Naveen A. Reddy

University of California, Riverside, Riverside, California, United States

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Publications (97)381.75 Total impact

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    ABSTRACT: We present a study of the H$\alpha$ gas kinematics for 179 star-forming galaxies at $z\sim2$ from the MOSFIRE Deep Evolution Field survey. We have developed models to interpret the kinematic measurements from fixed-angle multi-object spectroscopy, using structural parameters derived from CANDELS HST/F160W imaging. For 35 galaxies we measure resolved rotation with a median $(V/\sigma_{v,0})_{R_E}=2.11$. We derive dynamical masses from the kinematics and sizes and compare them to baryonic masses, with gas masses estimated from Balmer decrement corrected H$\alpha$ star formation rates (SFRs) and the Kennicutt-Schmidt relation. When assuming that galaxies with and without observed rotation have the same median $(V/\sigma_{v,0})_{R_E}$, we find good agreement between the dynamical and baryonic masses, with a scatter of $\sigma_{RMS}=0.338$ dex and a median offset of $\Delta\log_{10}M=0.04$ dex. This comparison implies a low dark matter fraction (8% within an effective radius) for a Chabrier initial mass function (IMF), and disfavors a Salpeter IMF. Moreover, the requirement that $M_{dyn}/M_{baryon}$ for galaxies without observed rotation should be independent of inclination yields a median value of $(V/\sigma_{v,0})_{R_E}= 2.1$. If instead we assume that galaxies without resolved rotation are ellipticals, the masses are also in reasonable agreement ($\Delta\log_{10}M=-0.06$ dex, $\sigma_{RMS}=0.364$ dex). The inclusion of gas masses is critical in this comparison; if gas masses are excluded there is an increasing trend of $M_{dyn}/M_{*}$ with higher specific SFR (SSFR). Furthermore, we find indications that $V/\sigma$ decreases with increasing H$\alpha$ SSFR for our full sample, which may reflect disk settling. The active galactic nuclei in our sample have a similar distribution in $M_{dyn}-M_{baryon}$ as the primary sample, which suggests the kinematics describe the host galaxies.
    No preview · Article · Nov 2015
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    ABSTRACT: Using observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey, we investigate the physical conditions of star-forming regions in $z\sim2.3$ galaxies, specifically the electron density and ionization state. From measurements of the [O II]$\lambda\lambda$3726,3729 and [S II]$\lambda\lambda$6716,6731 doublets, we find a median electron density of $\sim250$ cm$^{-3}$ at $z\sim2.3$, an increase of an order of magnitude compared to measurements of galaxies at $z\sim0$. While $z\sim2.3$ galaxies are offset towards significantly higher O$_{32}$ values relative to local galaxies at fixed stellar mass, we find that the high-redshift sample follows a similar distribution to the low-metallicity tail of the local distribution in the O$_{32}$ vs. R$_{23}$ and O3N2 diagrams. Based on these results, we propose that $z\sim2.3$ star-forming galaxies have the same ionization parameter as local galaxies at fixed metallicity. In combination with simple photoionization models, the position of local and $z\sim2.3$ galaxies in excitation diagrams suggests that there is no significant change in the hardness of the ionizing spectrum at fixed metallicity from $z\sim0$ to $z\sim2.3$. We find that $z\sim2.3$ galaxies show no offset compared to low-metallicity local galaxies in emission line ratio diagrams involving only lines of hydrogen, oxygen, and sulfur, but show a systematic offset in diagrams involving [N II]$\lambda$6584. We conclude that the offset of $z\sim2.3$ galaxies from the local star-forming sequence in the [N II] BPT diagram is primarily driven by elevated N/O at fixed O/H compared to local galaxies. These results suggest that the local gas-phase and stellar metallicity sets the ionization state of star-forming regions at $z\sim0$ and $z\sim2$.
    Full-text · Article · Sep 2015 · The Astrophysical Journal
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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.
    Full-text · Article · Jul 2015 · The Astrophysical Journal
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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.
    Preview · Article · Jun 2015 · The Astrophysical Journal
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    ABSTRACT: The TMT Detailed Science Case describes the transformational science that the Thirty Meter Telescope will enable. Planned to begin science operations in 2024, TMT will open up opportunities for revolutionary discoveries in essentially every field of astronomy, astrophysics and cosmology, seeing much fainter objects much more clearly than existing telescopes. Per this capability, TMT's science agenda fills all of space and time, from nearby comets and asteroids, to exoplanets, to the most distant galaxies, and all the way back to the very first sources of light in the Universe. More than 150 astronomers from within the TMT partnership and beyond offered input in compiling the new 2015 Detailed Science Case. The contributing astronomers represent the entire TMT partnership, including the California Institute of Technology (Caltech), the Indian Institute of Astrophysics (IIA), the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the National Astronomical Observatory of Japan (NAOJ), the University of California, the Association of Canadian Universities for Research in Astronomy (ACURA) and US associate partner, the Association of Universities for Research in Astronomy (AURA).
    Full-text · Article · May 2015
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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]
