination: edge-on systems will have larger values compared to face-on systems, where only the random component contributes to the measured velocity dispersion (see, e.g., van Dokkum et al 2015 andPrice et al 2016 for applications of this method to unresolved emission lines). This change in the measured velocity dispersion will cause, in turn, a corresponding change in the dynamical mass M dyn .…”

“…In particular, our detection of a significant rotational component suggests that some progenitors must be rotating disks. Compact star-forming galaxies atz 2, for example, show tentative evidence of rotation (van Dokkum et al 2015). If these are credible progenitors, the quenching process must preserve some fraction of the rotational motion.…”