Metallicity Gradient of a Lensed Face-on Spiral Galaxy at Redshift 1.49

We present the first metallicity gradient measurement for a grand-design face-on spiral galaxy at z~1.5. This galaxy has been magnified by a factor of 22$\times$ by a massive, X-ray luminous galaxy cluster MACS\,J1149.5+2223 at z=0.544. Using the Laser Guide Star Adaptive Optics aided integral field spectrograph OSIRIS on KECK II, we target the Halpha emission and achieve a spatial resolution of 0.1", corresponding to a source plane resolution of 170 pc. The galaxy has well-developed spiral arms and the nebular emission line dynamics clearly indicate a rotationally supported disk with V_{rot}/\sigma~4. The best-fit disk velocity field model yields a maximum rotation of V_{rot} sin{i}=150$\pm$15 km s^{-1}, and a dynamical mass of M_{dyn}=1.3$\pm0.2\times10^{10}csc^2(i) M_{\odot} (within 2.5\,kpc), where the inclination angle i=45$\pm10^{\circ}$. Based on the [NII] and Halpha ratios, we measured the radial chemical abundance gradient from the inner hundreds of parsecs out to ~5 kpc. The slope of the gradient is -0.16$\pm$0.02 dex kpc$^{-1}$, significantly steeper than the gradient of late-type or early-type galaxies in the local universe. If representative of disk galaxies at z~1.5, our results support an "inside-out" disk formation scenario in which early infall/collapse in the galaxy center builds a chemically enriched nucleus, followed by slow enrichment of the disk over the next 9 Gyr.Comment: 13page, 4 figures, ApJL in press (updated version after proof

Non-equilibrium dissociating nitrogen flow over a wedge

Experimental results for dissociating nitrogen flow over a wedge, obtained in a free-piston shock tunnel, are described. Interferograms of the flow show clearly the curvature of the shock wave and the rise in fringe shift after the shock associated with the dissociation. It is shown that the shock curvature at the tip of the wedge can be used to calculate the initial dissociation rate and that it is a more sensitive indication of the rate than can be obtained from fringe shift measurements under the prevailing experimental conditions. Because the freestream dissociation fraction can be adjusted in the shock tunnel, the dependence on atomic nitrogen concentration of the dissociation rate can be determined by the shock curvature method. A detailed calculation of the flow field by an inverse method, starting from the measured shock shape, shows good agreement with experiments

Galaxy Mergers and the Mass-Metallicity Relation: Evidence for Nuclear Metal Dilution and Flattened Gradients from Numerical Simulations

Recent results comparing interacting galaxies to the mass-metallicity relation show that their nuclear oxygen abundances are unexpectedly low. We present analysis of N-body/SPH numerical simulations of equal-mass mergers that confirm the hypothesis that these underabundances are accounted for by radial inflow of low-metallicity gas from the outskirts of the two merging galaxies. The underabundances arise between first and second pericenter, and the simulated abundance dilution is in good agreement with observations. The simulations further predict that the radial metallicity gradients of the disk galaxies flatten shortly after first passage, due to radial mixing of gas. These predictions will be tested by future observations of the radial metallicity distributions in interacting galaxies.Comment: ApJ Letters, in press; 6 pages, 5 figure

Gas-Phase Oxygen Gradients in Strongly Interacting Galaxies: I. Early-Stage Interactions

A consensus is emerging that interacting galaxies show depressed nuclear gas metallicities compared to isolated star-forming galaxies. Simulations suggest that this nuclear underabundance is caused by interaction-induced inflow of metal-poor gas, and that this inflow concurrently flattens the radial metallicity gradients in strongly interacting galaxies. We present metallicities of over 300 HII regions in a sample of 16 spirals that are members of strongly interacting galaxy pairs with mass ratio near unity. The deprojected radial gradients in these galaxies are about half of those in a control sample of isolated, late-type spirals. Detailed comparison of the gradients with simulations show remarkable agreement in gradient distributions, the relationship between gradients and nuclear underabundances, and the shape of profile deviations from a straight line. Taken together, this evidence conclusively demonstrates that strongly interacting galaxies at the present day undergo nuclear metal dilution due to gas inflow, as well as significant flattening of their gas-phase metallicity gradients, and that current simulations can robustly reproduce this behavior at a statistical level.Comment: Accepted for publication in Ap