    Full-text · Article · Apr 2015 · The Astrophysical Journal
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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.
    Full-text · Article · Mar 2015 · The Astrophysical Journal
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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.
    No preview · Article · Mar 2015 · The Astrophysical Journal
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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.
    Full-text · Article · Dec 2014 · The Astrophysical Journal Supplement Series
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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.
    No preview · Article · Oct 2014 · The Astrophysical Journal
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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.
    Full-text · Article · Sep 2014 · The Astrophysical Journal
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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.
    Full-text · Article · Sep 2014
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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]
    Full-text · Article · Aug 2014
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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.
    Full-text · Article · Aug 2014 · The Astrophysical Journal
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    ABSTRACT: We present initial results from a large near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey (KBSS) using the recently-commissioned near-IR multi-object spectrometer MOSFIRE on the W.M. Keck Observatory Keck 1 telescope. Within the current KBSS-MOSFIRE sample, we focus on a representative sub-sample of 179 galaxies with redshifts 2.0 ≤ z ≤ 2.6, star-formation rates (SFRs) 5 ≾ SFR ≾ 150 M_☉ yr^(-1), and stellar masses 8.8 ≾ log(M_•/M_☉) ≾ 11.5. Most of the sample has high quality MOSFIRE spectra in both H and K-band atmospheric windows, allowing sensitive measurements of [O III]λλ4960, 5008, Hβ, [N II]λ6585, and Hɑ 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]/Hɑ and [OIII]/Hβ as compared to local galaxies. Using photoionization models, we argue that the offset of the z ~ 2.3 BPT locus relative to that at z ~ 0 is caused by both higher ionization parameter and harder stellar ionizing radiation field than applies to most local galaxies. Because of the higher overall excitation at z ~ 2.3, the position of a galaxy along the star-forming BPT locus is surprisingly insensitive to gas-phase oxygen abundance. The locus of [OIII]/Hβ vs. [NII]/Hɑ is most easily reproduced by models in which the effective ionizing radiation field is characterized by T_(eff) = 50000-60000 K, the gas-phase oxygen abundances lie in the range 0.2 < Z/Z_☉ ≤ 1.0, and the ratio of gas-phase N to O is close to the solar value. Such high sustained T_(eff) is not easily produced by standard population synthesis models, but is an expected consequence if massive binaries and/or rapid stellar rotation are important for the evolution of main sequence O-stars; the same phenomena may also account for enhanced production of primary N (and thus high N/O). We assess the applicability of commonly-used strong line indices for estimating the gas-phase metallicity of high redshift galaxies, as well as their likely systematic biases. The empirical correlation between M_• and strong-line-inferred oxygen abundance (the “MZR”) at z ~ 2.3 is as tight as for local galaxy samples (with intrinsic metallicity scatter of only 0.09 dex at a given M_•), but is offset to lower oxygen abundance by 0.34-0.38 dex, apparently independent of M_•, compared to a z ≃ 0 MZR using the same strong-line metallicity indicator. Over the mass range well-covered by the current sample (9 ≾ log(M_•/M_☉) ≾ 11), the MZR slope is shallow, with 12+log(O/H) ∝ 0.12 log(M_•/M_☉); a similarly shallow slope is observed for the analogous z ' 0 MZRs when evaluated over the same range in M_•. We investigated whether including SFR as an additional parameter in the MZR lowers the scatter, as found for local star-forming galaxy samples (the “fundamental metallicity relation”), finding that the dependence of inferred gas-phase metallicity on SFR at a given M_• is much weaker at high redshift than at z ~ 0, indicating that z ~ 2.3 galaxies do not adhere to the same “fundamental metallicity relation” as star-forming galaxies at low redshift. Taken together, our results suggest that the small observed scatter between M_• and inferred metallicity (the MZR) may be a by-product of a more fundamental relationship between a galaxy’s stellar mass and the ionization/excitation of its H II regions.
    Full-text · Article · May 2014 · The Astrophysical Journal
  • Irene Shivaei · N. Reddy
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    ABSTRACT: From a unique sample of 262 UV-selected galaxies with spectroscopic redshifts at z˜2 and rest-UV through Spitzer/IRAC near-IR photometry, we investigate their star-formation rates and dust attenuation based on multiple diagnostics. The sample includes galaxies at redshifts 2.08≤z≤2.51, where H-alpha falls in the K-band, and where the H-alpha flux can be estimated by comparing the K-band photometry to stellar population model fits to the UV- to near-IR photometry. This technique has the advantage of being immune to uncertain corrections for slit loss that can affect spectroscopic measurements of H-alpha. Comparing the H-alpha SFRs with those from the UV shows a general agreement between the two tracers if we assume that the nebular lines are attenuated by the same amount as the stellar continuum. We use the H-alpha, UV, and Spitzer/MIPS 24 micron data (where available) to examine the bolometric SFRs and recipes for dust corrections at high redshift.
