Alice E. Shapley

University of California, Los Angeles, Los Ángeles, California, United States

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Publications (188)801.93 Total impact

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    ABSTRACT: We present results on the SFR-$M_*$ relation (i.e., the "main sequence") among star-forming galaxies at $1.37\leq z \leq2.61$ using the MOSFIRE Deep Evolution Field (MOSDEF) survey. Based on a sample of 261 star-forming galaxies with observations of H$\alpha$ and H$\beta$ emission lines, we have estimated robust dust-corrected instantaneous star-formation rates (SFRs) over a large dynamic range in stellar mass ($\sim 10^{9.0}-10^{11.5}M_\odot$). We find a tight correlation between SFR(H$\alpha$) and $M_*$ with an intrinsic scatter of 0.36 dex, 0.05 dex larger than that of UV-based SFRs. This increased scatter is consistent with predictions from numerical simulations of 0.03 - 0.1 dex, and is attributed to H$\alpha$ more accurately tracing SFR variations. The slope of the $\log(\text{SFR})-\log(M_*)$ relation, using SFR(H$\alpha$), at $1.4< z<2.6$ and over the stellar mass range of $10^{9.5}$ to $10^{11.5}M_\odot$ is $0.65\pm 0.09$. We find that different assumptions for the dust correction, such as using the stellar $E(B-V)$ with a Calzetti et al. (2000) attenuation curve, as well as the sample biases against red and dusty star-forming galaxies at large masses, could yield steeper slopes. Moreover, not correcting the Balmer emission line fluxes for the underlying Balmer absorption results in overestimating the dust extinction of H$\alpha$ and SFR(H$\alpha$) at the high-mass end by 2.1 (2.5) at $10^{10.6} M_\odot$ ($10^{11.1} M_\odot$) and artificially increases the slope of the main-sequence. The shallower main-sequence slope found here compared to that of galaxy evolution simulations may be indicative of different feedback processes governing the low- and/or high-mass end of the main sequence.
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    David R. Law · Alice E. Shapley · Jade Checlair · Charles C. Steidel
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    ABSTRACT: We use Hubble Space Telescope Wide-Field Camera 3 (HST/WFC3) rest-frame optical imaging to select a pilot sample of star-forming galaxies in the redshift range z = 2.00-2.65 whose multi-component morphologies are consistent with expectations for major mergers. We follow up this sample of major merger candidates with Keck/NIRSPEC longslit spectroscopy obtained in excellent seeing conditions (FWHM ~ 0.5 arcsec) to obtain Halpha-based redshifts of each of the morphological components in order to distinguish spectroscopic pairs from false pairs created by projection along the line of sight. Of six pair candidates observed, companions (estimated mass ratios 5:1 and 7:1) are detected for two galaxies down to a 3sigma limiting emission-line flux of ~ 10^{-17} erg/s/cm2. This detection rate is consistent with a ~ 50% false pair fraction at such angular separations (1-2 arcsec), and with recent claims that the star-formation rate (SFR) can differ by an order of magnitude between the components in such mergers. The two spectroscopic pairs identified have total SFR, SFR surface densities, and stellar masses consistent on average with the overall z ~ 2 star forming galaxy population.
    The Astrophysical Journal 07/2015; 808(2). DOI:10.1088/0004-637X/808/2/160 · 6.28 Impact Factor
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    ABSTRACT: We present $U_{336}V_{606}J_{125}H_{160}$ follow-up $HST$ observations of 16 $z\sim3$ candidate LyC emitters in the HS1549+1933 field. With these data, we obtain high spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 1500\AA\ to 900\AA\ flux-density ratio of $(F_{UV}/F_{LyC})_{obs}=4.0\pm2.0$, compatible with predictions from stellar population synthesis models. Neglecting IGM absorption, this ratio corresponds to lower limits to the relative (absolute) escape fraction of $f_{esc,rel}^{MD5b}=75\%\pm38\%$ ($f_{esc,abs}^{MD5b}=14\%\pm7\%$). The stellar population fit to MD5b indicates an age of $\lesssim50$Myr, which is in the youngest 10% of the $HST$ sample and the youngest third of typical $z\sim3$ Lyman break galaxies, and may be a contributing factor to its LyC detection. We obtain a revised, contamination-free estimate for the comoving specific ionizing emissivity at $z=2.85$, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. Our results show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from $z\sim3$ star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination of foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in high-redshift galaxies.
