The Star Formation Rate-Density Relationship at Redshift 3

We study the star formation rate (SFR) as a function of environment for UV-selected Lyman break galaxies (LBGs) at redshift 3. From deep [μ (sky) ≃ 27.6] UBVI MOSAIC images, covering a total of 0.90 deg , we select 334 LBGs in slices 100 h Mpc (comoving) deep spanning the redshift range 2.9 \u3c z \u3c 3.4 based on Bayesian photometric redshifts that include the I magnitude as a prior. The slice width (100 h Mpc) corresponds to the photometric redshift accuracy (Δ ∼ 0.15). We used mock catalogs from the GIF2 cosmological simulations to show that this redshift resolution is sufficient to statistically differentiate the high-density regions from the low-density regions using ∑ , the projected density to the fifth nearest neighbor. These mock catalogs have a redshift depth of 110 h Mpc, similar to our slice width. The large area of the MOSAIC images, ∼40 × 40 Mpc (comoving) per field, allows us to measure the SFR from the dust-corrected UV continuum as a function of ∑ . In contrast to low-redshift galaxies, we find that the SFR (or UV luminosity) of LBGs at z = 3 shows no detectable dependence on environment over 2 orders of magnitude in densities. To test the significance of our result, we use Monte Carlo simulations (from the mock catalogs) and the same projected density estimators that we applied to our data. We find that we can reject the steep z = 0 SFR-density relation at the 5 σ level. We conclude that the SFR-density relation at z = 3 must be at least 3.6 times flatter than it is locally; i.e., the SFR of LBGs is significantly less dependent on environment than the SFR of local star-forming galaxies. We find that the rest-frame UV colors are also independent of environment

A SINFONI Integral Field Spectroscopy Survey for Galaxy Counterparts to Damped Lyman-alpha Systems - II. Dynamical Properties of the Galaxies towards Q0302-223 and Q1009-0026

Details of processes through which galaxies convert their gas into stars need to be studied in order to obtain a complete picture of galaxy formation. One way to tackle these phenomena is to relate the HI gas and the stars in galaxies. Here, we present dynamical properties of Damped and sub-Damped Lyman-alpha Systems identified in H-alpha emission with VLT/SINFONI at near infra-red wavelengths. While the DLA towards Q0302-223 is found to be dispersion-dominated, the sub-DLA towards Q1009-0026 shows clear signatures of rotation. We use a proxy to circular velocity to estimate the mass of the halo in which the sub-DLA resides and find M_halo=10^12.6 M_sun. We also derive dynamical masses of these objects, and find M_dyn=10^10.3 M_sun and 10^10.9 M_sun. For one of the two systems (towards Q0302-223), we are able to derive a stellar mass of M_*=10^9.5 M_sun from Spectral Energy Distribution fit. The gas fraction in this object is 1/3rd, comparable to similar objects at these redshifts. Our work illustrates that detailed studies of quasar absorbers can offer entirely new insights into our knowledge of the interaction between stars and the interstellar gas in galaxies.Comment: 6 pages, 2 figures. Accepted for publication in MNRA

Star-Forming Galaxies at z~2: An Emerging Picture of Galaxy Dynamics and Assembly

In these proceedings, we summarize recent results from our "SINS" VLT/SINFONI integral-field survey, focusing on the 52 detected UV/optically-selected star-forming galaxies at z~2. Our H-alpha emission-line imaging and kinematic data of these systems illustrates that a substantial fraction (> 1/3) of these galaxies are large, rotating disks and that these disks are clumpy, thick, and forming stars rapidly. We compare these systems to local disk scaling relations and find that the backbones of these relations are already in place at z~2. Detailed analysis of the large disks in our sample provides strong evidence that this population cannot result from a merger-dominated formation history and instead must be assembled by the smooth but rapid inflow of gas along filaments. These systems will then secularly evolve from clump-dominated disks to bulge-dominated disks on short timescales, a phenomenon that is observed in our SINS observations and is consistent with predictions from numerical simulations. These results provide new and exciting insights into the formation of bulge-dominated galaxies in the local Universe.Comment: 7 pages, to appear in the proceedings of "Galaxy Evolution: Emerging Insights and Future Challenges," Austin, TX, 11-14 Nov 200

