B. Epinat

HYDRODYNAMICS OF HIGH-REDSHIFT GALAXY COLLISIONS: FROM GAS-RICH DISKS TO DISPERSION-DOMINATED MERGERS AND COMPACT SPHEROIDS

Disk galaxies at high redshift (z ~ 2) are characterized by high fractions of cold gas, strong turbulence, and giant star-forming clumps. Major mergers of disk galaxies at high redshift should then generally involve such turbulent clumpy disks. Merger simulations, however, model the interstellar medium as a stable, homogeneous, and thermally pressurized medium. We present the first merger simulations with high fractions of cold, turbulent, and clumpy gas. We discuss the major new features of these models compared to models where the gas is artificially stabilized and warmed. Gas turbulence, which is already strong in high-redshift disks, is further enhanced in mergers. Some phases are dispersion dominated, with most of the gas kinetic energy in the form of velocity dispersion and very chaotic velocity fields, unlike merger models using a thermally stabilized gas. These mergers can reach very high star formation rates, and have multi-component gas spectra consistent with SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas are also characterized by highly dissipative gas collapse to the center of mass, with the stellar component following in a global contraction. The final galaxies are early type with relatively small radii and high Sersic indices, like high-redshift compact spheroids. The mass fraction in a disk component that survives or re-forms after a merger is severely reduced compared to models with stabilized gas, and the formation of a massive disk component would require significant accretion of external baryons afterwards. Mergers thus appear to destroy extended disks even when the gas fraction is high, and this lends further support to smooth infall as the main formation mechanism for massive disk galaxies.

GHASP: an Hα kinematic survey of spiral and irregular galaxies – V. Dark matter distribution in 36 nearby spiral galaxies

The results obtained from a study of the mass distribution of 36 spiral galaxies are presented. The galaxies were observed using Fabry-Perot interferometry as part of the GHASP survey. The main aim of obtaining high resolution H alpha 2D velocity fields is to define more accurately the rising part of the rotation curves which should allow to better constrain the parameters of the mass distribution. The H alpha velocities were combined with low resolution HI data from the literature, when available. Combining the kinematical data with photometric data, mass models were derived from these rotation curves using two different functional forms for the halo: an isothermal sphere and an NFW profile. For the galaxies already modeled by other authors, the results tend to agree. Our results point at the existence of a constant density core in the center of the dark matter halos rather than a cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all types the result already obtained by other authors studying dwarf and LSB galaxies but would necessitate a larger sample of galaxies to conclude more strongly. Whatever model is used (ISO or NFW), small core radius halos have higher central densities, again for all morphological types. We confirm different halo scaling laws, such as the correlations between the core radius and the central density of the halo with the absolute magnitude of a galaxy: low luminosity galaxies have small core radius and high central density. We find that the product of the central density with the core radius of the dark matter halo is nearly constant, whatever the model and whatever the absolute magnitude of the galaxy. This suggests that the halo surface density is independent from the galaxy type.

The long lives of giant clumps and the birth of outflows in gas-rich galaxies at high redshift

Star-forming disk galaxies at high redshift are often subject to violent disk instability, characterized by giant clumps whose fate is yet to be understood. The main question is whether the clumps disrupt within their dynamical timescale (\textless= 50 Myr), like the molecular clouds in todays galaxies, or whether they survive stellar feedback for more than a disk orbital time (approximate to 300 Myr) in which case they can migrate inward and help building the central bulge. We present 3.5-7 pc resolution adaptive mesh refinement simulations of high-redshift disks including photoionization, radiation pressure, and supernovae feedback. Our modeling of radiation pressure determines the mass loading and initial velocity of winds from basic physical principles. We find that the giant clumps produce steady outflow rates comparable to and sometimes somewhat larger than their star formation rate, with velocities largely sufficient to escape the galaxy. The clumps also lose mass, especially old stars, by tidal stripping, and the stellar populations contained in the clumps hence remain relatively young (\textless= 200 Myr), as observed. The clumps survive gaseous outflows and stellar loss, because they are wandering in gas-rich turbulent disks from which they can reaccrete gas at high rates compensating for outflows and tidal stripping, overall keeping realistic and self-regulated gaseous and stellar masses. The outflow and accretion rates have specific timescales of a few 10(8) yr, as opposed to rapid and repeated dispersion and reformation of clumps. Our simulations produce gaseous outflows with velocities, densities, and mass loading consistent with observations, and at the same time suggest that the giant clumps survive for hundreds of Myr and complete their migration to the center of high-redshift galaxies. These long-lived clumps are gas-dominated and contain a moderate mass fraction of stars; they drive inside-out disk evolution, thickening, spheroid growth, and fueling of the central black hole.

