E. K. S. Hicks

THE SINS SURVEY OF z ∼ 2 GALAXY KINEMATICS: PROPERTIES OF THE GIANT STAR-FORMING CLUMPS ∗

We have studied the properties of giant star-forming clumps in five z ~ 2 star-forming disks with deep SINFONI AO spectroscopy at the ESO VLT. The clumps reside in disk regions where the Toomre Q-parameter is below unity, consistent with their being bound and having formed from gravitational instability. Broad H?/[N II] line wings demonstrate that the clumps are launching sites of powerful outflows. The inferred outflow rates are comparable to or exceed the star formation rates, in one case by a factor of eight. Typical clumps may lose a fraction of their original gas by feedback in a few hundred million years, allowing them to migrate into the center. The most active clumps may lose much of their mass and disrupt in the disk. The clumps leave a modest imprint on the gas kinematics. Velocity gradients across the clumps are 10-40 km s?1 kpc?1, similar to the galactic rotation gradients. Given beam smearing and clump sizes, these gradients may be consistent with significant rotational support in typical clumps. Extreme clumps may not be rotationally supported; either they are not virialized or they are predominantly pressure supported. The velocity dispersion is spatially rather constant and increases only weakly with star formation surface density. The large velocity dispersions may be driven by the release of gravitational energy, either at the outer disk/accreting streams interface, and/or by the clump migration within the disk. Spatial variations in the inferred gas phase oxygen abundance are broadly consistent with inside-out growing disks, and/or with inward migration of the clumps.

THE SINS SURVEY: MODELING THE DYNAMICS OF z ∼ 2 GALAXIES AND THE HIGH-z TULLY-FISHER RELATION*

We present the modeling of SINFONI integral field dynamics of 18 star-forming galaxies at z ~ 2 from Hα line emission. The galaxies are selected from the larger sample of the SINS survey, based on the prominence of ordered rotational motions with respect to more complex merger-induced dynamics. The quality of the data allows us to carefully select systems with kinematics dominated by rotation, and to model the gas dynamics across the whole galaxy using suitable exponential disk models. We obtain a good correlation between the dynamical mass and the stellar mass, finding that large gas fractions (M gas ≈ M *) are required to explain the difference between the two quantities. We use the derived stellar mass and maximum rotational velocity V max from the modeling to construct for the first time the stellar mass Tully-Fisher relation at z ~ 2.2. The relation obtained shows a slope similar to what is observed at lower redshift, but we detect an evolution of the zero point. We find that at z ~ 2.2 there is an offset in log(M *) for a given rotational velocity of 0.41 ± 0.11 with respect to the local universe. This result is consistent with the predictions of the latest N-body/hydrodynamical simulations of disk formation and evolution, which invoke gas accretion onto the forming disk in filaments and cooling flows. This scenario is in agreement with other dynamical evidence from SINS, where gas accretion from the halo is required to reproduce the observed properties of a large fraction of the z ~ 2 galaxies. Based on observations obtained at the Very Large Telescope (VLT) of the European Southern Observatory, Paranal, Chile, in the context of guaranteed time programs 073.B-9018, 074.A-9011, 075.A-0466, 076.A-0527, 077.A-0576, 078.A-0600, 078.A-0055, 079.A-0341, 080.A-0330, and 080.A-0635.

Mergers and Mass Accretion Rates in Galaxy Assembly: The Millennium Simulation Compared to Observations of z ≈ 2 Galaxies

