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Register a new userThe SINS/zC-SINF Survey of z~2 Galaxy Kinematics: Rest-frame Morphology, Structure, and Colors from Near-infrared Hubble Space Telescope Imaging
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We present the analysis of HST $J$- and $H$-band imaging for 29 galaxies on the star-forming main sequence at $zsim2$, which have Adaptive Optics VLT SINFONI integral field spectroscopy from our SINS/zC-SINF program. The SINFONI H$alpha$ data resolve the on-going star-formation and the ionized gas kinematics on scales of $1-2$ kpc; the near-IR images trace the galaxies rest-frame optical morphologies and distributions of stellar mass in old stellar populations at a similar resolution. The global light profiles of most galaxies show disk-like properties well described by a single Sersic profile with $nsim1$, with only $sim15%$ requiring a high $n>3$ Sersic index, all more massive than $10^{10}M_odot$. In bulge+disk fits, about $40%$ of galaxies have a measurable bulge component in the light profiles, with $sim15%$ showing a substantial bulge-to-total ratio $B/Tge0.3$. This is a lower limit to the frequency of $zsim2$ massive galaxies with a developed bulge component in stellar mass because it could be hidden by dust and/or outshined by a thick actively star-forming disk component. The galaxies rest-optical half-light radii range between $1-7$ kpc, with a median of 2.1 kpc, and lie slightly above the size-mass relation at these epochs reported in the literature. This is attributed to differences in sample selection and definitions of size and/or mass measurements. The $(u-g)_{rest}$ color gradient and scatter within individual $zsim2$ massive galaxies with $ge10^{11}M_odot$ are as high as in $z=0$ low-mass, late-type galaxies, and are consistent with the high star-formation rates of massive $zsim2$ galaxies being sustained at large galactocentric distances.
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As part of the SINS/zC-SINF surveys of high-z galaxy kinematics, we derive the radial distributions of H-alpha surface brightness, stellar mass surface density, and dynamical mass at ~2 kpc resolution in 19 z~2 star-forming disks with deep SINFONI AO spectroscopy at the ESO VLT. From these data we infer the radial distribution of the Toomre Q-parameter for these main-sequence star forming galaxies (SFGs), covering almost two decades of stellar mass (10^9.6 to 10^11.5 solar masses). In more than half of our SFGs, the H-alpha distributions cannot be fit by a centrally peaked distribution, such as an exponential, but are better described by a ring, or the combination of a ring and an exponential. At the same time the kinematic data indicate the presence of a mass distribution more centrally concentrated than a single exponential distribution for 5 of the 19 galaxies. The resulting Q-distributions are centrally peaked for all, and significantly exceed unity there for three quarters of the SFGs. The occurrence of H-alpha rings and of large nuclear Q-values is strongly correlated, and is more common for the more massive SFGs. While our sample is small and there remain substantial uncertainties and caveats, our observations are consistent with a scenario in which cloud fragmentation and global star formation are secularly suppressed in gas rich high-z disks from the inside out, as the central stellar mass density of the disks grows.
We present the SINS/zC-SINF AO survey of 35 star-forming galaxies, the largest sample with deep adaptive optics-assisted (AO) near-infrared integral field spectroscopy at z~2. The observations, taken with SINFONI at the Very Large Telescope, resolve the Ha and [NII] line emission and kinematics on scales of ~1.5 kpc. In stellar mass, star formation rate, rest-optical colors and size, the AO sample is representative of its parent seeing-limited sample and probes the massive (M* ~ 2x10^9 - 3x10^11 Msun), actively star-forming (SFR ~ 10-600 Msun/yr) part of the z~2 galaxy population over a wide range in colors ((U-V)_rest ~ 0.15-1.5 mag) and half-light radii (R_e,H ~ 1-8.5 kpc). The sample overlaps largely with the main sequence of star-forming galaxies in the same redshift range to a similar K_AB = 23 magnitude limit; it has ~0.3 dex higher median specific SFR, ~0.1 mag bluer median (U-V)_rest color, and ~10% larger median rest-optical size. We describe the observations, data reduction, and extraction of basic flux and kinematic properties. With typically 3-4 times higher resolution and 4-5 times longer integrations (up to 23hr) than the seeing-limited datasets of the same objects, the AO data reveal much more detail in morphology and kinematics. The now complete AO observations confirm the majority of kinematically-classified disks and the typically elevated disk velocity dispersions previously reported based on subsets of the data. We derive typically flat or slightly negative radial [NII]/Ha gradients, with no significant trend with global galaxy properties, kinematic nature, or the presence of an AGN. Azimuthal variations in [NII]/Ha are seen in several sources and are associated with ionized gas outflows, and possible more metal-poor star-forming clumps or small companions. [Abridged]
