Charting the evolution of the ages and metallicities of massive galaxies since z=0.7

The stellar populations of intermediate-redshift galaxies can shed light onto the growth of massive galaxies in the last 8 billion years. We perform deep, multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70 galaxies in the E-CDFS with redshift 0.6522.7 and stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous low-redshift studies, we constrain the stellar mass, mean stellar age and stellar metallicity of individual galaxies from stellar absorption features. We characterize for the first time the dependence of stellar metallicity and age on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming galaxies separately. These relations for the whole sample have a similar shape as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex in metallicity, at odds with simple passive evolution. We find that no additional star formation and chemical enrichment are required for z=0.7 quiescent galaxies to evolve into the present-day quiescent population. However, this must be accompanied by the quenching of a fraction of z=0.7 Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those of quiescent galaxies, thus increasing the scatter in age without affecting the metallicity distribution. However rapid quenching of the entire population of massive star-forming galaxies at z=0.7 would be inconsistent with the age/metallicity--mass relation for the population as a whole and with the metallicity distribution of star-forming galaxies only, which are on average 0.12 dex less metal-rich than their local counterparts. This indicates chemical enrichment until the present in at least a fraction of the z=0.7 massive star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure

On the Reported Death of the MACHO Era

We present radial velocity measurements of four wide halo binary candidates from the sample in Chaname & Gould (2004; CG04) which, to date, is the only sample containing a large number of such candidates. The four candidates that we have observed have projected separations >0.1 pc, and include the two widest binaries from the sample, with separations of 0.45 and 1.1 pc. We confirm that three of the four CG04 candidates are genuine, including the one with the largest separation. The fourth candidate, however, is spurious at the 5-sigma level. In the light of these measurements we re-examine the implications for MACHO models of the Galactic halo. Our analysis casts doubt on what MACHO constraints can be drawn from the existing sample of wide halo binaries.Comment: 6 Pages, 4 Figures, Accepted for MNRAS Letter

The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) I: The Sample and the Spectra

We introduce Project MEGaSaURA: The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas. MEGaSaURA comprises medium-resolution, rest-frame ultraviolet spectroscopy of N=15 bright gravitationally lensed galaxies at redshifts of 1.68$<$z$<$3.6, obtained with the MagE spectrograph on the Magellan telescopes. The spectra cover the observed-frame wavelength range $3200 < \lambda_o < 8280$ \AA ; the average spectral resolving power is R=3300. The median spectrum has a signal-to-noise ratio of $SNR=21$ per resolution element at 5000 \AA . As such, the MEGaSaURA spectra have superior signal-to-noise-ratio and wavelength coverage compared to what COS/HST provides for starburst galaxies in the local universe. This paper describes the sample, the observations, and the data reduction. We compare the measured redshifts for the stars, the ionized gas as traced by nebular lines, and the neutral gas as traced by absorption lines; we find the expected bulk outflow of the neutral gas, and no systemic offset between the redshifts measured from nebular lines and the redshifts measured from the stellar continuum. We provide the MEGaSaURA spectra to the astronomical community through a data release.Comment: Resubmitted to AAS Journals. Data release will accompany journal publication. v2 addresses minor comments from refere