MASSIV: Mass Assembly Survey with SINFONI in VVDS - II. Kinematics and close environment classification

Context. Processes driving mass assembly are expected to evolve on different timescales along cosmic time. A transition might happen around z similar to 1 as the cosmic star formation rate starts its decrease. Aims. We aim to identify the dynamical nature of galaxies in a representative sample to be able to infer and compare the mass assembly mechanisms across cosmic time. Methods. We present an analysis of the kinematics properties of 50 galaxies with redshifts 0.9 \textless z \textless 1.6 from the MASSIV sample observed with SINFONI/VLT with a mass range from 4.5 x 10(9) M-circle dot to 1.7 x 10(11) M-circle dot and a star formation rate from 6 M-circle dot yr(-1) to 300 M-circle dot yr(-1). This is the largest sample with 2D kinematics in this redshift range. We provide a classification based on kinematics as well as on close galaxy environment. Results. We find that a significant fraction of galaxies in our sample (29%) experience merging or have close companions that may be gravitationally linked. This places a lower limit on the fraction of interacting galaxies because ongoing mergers are probably also present but harder to identify. We find that at least 44% of the galaxies in our sample display ordered rotation, whereas at least 35% are non-rotating objects. All rotators except one are compatible with rotation-dominated (V-max/sigma \textgreater 1) systems. Non-rotating objects are mainly small objects (R-e \textless 4 kpc). They show an anti-correlation of their velocity dispersion and their effective radius. These low-mass objects (log M-star \textless 10.5) may be ongoing mergers in a transient state, galaxies with only one unresolved star-forming region, galaxies with an unstable gaseous phase or, less probably, spheroids. Combining our sample with other 3D-spectroscopy samples, we find that the local velocity dispersion of the ionized gas component decreases continuously from z similar to 3 to z = 0. The proportion of disks also seems to be increasing in star-forming galaxies when the redshift decreases. The number of interacting galaxies seems to be at a maximum at z similar to 1.2. Conclusions. These results draw a picture in which cold gas accretion may still be efficient at z similar to 1.2 but in which mergers may play a much more significant role at z similar to 1.2 than at higher redshift. From a dynamical point of view, the redshift range 1 \textless z \textless 2 therefore appears as a transition period in the galaxy mass assembly process(star star star star).

Evidence for strong dynamical evolution in disc galaxies through the last 11 Gyr. GHASP VIII – a local reference sample of rotating disc galaxies for high-redshift studies

Due to their large distances, high-redshift galaxies are observed at a very low spatial resolution. In order to disentangle the evolution of galaxy kinematics from low-resolution effects, we have used Fabry-Perot 3D Hα data cubes of 153 nearby isolated galaxies selected from the Gassendi Hα survey of SPirals (GHASP) to simulate data cubes of galaxies at redshift z = 1.7 using a pixel size of 0.125 arcsec and a 0.5 arcsec seeing. We have derived Hα flux, velocity and velocity dispersion maps. From these data, we show that the inner velocity gradient is lowered and is responsible for a peak in the velocity dispersion map. This signature in the velocity dispersion map can be used to make a kinematical classification, but misses 30 per cent of the regular rotating discs in our sample. Toy models of rotating discs have been built to recover the kinematical parameters and the rotation curves from low-resolution data. The poor resolution makes the kinematical inclination uncertain and the position of galaxy centre difficult to recover. The position angle of the major axis is retrieved with an accuracy higher than 5° for 70 per cent of the sample. Toy models also enable us to retrieve statistically the maximum velocity and the mean velocity dispersion of galaxies with a satisfying accuracy. This validates the use of the Tully-Fisher relation for high-redshift galaxies, but the loss of resolution induces a lower slope of the relation despite the beam smearing corrections. We conclude that the main kinematic parameters are better constrained for galaxies with an optical radius at least as large as three times the seeing. The simulated data have been compared to actual high-redshift galaxy data observed with VLT/SINFONI, Keck/OSIRIS and VLT/GIRAFFE in the redshift range 3 > z > 0.4, allowing us to follow galaxy evolution from 11 to 4 Gyr. For rotation-dominated galaxies, we find that the use of the velocity dispersion central peak as a signature of rotating discs may misclassify slow and solid body rotators. This is the case for ∼30 per cent of our sample. We show that the projected local data cannot reproduce the high velocity dispersion observed in high-redshift galaxies except when no beam smearing correction is applied. This unambiguously means that, unlike local evolved galaxies, there exists at high redshift at least a population of disc galaxies for which a large fraction of the dynamical support is due to random motions. We should nevertheless ensure that these features are not due to important selection biases before concluding that the formation of an unstable and transient gaseous disc is a general galaxy formation process.

