3,010 research outputs found
The Spectroscopic Properties of Ly{\alpha}-Emitters at z 2.7: Escaping Gas and Photons from Faint Galaxies
We present a spectroscopic survey of 318 faint , ,
Ly{\alpha}-emission-selected galaxies (LAEs) at 2.5<z<3. A sample of 32 LAEs
with rest-frame optical spectra from Keck/MOSFIRE are used to interpret the LAE
spectra in the context of their systemic redshifts. We find that the Ly{\alpha}
emission of LAEs is typically less spectrally extended than among samples of
more luminous continuum-selected galaxies (LBGs) at similar redshifts. Using
the MOSFIRE subsample, we find that the peak of the Ly{\alpha} line is shifted
by +200 km/s with respect to systemic across a diverse set of galaxies
including both LAEs and LBGs. We also find a small number of objects with
significantly blueshifted Ly{\alpha} emission, a potential indicator of
accreting gas. The Ly{\alpha}-to-H{\alpha} line ratios suggest that the LAEs
have Ly{\alpha} escape fractions %,
significantly higher than typical LBG samples. Using redshifts calibrated by
our MOSFIRE sample, we construct composite LAE spectra, finding the first
evidence for metal-enriched outflows in such intrinsically-faint high-redshift
galaxies. These outflows have smaller continuum covering fractions and velocities km/s, km/s than those associated with typical LBGs, suggesting that
gas covering fraction is a likely driver of the high Ly{\alpha} and
Ly-continuum escape fractions of LAEs. Our results suggest a similar scaling of
outflow velocity with star formation rate as is observed at lower redshifts
and indicate that a substantial
fraction of gas is ejected with
The Rest-frame Optical Spectroscopic Properties of Lyα-emitters at z~2.5: The Physical Origins of Strong Lyα Emission
We present the rest-frame optical spectroscopic properties of 60 faint (R_(AB) ~ 27; L ~ 0.1 L_*) Lyα-selected galaxies (LAEs) at z ≈ 2.56. These LAEs also have rest-UV spectra of their Lyα emission line morphologies, which trace the effects of interstellar and circumgalactic gas on the escape of Lyα photons. We find that the LAEs have diverse rest-optical spectra, but their average spectroscopic properties are broadly consistent with the extreme low-metallicity end of the populations of continuum-selected galaxies selected at z ≈ 2–3. In particular, the LAEs have extremely high [O iii] λ5008/Hβ ratios (log([O iii]/Hβ) ~ 0.8) and low [N ii] λ6585/Hα ratios (log([N ii]/Hα) < 1.15). Coupled with a detection of the [O iii] λ4364 auroral line, these measurements indicate that the star-forming regions in faint LAEs are characterized by high electron temperatures (T_e ≈ 1.8 × 10^4 K), low oxygen abundances (12 + log(O/H) ≈ 8.04, Z_(neb) ≈ 0.22Z_⊙), and high excitations with respect to their more luminous continuum-selected analogs. Several of our faintest LAEs have line ratios consistent with even lower metallicities, including six with 12 + log(O/H) ≈ 6.9–7.4 (Z_(neb) ≈ 0.02–0.05Z_⊙). We interpret these observations in light of new models of stellar evolution (including binary interactions) that have been shown to produce long-lived populations of hot, massive stars at low metallicities. We find that strong, hard ionizing continua are required to reproduce our observed line ratios, suggesting that faint galaxies are efficient producers of ionizing photons and important analogs of reionization-era galaxies. Furthermore, we investigate the physical trends accompanying Lyα emission across the largest current sample of combined Lyα and rest-optical galaxy spectroscopy, including both the 60 KBSS-Lyα LAEs and 368 more luminous galaxies at similar redshifts. We find that the net Lyα emissivity (parameterized by the Lyα equivalent width) is strongly correlated with nebular excitation and ionization properties and weakly correlated with dust attenuation, suggesting that metallicity plays a strong role in determining the observed properties of these galaxies by modulating their stellar spectra, nebular excitation, and dust content
Detection of hot, metal-enriched outflowing gas around 2.3 star-forming galaxies in the Keck Baryonic Structure Survey