    The Astrophysical Journal 06/2015; 810(2). DOI:10.1088/0004-637X/810/2/107 · 6.28 Impact Factor
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    ABSTRACT: Narrowband imaging of the rest-frame Lyman continuum (LyC) of galaxies at has produced a large number of candidate LyC-emitting galaxies. These samples are contaminated by galaxies at lower redshift. To better understand LyC escape, we need an uncontaminated sample of galaxies that emit strongly in the LyC. Here we present deep Hubble Space Telescope imaging of five bright galaxies at that had previously been identified as candidate LyC emitters with ground-based images. The WFC3 F336W images probe the LyC of galaxies at and provide an order-of-magnitude increase in spatial resolution over ground-based imaging. The non-ionizing UV images often show multiple galaxies (or components) within of the candidate LyC emission seen from the ground. In each case, only one of the components is emitting light in the F336W filter, which would indicate LyC escape if that component is at . We use Keck/NIRSPEC near-IR spectroscopy to measure redshifts of these components to distinguish LyC emitters from foreground contamination. We find that two candidates are low-redshift contaminants, one candidate had a previously misidentified redshift, and the other two cannot be confirmed as LyC emitters. The level of contamination is consistent with previous estimates. For the galaxies with , we derive strong limits on the relative escape fraction between 0.07 and 0.09. We still do not have a sample of definitive LyC emitters, and a much larger study of low-luminosity galaxies is required. The combination of high-resolution imaging and deep spectroscopy is critical for distinguishing LyC emitters from foreground contaminants.
    The Astrophysical Journal 05/2015; 804(1). DOI:10.1088/0004-637X/804/1/17 · 6.28 Impact Factor
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    ABSTRACT: Most present-day galaxies with stellar masses $\geq10^{11}$ solar masses show no ongoing star formation and are dense spheroids. Ten billion years ago, similarly massive galaxies were typically forming stars at rates of hundreds solar masses per year. It is debated how star formation ceased, on which timescales, and how this "quenching" relates to the emergence of dense spheroids. We measured stellar mass and star-formation rate surface density distributions in star-forming galaxies at redshift 2.2 with $\sim1$ kiloparsec resolution. We find that, in the most massive galaxies, star formation is quenched from the inside out, on timescales less than 1 billion years in the inner regions, up to a few billion years in the outer disks. These galaxies sustain high star-formation activity at large radii, while hosting fully grown and already quenched bulges in their cores.
    Science 04/2015; 348(6232). DOI:10.1126/science.1261094 · 31.48 Impact Factor
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    ABSTRACT: 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]
    The Astrophysical Journal 04/2015; 806(2). DOI:10.1088/0004-637X/806/2/259 · 6.28 Impact Factor
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    ABSTRACT: We use a sample of 262 spectroscopically confirmed star-forming galaxies at redshifts $2.08\leq z\leq 2.51$ to compare H$\alpha$, UV, and IR star-formation-rate diagnostics and to investigate the dust properties of the galaxies. At these redshifts, the H$\alpha$ line shifts to the $K_{s}$-band. By comparing $K_{s}$-band photometry to underlying stellar population model fits to other UV, optical, and near-infrared data, we infer the H$\alpha$ flux for each galaxy. We obtain the best agreement between H$\alpha$- and UV-based SFRs if we assume that the ionized gas and stellar continuum are reddened by the same value and that the Calzetti attenuation curve is applied to both. Aided with MIPS 24$\mu$m data, we find that an attenuation curve steeper than the Calzetti curve is needed to reproduce the observed IR/UV ratios of galaxies younger than 100 Myr. Furthermore, using the bolometric star-formation rate inferred from the UV and mid-IR data (SFR$_{IR}$+SFR$_{UV}$), we calculated the conversion between the H$\alpha$ luminosity and SFR to be $(7.5\pm1.3) \times 10^{-42}$ for a Salpeter IMF, which is consistent with the Kennicutt (1998) conversion. The derived conversion factor is independent of any assumption of the dust correction and is robust to stellar population model uncertainties.