Recent observations of UV/optically selected, massive star-forming galaxies at z ≈ 2 indicate that the baryonic mass assembly and star formation history is dominated by continuous rapid accretion of gas and internal secular evolution, rather than by major mergers. We use the Millennium Simulation to build new halo merger trees and extract halo merger fractions and mass accretion rates. We find that, even for halos not undergoing major mergers, the mass accretion rates are plausibly sufficient to account for the high star formation rates observed in z ≈ 2 disks. On the other hand, the fraction of major mergers in the Millennium Simulation is sufficient to account for the number counts of submillimeter galaxies (SMGs), in support of observational evidence that these are major mergers. When following the fate of these two populations in the Millennium Simulation to z = 0, we find that subsequent mergers are not frequent enough to convert all z ≈ 2 turbulent disks into elliptical galaxies at z = 0. Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at z ≈ 2 into observed massive cluster ellipticals at z = 0. We argue therefore, that secular and internal evolution must play an important role in the evolution of a significant fraction of z ≈ 2 UV/optically and submillimeter-selected galaxy populations.

THE ROLE OF MOLECULAR GAS IN OBSCURING SEYFERT ACTIVE GALACTIC NUCLEI

In a sample of local active galactic nuclei (AGNs) studied at a spatial resolution on the order of 10 pc, we show that the interstellar medium traced by the molecular hydrogen ? = 1-0 S(1) line at 2.1 ?m forms a geometrically thick, clumpy disk. The kinematics of the molecular gas reveals general rotation, although an additional significant component of random bulk motion is required by the high local velocity dispersion. The size scale of the typical gas disk is found to have a radius of ~30 pc with a comparable vertical height. Within this radius, the average gas mass is estimated to be ~107?M ? based on a typical gas mass fraction of 10%, which suggests column densities of N H ~ 5 ? 1023 cm?2. Extinction of the stellar continuum within this same region suggests lower column densities of N H ~2 ? 1022 cm?2, indicating that the gas distribution on these scales is dominated by dense clumps. In half of the observed Seyfert galaxies, this lower column density is still great enough to obscure the AGN at optical/infrared wavelengths. We conclude, based on the spatial distribution, kinematics, and column densities that the molecular gas observed is spatially mixed with the nuclear stellar population and is likely to be associated with the outer extent of any smaller scale nuclear obscuring structure. Furthermore, we find that the velocity dispersion of the molecular gas is correlated with the star formation rate per unit area, suggesting a link between the two phenomena, and that the gas surface density follows known Schmidt-Kennicutt relations. The molecular/dusty structure on these scales may be dynamic since it is possible that the velocity dispersion of the gas, and hence the vertical disk height, is maintained by a short, massive inflow of material into the nuclear region and/or by intense, short-lived nuclear star formation.

MOLECULAR GAS STREAMERS FEEDING AND OBSCURING THE ACTIVE NUCLEUS OF NGC 1068

We report the first direct observations of neutral, molecular gas streaming in the nucleus of NGC 1068 on scales of <30 pc using SINFONI near-infrared integral field spectroscopy. At a resolution of 0075, the flux map of 2.12 μm 1-0 S(1) molecular hydrogen emission around the nucleus in the central arcsec reveals two prominent linear structures leading to the active galactic nucleus from the north and south. The kinematics of the gas in these features are dominated by noncircular motions and indicate that material streams toward the nucleus on highly elliptical or parabolic trajectories, whose orientations are compatible with that of the disk plane of the galaxy. We interpret the data as evidence for fueling of gas to the central region. The radial transport rate from ~30 pc to a few parsecs from the nucleus is ~15 M ☉ yr–1. One of the infalling clouds lies directly in front of the central engine. We interpret it as a tidally disrupted streamer that forms the optically thick outer part of an amorphous clumpy molecular/dusty structure which contributes to the nuclear obscuration.

