Metal-poor, Strongly Star-Forming Galaxies in the DEEP2 Survey: The Relationship between Stellar Mass, Temperature-based Metallicity, and Star Formation Rate

We report on the discovery of 28 $z\approx0.8$ metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [OIII]$\lambda$4363 emission line, which provides a "direct" measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFRs, with stellar mass estimates from modeling the optical photometry. We find that these galaxies are $1.05\pm0.61$ dex above the z~1 stellar mass-SFR relation, and $0.23\pm0.23$ dex below the local mass-metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains (0.29 dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of $\approx100^{+310}_{-75}$ Myr that suggests stochastic star formation. Combining our sample with other z~1 metal-poor galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar mass) that is significant at 94.4% confidence. We interpret this positive correlation as recent star formation that has enriched the gas, but has not had time to drive the metal-enriched gas out with feedback mechanisms.Comment: Resubmitted to ApJ on March 6, 2015. Revised to discuss selection biases and methodologies, and address the former by including more metal-rich galaxies with robust non-detections of [OIII]4363. Primary results on FMR analyses are unchanged. Additional figures are included to illustrate selection biases; previous figures have been revised to improve presentatio

The Wyoming Survey for H-alpha. III. H-alpha Luminosity Functions at z ~ 0.16, 0.24, 0.32, and 0.40

The Wyoming Survey for H-alpha, or WySH, is a large-area, ground-based imaging survey for H-alpha-emitting galaxies at redshifts of z ~ 0.16, 0.24, 0.32, and 0.40. The survey spans up to four square degrees in a set of fields of low Galactic cirrus emission, using twin narrowband filters at each epoch for improved stellar continuum subtraction. H-alpha luminosity functions are presented for each Delta(z) ~ 0.02 epoch based on a total of nearly 1200 galaxies. These data clearly show an evolution with lookback time in the volume-averaged cosmic star formation rate. Integrals of Schechter fits to the incompleteness- and extinction-corrected H-alpha luminosity functions indicate star formation rates per co-moving volume of 0.010, 0.013, 0.020, 0.022 h_70 M_sun yr^{-1} Mpc^{-3} at z ~ 0.16, 0.24, 0.32, and 0.40, respectively. Statistical and systematic measurement uncertainties combined are on the order of 25% while the effects of cosmic variance are at the 20% level. The bulk of this evolution is driven by changes in the characteristic luminosity L_* of the H-alpha luminosity functions, with L_* for the earlier two epochs being a factor of two larger than L_* at the latter two epochs; it is more difficult with this data set to decipher systematic evolutionary differences in the luminosity function amplitude and faint-end slope. Coupling these results with a comprehensive compilation of results from the literature on emission line surveys, the evolution in the cosmic star formation rate density over 0 < z < 1.5 is measured to be rho_dot_SFR(z) = rho_dot_SFR(0) (1+z)^{3.4+/-0.4}.Comment: Accepted for publication in ApJ Letter

Global Star Formation Rate Density over 0.7<z<1.9

We determine the global star formation rate density at 0.7<z<1.9 using emission-line selected galaxies identified in Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrograph (HST-NICMOS) grism spectroscopy observations. Observing in pure parallel mode throughout HST Cycles 12 and 13, our survey covers ~104 arcmin2 from which we select 80 galaxies with likely redshifted Ha emission lines. In several cases, a somewhat weaker [OIII] doublet emission is also detected. The Ha luminosity range of the emission-line galaxy sample is 4.4 x 10^41 < L(Ha) < 1.5 x 10^43 erg/s. In this range, the luminosity function is well described by a Schechter function with phi* = (4.24\pm3.55) x 10^-3 Mpc^-3, L* = (2.88\pm1.58) x 10^42 erg/s, and alpha = -1.39\pm0.43. We derive a volume-averaged star formation rate density of 0.138\pm0.058 Msun/yr/Mpc3 at z=1.4 without an extinction correction. Subdividing the redshift range, we find star formation rate densities of 0.088\pm0.056 Msun/yr/Mpc3 at z=1.1 and 0.265\pm0.174 Msun/yr/Mpc3 at z=1.6. The overall star formation rate density is consistent with previous studies using Ha when the same average extinction correction is applied, confirming that the cosmic peak of star formation occurs at z>1.5.Comment: Accepted for publication in Ap