    The Astrophysical Journal 03/2015; 804(2). DOI:10.1088/0004-637X/804/2/149 · 6.28 Impact Factor
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    ABSTRACT: We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on rest-frame optical active galactic nucleus (AGN) identification and completeness at z ~ 2.3. With our sample of 50 galaxies and 10 X-ray and IR-selected AGNs with measured Hβ, [O III], Hα, and N II emission lines, we investigate the location of AGNs in the BPT, MEx (mass-excitation), and CEx (color-excitation) diagrams. We find that th BPT diagram works well to identify AGNs at z ~ 2.3 and that the z ~ 0 AGN/star-forming galaxy classifications do not need to shift substantially at z ~ 2.3 to robustly separate these populations. However, the MEx diagram fails to identify all of the AGN identified in the BPT diagram, and the CEx diagram is substantially contaminated at high redshift. We further show that AGN samples selected using the BPT diagram have selection biases in terms of both host stellar mass and stellar population, in that AGNs in low mass and/or high specific star formation rate galaxies are difficult to identify using the BPT diagram. These selection biases become increasingly severe at high redshift, such that optically selected AGN samples at high redshift will necessarily be incomplete. We also find that the gas in the narrow-line region appears to be more enriched than gas in the host galaxy for at least some MOSDEF AGNs. However, AGNs at z ~ 2 are generally less enriched than local AGNs with the same host stellar mass.
    The Astrophysical Journal 03/2015; 801(1). DOI:10.1088/0004-637X/801/1/35 · 6.28 Impact Factor
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    ABSTRACT: Narrow-band imaging of the rest-frame Lyman continuum (LyC) of galaxies at z~3.1 has produced a large number of candidate LyC-emitting galaxies. These samples are contaminated by galaxies at lower redshift. To better understand LyC escape, we need an uncontaminated sample of galaxies that emit strongly in the LyC. Here we present deep Hubble imaging of five bright galaxies at z~3.1 that had previously been identified as candidate LyC-emitters with ground-based images. The WFC3 F336W images probe the LyC of galaxies at z>3.06 and provide an order-of-magnitude increase in spatial resolution over ground-based imaging. The non-ionizing UV images often show multiple galaxies (or components) within ~1'' of the candidate LyC emission seen from the ground. In each case, only one of the components is emitting light in the F336W filter, which would indicate LyC escape if that component is at z>3.06. We use Keck/NIRSPEC near-IR spectroscopy to measure redshifts of these components to distinguish LyC-emitters from foreground contamination. We find that two candidates are low redshift contaminants, one candidate had a previously misidentified redshift, and the other two cannot be confirmed as LyC-emitters. The level of contamination is consistent with previous estimates. For the galaxies with z>3.06, we derive strong 1 sigma limits on the relative escape fraction between 0.07 and 0.09. We still do not have a sample of definitive LyC-emitters, and a much larger study of low luminosity galaxies is required. The combination of high resolution imaging and deep spectroscopy is critical for distinguishing LyC-emitters from foreground contaminants.
