Astronomy

Permanent link for this communityhttps://hdl.handle.net/2022/12958

Browse

Browsing Astronomy by Subject "galaxies: high-redshift"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Average metallicity and star formation rate of Ly$\alpha$ emitters probed by a triple narrowband survey*
    (The American Astronomical Society, 2012) Nakajima, K.; Ouchi, M.; Shimasaku, K.; Ono, Y.; Lee, J.C.; Foucad, S.; Ly, C.; Dale, D.A.; Salim, S.; Finn, R.; Almaini, O.; Okamura, S.
    We present the average metallicity and star formation rate (SFR) of $Ly\alpha$ emitters (LAEs) measured from our large-area survey with three narrowband (NB) filters covering the $Ly\alpha, \big[O II\big]\lambda3727$, and $\text{H}\alpha+\big[\text{N} II\big]$ lines of LAEs at $z = 2.2$. We select 919 $z = 2.2$ LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS spectroscopy. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM near-infrared NB filters whose central wavelengths are matched to redshifted $\big[\text{O} II\big]$ and $\text{H}\alpha$ nebular lines, respectively. By stacking the near-infrared images of the LAEs, we successfully obtain average nebular-line fluxes of LAEs, the majority of which are too faint to be identified individually by NB imaging or deep spectroscopy. The stacked object has an Hα luminosity of $1.7 × 10^{42} erg s^{–1}$ corresponding to an SFR of $14 M_{\odot} yr6{–1}$. We place, for the first time, a firm lower limit to the average metallicity of LAEs of $Z \gtrsim 0.09 Z_{\odot} \big(2\sigma\big)$ based on the $\big[\text{O} II\big]/\big(\text{H}\alpha+\big[\text{N} II\big]\big)$ index together with photoionization models and empirical relations. This lower limit of metallicity rules out the hypothesis that LAEs, so far observed at $z \sim 2$, are extremely metal-poor $\big(Z < 2 × 10–2 Z_{\odot}\big)$ galaxies at the $4\sigma$ level. This limit is higher than a simple extrapolation of the observed mass-metallicity relation of $z \sim 2$ UV-selected galaxies toward lower masses $\big(5 × 108 M_{\odot}\big)$, but roughly consistent with a recently proposed fundamental mass-metallicity relation when the LAEs' relatively low SFR is taken into account. The $\text{H}\alpha$ and $\text{Ly}\alpha$ luminosities of our NB-selected LAEs indicate that the escape fraction of $\text{Ly}\alpha$ photons is $\sim12%-30%$, much higher than the values derived for other galaxy populations at $z \sim 2$.
  • Thumbnail Image
    Item
    Calibrating the star formation rate at $z \sim 1$ from optical data
    (The American Astronomical Society, 2012) Mostek, N.; Coil, A.L.; Moustakas, J.; Salim, S.; Weiner, B.J.
    We present a star formation rate (SFR) calibration based on optical data that is consistent with average observed rates in both the red and blue galaxy populations at $z \sim 1$. The motivation for this study is to calculate SFRs for DEEP2 Redshift Survey galaxies in the $0.7 < z < 1.4$ redshift range, but our results are generally applicable to similar optically selected galaxy samples without requiring UV or IR data. Using SFR fits from UV/optical spectral energy distributions (SEDs) in the All-Wavelength Extended Groth Strip International Survey, we explore the behavior of rest-frame B-band magnitude, observed [O II] luminosity, and rest-frame color with SED-fit SFR for both red sequence and blue cloud galaxies. The resulting SFR calibration is based on three optical-band observables: $M_{B} , \big(U – B\big)$, and $\big(B – V\big)$. The best-fit linear relation produces residual errors of 0.3 dex rms scatter for the full color-independent sample with minimal correlated residual error in $L\big[O_{II}\big]$ or stellar mass. We then compare the calibrated $z \sim 1$ SFRs to two diagnostics that use $L\big[O_{II}\big]$ as a tracer in local galaxies and correct for dust extinction at intermediate redshifts through either galaxy $B$-band luminosity or stellar mass. We find that an$ L\big[O_{II}\big]-M_{B}$ SFR calibration commonly used in the literature agrees well with our calculated SFRs after correcting for the average B-band luminosity evolution in $L_{*}$ galaxies. However, we find better agreement with a local $L\big[O_{II}\big]$-based SFR calibration that includes stellar mass to correct for reddening effects, indicating that stellar mass is a better tracer of dust extinction for all galaxy types and less affected by systematic evolution than galaxy luminosity from $z = 1$ to the current epoch.