The [O II] lambda 3727 Luminosity Function at z ~ 1

We measure the evolution of the [OII]lambda 3727 luminosity function at 0.75<z<1.45 using high-resolution spectroscopy of ~14,000 galaxies observed by the DEEP2 galaxy redshift survey. We find that brighter than L_{OII}=10^{42} erg s^(-1) the luminosity function is well-represented by a power law dN/dL ~ L^{\alpha} with slope \alpha ~ -3. The number density of [OII] emitting galaxies above this luminosity declines by a factor of >~2.5 between z ~ 1.35 and z ~ 0.84. In the limit of no number-density evolution, the characteristic [OII] luminosity, L^*_[OII], defined as the luminosity where the space density equals 10^{-3.5} dex^{-1} Mpc^{-3}, declines by a factor of ~1.8 over the same redshift interval. Assuming that L_[OII] is proportional to the star-formation rate (SFR), and negligible change in the typical dust attenuation in galaxies at fixed [OII] luminosity, the measured decline in L^*_[OII] implies a ~25% per Gyr decrease in the amount of star formation in galaxies during this epoch. Adopting a faint-end power-law slope of -1.3\pm0.2, we derive the comoving SFR density in four redshift bins centered around z~1 by integrating the observed [OII] luminosity function using a local, empirical calibration between L_[OII] and SFR, which statistically accounts for variations in dust attenuation and metallicity among galaxies. We find that our estimate of the SFR density at z~1 is consistent with previous measurements based on a variety of independent SFR indicators.Comment: 10 pages, 6 figures, 2 tables, resubmitted to ApJ, in emulateapj style. Comparison with narrow-band observations added. Wavelength coverage included into complete function, little effects. The data is available on http://bias.cosmo.fas.nyu.edu/galevolution

Nebular Attenuation in H\alpha-selected Star-forming Galaxies at z=0.8 from the NewH\alpha\ Survey

We present measurements of the dust attenuation of H\alpha-selected emission-line galaxies at z=0.8 from the NewH\alpha\ narrowband survey. The analysis is based on deep follow-up spectroscopy with Magellan/IMACS, which captures the strong rest-frame optical emission lines from [OII] \lambda 3727 to [OIII] \lambda 5007. The spectroscopic sample used in this analysis consists of 341 confirmed H\alpha\ emitters. We place constraints on the AGN fraction using diagnostics which can be applied at intermediate redshift. We find that at least 5% of the objects in our spectroscopic sample can be classified as AGN and 2% are composite, i.e. powered by a combination of star-formation and AGN activity. We measure the dust attenuation for individual objects from the ratios of the higher order Balmer lines. The H\beta\ and H\gamma\ pair of lines is detected with S/N>5 in 55 individual objects and the H\beta\ and H\delta\ pair is detected in 50 individual objects. We also create stacked spectra to probe the attenuation in objects without individual detections. The median attenuation at H\alpha\ based on the objects with individually detected lines is A(H\alpha)=0.9+-1.0 magnitudes, in good agreement with the attenuation found in local samples of star-forming galaxies. We find that the z=0.8 galaxies occupy a similar locus of attenuation as a function of magnitude, mass and SFR as a comparison sample drawn from the SDSS DR4. Both the results from the individual z=0.8 galaxies and from the stacked spectra show consistency with the mass -- attenuation and SFR -- attenuation relations found in the local Universe, indicating that these relations are also applicable at intermediate redshift.Comment: Submitted to AJ. Revised per referee's comment

Large Area Survey for z=7 Galaxies in SDF and GOODS-N: Implications for Galaxy Formation and Cosmic Reionization

We present results of our large-area survey for z'-band dropout galaxies at z=7 in a 1568 arcmin^2 sky area covering the SDF and GOODS-N fields. Combining our ultra-deep Subaru/Suprime-Cam z'- and y-band (lambda_eff=1um) images with legacy data of Subaru and HST, we have identified 22 bright z-dropout galaxies down to y=26, one of which has a spectroscopic redshift of z=6.96 determined from Lya emission. The z=7 luminosity function (LF) yields the best-fit Schechter parameters of phi*=0.69 +2.62/-0.55 x10^(-3) Mpc^(-3), Muv*=-20.10 +/-0.76 mag, and alpha=-1.72 +/-0.65, and indicates a decrease from z=6 at a >95% confidence level. This decrease is beyond the cosmic variance in our two fields, which is estimated to be a factor of <~2. We have found that the cosmic star formation rate density drops from the peak at z=2-3 to z=7 roughly by a factor of ~10 but not larger than ~100. A comparison with the reionization models suggests either that the Universe could not be totally ionized by only galaxies at z=7, or more likely that properties of galaxies at z=7 are different from those at low redshifts having, e.g., a larger escape fraction (>~0.2), and/or a flatter IMF. Our SDF z-dropout galaxies appear to form 60-Mpc long filamentary structures, and the z=6.96 galaxy with Lya emission is located at the center of an overdense region consisting of four UV bright dropout candidates, which might suggest an existence of a well-developed ionized bubble at z=7.Comment: 20 pages; ApJ in press, measurements improved with HST/WFC3 data point

Stellar Populations of Lyman Break Galaxies at z=1-3 in the HST/WFC3 Early Release Science Observations

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z=1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 sq. arcmin in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z=1-3 are selected using dropout selection criteria similar to high redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout selected LBGs is accurate to within few percent; (2) the UV spectral slope (beta) is redder than at high redshift (z>3), where LBGs are less dusty; (3) on average, LBGs at z=1-3 are massive, dustier and more highly star-forming, compared to LBGs at higher redshifts with similar luminosities (0.1L*<~L<~2.5L*), though their median values are similar within 1-sigma uncertainties. This could imply that identical dropout selection technique, at all redshifts, find physically similar galaxies; and (4) stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of ~0.46, and star-formation rates are proportional to their stellar masses with a logarithmic slope of ~0.90. These relations hold true --- within luminosities probed in this study --- for LBGs from z~1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z~2, but to avoid any selection biases, and for direct comparison with LBGs at z>3, a true Lyman break selection at z~2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties, and their evolution.Comment: Accepted for publication in ApJ (29 pages, 9 figures