Compton Echoes from Gamma-ray Bursts

Recent observations of gamma-ray bursts (GRBs) have provided growing evidence for collimated outflows and emission, and strengthened the connection between GRBs and supernovae. If massive stars are the progenitors of GRBs, the hard photon pulse will propagate in the pre-burst, dense environment. Circumstellar material will Compton scatter the prompt GRB radiation and give rise to a reflection echo. We calculate luminosities, spectra, and light curves of such Compton echoes in a variety of emission geometries and ambient gas distributions, and show that the delayed hard X-ray flash from a pulse propagating into a red supergiant wind could be detectable by Swift out to z~0.2. Independently of the gamma-ray spectrum of the prompt burst, reflection echoes will typically show a high-energy cutoff between m_ec^2/2 and m_ec^2 because of Compton downscattering. At fixed burst energy per steradian, the luminosity of the reflected echo is proportional to the beaming solid angle, Omega_b, of the prompt pulse, while the number of bright echoes detectable in the sky above a fixed limiting flux increases as Omega_b^{1/2}, i.e. it is smaller in the case of more collimated jets. The lack of an X-ray echo at one month delay from the explosion poses severe constraints on the possible existence of a lateral GRB jet in SN 1987A. The late r-band afterglow observed in GRB990123 is fainter than the optical echo expected in a dense red supergiant environment from a isotropic prompt optical flash. Significant MeV delayed emission may be produced through the bulk Compton (or Compton drag) effect resulting from the interaction of the decelerating fireball with the scattered X-ray radiation.Comment: LaTeX, 18 pages, 4 figures, revised version accepted for publication in the Ap

Demonstrating Diversity in Star Formation Histories with the CSI Survey

We present coarse but robust star formation histories (SFHs) derived from spectro-photometric data of the Carnegie-Spitzer-IMACS Survey, for 22,494 galaxies at 0.3<z<0.9 with stellar masses of 10^9 Msun to 10^12 Msun. Our study moves beyond "average" SFHs and distribution functions of specific star formation rates (sSFRs) to individually measured SFHs for tens of thousands of galaxies. By comparing star formation rates (SFRs) with timescales of 10^10, 10^9, and 10^8 years, we find a wide diversity of SFHs: 'old galaxies' that formed most or all of their stars early; galaxies that formed stars with declining or constant SFRs over a Hubble time, and genuinely 'young galaxies' that formed most of their stars since z=1. This sequence is one of decreasing stellar mass, but, remarkably, each type is found over a mass range of a factor of 10. Conversely, galaxies at any given mass follow a wide range of SFHs, leading us to conclude that: (1) halo mass does not uniquely determine SFHs; (2) there is no 'typical' evolutionary track; and (3) "abundance matching" has limitations as a tool for inferring physics. Our observations imply that SFHs are set at an early epoch, and that--for most galaxies--the decline and cessation of star formation occurs over a Hubble-time, without distinct "quenching" events. SFH diversity is inconsistent with models where galaxy mass, at any given epoch, grows simply along relations between SFR and stellar mass, but is consistent with a 2-parameter lognormal form, lending credence to this model from a new and independent perspective.Comment: 17 pages, 10 figures; accepted by ApJ; version 2 - no substantive changes; clarifications and correction

Ensemble v-representable ab-initio density functional calculation of energy and spin in atoms: atest of exchange-correlation approximations

The total energies and the spin states for atoms and their first ions with Z = 1-86 are calculated within the the local spin-density approximation (LSDA) and the generalized-gradient approximation (GGA) to the exchange-correlation (xc) energy in density-functional theory. Atoms and ions for which the ground-state density is not pure-state v-representable, are treated as ensemble v- representable with fractional occupations of the Kohn-Sham system. A newly developed algorithm which searches over ensemble v-representable densities [E. Kraisler et al., Phys. Rev. A 80, 032115 (2009)] is employed in calculations. It is found that for many atoms the ionization energies obtained with the GGA are only modestly improved with respect to experimental data, as compared to the LSDA. However, even in those groups of atoms where the improvement is systematic, there remains a non-negligible difference with respect to the experiment. The ab-initio electronic configuration in the Kohn-Sham reference system does not always equal the configuration obtained from the spectroscopic term within the independent-electron approximation. It was shown that use of the latter configuration can prevent the energy-minimization process from converging to the global minimum, e.g. in lanthanides. The spin values calculated ab-initio fit the experiment for most atoms and are almost unaffected by the choice of the xc-functional. Among the systems with incorrectly obtained spin there exist some cases (e.g. V, Pt) for which the result is found to be stable with respect to small variations in the xc-approximation. These findings suggest a necessity for a significant modification of the exchange-correlation functional, probably of a non-local nature, to accurately describe such systems. PACS numbers: 31.15.