Stellar and Molecular Gas Kinematics Of NGC?1097: Inflow Driven by a Nuclear Spiral

We present spatially resolved distributions and kinematics of the stars and molecular gas in the central 320 pc of NGC?1097. The stellar continuum confirms the previously reported three-arm spiral pattern extending into the central 100 pc. The stellar kinematics and the gas distribution imply this is a shadowing effect due to extinction by gas and dust in the molecular spiral arms. The molecular gas kinematics show a strong residual (i.e., non-circular) velocity, which is manifested as a two-arm kinematic spiral. Linear models indicate that this is the line-of-sight velocity pattern expected for a density wave in gas that generates a three-arm spiral morphology. We estimate the inflow rate along the arms. Using hydrodynamical models of nuclear spirals, we show that when deriving the accretion rate into the central region, outflow in the disk plane between the arms has to be taken into account. For NGC?1097, despite the inflow rate along the arms being ~ 1.2 M ? yr?1, the net gas accretion rate to the central few tens of parsecs is much smaller. The numerical models indicate that the inflow rate could be as little as ~ 0.06 M ? yr?1. This is sufficient to generate recurring starbursts, similar in scale to that observed, every 20-150 Myr. The nuclear spiral represents a mechanism that can feed gas into the central parsecs of the galaxy, with the gas flow sustainable for timescales of a gigayear.

The zCOSMOS-SINFONI Project I: Sample Selection and Natural-Seeing Observations

The zCOSMOS-SINFONI project is aimed at studying the physical and kinematical properties of a sample of massive z ~ 1.4-2.5 star-forming galaxies, through SINFONI near-infrared integral field spectroscopy (IFS), combined with the multiwavelength information from the zCOSMOS (COSMOS) survey. The project is based on one hour of natural-seeing observations per target, and adaptive optics (AO) follow-up for a major part of the sample, which includes 30 galaxies selected from the zCOSMOS/VIMOS spectroscopic survey. This first paper presents the sample selection, and the global physical characterization of the target galaxies from multicolor photometry, i.e., star formation rate (SFR), stellar mass, age, etc. The Hα integrated properties, such as, flux, velocity dispersion, and size, are derived from the natural-seeing observations, while the follow-up AO observations will be presented in the next paper of this series. Our sample appears to be well representative of star-forming galaxies at z ~ 2, covering a wide range in mass and SFR. The Hα integrated properties of the 25 Hα detected galaxies are similar to those of other IFS samples at the same redshifts. Good agreement is found among the SFRs derived from Hα luminosity and other diagnostic methods, provided the extinction affecting the Hα luminosity is about twice that affecting the continuum. A preliminary kinematic analysis, based on the maximum observed velocity difference across the source and on the integrated velocity dispersion, indicates that the sample splits nearly 50-50 into rotation-dominated and velocity-dispersion-dominated galaxies, in good agreement with previous surveys.

Shocked Superwinds from the z ~ 2 Clumpy Star-forming Galaxy, ZC406690

We have obtained high-resolution data of the z � 2 ring-like, clumpy star-forming galaxy (SFG) ZC406690 using the VLT/SINFONI with AO (in K-band) and in seeing-limited mode (in H- and J-band). Our data includes all of the main strong optical emission lines: [OII], [OIII], Hα, Hβ, [NII], and [SII]. We find broad, blueshifted Hα and [OIII] emission line wings in the spectra of the galaxy’s massive, star-forming clumps (σ � 85 km s −1 ) and even broader wings (up to 70% of the total Hα flux, with σ � 290 km s −1 ) in regions spatially offset from the clumps by � 2 kpc. The broad emission likely originates from large-scale outflows with mass outflow rates from individual clumps that are 1–8x the SFR of the clumps. Based on emission line ratio diagnostics ([NII]/Hα and [SII]/Hα) and photoionization and shock models, we find that the emission from the clumps is due to a combination of photoionization from the star-forming regions and shocks generated in the outflowing component, with 5–30% of the emission deriving from shocks. In terms of the ionization parameter (6x10 7 -10 8 cm/s, based on both the SFR and the O32 ratio), density (local electron densities of 300–1800 cm −3 in and around the clumps, and ionized gas column densities of 1200–8000 M⊙/pc 2 ), and SFR (10–40 M⊙ yr −1 ), these clumps more closely resemble nuclear starburst regions of local ULIRGs and dwarf irregulars than HII regions in local galaxies. However, the star-forming clumps are not located in the nucleus as in local starburst galaxies but instead are situated in a ring several kpc from the center of their high-redshift host galaxy, and have an overall disk-like morphology. The two brightest clumps are quite different in terms of their internal properties, energetics and relative ages, and thus we are given a glimpse at two different stages in the formation and evolution of rapidly star-forming giant clumps at high-z. Subject headings: galaxies: high redshift – galaxies: evolution – galaxies: emission lines – galaxies: star formation – ISM: jets and outflows