MASSIV: Mass Assembly Survey with SINFONI in VVDS - III. Evidence for positive metallicity gradients in z ~ 1.2 star-forming galaxies

Aims. The estimate of radial abundance gradients in high-redshift galaxies allows to constrain their star formation history and their interplay with the surrounding intergalactic medium. Methods. We present VLT/SINFONI integral-field spectroscopy of a first sample of 50 galaxies at z similar to 1.2 in the MASSIV survey. Using the N2 ratio between the [N II]6584 and H alpha rest-frame optical emission lines as a proxy for oxygen abundance in the interstellar medium, we measured the metallicity of the sample galaxies. We developed a tool to extract spectra in annular regions, leading to a spatially resolved estimate of the oxygen abundance in each galaxy. We were able to derive a metallicity gradient for 26 galaxies in our sample and discovered a significant fraction of galaxies with a “positive” gradient. Using a simple chemical evolution model, we derived infall rates of pristine gas onto the disks. Results. Seven galaxies display a positive gradient at a high confidence level. Four out of these are interacting, and one is a chain galaxy. We suggest that interactions might be responsible for shallowing and even inverting the abundance gradient. We also identify two interesting correlations in our sample: a) galaxies with higher gas velocity dispersion have shallower/positive gradients; and b) metal-poor galaxies tend to show a positive gradient, whereas metal-rich ones tend to show a negative one. This last observation can be explained by the infall of metal-poor gas into the center of the disks. We address the question of the origin of this infall under the influence of gas flows triggered by interactions and/or cold gas accretion. All the data published in this paper are publicly available at the time of publication following this link: http://cosmosdb.lambrate.inaf.it/VVDS-SINFONI.

The MUSE 3D view of the Hubble Deep Field South

We observed Hubble Deep Field South with the new panoramic integral-field spectrograph MUSE that we built and have just commissioned at the VLT. The data cube resulting from 27 h of integration covers one arcmin(2) field of view at an unprecedented depth with a 1 sigma emission-line surface brightness limit of 1 x 10(-19) erg s(-1) cm(-2) arcsec(-2), and contains similar to 90 000 spectra. We present the combined and calibrated data cube, and we performed a first-pass analysis of the sources detected in the Hubble Deep Field South imaging. We measured the redshifts of 189 sources up to a magnitude I-814 = 29.5, increasing the number of known spectroscopic redshifts in this field by more than an order of magnitude. We also discovered 26 Ly alpha emitting galaxies that are not detected in the HST WFPC2 deep broad-band images. The intermediate spectral resolution of 2.3 angstrom allows us to separate resolved asymmetric Ly alpha emitters, [O II] 3727 emitters, and C III] 1908 emitters, and the broad instantaneous wavelength range of 4500 angstrom helps to identify single emission lines, such as [O III] 5007, H beta, and H alpha, over a very wide redshift range. We also show how the three-dimensional information of MUSE helps to resolve sources that are confused at ground-based image quality. Overall, secure identifications are provided for 83% of the 227 emission line sources detected in the MUSE data cube and for 32% of the 586 sources identified in the HST catalogue. The overall redshift distribution is fairly flat to z = 6.3, with a reduction between z = 1.5 to 2.9, in the well-known redshift desert. The field of view of MUSE also allowed us to detect 17 groups within the field. We checked that the number counts of [O II] 3727 and Ly alpha emitters are roughly consistent with predictions from the literature. Using two examples, we demonstrate that MUSE is able to provide exquisite spatially resolved spectroscopic information on the intermediate-redshift galaxies present in the field. This unique data set can be used for a wide range of follow-up studies. We release the data cube, the associated products, and the source catalogue with redshifts, spectra, and emission-line fluxes.

