From Rings to Bulges: Evidence for Rapid Secular Galaxy Evolution atz∼ 2 from Integral Field Spectroscopy in the SINS Survey

Abstract

We present Hα integral field spectroscopy of well resolved, UV/optically selected z~2 star-forming galaxies as part of the SINS survey with SINFONI on the ESO VLT.Our laser guide star adaptive optics and good seeing data show the presence of turbulent rotating star forming rings/disks, plus central bulge/inner disk components, whose mass fractions relative to total dynamical mass appears to scale with [NII]/Hα flux ratio and 'star formation' age. We propose that the buildup of the central disks and bulges of massive galaxies at z~2 can be driven by the early secular evolution of gas-rich 'proto'-disks. High redshift disks exhibit large random motions. This turbulence may in part be stirred up by the release of gravitational energy in the rapid 'cold' accretion flows along the filaments of the cosmic web. As a result dynamical friction and viscous processes proceed on a time scale of <1 Gyr, at least an order of magnitude faster than in z~0 disk galaxies. Early secular evolution thus drives gas and stars into the central regions and can build up exponential disks and massive bulges, even without major mergers. Secular evolution along with increased efficiency of star formation at high surface densities may also help to account for the short time scales of the stellar buildup observed in massive galaxies at z~2.

Early formation of massive, compact, spheroidal galaxies with classical profiles by violent disc instability or mergers

“…These are galaxies with stellar masses between M * = 10 10 and 10 11 M ⊙ at redshifts z = 1 − 4, a few Gyrs period during which more than half of the present-day stellar mass was formed (Dickinson et al 2003;Reddy et al 2008;Madau & Dickinson 2014). A significant fraction of the massive star-forming galaxies can be characterized as thick, turbulent, extended, rotating discs that are highly perturbed by transient elongated features and giant clumps (Genzel et al 2006;Elmegreen & Elmegreen 2006;Genzel et al 2008;Stark et al 2008;Law et al 2009;Förster Schreiber et al 2009Wuyts et al 2012;Guo et al 2012Guo et al , 2014. First dubbed "clump-cluster" and "chain" galaxies, based on their face-on or edge-on rest-frame UV images (Cowie et al 1995;van den Bergh 1996;Elmegreen et al 2004Elmegreen et al , 2005Elmegreen et al , 2007, a significant fraction of these galaxies (> 50% for the more massive ones) are confirmed by spectroscopic measurements to be rotating ⋆ E-mail: daniel.ceverino@uam.es and extended discs (Genzel et al 2006;Shapiro et al 2008;Förster Schreiber et al 2009).…”

A magnified view of galaxy formation

“…Later that year, the first observations of a lensed galaxy at z = 3.2 showed clear rotation with ∼200 parsec spatial resolution [34], sufficient to characterize the rising inner rotation curve. Early AO-assisted observations achieved ∼1 kpc resolution and likewise found rotational kinematics in more massive non-lensed galaxies [35,36]. These early results are exemplary of the role that lensed galaxies have continued to play in understanding the internal structure of galaxies at high redshift.…”

“…Galaxies across a wide range of stellar mass, size, and V rot /σ consistently have values of Q near or below unity indicating that they are likely unstable to localized gravitational collapse [36,11,40]. The Jeans scale for collapse, L J , is consistent with the sizes of the largest star-forming clumps although the uncertainties are of order a factor of two.…”

“…Figure 1 shows a representative example of clumpy structure. In the most massive galaxies, these clumps can reach ∼1 kpc in size such that they are marginally resolved [45,36,46]. Targeted studies of lensed galaxies, including Hα narrowband imaging with Hubble, have resolved clumps spanning over a decade in size and nearly three decades in luminosity [11,47,42] as shown in…”

Spectroscopy and the Age of Giant Telescopes

“…Fundamental work in the field began with local and unresolved galaxies, leading to studies of star formation and dust as global properties [20]. Extensive work has now been done to resolve local star formation spatially (see figure 3) in the optical [21][22], directly measuring gas at radio and UV wavelengths [23], and evolution of galaxies out to z=2 [24]. Working with spatially resolved data identifies the root cause of the physical conditions in the galaxy as well as the possible external influences such as winds and filamentary streams.…”