    No preview · Article · Jan 2014
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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.
    No preview · Article · Jan 2014
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    ABSTRACT: We present Keck/MOSFIRE observations of the role of environment in the formation of galaxies at z~2. Using K-band spectroscopy of H-alpha and [N II] emission lines, we have analyzed the metallicities of galaxies within and around a z=2.3 protocluster discovered in the HS1700+643 field. Our main sample consists of 23 protocluster and 20 field galaxies with estimates of stellar masses and gas-phase metallicities based on the N2 strong-line metallicity indicator. With these data we have examined the mass-metallicity relation (MZR) with respect to environment at z~2. We find that field galaxies follow the well-established trend between stellar mass and metallicity, such that more massive galaxies have larger metallicities. The protocluster galaxies, however, do not exhibit a dependence of metallicity on mass, with the low-mass protocluster galaxies showing an enhancement in metallicity compared to field galaxies spanning the same mass range. A comparison with galaxy formation models suggests that the mass-dependent environmental trend we observed can be qualitatively explained in the context of the recycling of "momentum-driven" galaxy wind material. Accordingly, winds are recycled on a shorter timescale in denser environments, leading to an enhancement in metallicity at fixed mass for all but the most massive galaxies. Future hydrodynamical simulations of z~2 overdensities matching the one in the HS1700 field will be crucial for understanding the origin of the observed environmental trend in detail.
    Full-text · Article · Jun 2013 · The Astrophysical Journal
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    ABSTRACT: We present results from a survey for z~2.85 Lyman-Continuum (LyC) emission in the HS1549+1933 field and place constraints on the amount of ionizing radiation escaping from star-forming galaxies. Using a custom narrowband filter (NB3420) tuned to wavelengths just below the Lyman limit at z>=2.82, we probe the LyC spectral region of 49 Lyman break galaxies (LBGs) and 91 Lya-emitters (LAEs) spectroscopically confirmed at z>=2.82. Four LBGs and seven LAEs are detected in NB3420. Using V-band data probing the rest-frame non-ionizing UV, we observe that many NB3420-detected galaxies exhibit spatial offsets between their LyC and non-ionizing UV emission and are characterized by extremely blue NB3420-V colors, corresponding to low ratios of non-ionizing to ionizing radiation (F_UV/F_LyC) that are in tension with current stellar population synthesis models. We measure average values of F_UV/F_LyC for our LBG and LAE samples, correcting for foreground galaxy contamination and HI absorption in the IGM. We find (F_UV/F_LyC)_corr^LBG=82 +/- 45 and (F_UV/F_LyC)_corr^LAE=7.4 +/- 3.6. These flux-density ratios correspond respectively to relative LyC escape fractions of f_esc,rel^LBG=5-8% and f_esc,rel^LAE=18-49%, absolute LyC escape fractions of f_esc^LBG=1-2% and f_esc^LAE=5-15%, and a comoving LyC emissivity from star-forming galaxies of 8.8-15.0 x 10^24 ergs/s/Hz/Mpc^3. In order to study the differential properties of galaxies with and without LyC detections, we analyze narrowband Lya imaging and rest-frame near-infrared imaging, finding that while LAEs with LyC detections have lower Lya equivalent widths on average, there is no substantial difference in the rest-frame near-infrared colors of LBGs or LAEs with and without LyC detections. These preliminary results are consistent with an orientation-dependent model where LyC emission escapes through cleared paths in a patchy ISM.
    Preview · Article · Jun 2013 · The Astrophysical Journal
  • Irene Shivaei · N. Reddy
    [Show abstract] [Hide abstract]
    ABSTRACT: We use a unique sample of 262 star-forming galaxies selected during the epoch of peak cosmic star formation to infer star-formation rates and dust reddening, and to investigate the validity of the local star-formation calibrations at high redshift. The sample contains galaxies at redshifts 2.08<z<2.51, where the K-band photometry is affected by H-alpha. By comparing the K-band photometry to the underlying stellar population model fit to the optical and IRAC data, we are able to infer an H-alpha flux for each galaxy. These H-alpha fluxes are combined with UV continuum and Spitzer/MIPS 24 micron measurements in order to examine the bolometric star-formation rates and dust attenuation. We discuss the comparison of these three complementary star-formation rate indicators in the context of the star-formation histories of the galaxies, and the implications for inferring star-formation rates and reddening at high redshift.
    No preview · Article · Jan 2013

Publication Stats

6k Citations
381.75 Total Impact Points

Institutions

  • 2011-2015
    • University of California, Riverside
      • Department of Physics and Astronomy
      Riverside, California, United States
  • 2014
    • University of Toronto
      • Dunlap Institute for Astronomy and Astrophysics
      Toronto, Ontario, Canada
    • University of California, Berkeley
      Berkeley, California, United States
  • 2008-2010
    • National Optical Astronomy Observatory
      Tucson, Arizona, United States
  • 2003-2008
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States
  • 2005
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, Maryland, United States
  • 2001
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States
  • 1999
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States