We report the detection of ubiquitous powerful nuclear outflows in massive (> 10^11 Msun) z~2 star-forming galaxies (SFGs), which are plausibly driven by an Active Galactic Nucleus (AGN). The sample consists of the eight most massive SFGs from our SINS/zC-SINF survey of galaxy kinematics with the imaging spectrometer SINFONI, six of which have sensitive high-resolution adaptive optics (AO) assisted observations. All of the objects are disks hosting a significant stellar bulge. The spectra in their central regions exhibit a broad component in Halpha and forbidden [NII] and [SII] line emission, with typical velocity FWHM ~ 1500 km/s, [NII]/Halpha ratio ~ 0.6, and intrinsic extent of 2 - 3 kpc. These properties are consistent with warm ionized gas outflows associated with Type 2 AGN, the presence of which is confirmed via independent diagnostics in half the galaxies. The data imply a median ionized gas mass outflow rate of ~ 60 Msun/yr and mass loading of ~ 3. At larger radii, a weaker broad component is detected but with lower FWHM ~ 485 km/s and [NII]/Halpha ~ 0.35, characteristic for star formation-driven outflows as found in the lower-mass SINS/zC-SINF galaxies. The high inferred mass outflow rates and frequent occurrence suggest the nuclear outflows efficiently expel gas out of the centers of the galaxies with high duty cycles, and may thus contribute to the process of star formation quenching in massive galaxies. Larger samples at high masses will be crucial to confirm the importance and energetics of the nuclear outflow phenomenon, and its connection to AGN activity and bulge growth.
We investigate the relationship between star formation activity and outflow properties on kiloparsec scales in a sample of 28 star forming galaxies at $zsim$ 2-2.6, using adaptive optics assisted integral field observations from SINFONI on the VLT. The narrow and broad components of the H$alpha$ emission are used to simultaneously determine the local star formation rate surface density ($Sigma_{rm SFR}$), and the outflow velocity $v_{rm out}$ and mass outflow rate $dot{M}_{rm out}$, respectively. We find clear evidence for faster outflows with larger mass loading factors at higher $Sigma_{rm SFR}$. The outflow velocities scale as $v_{rm out}$ $propto$ $Sigma_{rm SFR}^{0.34 pm 0.10}$, which suggests that the outflows may be driven by a combination of mechanical energy released by supernova explosions and stellar winds, as well as radiation pressure acting on dust grains. The majority of the outflowing material does not have sufficient velocity to escape from the galaxy halos, but will likely be re-accreted and contribute to the chemical enrichment of the galaxies. In the highest $Sigma_{rm SFR}$ regions the outflow component contains an average of $sim$45% of the H$alpha$ flux, while in the lower $Sigma_{rm SFR}$ regions only $sim$10% of the H$alpha$ flux is associated with outflows. The mass loading factor, $eta$ = $dot{M}_{rm out}$/SFR, is positively correlated with $Sigma_{rm SFR}$ but is relatively low even at the highest $Sigma_{rm SFR}$: $eta lesssim$ 0.5 $times$ (380 cm$^{-3}$/n$_e$). This may be in tension with the $eta$ $gtrsim$ 1 required by cosmological simulations, unless a significant fraction of the outflowing mass is in other gas phases and has sufficient velocity to escape the galaxy halos.
We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 < z < 8. We use new wide-field near-infrared data in GOODS-S from the CANDELS, HUDF09 and ERS programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z > 3.5, including 113 at z = 7 to 8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. The median value of beta evolves significantly from -1.82 (+0.00,-0.04) at z = 4, to -2.37 (+0.26,-0.06) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68 (+0.39,-0.24) (~ -2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require exotic stellar populations to explain their colors. This evolution can be explained by an increase in dust extinction, with the timescale consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2-sigma) correlation between beta and M_UV when measuring M_UV at a consistent rest-frame wavelength of 1500 A. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z > 7. Thus the stellar-mass - metallicity relation, previously observed up to z ~ 3, may extend out to z = 7 - 8.
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