Brian Siana

The MOSDEF Survey: Mass, Metallicity, and Star-formation Rate at z ~ 2.3

© 2015. The American Astronomical Society. All rights reserved. We present results on the z ∼ 2.3 mass-metallicity relation (MZR) using early observations from the MOSFIRE Deep Evolution Field survey. We use an initial sample of 87 star-forming galaxies with spectroscopic coverage of Hβ, [OIII] λ5007, Hα, and [NII] λ6584 rest-frame optical emission lines, and estimate the gas-phase oxygen abundance based on the N2 and O3N2 strong-line indicators. We find a positive correlation between stellar mass and metallicity among individual z ∼ 2.3 galaxies using both the N2 and O3N2 indicators. We also measure the emission-line ratios and corresponding oxygen abundances for composite spectra in bins of stellar mass. Among composite spectra, we find a monotonic increase in metallicity with increasing stellar mass, offset ∼0.15-0.3 dex below the local MZR. When the sample is divided at the median star-formation rate (SFR), we do not observe significant SFR dependence of the z ∼ 2.3 MZR among either individual galaxies or composite spectra. We furthermore find that z ∼ 2.3 galaxies have metallicities ∼0.1 dex lower at a given stellar mass and SFR than is observed locally. This offset suggests that high-redshift galaxies do not fall on the local fundamental metallicity relation among stellar mass, metallicity, and SFR, and may provide evidence of a phase of galaxy growth in which the gas reservoir is built up due to inflow rates that are higher than star-formation and outflow rates. However, robust conclusions regarding the gas-phase oxygen abundances of high-redshift galaxies await a systematic reappraisal of the application of locally calibrated metallicity indicators at high redshift.

THE MOSDEF SURVEY: MEASUREMENTS OF BALMER DECREMENTS AND THE DUST ATTENUATION CURVE AT REDSHIFTS z ∼ 1.4–2.6*

We present results on the dust attenuation curve of z~2 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Our sample consists of 224 star-forming galaxies with nebular spectroscopic redshifts in the range z= 1.36-2.59 and high S/N measurements of, or upper limits on, the H-alpha and H-beta emission lines obtained with Keck/MOSFIRE. We construct composite SEDs of galaxies in bins of specific SFR and Balmer optical depth in order to directly constrain the dust attenuation curve from the UV through near-IR for typical star-forming galaxies at high redshift. Our results imply an attenuation curve that is very similar to the SMC extinction curve at wavelengths redward of 2500 Angstroms. At shorter wavelengths, the shape of the curve is identical to that of the Calzetti relation, but with a lower normalization (R_V). Hence, the new attenuation curve results in SFRs that are ~20% lower, and log stellar masses that are 0.16 dex lower, than those obtained with the Calzetti attenuation curve. Moreover, we find that the difference in the reddening---and the total attenuation---of the ionized gas and stellar continuum correlates strongly with SFR, such that for dust-corrected SFRs larger than 20 Msun/yr assuming a Chabrier IMF, the nebular emission lines suffer an increasing degree of obscuration relative to the continuum. A simple model that can account for these trends is one in which the UV through optical stellar continuum is dominated by a population of less reddened stars, while the nebular line and bolometric luminosities become increasingly dominated by dustier stellar populations for galaxies with large SFRs, as a result of the increased dust enrichment that accompanies such galaxies. Consequently, UV- and SED-based SFRs may underestimate the total SFR at even modest levels of ~20 Msun/yr. [Abridged]

A REFINED ESTIMATE OF THE IONIZING EMISSIVITY FROM GALAXIES AT z {approx_equal} 3: SPECTROSCOPIC FOLLOW-UP IN THE SSA22a FIELD

We investigate the contribution of star-forming galaxies to the ionizing background at z ~ 3, building on previous work based on narrowband (NB3640) imaging in the SSA22a field. We use new Keck/LRIS spectra of Lyman break galaxies (LBGs) and narrowband-selected Lyα emitters (LAEs) to measure redshifts for 16 LBGs and 87 LAEs at z > 3.055, such that our NB3640 imaging probes the Lyman-continuum (LyC) region. When we include the existing set of spectroscopically confirmed LBGs, our total sample with z > 3.055 consists of 41 LBGs and 91 LAEs, of which 9 LBGs and 20 LAEs are detected in our NB3640 image. With our combined imaging and spectroscopic data sets, we critically investigate the origin of NB3640 emission for detected LBGs and LAEs. We remove from our samples three LBGs and three LAEs with spectroscopic evidence of contamination of their NB3640 flux by foreground galaxies and statistically model the effects of additional, unidentified foreground contaminants. The resulting contamination and LyC-detection rates, respectively, are 62% ± 13% and 8% ± 3% for our LBG sample, and 47% ± 10% and 12% ± 2% for our LAE sample. The corresponding ratios of non-ionizing UV to LyC flux density, corrected for intergalactic medium (IGM) attenuation, are 18.0^(+34.8)_(–7.4) for LBGs and 3.7^(+2.5)_(–1.1) for LAEs. We use these ratios to estimate the total contribution of star-forming galaxies to the ionizing background and the hydrogen photoionization rate in the IGM, finding values larger than, but consistent with, those measured in the Lyα forest. Finally, the measured UV to LyC flux-density ratios imply model-dependent LyC escape fractions of f^(LyC)_(esc) ~ 5%-7% for our LBG sample and f^(LyC)_(esc) ~ 10%-30% for our fainter LAE sample.