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    ABSTRACT: In this paper we present the MOSFIRE Deep Evolution Field (MOSDEF) survey. The MOSDEF survey aims to obtain moderate-resolution (R=3000-3650) rest-frame optical spectra (~3700-7000 Angstrom) for ~1500 galaxies at 1.37<z<3.80 in three well-studied CANDELS fields: AEGIS, COSMOS, and GOODS-N. Targets are selected in three redshift intervals: 1.37<z<1.70, 2.09<z<2.61, and 2.95<z<3.80, down to fixed H_AB (F160W) magnitudes of 24.0, 24.5 and 25.0, respectively, using the photometric and spectroscopic catalogs from the 3D-HST survey. We target both strong nebular emission lines (e.g., [OII], Hbeta, [OIII], 5008, Halpha, [NII], and [SII]) and stellar continuum and absorption features (e.g., Balmer lines, Ca-II H and K, Mgb, 4000 Angstrom break). Here we present an overview of our survey, the observational strategy, the data reduction and analysis, and the sample characteristics based on spectra obtained during the first 24 nights. To date, we have completed 21 masks, obtaining spectra for 591 galaxies. For ~80% of the targets we identify and measure multiple emission or absorption lines. In addition, we confirm 55 additional galaxies, which were serendipitously detected. The MOSDEF galaxy sample includes unobscured star-forming, dusty star-forming, and quiescent galaxies and spans a wide range in stellar mass (~10^9-10^11.5 Msol) and star formation rate (~0-10^4 Msol/yr). The spectroscopically confirmed sample is roughly representative of an H-band limited galaxy sample at these redshifts. With its large sample size, broad diversity in galaxy properties, and wealth of available ancillary data, MOSDEF will transform our understanding of the stellar, gaseous, metal, dust, and black hole content of galaxies during the time when the universe was most active.
    The Astrophysical Journal Supplement Series 12/2014; 218(2). DOI:10.1088/0067-0049/218/2/15 · 14.14 Impact Factor
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    ABSTRACT: We present the analysis of HST $J$- and $H$-band imaging for 29 galaxies on the star-forming main sequence at $z\sim2$, which have Adaptive Optics VLT SINFONI integral field spectroscopy from our SINS/zC-SINF program. The SINFONI H$\alpha$ data resolve the on-going star-formation and the ionized gas kinematics on scales of $1-2$ kpc; the near-IR images trace the galaxies' rest-frame optical morphologies and distributions of stellar mass in old stellar populations at a similar resolution. The global light profiles of most galaxies show disk-like properties well described by a single S\'ersic profile with $n\sim1$, with only $\sim15%$ requiring a high $n>3$ S\'ersic index, all more massive than $10^{10}M_\odot$. In bulge+disk fits, about $40%$ of galaxies have a measurable bulge component in the light profiles, with $\sim15%$ showing a substantial bulge-to-total ratio $B/T\ge0.3$. This is a lower limit to the frequency of $z\sim2$ massive galaxies with a developed bulge component in stellar mass because it could be hidden by dust and/or outshined by a thick actively star-forming disk component. The galaxies' rest-optical half-light radii range between $1-7$ kpc, with a median of 2.1 kpc, and lie slightly above the size-mass relation at these epochs reported in the literature. This is attributed to differences in sample selection and definitions of size and/or mass measurements. The $(u-g)_{rest}$ color gradient and scatter within individual $z\sim2$ massive galaxies with $\ge10^{11}M_\odot$ are as high as in $z=0$ low-mass, late-type galaxies, and are consistent with the high star-formation rates of massive $z\sim2$ galaxies being sustained at large galactocentric distances.
    The Astrophysical Journal 11/2014; 802(2). DOI:10.1088/0004-637X/802/2/101 · 6.28 Impact Factor
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    ABSTRACT: We study the Lyα profiles of 36 spectroscopically detected Lyα-emitters (LAEs) at z ~ 2-3, using Keck MOSFIRE to measure systemic redshifts and velocity dispersions from rest-frame optical nebular emission lines. The sample has a median optical magnitude , and ranges from to , corresponding to rest-frame UV absolute magnitudes M UV –22 to M UV > –18.2. Dynamical masses range from M dyn < 1.3 × 108M ☉ to M dyn = 6.8 × 109M ☉, with a median value of M dyn = 6.3 × 108M ☉. Thirty of the 36 Lyα emission lines are redshifted with respect to the systemic velocity with at least 1σ significance, and the velocity offset with respect to systemic Δv Lyα is correlated with the -band magnitude, M UV, and the velocity dispersion measured from nebular emission lines with >3σ significance: brighter galaxies with larger velocity dispersions tend to have larger values of Δv Lyα. We also make use of a comparison sample of 122 UV-color-selected galaxies at z ~ 2, all with Lyα emission and systemic redshifts measured from nebular emission lines. Using the combined LAE and comparison samples for a total of 158 individual galaxies, we find that Δv Lyα is anti-correlated with the Lyα equivalent width with 7σ significance. Our results are consistent with a scenario in which the Lyα profile is determined primarily by the properties of the gas near the systemic redshift; in such a scenario, the opacity to Lyα photons in lower mass galaxies may be reduced if large gaseous disks have not yet developed and if the gas is ionized by the harder spectrum of young, low metallicity stars.