We use quasar absorption lines to study the physical conditions in the
circumgalactic medium of redshift star-forming galaxies taken
from the Keck Baryonic Structure Survey (KBSS). In Turner et al. 2014 we used
the pixel optical depth technique to show that absorption by HI and the metal
ions OVI, NV, CIV, CIII and SiIV is strongly enhanced within km/s and projected distances proper kpc from
sightlines to the background quasars. Here we demonstrate that the OVI
absorption is also strongly enhanced at fixed HI, CIV, and SiIV optical depths,
and that this enhancement extends out to km/s. At fixed HI the
increase in the median OVI optical depth near galaxies is 0.3-0.7 dex and is
detected at 2--3- confidence for all seven HI bins that have
. We use ionization models to show that the
observed strength of OVI as a function of HI is consistent with enriched,
photoionized gas for pixels with . However, for pixels
with this would lead to implausibly high
metallicities at low densities if the gas were photoionized by the background
radiation. This indicates that the galaxies are surrounded by gas that is
sufficiently hot to be collisionally ionized (K) and that a
substantial fraction of the hot gas has a metallicity of
solar. Given the high metallicity and large velocity extent (out to
) of this gas, we conclude that we have detected
hot, metal enriched outflows arising from star-forming galaxies.Comment: Accepted for publication in MNRAS, 14 pages, 9 figures (not including
appendices
Q1549-C25: A Clean Source of Lyman-Continuum Emission at
We present observations of Q1549-C25, an ~L* star-forming galaxy at z=3.15
for which Lyman-continuum (LyC) radiation is significantly detected in deep
Keck/LRIS spectroscopy. We find no evidence for contamination from a
lower-redshift interloper close to the line of sight in the high
signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25
in V_606, J_125, and H_160 Hubble Space Telescope (HST) imaging reveals that
the object consists of a single, isolated component within 1". In combination,
these data indicate Q1549-C25 as a clean spectroscopic detection of LyC
radiation, only the second such object discovered to date at z~3. We model the
spectral energy distribution (SED) of Q1549-C25, finding evidence for
negligible dust extinction, an age (assuming continuous star formation) of ~1
Gyr, and a stellar mass of M_*=7.9x10^9 M_sun. Although it is not possible to
derive strong constraints on the absolute escape fraction of LyC emission,
f_esc(LyC), from a single object, we use simulations of intergalactic and
circumgalactic absorption to infer f_esc(LyC)>=0.51 at 95% confidence. The
combination of deep Keck/LRIS spectroscopy and HST imaging is required to
assemble a larger sample of objects like Q1549-C25, and obtain robust
constraints on the average f_esc(LyC) at z~3 and beyond.Comment: 6 pages, 5 figures, accepted to ApJ Letter
A High Fraction of Ly-alpha-Emitters Among Galaxies with Extreme Emission Line Ratios at z ~ 2
Star-forming galaxies form a sequence in the [OIII]/H-beta vs. [NII]/H-alpha
diagnostic diagram, with low metallicity, highly ionized galaxies falling in
the upper left corner. Drawing from a large sample of UV-selected star-forming
galaxies at z~2 with rest-frame optical nebular emission line measurements from
Keck-MOSFIRE, we select the extreme ~5% of the galaxies lying in this upper
left corner, requiring log([NII]/H-alpha) =
0.75. These cuts identify galaxies with 12 + log(O/H) <~ 8.0, when oxygen
abundances are measured via the O3N2 diagnostic. We study the Ly-alpha
properties of the resulting sample of 14 galaxies. The mean (median) rest-frame
Ly-alpha equivalent width is 39 (36) A, and 11 of the 14 objects (79%) are
Ly-alpha-emitters (LAEs) with W_Lya > 20 A. We compare the equivalent width
distribution of a sample of 522 UV-selected galaxies at 2.0<z<2.6 identified
without regard to their optical line ratios; this sample has mean (median)
Ly-alpha equivalent width -1 (-4) A, and only 9% of these galaxies qualify as
LAEs. The extreme galaxies typically have lower attenuation at Ly-alpha than
those in the comparison sample, and have ~50% lower median oxygen abundances.