Outliers from the Mass--Metallicity Relation II: A Sample of Massive Metal-Poor Galaxies from SDSS

We present a sample of 42 high-mass low-metallicity outliers from the mass--metallicity relation of star-forming galaxies. These galaxies have stellar masses that span log(M_*/M_sun) ~9.4 to 11.1 and are offset from the mass--metallicity relation by -0.3 to -0.85 dex in 12+log(O/H). In general, they are extremely blue, have high star formation rates for their masses, and are morphologically disturbed. Tidal interactions are expected to induce large-scale gas inflow to the galaxies' central regions, and we find that these galaxies' gas-phase oxygen abundances are consistent with large quantities of low-metallicity gas from large galactocentric radii diluting the central metal-rich gas. We conclude with implications for deducing gas-phase metallicities of individual galaxies based solely on their luminosities, specifically in the case of long gamma-ray burst host galaxies.Comment: Accepted for publication in ApJ; 11 pages, 11 figure

A universal, turbulence-regulated star formation law: from Milky Way clouds to high-redshift disk and starburst galaxies

Whilst the star formation rate (SFR) of molecular clouds and galaxies is key in understanding galaxy evolution, the physical processes which determine the SFR remain unclear. This uncertainty about the underlying physics has resulted in various different star formation laws, all having substantial intrinsic scatter. Extending upon previous works that define the column density of star formation (Sigma_SFR) by the gas column density (Sigma_gas), we develop a new universal star formation (SF) law based on the multi-freefall prescription of gas. This new SF law relies predominantly on the probability density function (PDF) and on the sonic Mach number of the turbulence in the star-forming clouds. By doing so we derive a relation where the star formation rate (SFR) correlates with the molecular gas mass per multi-freefall time, whereas previous models had used the average, single-freefall time. We define a new quantity called maximum (multi-freefall) gas consumption rate (MGCR) and show that the actual SFR is only about 0.4% of this maximum possible SFR, confirming the observed low efficiency of star formation. We show that placing observations in this new framework (Sigma_SFR vs. MGCR) yields a significantly improved correlation with 3-4 times reduced scatter compared to previous SF laws and a goodness-of-fit parameter R^2=0.97. By inverting our new relationship, we provide sonic Mach number predictions for kpc-scale observations of Local Group galaxies as well as unresolved observations of local and high-redshift disk and starburst galaxies that do not have independent, reliable estimates for the turbulent cloud Mach number.Comment: 6 pages, 2 figures, Accepted for publication in ApJ Letters, Movie available here: http://www.mso.anu.edu.au/~chfeder/pubs/universal_sf_law/universal_sf_law.htm

Metallicity Gradients and Gas Flows in Galaxy Pairs

We present the first systematic investigation into the metallicity gradients in galaxy close pairs. We determine the metallicity gradients for 8 galaxies in close pairs using HII region metallicities obtained with high signal-to-noise multi-slit observations with the Keck LRIS Spectrograph. We show that the metallicity gradients in close pairs are significantly shallower than gradients in isolated spiral galaxies such as the Milky Way, M83, and M101. These observations provide the first solid evidence that metallicity gradients in interacting galaxies are systematically different from metallicity gradients in isolated spiral galaxies. Our results suggest that there is a strong relationship between metallicity gradients and the gas dynamics in galaxy interactions and mergers.Comment: Accepted for publication in ApJL. 6 pages, 3 figures, 1 table. Article with full resolution figures can be obtained from http://www.ifa.hawaii.edu/~kewley/Gradients.pd

The spectral characteristics of the 2dFGRS-NVSS galaxies

We have analysed the 2dF spectra of a sample of galaxies common to the 2dF galaxy redshift survey (2dFGRS, Colless 1999) and the NRAO VLA sky survey (NVSS, Condon et al. 1998). Our sample comprises 88 galaxies selected by Sadler et al. (1999) from 30 2dFGRS fields observed in 1998. In this paper we discuss how this and future, much larger, samples of 2dFGRS-NVSS galaxies can be interpreted via analysis of those galaxies with strong narrow emission lines. Using diagnostic line ratio measurements we confirm the majority of the eyeball classifications of Sadler et al. (1999), although many galaxies show evidence of being `composite' galaxies - a mixture of AGN plus starburst components.Comment: 11 pages, 5 figures, accepted for publication in PAS