H. Jabran Zahid

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 eight galaxies in close pairs using H II 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.

THE CHEMICAL EVOLUTION OF STAR-FORMING GALAXIES OVER THE LAST 11 BILLION YEARS

We calculate the stellar mass-metallicity relation at five epochs ranging to z {approx} 2.3. We quantify evolution in the shape of the mass-metallicity relation as a function of redshift; the mass-metallicity relation flattens at late times. There is an empirical upper limit to the gas-phase oxygen abundance in star-forming galaxies that is independent of redshift. From examination of the mass-metallicity relation and its observed scatter, we show that the flattening at late times is a consequence of evolution in the stellar mass where galaxies enrich to this empirical upper metallicity limit; there is also evolution in the fraction of galaxies at a fixed stellar mass that enrich to this limit. The stellar mass where metallicities begin to saturate is {approx}0.7 dex smaller in the local universe than it is at z {approx} 0.8.

Metallicity gradients in local field star-forming galaxies: insights on inflows, outflows, and the coevolution of gas, stars and metals

We present metallicity gradients in 49 local field star-form ing galaxies. We derive gas- phase oxygen abundances using two widely adopted metallicity calibrations based on the (O III)/H�, (N II)/Hand (N II)/(O II) line ratios. The two derived metallicity gradients are usually in good agreement within ±0.14 dex R −1 25 (R25 is the B-band iso-photoal radius), but the metallicity gradients can differ significantly when the ionisation parameters change systematically with radius. We investigate the metallicit y gradients as a function of stellar mass (8 MB > −22). When the metallicity gradients are expressed in dex kpc −1 , we show that galaxies with lower mass and luminosity, on average, have steeper metallicity gradients. When the metallicity gradients are expressed in dex R −1 25 , we find no correlation between the metallicity gradients, and stellar mass and luminosity. We provide a local benchmark metallicity gradient of field star-forming galaxies useful for comparison with studies at high redshifts. We investigate the origin of the local benchmark gradient using simple chemical evolution models and observed gas and stellar surface density profiles in nearby field spira l galaxies. Our models suggest that the local benchmark gradient is a direct result of the co evolution of gas and stellar disk under virtually closed-box chemical evolution when the stellar-to-gas mass ratio becomes high (≫ 0.3). These models imply low current mass accretion rates (. 0.3 × SFR), and low mass outflow rates ( . 3 × SFR) in local field star-forming galaxies.

A large excess in apparent solar oblateness due to surface magnetism.

The shape of the Sun subtly reflects its rotation and internal flows. The surface rotation rate, ∼2 kilometers per second at the equator, predicts an oblateness (equator-pole radius difference) of 7.8 milli–arc seconds, or ∼0.001%. Observations from the Reuven Ramaty High-Energy Solar Spectroscopic Imager satellite show unexpectedly large flattening, relative to the expectation from surface rotation. This excess is dominated by the quadrupole term and gives a total oblateness of 10.77 ± 0.44 milli–arc seconds. The position of the limb correlates with a sensitive extreme ultraviolet proxy, the 284 angstrom limb brightness. We relate the larger radius values to magnetic elements in the enhanced network and use the correlation to correct for it as a systematic error term in the oblateness measurement. The corrected oblateness of the nonmagnetic Sun is 8.01 ± 0.14 milli–arc seconds, which is near the value expected from rotation.

Agriculture, population growth, and statistical analysis of the radiocarbon record

Significance We statistically analyze the radiocarbon record and show that early farming societies in Europe grew at the same rate as contemporaneous foraging societies in North America. Thus, our results challenge the commonly held view that the advent of agriculture was linked to accelerated growth of the human population. The same rates of prehistoric population growth measured worldwide suggest that the global climate and/or biological factors intrinsic to the species and not factors related to the regional environment or subsistence practices regulated the growth of the human population for most of the last 12,000 y. This study demonstrates that statistical analysis of the radiocarbon record is a robust quantitative approach for studying prehistoric human demography. The human population has grown significantly since the onset of the Holocene about 12,000 y ago. Despite decades of research, the factors determining prehistoric population growth remain uncertain. Here, we examine measurements of the rate of growth of the prehistoric human population based on statistical analysis of the radiocarbon record. We find that, during most of the Holocene, human populations worldwide grew at a long-term annual rate of 0.04%. Statistical analysis of the radiocarbon record shows that transitioning farming societies experienced the same rate of growth as contemporaneous foraging societies. The same rate of growth measured for populations dwelling in a range of environments and practicing a variety of subsistence strategies suggests that the global climate and/or endogenous biological factors, not adaptability to local environment or subsistence practices, regulated the long-term growth of the human population during most of the Holocene. Our results demonstrate that statistical analyses of large ensembles of radiocarbon dates are robust and valuable for quantitatively investigating the demography of prehistoric human populations worldwide.

The chemical evolution of local star forming galaxies: Radial profiles of ISM metallicity, gas mass, and stellar mass and constraints on galactic accretion and winds

The radially averaged metallicity distribution of the ISM and the young stellar population of a sample of 20 disk galaxies is investigated by means of an analytical chemical evolution model which assumes constant ratios of galactic wind mass loss and accretion mass gain to star formation rate. Based on this model the observed metallicities and their gradients can be described surprisingly well by the radially averaged distribution of the ratio of stellar mass to ISM gas mass. The comparison between observed and model predicted metallicity is used to constrain the rate of mass loss through galactic wind and accretion gain in units of the star formation rate. Three groups of galaxies are found: galaxies with either mostly winds and only weak accretion, or mostly accretion and only weak winds, and galaxies where winds are roughlybalancedbyaccretion.Thethreegroupsaredistinct inthepropertiesoftheir gasdisks. Galaxies with approximately equal rates of mass-loss and accretion gain have low metallicity, atomic hydrogen dominated gas disks with a flat spatial profile. The other two groups have gas disks dominated by molecular hydrogen out to 0.5 to 0.7 isophotal radii and show a radial exponential decline, which is on average steeper for the galaxies with small accretion rates. The rates of accretion (. 1.0×SFR) and outflow (. 2.4×SFR) are relatively low. The latter depend on the calibration of the zero point of the metallicity determination from the use of HII region strong emission lines.

Quiescent Compact Galaxies at Intermediate Redshift in the COSMOS Field. The Number Density

We investigate the evolution of compact galaxy number density over the redshift range

A RISE IN THE IONIZING PHOTONS IN STAR-FORMING GALAXIES OVER THE PAST 8 BILLION YEARS

0.2 1

Empirical constraints for the magnitude and composition of galactic winds

for equivalently selected compact samples. Small variations in the abundance of the COSMOS compact sources as a function of redshift correspond to known structures in the field. The constancy of the compact galaxy number density is robust and insensitive to the compactness threshold or the stellar mass range (for

COMPARING DENSE GALAXY CLUSTER REDSHIFT SURVEYS WITH WEAK-LENSING MAPS

M_\ast>10^{10}\, M_\odot