    The Astrophysical Journal 10/2014; 795(1):33. DOI:10.1088/0004-637X/795/1/33 · 6.28 Impact Factor
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    ABSTRACT: We present results on the excitation properties of z~2.3 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) Survey. With its coverage of the full suite of strongrest-frame optical emission lines, MOSDEF provides an unprecedented view of the rest-frame optical spectra of a representative sample of distant star-forming galaxies. We investigate the locations of z~2.3 MOSDEF galaxies in multiple emission-line diagnostic diagrams. These include the [OIII]/Hb vs. [NII]/Ha (O3N2) and [OIII]/Hb vs. [SII]/Ha (O3S2) "BPT" diagrams, as well as the O_32 vs. R_23 (O32R23) excitation diagram. We recover the well-known offset in the star-forming sequence of high-redshift galaxies in the O3N2 BPT diagram relative to Sloan Digital Sky Survey star-forming galaxies. However, the shift for our rest-frame optically selected sample is less significant than for rest-frame-UV selected and emission-line selected galaxies at z~2. Furthermore, we find that the offset is mass-dependent, only appearing within the low-mass half of the z~2.3 MOSDEF sample, where galaxies are shifted towards higher [NII]/Ha at fixed [OIII]/Hb. Within the O3S2 and O32R23 diagrams, we find that z~2.3 galaxies are distributed like local ones, and therefore attribute the shift in the O3N2 BPT diagram to elevated N/O abundance ratios among lower-mass (M_*<10^10 M_sun) high-redshift galaxies. The variation in N/O ratios calls into question the use at high redshift of oxygen abundance indicators based on nitrogen lines, but the apparent invariance with redshift of the excitation sequence in the O32R23 diagram paves the way for using the combination of O_{32 and R_23 as an unbiased metallicity indicator over a wide range in redshift. This indicator will allow for an accurate characterization of the shape and normalization of the mass-metallicity relationship over more than 10 Gyr.
    The Astrophysical Journal 09/2014; 801(2). DOI:10.1088/0004-637X/801/2/88 · 6.28 Impact Factor
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    ABSTRACT: We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on rest-frame optical AGN identification and completeness at z~2.3. With our sample of 50 galaxies and 10 X-ray and IR-selected AGN with measured H-beta, [OIII], H-alpha, and [NII] emission lines, we investigate the location of AGN in the BPT, MEx (mass-excitation), and CEx (color-excitation) diagrams. We find that the BPT diagram works well to identify AGN at z~2.3 and that the z~0 AGN/star-forming galaxy classifications do not need to shift substantially at z~2.3 to robustly separate these populations. However, the MEx diagram likely suffers from contamination from star-forming galaxies at high redshift and fails to identify all of the AGN identified in the BPT diagram. The CEx diagram does not work well at high redshift. We further show that AGN samples selected using the BPT diagram have strong selection biases in terms of both host stellar mass and stellar population, in that AGN in low mass and/or high specific star formation rate galaxies can not be identified using the BPT diagram at either low or high redshift. These selection biases become increasingly severe at high redshift, such that optically-selected AGN samples at high redshift will necessarily be incomplete. We also find that the gas in the narrow-line region appears to be more enriched than gas in the host galaxy for at least some MOSDEF AGN. However, AGN at z~2 are generally less enriched than local AGN with the same host stellar mass.