High star formation activity in the central region of a distant cluster at z=1.46

We present an unbiased deep [OII] emission survey of a cluster XMMXCS J2215.9-1738 at z=1.46, the most distant cluster to date with a detection of extended X-ray emission. With wide-field optical and near-infrared cameras (Suprime-Cam and MOIRCS, respectively) on Subaru telescope, we performed deep imaging with a narrow-band filter NB912 (lambda_c=9139A, Delta_lambda=134A) as well as broad-band filters (B, z', J and Ks). From the photometric catalogues, we have identified 44 [OII] emitters in the cluster central region of 6'x6' down to a dust-free star formation rate of 2.6 Msun/yr (3 sigma). Interestingly, it is found that there are many [OII] emitters even in the central high density region. In fact, the fraction of [OII] emitters to the cluster members as well as their star formation rates and equivalent widths stay almost constant with decreasing cluster-centric distance up to the cluster core. Unlike clusters at lower redshifts (z<1) where star formation activity is mostly quenched in their central regions, this higher redshift 2215 cluster shows its high star formation activity even at its centre, suggesting that we are beginning to enter the formation epoch of some galaxies in the cluster core eventually. Moreover, we find a deficit of galaxies on the red sequence at magnitudes fainter than ~M*+0.5 on the colour-magnitude diagram. This break magnitude is brighter than that of lower redshift clusters, and it is likely that we are seeing the formation phase of more massive red galaxies in the cluster core at z~1. These results may indicate inside-out and down-sizing propagation of star formation activity in the course of cluster evolution.Comment: 12 pages, 9 figures, 1 table, accepted for publication in MNRA

MOIRCS Deep Survey. VIII. Evolution of Star Formation Activity as a Function of Stellar Mass in Galaxies since z~3

We study the evolution of star formation activity of galaxies at 0.5<z<3.5 as a function of stellar mass, using very deep NIR data taken with Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope in the GOODS-North region. The NIR imaging data reach K ~ 23-24 Vega magnitude and they allow us to construct a nearly stellar mass-limited sample down to ~ 10^{9.5-10} Msun even at z~3. We estimated star formation rates (SFRs) of the sample with two indicators, namely, the Spitzer/MIPS 24um flux and the rest-frame 2800A luminosity. The SFR distribution at a fixed Mstar shifts to higher values with increasing redshift at 0.5<z<3.5. More massive galaxies show stronger evolution of SFR at z>~1. We found galaxies at 2.5<z<3.5 show a bimodality in their SSFR distribution, which can be divided into two populations by a constant SSFR of ~2 Gyr^{-1}. Galaxies in the low-SSFR group have SSFRs of ~ 0.5-1.0 Gyr^{-1}, while the high-SSFR population shows ~10 Gyr^{-1}. The cosmic SFRD is dominated by galaxies with Mstar = 10^{10-11} Msun at 0.5<z<3.5, while the contribution of massive galaxies with Mstar = 10^{11-11.5} Msun shows a strong evolution at z>1 and becomes significant at z~3, especially in the case with the SFR based on MIPS 24um. In galaxies with Mstar = 10^{10-11.5} Msun, those with a relatively narrow range of SSFR (<~1 dex) dominates the cosmic SFRD at 0.5<z<3.5. The SSFR of galaxies which dominate the SFRD systematically increases with redshift. At 2.5<z<3.5, the high-SSFR population, which is relatively small in number, dominates the SFRD. Major star formation in the universe at higher redshift seems to be associated with a more rapid growth of stellar mass of galaxies.Comment: 16 pages, 13 figures, accepted for publication in Ap

Determination of the intrinsic velocity field in the M87 jet

A new method to estimate the Doppler beaming factor of relativistic large-scale jet regions is presented. It is based on multiwaveband fitting to radio-to-X-ray continua with synchrotron spectrum models. Combining our method with available observational data of proper motions, we derive the intrinsic velocity as well as the viewing angles to the line of sight for eight knotty regions down the M87 jet. The results favor the 'modest beaming' scenario along the jet, with Doppler factors varying between 2-5. The inner jet of M87 suffers sharp deceleration, and the intrinsic speed remains roughly constant down the outer jet. The orientation of the inner jet regions is fully consistent with the result of 10deg-19deg to the line of sight suggested by previous Hubble Space Telescope (HST) proper motion studies of the M87 jet. The outer jet, however, shows systematic deflection off the inner jet to much smaller inclination (<<10deg). Further calculation of knot A suggests this deflection can be regarded as evidence that the outer jet suffers some departure from equipartition. The nucleus region of the M87 jet should have a viewing angle close to its first knot HST-1, i.e. ~15deg, which favors the idea that M87 may be a misaligned blazar. This work provides some hints about the overall dynamics of this famous extragalactic jet.Comment: 11 pages, 3 figures, 4 tables, MNRAS, 2009, in pres