GHASP: an Hα kinematic survey of 203 spiral and irregular galaxies – VII. Revisiting the analysis of Hα data cubes for 97 galaxies

The GHASP survey (Gassendi HAlpha survey of SPirals) consists of 3D Ha data cubes for 203 spiral and irregular galaxies, covering a large range in morphological types and absolute magnitudes, for kinematics analysis. It is the largest sample of Fabry-Perot data published up to now. In order to provide an homogenous sample, reduced and analyzed using the same procedure, we present in this paper the new reduction and analysis for a set of 97 galaxies already published in previous papers but now using the new data reduction procedure adopted for the whole sample. The GHASP survey is now achieved and the whole sample is reduced using adaptive binning techniques based on Voronoi tessellations. We have derived Ha data cubes from which are computed Ha maps, radial velocity fields as well as residual velocity fields, position-velocity diagrams, rotation curves and kinematical parameters for almost all galaxies. The rotation curves, the kinematical parameters and their uncertainties are computed homogeneously using the new method based on the power spectrum of the residual velocity field. This paper provides the kinematical parameters for the whole sample. For the first time, the integrated Ha profiles have been computed and are presented for the whole sample. The total Ha fluxes deduced from these profiles have been used in order to provide a flux calibration for the 203 GHASP galaxies. This paper confirms the conclusions already drawn from half the sample concerning (i) the increased accuracy of position angles measurements using kinematical data, (ii) the difficulty to have robust determinations of both morphological and kinematical inclinations in particular for low inclination galaxies and (iii) the very good agreement between the Tully-Fisher relationship derived from our data and previous determinations.

MASSIV: Mass Assemby Survey with SINFONI in VVDS I. Survey description and global properties of the 0.9 \textless z \textless 1.8 galaxy sample

Understanding how galaxies evolve and assemble their mass across cosmic time is still a fundamental unsolved issue. To get insight into the various processes of galaxy mass assembly, the Mass Assembly Survey with SINFONI in VVDS (MASSIV) aims at probing the kinematical and chemical properties of a significant and representative sample of high-redshift (0.9 5 Msun/yr. Together with the size of the sample, the spatially-resolved SINFONI data therefore enables us to discuss global, volume averaged, galaxy kinematic and chemical properties all accross the mass and SFR range of the survey to derive robust conclusions on galaxy mass assembly over cosmological timescales.

MASSIV: Mass Assembly Survey with SINFONI in VVDS. IV. Fundamental relations of star-forming galaxies at 1 < z < 1.6

Aims. How mass assembly occurs in galaxies and which process(es) contribute to such activity are some of the main questions highly debated in galaxy formation and evolution theories. This has motivated our survey MASSIV (Mass Assembly Survey with SINFONI in VVDS) of 0.9< z< 1.9 star-forming galaxies selected from the purely flux-limit ed VVDS redshift survey. Methods. We evaluate the characteristic size and stellar mass of 45 MASSIV galaxies at 1< z< 1.6 and we use the internal dynamics obtained with the SINFONI/VLT-ESO integral field spectrograph, in order to derive the s tellar mass - size - velocity relations. We use the Kennicutt-Schmidt formulation to estimate the gas content and to compute its contribution to the total baryonic mass in MASSIV galaxies. Results. For the first time we obtain the relations between galaxy size , mass, and internal velocity, and the baryonic Tully-Fisher relation, from a statistically representative sample of st ar-forming galaxies at 1< z< 1.6. We derive a dynamical mass which is in good agreement with rotating galaxies containing a gas fraction of∼ 20%, that is perfectly consistent with the content derived using the Kennicutt-Schmidt formulation and corresponding to the expected evolution. Non-rotating galaxies are more compact in their extent of the stellar component, and less massive than rotators, but not statistically different in their gas extent. We obtain a marginal evolution in the size - stellar mass and size - velocity relat ions with discs being evenly smaller with cosmic time at fixed stellar mass or velocity, and less massive for a given velocity with respect to the local Universe. This result does not imply an abnormal evolution in the galactic spin as previously reported. The s catter of the Tully-Fisher relation is reduced introducing the S 05 index, that is interpreted with the increasing contribution to galacti c kinematics of turbulent motions with cosmic time. We report a persisting scatter for rotators in our relations, that we suggest to be i ntrinsic, and possibly caused by complex physical mechanism(s) at work in our stellar mass/luminosity regime and redshift range. Conclusions. Our results consistently point towards a mild, net evolutio n of these relations, comparable to what is predicted by cosmological simulations of disc formation. In a conflictua l picture where earlier studies reported discrepant result s, our findings put on firmer ground the lack of an influential transformation of t he fundamental relations of star-forming galaxies for at le ast 8 Gyr and a dark halo strongly coupled with galactic spectrophotometric properties.