The growth of dark matter halos: evidence for significant smooth accretion

We study the growth of dark matter halos in the concordance LCDM cosmology using several N-body simulations of large cosmological volumes. We build merger trees from the Millennium and Millennium-II simulations, covering a range 10^9-10^15 Msun in halo mass and 1-10^5 in merger mass ratio. Our algorithm takes special care of halo fragmentation and ensures that the mass contribution of each merger to halo growth is only counted once. This way the integrated merger rate converges and we can consistently determine the contribution of mergers of different mass ratios to halo growth. We find that all resolved mergers, up to mass ratios of 10^5, contribute only ~60% of the total halo mass growth, while major mergers are subdominant, e.g. mergers with mass ratios smaller than 3:1 (10:1) contribute only ~20% (~30%). This is verified with an analysis of two additional simulation boxes, where we follow all particles individually throughout cosmic time. Our results are also robust against using several halo definitions. Under the assumption that the power-law behaviour of the merger rate at large mass ratios can be extrapolated to arbitrarily large mass ratios, it is found that, independently of halo mass, ~40% of the mass in halos comes from genuinely smooth accretion of dark matter that was never bound in smaller halos. We discuss possible implications of our findings for galaxy formation. One implication, assuming as is standard that the pristine intergalactic medium is heated and photoionized by UV photons, is that all halos accrete >40% of their baryons in smooth "cold" T>~10^4K gas, rather than as warm, enriched or clumpy gas or as stars.Comment: 11 pages, 9 figures. Accepted for publication in the Astrophysical Journa

Towards Understanding the Mass-Metallicity relation of Quasar Absorbers: Evidence for bimodality and consequences

One way to characterize and understand HI-selected galaxies is to study their metallicity properties. In particular, we show that the metallicity of absorbers is a bivariate function of the HI column density (NHI) and the MgII equivalent width (Wr(2796)). Thus, a selection upon Wr(2796) is not equivalent to a HI selection for intervening absorbers. A direct consequence for damped absorbers with NHI>20.3 from the bivariate metallicity distribution is that any correlation between the metallicity [X/H] and velocity width (or Wr(2796)) arises from the HI cut and therefore can not be interpreted as a signature of the mass-metallicity relation akin to normal field galaxies. Thus DLA samples are intrinsically heterogeneous. On the other hand, a sample of MgII-selected absorbers, which are statistically dominated by lowest NHI systems (sub-DLAs), are found to have a more uniform metallicity distribution. We postulate that the bivariate metallicity distribution can be explained by two different physical origins of absorbers, namely sight-lines through the ISM of small galaxies and sight-lines through out-flowing material. Several published results follow from this bivariate [X/H] distribution such as (a) the two classes of DLAs, reported by Wolfe et al., and (b) the constant dust-to-gas ratio for MgII-absorbers.Comment: 6 pages, 4 figures, accepted for publication in MNRA

MusE GAs FLOw and Wind (MEGAFLOW) I: First MUSE results on background quasars

The physical properties of galactic winds are one of the keys to understand galaxy formation and evolution. These properties can be constrained thanks to background quasar lines of sight (LOS) passing near star-forming galaxies (SFGs). We present the first results of the MusE GAs FLOw and Wind (MEGAFLOW) survey obtained of 2 quasar fields which have 8 MgII absorbers of which 3 have rest-equivalent width greater than 0.8 \AA. With the new Multi Unit Spectroscopic Explorer (MUSE) spectrograph on the Very Large Telescope (VLT), we detect 6 (75$\%$) MgII host galaxy candidates withing a radius of 30 arcsec from the quasar LOS. Out of these 6 galaxy--quasar pairs, from geometrical arguments, one is likely probing galactic outflows, two are classified as "ambiguous", two are likely probing extended gaseous disks and one pair seems to be a merger. We focus on the wind$-$pair and constrain the outflow using a high resolution quasar spectra from Ultraviolet and Visual Echelle Spectrograph (UVES). Assuming the metal absorption to be due to gas flowing out of the detected galaxy through a cone along the minor axis, we find outflow velocities of the order of $\approx$ 150 km/s (i.e. smaller than the escape velocity) with a loading factor, $\eta =\dot M_{\rm out}/$SFR, of $\approx$ 0.7. We see evidence for an open conical flow, with a low-density inner core. In the future, MUSE will provide us with about 80 multiple galaxy$-$quasar pairs in two dozen fields.Comment: 20 pages, 16 figures, accepted for publication in Ap

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.Comment: 5 pages, 4 figures, Accepted for publication in Ap

