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Register a new userThe [OII]3727 Luminosity function and Star Formation Rate at z~1.2 in the COSMOS 2 Square-degree Field and the Subaru Deep Field
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We have carried out a wide-field imaging survey for [OII]3727 emitting galaxies at z~1.2 in the HST COSMOS 2 square degree field using the Suprime-Cam on the Subaru Telescope. The survey covers a sky area of 6700 arcmin^2 in the COSMOS field, and a redshift range between 1.17 and 1.20 (Delta_z = 0.03), corresponding to a survey volume of 5.56*10^5 Mpc^3. We obtain a sample of 3176 [OII] emitting galaxies with observed emission-line equivalent widths greater than 26 AA. Since our survey tends to sample brighter [OII]3727 emitting galaxies, we also analyze a sample of fainter [OII]3727 emitting galaxies found in the Subaru Deep Field (SDF). We find an extinction-corrected [OII] luminosity density of 10^{40.35^+0.08_-0.06} ergs s^-1 Mpc-3, corresponding to star formation rate density of 0.32^+0.06_-0.04 M_sun yr-1 Mpc^-3 in the COSMOS field at z~1.2. This is the largest survey for [OII]3727 emitters beyond z=1 currently available.
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To derive a new H$alpha$ luminosity function and to understand the clustering properties of star-forming galaxies at $z approx 0.24$, we have made a narrow-band imaging survey for H$alpha$ emitting galaxies in the HST COSMOS 2 square degree field. We used the narrow-band filter NB816 ($lambda_c = 8150$ AA, $Delta lambda = 120$ AA) and sampled H$alpha$ emitters with $EW_{rm obs}(rm Halpha + [Ntextsc{ii}]) > 12$ AA in a redshift range between $z=0.233$ and $z=0.251$ corresponding to a depth of 70 Mpc. We obtained 980 H$alpha$ emitting galaxies in a sky area of 5540 arcmin$^2$, corresponding to a survey volume of $3.1 times 10^4 {rm Mpc^3}$. We derive a H$alpha$ luminosity function with a best-fit Schechter function parameter set of $alpha = -1.35^{+0.11}_{-0.13}$, $logphi_* = -2.65^{+0.27}_{-0.38}$, and $log L_* ({rm erg s^{-1}}) = 41.94^{+0.38}_{-0.23}$. The H$alpha$ luminosity density is $2.7^{+0.7}_{-0.6} times 10^{39}$ ergs s$^{-1}$ Mpc$^{-3}$. After subtracting the AGN contribution (15 %) to the H$alpha$ luminosity density, the star formation rate density is evaluated as $1.8^{+0.7}_{-0.4} times 10^{-2}$ $M_{sun}$ yr$^{-1}$ Mpc$^{-3}$. The angular two-point correlation function of H$alpha$ emitting galaxies of $log L({rm Halpha}) > 39.8$ is well fit by a power law form of $w(theta) = 0.013^{+0.002}_{-0.001} theta^{-0.88 pm 0.03}$, corresponding to the correlation function of $xi(r) = (r/1.9{rm Mpc})^{-1.88}$. We also find that the H$alpha$ emitters with higher H$alpha$ luminosity are more strongly clustered than those with lower luminosity.
The measurement of the Star Formation Rate density of the Universe is of prime importance in understanding the formation and evolution of galaxies. The [OII]3727 emission line flux, easy to measure up to z~1.4 within deep redshift surveys in the optical and up to z~5.4 in the near infrared, offers a reliable means of characterizing the star formation properties of high-z objects. In order to provide the high-z studies with a local reference, we have measured total [OII]3727 fluxes for the well analyzed local sample of star-forming galaxies from the Universidad Complutense de Madrid Survey. This data is used to derive the [OII]3727 luminosity function for local star-forming galaxies. When compared with similar luminosity densities published for redshift up to z~1, the overall evolution already observed in the star formation activity of the Universe is confirmed.
We examine the faint-end slope of the rest-frame V-band luminosity function (LF), with respect to galaxy spectral type, of field galaxies with redshift z<0.5, using a sample of 80,820 galaxies with photometric redshifts in the Cosmic Evolution Survey (COSMOS) field. For all galaxy spectral types combined, the LF slope, alpha, ranges from -1.24 to -1.12, from the lowest redshift bin to the highest. In the lowest redshift bin (0.02<z<0.1), where the magnitude limit is M(V) ~ -13, the slope ranges from ~ -1.1 for galaxies with early-type spectral energy distributions (SEDs), to ~ -1.9 for galaxies with low-extinction starburst SEDs. In each galaxy SED category (Ell, Sbc, Scd/Irr, and starburst), the faint-end slopes grow shallower with increasing redshift; in the highest redshift bin (0.4<z<0.5), the slope is ~ -0.5 and ~ -1.3 for early-types and starbursts respectively. The steepness of alpha at lower redshift could be qualitatively explained by large numbers of faint dwarf galaxies, perhaps of low surface brightness, which are not detected at higher redshifts.
Abridged: Subaru Deep Field line-emitting galaxies in four narrow-band filters at low and intermediate redshifts are presented. Broad-band colors, follow-up optical spectroscopy, and multiple NB filters are used to distinguish Ha, [O II], and [O III] emitters between redshifts of 0.07 and 1.47 to construct their luminosity functions (LFs). These LFs are derived down to faint magnitudes, which allows for a more accurate determination of the faint end slope. With a large (N~200-900) sample for each redshift interval, a Schechter profile is fitted to each LF. Prior to dust extinction corrections, the [O III] and [O II] LFs reported in this paper agree reasonably well with those of Hippelein et al. The z=0.08 Ha LF, which reaches two orders of magnitude fainter than Gallego et al., is steeper by 25%. This indicates that there are more low luminosity star-forming galaxies for z<0.1. The faint end slope alpha and phi_{star} show a strong evolution with redshift while L_{star} show little evolution. The evolution in alpha indicates that low-luminosity galaxies have a stronger evolution compared to brighter ones. Integrated star formation rate densities are derived via Ha, [O III], and [O II] for 0.07<z<1.47. A steep increase in the star-formation rate density, as a function of redshift, is seen for 0.4<z<0.9. For z>1, the star-formation rate densities are more or less constant. The latter is consistent with previous UV and [O II] measurements. Below z<0.4, the SFR densities are consistent with several Ha, [O II], and UV measurements, but other measurements are a factor of two higher. For example, the z=0.066-0.092 Ha LF agrees with those of Jones & Bland-Hawthorn, but at z=0.24 and 0.40, their number density is higher by a factor of two. This discrepancy can be explained by cosmic variance.
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 -1.3pm0.2, 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.
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