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    ABSTRACT: We study the Lya profiles of 36 Lya-emitters (LAEs) at z~2-3, taking advantage of the sensitivity and efficiency of Keck MOSFIRE to measure systemic redshifts from rest-frame optical nebular emission lines. The galaxies were selected via narrowband imaging of their Lya emission, and have spectroscopic measurements of their Lya profiles from Keck LRIS. The LAE sample has a median optical magnitude R = 26.0, and ranges from R = 23 to R > 27, corresponding to rest-frame UV absolute magnitudes M_UV = -22 to M_UV > -18.2. The vast majority of Lya emission lines in this sample are redshifted with respect to the systemic velocity, and the velocity offset with respect to systemic dv_Lya is correlated with R-band magnitude, M_UV, and the velocity dispersion measured from nebular emission lines with >3 sigma significance: brighter galaxies with larger velocity dispersions tend to have larger values of dv_Lya. We also make use of a comparison sample of 122 UV-color-selected R < 25.5 galaxies at z~2, all with Lya emission and systemic redshifts measured from nebular emission lines. Using the combined LAE and comparison samples for a total of 158 individual galaxies, we find that dv_Lya is anti-correlated with the Lya equivalent width with 7 sigma significance. Our results are consistent with a scenario in which the Lya profile is determined primarily by the properties of the gas near the systemic redshift; in such a scenario, the opacity to Lya photons in lower mass galaxies may be reduced if large gaseous disks have not yet developed and if the gas is ionized by the harder spectrum of young, low metallicity stars. [Abridged]
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    ABSTRACT: We present results on the z~2.3 mass-metallicity relation (MZR) using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. We use an initial sample of 87 star-forming galaxies with spectroscopic coverage of H\beta, [OIII]\lambda 5007, H\alpha, and [NII]\lambda 6584 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 Astrophysical Journal 08/2014; 799(2). DOI:10.1088/0004-637X/799/2/138 · 6.28 Impact Factor
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    ABSTRACT: We present initial results of a large near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey (KBSS) using the recently-commissioned MOSFIRE instrument on the Keck 1 10m telescope. We focus on 179 galaxies with redshifts 2.0 < z < 2.6, most of which have high-quality MOSFIRE spectra in both H and K-band atmospheric windows, allowing sensitive measurements of [OIII]4960,5008, H-beta, [NII]6585, and H-alpha emission lines. We show unambiguously that the locus of z~2.3 galaxies in the "BPT" nebular diagnostic diagram exhibits an almost entirely disjoint, yet similarly tight, relationship between the line ratios [NII]/Halpha and [OIII]/Hbeta as compared to local galaxies. We argue that the offset of the z~2.3 BPT locus relative to that at z~0 is caused primarily by higher excitation (driven by both higher ionization parameter and harder stellar ionizing radiation field) than applies to most local galaxies. Also unlike nearby counterparts, a z~2.3 galaxy's position along the BPT locus is surprisingly insensitive to gas-phase O/H. The observed emission line ratios are most easily reproduced by models in which the stellar ionizing radiation field has Teff=50000-60000 K, gas-phase O/H in the range 0.2 < Z/Zsun < 1.0, and gas-phase N/O close to solar. Such high sustained Teff are not easily produced by standard population synthesis models, but are expected if massive binaries and/or rapid stellar rotation are important for the evolution of main sequence O-stars in typical high-redshift galaxies. We assess the applicability of commonly-used strong line indices for estimating gas-phase metallicities of high redshift galaxies, as well as their likely systematic biases. The empirical correlation between M* and inferred metallicity (the "MZR") at z~2.3 is as tight as for local galaxy samples, but is offset to lower metallicity (at all M*) by ~0.35 dex (abridged)
    The Astrophysical Journal 05/2014; 795(2). DOI:10.1088/0004-637X/795/2/165 · 6.28 Impact Factor
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    ABSTRACT: The MOSFIRE Deep Evolution Field (MOSDEF) survey is using the MOSFIRE instrument on the Keck I telescope to obtain more than 1700 rest-frame optical spectra of galaxies from redshift 1.5 to 3.6 over four years. We are using the first 200 spectra to investigate the prevalence of outflows measured in emission in broad components of the nebular emission lines. We create stacks of galaxies based on properties such as star formation rate, stellar mass, and star formation rate surface density (controlling for the presence of AGNs) in order to study how outflow strength depends on these factors. Additionally, we will stack spectra by redshift to understand how outflows change over time. We will present the results of two component (narrow and broad) fits to the nebular emission and compare to theoretical predictions of outflow efficiency.