The clustering of Luminous Red Galaxies around MgII absorbers

We study the cross-correlation between 212 MgII quasar absorption systems and \~20,000 Luminous Red Galaxies (LRGs) selected from the Sloan Digital Sky Survey Data Release 1 in the redshift range 0.4<z<0.8. The MgII systems were selected to have 2796 & 2803 rest-frame equivalent widths >=1.0 Angstrom and identifications confirmed by the FeII 2600 or MgI 2852 lines. Over comoving scales 0.05--13 h^-1 Mpc, the MgII--LRG cross-correlation has an amplitude 0.69+/-0.09 times that of the LRG--LRG auto-correlation. Since LRGs have halo-masses greater than 3.5 x 10^12 solar masses for M_R<-21, this relative amplitude implies that the absorber host-galaxies have halo-masses greater than 2--8 x 10^11 Msun. For 10^13 Msun LRGs, the absorber host-galaxies have halo-masses 0.5--2.5 x 10^12 Msun. Our results appear consistent with those of Steidel et al. (1994) who found that MgII absorbers with W_r>=0.3 Angstrom are associated with ~0.7 L^*_B galaxies.Comment: 7 pages, 3 figs; Accepted for publication in MNRAS Letters; Extended version with Appendix; Text version of MgII absorber catalogue (Table 1) can be found at http://www.ast.cam.ac.uk/~mim/pub.html. Minor changes to match the published tex

New perspectives on strong z=0.5 MgII absorbers: are halo-mass and equivalent width anti-correlated?

We measure the mean halo-mass of z=0.5 MgII absorbers using the cross-correlation (over co-moving scales 0.05-13h^{-1}Mpc) between 1806 MgII quasar absorption systems and ~250,000 Luminous Red Galaxies (LRGs), both selected from the SDSS DR3. The MgII systems have rest-frame equivalent widths W_r(2796)>=0.3A. From the ratio of the MgII-LRG cross-correlation to the LRG-LRG auto-correlation, we find that the bias ratio between MgII absorbers and LRGs is 0.65+/-0.08, which implies that the absorber host-galaxies have a mean halo-mass 20-40 times smaller than that of the LRGs; the MgII absorbers have halos of mean mass =11.94+/-0.31(stat)+/-0.25(sys). We demonstrate that this statistical technique, which does not require any spectroscopic follow-up, does not suffer from contaminants such as stars or foreground and background galaxies. Finally, we find that the absorber halo-mass is anti-correlated with the equivalent width. If MgII absorbers were virialized in galaxy halos a positive M_h-W_r correlation would have been observed since W_r(2796) is a direct measure of the velocity spread of the MgII sub-components. Thus, our results demonstrate that the individual clouds of a MgII system are not virialized in the gaseous halos of the host-galaxies. We review past results in the literature on the statistics of MgII absorbers and find that they too require an M_h-W_r anti-correlation. When combined with measurements of the equivalent width distribution, the M_h-W_r anti-correlation naturally explains why absorbers with W_r(2796)>=2A are not seen at large impact parameters. We interpret the M_h-W_r anti-correlation within the starburst scenario where strong MgII absorbers are produced by supernovae-driven winds.Comment: 18 pages, 12 EPS figures, Accepted by MNRAS. Full table of MgII absorbers available at http://www.ast.cam.ac.uk/~mim/pub.html, minor changes to match the published tex

Ubiquitous giant Ly α\alpha nebulae around the brightest quasars at z∼3.5z\sim3.5 revealed with MUSE

Direct Ly $\alpha$ imaging of intergalactic gas at $z\sim2$ has recently revealed giant cosmological structures around quasars, e.g. the Slug Nebula (Cantalupo et al. 2014). Despite their high luminosity, the detection rate of such systems in narrow-band and spectroscopic surveys is less than 10%, possibly encoding crucial information on the distribution of gas around quasars and the quasar emission properties. In this study, we use the MUSE integral-field instrument to perform a blind survey for giant Ly $\alpha$ nebulae around 17 bright radio-quiet quasars at $3<z<4$ that does not suffer from most of the limitations of previous surveys. After data reduction and analysis performed with specifically developed tools, we found that each quasar is surrounded by giant Ly $\alpha$ nebulae with projected sizes larger than 100 physical kpc and, in some cases, extending up to 320 kpc. The circularly averaged surface brightness profiles of the nebulae appear very similar to each other despite their different morphologies and are consistent with power laws with slopes $\approx-1.8$. The similarity between the properties of all these nebulae and the Slug Nebula suggests a similar origin for all systems and that a large fraction of gas around bright quasars could be in a relatively "cold" (T$\sim$10$^4$K) and dense phase. In addition, our results imply that such gas is ubiquitous within at least 50 kpc from bright quasars at $3<z<4$ independently of the quasar emission opening angle, or extending up to 200 kpc for quasar isotropic emission.Comment: 19 pages, 9 figures, 3 Tables, accepted to Ap