Both factors are likely to facilitate the escape of Ly-alpha: in less dusty
galaxies Ly-alpha photons are less likely to be absorbed during multiple
scatterings, while the harder ionizing spectrum and higher ionization parameter
associated with strong, low metallicity star formation may reduce the covering
fraction or column density of neutral hydrogen, further easing Ly-alpha escape.
The use of nebular emission line ratios may prove useful in the identification
of galaxies with low opacity to Ly-alpha photons across a range of redshifts.Comment: 12 pages, 5 figures, 2 tables. Accepted for publication in Ap
Measuring the Physical Conditions in High-redshift Star-forming Galaxies: Insights from KBSS-MOSFIRE
We use photoionization models that are designed to reconcile the joint rest-UV-optical spectra of high-z star-forming galaxies to self-consistently infer the gas chemistry and nebular ionization and excitation conditions for ~150 galaxies from the Keck Baryonic Structure Survey (KBSS), using only observations of their rest-optical nebular spectra. We find that the majority of z ~ 2–3 KBSS galaxies are moderately O-rich, with an interquartile range in 12 + log(O/H) = 8.29–8.56, and have significantly sub-solar Fe enrichment, with an interquartile range of [Fe/H] = [−0.79, −0.53], which contributes additional evidence in favor of super-solar O/Fe in high-z galaxies. The model-inferred ionization parameters and N/O are strongly correlated with common strong-line indices (such as O32 and N2O2), with the latter exhibiting similar behavior to local extragalactic H ii regions. In contrast, diagnostics commonly used for measuring gas-phase O/H (such as N2 and O3N2) show relatively large scatter with the overall amount of oxygen present in the gas and behave differently than observed at z ~ 0. We provide a new calibration for using R23 to measure O/H in typical high-z galaxies, although it is most useful for relatively O-rich galaxies; combining O32 and R23 does not yield a more effective calibration. Finally, we consider the implications for the intrinsic correlations between physical conditions across the galaxy sample and find that N/O varies with O/H in high-z galaxies in a manner that is almost identical to local H ii regions. However, we do not find a strong anti-correlation between ionization parameter and metallicity (O/H or Fe/H) in high-z galaxies, which is one of the principal bases for using strong-line ratios to infer oxygen abundance
Predicting Lyα Emission from Galaxies via Empirical Markers of Production and Escape in the KBSS
Lyα emission is widely used to detect and confirm high-redshift galaxies and characterize the evolution of the intergalactic medium (IGM). However, many galaxies do not display Lyα emission in typical spectroscopic observations, and intrinsic Lyα emitters represent a potentially biased set of high-redshift galaxies. In this work, we analyze a set of 703 galaxies at 2 ≾ z ≾ 3 with both Lyα spectroscopy and measurements of other rest-frame ultraviolet and optical properties in order to develop an empirical model for Lyα emission from galaxies and understand how the probability of Lyα emission depends on other observables. We consider several empirical proxies for the efficiency of Lyα photon production, as well as the subsequent escape of these photons through their local interstellar medium. We find that the equivalent width of metal-line absorption and the O3 ratio of rest-frame optical nebular lines are advantageous empirical proxies for Lyα escape and production, respectively. We develop a new quantity, X_(LIS)^(O3), that combines these two properties into a single predictor of net Lyα emission, which we find describes ~90% of the observed variance in Lyα equivalent width when accounting for our observational uncertainties. We also construct conditional probability distributions demonstrating that galaxy selection based on measurements of galaxy properties yield samples of galaxies with widely varying probabilities of net Lyα emission. The application of the empirical models and probability distributions described here may be used to infer the selection biases of current galaxy surveys and evaluate the significance of high-redshift Lyα (non)detections in studies of reionization and the IGM
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