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    ABSTRACT: We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming (SFR ~6-7 Msol/yr) galaxy at z=1.433, magnified by a factor of 11.5 by the gravitational lensing produced by a massive foreground galaxy at z=0.741. We analysed the integrated physical properties of the HII regions of CSWA 20 using temperature- and density-sensitive emission lines. We find the abundance of oxygen to be ~1/7 of solar, while carbon is ~50 times less abundant than in the Sun. The unusually low C/O ratio may be an indication of a particularly rapid timescale of chemical enrichment. The wide wavelength coverage of X-shooter gives us access to five different methods for determining the metallicity of CSWA 20, three based on emission lines from HII regions and two on absorption features formed in the atmospheres of massive stars. All five estimates are in agreement, within the factor of ~2 uncertainty of each method. The interstellar medium of CSWA 20 only partially covers the star-forming region as viewed from our direction; in particular, absorption lines from neutrals and first ions are exceptionally weak. We find evidence for large-scale outflows of the interstellar medium (ISM) with speeds of up 750 km/s, similar to the values measured in other high-z galaxies sustaining much higher rates of star formation.
    Monthly Notices of the Royal Astronomical Society 11/2013; 440(2). DOI:10.1093/mnras/stu287 · 5.23 Impact Factor
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    ABSTRACT: We report the detection of ubiquitous powerful nuclear outflows in massive (> 10^11 Msun) z~2 star-forming galaxies (SFGs), which are plausibly driven by an Active Galactic Nucleus (AGN). The sample consists of the eight most massive SFGs from our SINS/zC-SINF survey of galaxy kinematics with the imaging spectrometer SINFONI, six of which have sensitive high-resolution adaptive optics (AO) assisted observations. All of the objects are disks hosting a significant stellar bulge. The spectra in their central regions exhibit a broad component in Halpha and forbidden [NII] and [SII] line emission, with typical velocity FWHM ~ 1500 km/s, [NII]/Halpha ratio ~ 0.6, and intrinsic extent of 2 - 3 kpc. These properties are consistent with warm ionized gas outflows associated with Type 2 AGN, the presence of which is confirmed via independent diagnostics in half the galaxies. The data imply a median ionized gas mass outflow rate of ~ 60 Msun/yr and mass loading of ~ 3. At larger radii, a weaker broad component is detected but with lower FWHM ~ 485 km/s and [NII]/Halpha ~ 0.35, characteristic for star formation-driven outflows as found in the lower-mass SINS/zC-SINF galaxies. The high inferred mass outflow rates and frequent occurrence suggest the nuclear outflows efficiently expel gas out of the centers of the galaxies with high duty cycles, and may thus contribute to the process of star formation quenching in massive galaxies. Larger samples at high masses will be crucial to confirm the importance and energetics of the nuclear outflow phenomenon, and its connection to AGN activity and bulge growth.
    The Astrophysical Journal 11/2013; 787(1). DOI:10.1088/0004-637X/787/1/38 · 6.28 Impact Factor

Publication Stats

10k Citations
801.93 Total Impact Points

Institutions

  • 2008–2014
    • University of California, Los Angeles
      • • Department of Physics and Astronomy
      • • Division of Astronomy & Astrophysics
      Los Ángeles, California, United States
  • 2013
    • Pierre and Marie Curie University - Paris 6
      • Institut d'astrophysique de Paris
      Lutetia Parisorum, Île-de-France, France
  • 2012
    • University of Toronto
      • Dunlap Institute for Astronomy and Astrophysics
      Toronto, Ontario, Canada
  • 2001–2012
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States
  • 2011
    • University of California, Santa Cruz
      Santa Cruz, California, United States
  • 2010
    • University of California, Davis
      • Department of Physics
      Davis, California, United States
  • 2006–2008
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2005–2008
    • University of California, Berkeley
      • Department of Astronomy
      Berkeley, CA, United States