HOW WELL CAN WE MEASURE THE INTRINSIC VELOCITY DISPERSION OF DISTANT DISK GALAXIES

The kinematics of distant galaxies from z = 0.1 to z > 2 play a key role in our understanding of galaxy evolution from early times to the present. One of the important parameters is the intrinsic, or local, velocity dispersion of a galaxy, which allows one to quantify the degree of non-circular motions such as pressure support. However, this is difficult to measure because the observed dispersion includes the effects of (often severe) beam smearing on the velocity gradient. Here we investigate four methods of measuring the dispersion that have been used in the literature, to assess their effectiveness at recovering the intrinsic dispersion. We discuss the biases inherent in each method, and apply them to model disk galaxies in order to determine which methods yield meaningful quantities and under what conditions. All the mean-weighted dispersion estimators are affected by (residual) beam smearing. In contrast, the dispersion recovered by fitting a spatially and spectrally convolved disk model to the data is unbiased by the beam smearing it is trying to compensate. Because of this, and because the bias it does exhibit depends only on the signal-to-noise ratio (S/N), it can be considered reliable. However, at very low S/N, all methods should be used with caution.

HIGH-REDSHIFT STAR-FORMING GALAXIES: ANGULAR MOMENTUM AND BARYON FRACTION, TURBULENT PRESSURE EFFECTS, AND THE ORIGIN OF TURBULENCE

The structure of a sample of high-redshift (z ~ 2), rotating galaxies with high star formation rates and turbulent gas velocities of σ ≈ 40-80 km s -1 is investigated. Fitting the observed disk rotational velocities and radii with a Mo et al. (MMW) model requires unusually large disk spin parameters λ d > 0.1 and disk-to-dark halo mass fractions of m d ≈ 0.2, close to the cosmic baryon fraction. The galaxies segregate into dispersion-dominated systems with 1 ≤ υ max /σ ≤ 3, maximum rotational velocities υ max ≤ 200 km s -1 , and disk half-light radii r 1/2 ≈ 1-3 kpc, and rotation-dominated systems with υ max > 200 km s -1 , υ max /σ > 3, and r 1/2 ≈ 4-8 kpc. For the dispersion-dominated sample, radial pressure gradients partly compensate the gravitational force, reducing the rotational velocities. Including this pressure effect in the MMW model, dispersion-dominated galaxies can be fitted well with spin parameters of λ d = 0.03-0.05 for high disk mass fractions of m d ≈ 0.2 and with λ d = 0.01-0.03 for m d = 0.05. These values are in good agreement with cosmological expectations. For the rotation-dominated sample, however, pressure effects are small and better agreement with theoretically expected disk spin parameters can only be achieved if the dark halo mass contribution in the visible disk regime (2-3 x r 1/2 ) is smaller than predicted by the MMW model. We argue that these galaxies can still be embedded in standard cold dark matter halos if the halos do not contract adiabatically in response to disk formation. In this case, the data favor models with small disk mass fractions of m d = 0.05 and disk spin parameters of λ d ≈ 0.035. It is shown that the observed high turbulent gas motions of the galaxies are consistent with a Toomre instability parameter Q = 1 which is equal to the critical value, expected for gravitational disk instability to be the major driver of turbulence. The dominant energy source of turbulence is then the potential energy of the gas in the disk.