Ultra-faint Ultraviolet Galaxies at z ~ 2 behind the Lensing Cluster A1689: The Luminosity Function, Dust Extinction, and Star Formation Rate Density

We have obtained deep ultraviolet imaging of the lensing cluster A1689 with the WFC3/UVIS camera onboard the Hubble Space Telescope in the F275W (30 orbits) and F336W (4 orbits) filters. These images are used to identify z ~ 2 star-forming galaxies via their Lyman break, in the same manner that galaxies are typically selected at z ≥ 3. Because of the unprecedented depth of the images and the large magnification provided by the lensing cluster, we detect galaxies 100× fainter than previous surveys at this redshift. After removing all multiple images, we have 58 galaxies in our sample in the range –19.5 = 0.15 mag. We assume the stars in these galaxies are metal poor (0.2 Z_☉) compared to their brighter counterparts (Z_☉), resulting in bluer assumed intrinsic UV slopes and larger derived values for dust extinction. The total UV luminosity density at z ~ 2 is 4.31^(+0.68)_(-0.60) × 10^(26) erg s^(–1) Hz^(–1) Mpc^(–3), more than 70% of which is emitted by galaxies in the luminosity range of our sample. Finally, we determine the global star formation rate density from UV-selected galaxies at z ~ 2 (assuming a constant dust extinction correction of 4.2 over all luminosities and a Kroupa initial mass function) of 0.148^(+0.023)_(-0.020) M_☉ yr^(–1) Mpc^(–3), significantly higher than previous determinations because of the additional population of fainter galaxies and the larger dust correction factors.

DUST EXTINCTION FROM BALMER DECREMENTS OF STAR-FORMING GALAXIES AT 0.75 z 1.5 WITH HUBBLE SPACE TELESCOPE/WIDE-FIELD-CAMERA 3 SPECTROSCOPY FROM THE WFC3 INFRARED SPECTROSCOPIC PARALLEL SURVEY

Spectroscopic observations of H and H emission lines of 128 star-forming galaxies in the redshift range 0:75 z 1:5 are presented. These data were taken with slitless spectroscopy using the G102 and G141 grisms of the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of the WFC3 Infrared Spectroscopic Parallel (WISP) survey. Interstellar dust extinction is measured from stacked spectra that cover the Balmer decrement (H /H ). We present dust extinction as a function of H luminosity (down to 3 10 41 erg s 1 ), galaxy stellar mass (reaching 4 10 8 M ), and rest-frame H equivalent width. The faintest galaxies are two times fainter in H luminosity than galaxies previously studied at z 1:5. An evolution is observed where galaxies of the same H luminosity have lower extinction at higher redshifts, whereas no evolution is found within our error bars with stellar mass. The lower H luminosity galaxies in our sample are found to be consistent with no dust extinction. We

Physical properties of emission-line galaxies at z ∼ 2 from near-infrared spectroscopy with magellan fire

We present results from near-infrared spectroscopy of 26 emission-line galaxies at z ~ 2.2 and z ~ 1.5 obtained with the Folded-port InfraRed Echellette (FIRE) spectrometer on the 6.5 m Magellan Baade telescope. The sample was selected from the WFC3 Infrared Spectroscopic Parallels survey, which uses the near-infrared grism of the Hubble Space Telescope Wide Field Camera 3 (WFC3) to detect emission-line galaxies over 0.3 ≾z ≾2.3. Our FIRE follow-up spectroscopy (R ~ 5000) over 1.0-2.5 μm permits detailed measurements of the physical properties of the z ~ 2 emission-line galaxies. Dust-corrected star formation rates for the sample range from ~5-100 M☉ yr^(–1) with a mean of 29 M☉ yr^(–1). We derive a median metallicity for the sample of 12 + log(O/H) = 8.34 or ~0.45 Z☉. The estimated stellar masses range from ~10^(8.5)-10^(9.5) M☉, and a clear positive correlation between metallicity and stellar mass is observed. The average ionization parameter measured for the sample, log U ≈–2.5, is significantly higher than what is found for most star-forming galaxies in the local universe, but similar to the values found for other star-forming galaxies at high redshift. We derive composite spectra from the FIRE sample, from which we measure typical nebular electron densities of ~100-400 cm^(–3). Based on the location of the galaxies and composite spectra on diagnostic diagrams, we do not find evidence for significant active galactic nucleus activity in the sample. Most of the galaxies, as well as the composites, are offset diagram toward higher [O III]/Hβ at a given [N II]/Hα, in agreement with other observations of z ≳1 star-forming galaxies, but composite spectra derived from the sample do not show an appreciable offset from the local star-forming sequence on the [O III]/Hβ versus [S II]/Hα diagram. We infer a high nitrogen-to-oxygen abundance ratio from the composite spectrum, which may contribute to the offset of the high-redshift galaxies from the local star-forming sequence in the [O III]/Hβ versus [N II]/Hα diagram. We speculate that the elevated nitrogen abundance could result from substantial numbers of Wolf-Rayet stars in starbursting galaxies at z ~ 2.

Ultraviolet emission lines in young low-mass galaxies at z ≃ 2: physical properties and implications for studies at z > 7

We present deep spectroscopy of 17 very low mass (M* similar or equal to 2.0 x 10(6)-1.4 x 10(9) M-circle dot) and low luminosity (M-UV similar or equal to -13.7 to -19.9) gravitationally lensed ga ...

THE MOSDEF SURVEY: EXCITATION PROPERTIES OF z ∼ 2.3 STAR-FORMING GALAXIES*

Author(s): Shapley, AE; Reddy, NA; Kriek, M; Freeman, WR; Sanders, RL; Siana, B; Coil, AL; Mobasher, B; Shivaei, I; Price, SH; Groot, LD | Abstract:

Spectroscopic detections of C iii] λ1909 Å at z ≃ 6–7: a new probe of early star-forming galaxies and cosmic reionization

Deep spectroscopic observations of z ≳ 6.5 galaxies have revealed a marked decline with increasing redshift in the detectability of Ly α emission. While this may offer valuable insight into the end of the reionization process, it presents a challenge to the detailed spectroscopic study of bright photometrically-selected distant sources now being found via deep Hubble Space Telescope imaging, and particularly those highly magnified sources viewed through foreground lensing clusters. In this paper, we demonstrate the validity of a new way forward via the detection of an alternative diagnostic line, Cu2009iii] λ1909 A, seen in spectroscopic exposures of a star-forming galaxy at zLyα = 6.029. We also report tentative detection of Cu2009iii] λ1909 A in a galaxy at z_Lyα = 7.213. The former 3.3σ detection is based on a 3.5 h XShooter spectrum of a bright (J125 = 25.2) gravitationally-lensed galaxy behind the cluster Abell 383. The latter 2.8σ detection is based on a 4.2 h MOSFIRE spectra of one of the most distant spectroscopically confirmed galaxies, GN-108036, with J140 = 25.2. Both targets were chosen for their continuum brightness and previously-known redshift (based on Ly α), ensuring that any Cu2009iii] emission would be located in a favourable portion of the near-infrared sky spectrum. Since the availability of secure Ly α redshifts significantly narrows the wavelength range where Cu2009iii] is sought, this increases confidence in these, otherwise, low-signal-to-noise ratio detections. We compare our Cu2009iii] and Ly α equivalent widths in the context of those found at z ≃ 2 from earlier work and discuss the motivation for using lines other than Ly α to study galaxies in the reionization era.

ON THE DETECTION OF IONIZING RADIATION ARISING FROM STAR-FORMING GALAXIES AT REDSHIFT z ∼ 3-4: LOOKING FOR ANALOGS OF “STELLAR RE-IONIZERS”

We use the spatially resolved, multi-band photometry in the GOODS South field acquired by the CANDELS project to constrain the nature of candidate Lyman continuum (LyC) emitters at redshift z ~ 3.7 identified using ultradeep imaging below the Lyman limit (1σ limit of ≈30 AB in a 2 diameter aperture). In 19 candidates out of a sample of 20 with flux detected at >3σ level, the light centroid of the candidate LyC emission is offset from that of the Lyman break galaxy (LBG) by up to 15. We fit the spectral energy distribution of the LyC candidates to spectral population synthesis models to measure photometric redshifts and the stellar population parameters. We also discuss the differences in the UV colors between the LBG and the LyC candidates, and how to estimate the escape fraction of ionizing radiation (f esc) in cases, like in most of our galaxies, where the LyC emission is spatially offset from the host galaxy. In all but one case we conclude that the candidate LyC emission is most likely due to lower redshift interlopers. Based on these findings, we argue that the majority of similar measurements reported in the literature need further investigation before it can be firmly concluded that LyC emission is detected. Our only surviving LyC candidate is an LBG at z = 3.795, which shows the bluest (B – V) color among LBGs at similar redshift, a stellar mass of M ~ 2 × 109 M ☉, weak interstellar absorption lines, and a flat UV spectral slope with no Lyα in emission. We estimate its f esc to be in the range 25%-100%, depending on the dust and intergalactic attenuation.