Lyα-emitting Galaxies Research Articles

J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons

We report the detection of the Lyman continuum (LyC) radiation of the compact star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a redshift of z=0.3690, is characterized by a high emission-line flux ratio O32=[OIII]5007/[OII]3727=11.5. The escape fraction of the LyC radiation fesc(LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Lya emission line is detected in the spectrum of J1154+2443 with a separation between the peaks Vsep of 199 km/s, one of the lowest known for Lya-emitting galaxies, implying a high fesc(Lya). Comparing the extinction-corrected Lya/Hb flux ratio with the case B value we find fesc(Lya) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O32 with increasing fesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length alpha=1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+logO/H=7.65+/-0.01, the lowest stellar mass M*=10^8.20 Msun, a similar star formation rate SFR=18.9 Msun/yr and a high specific SFR of 1.2x10^-7 yr^-1.

First Spectroscopic Confirmations of z ∼ 7.0 Lyα Emitting Galaxies in the LAGER Survey

Abstract Narrowband imaging is a highly successful approach for finding large numbers of high-redshift Lyα emitting galaxies (LAEs) up to z ∼ 6.6. However, at z ≳ 7 there are as of yet only three narrowband selected LAEs with spectroscopic confirmations (two at z ∼ 6.9–7.0, one at z ∼ 7.3), which hinders extensive studies on cosmic reionization and galaxy evolution at this key epoch. We have selected 23 candidate z ∼ 6.9 LAEs in COSMOS field with the large area narrowband survey Lyman-Alpha Galaxies at the End of Reionization (LAGER). In this work, we present spectroscopic follow-up observations of 12 candidates using the Inamori Magellan Areal Camera and Spectrograph on Magellan. For nine of these, the observations are sufficiently deep to detect the expected lines. Lyα emission lines are identified in six sources (yielding a success rate of 2/3), including three luminous LAEs with Lyα luminosities of L Lyα ∼ 1043.5 erg s−1, the highest among known spectroscopically confirmed galaxies at ≳7.0. This triples the sample size of spectroscopically confirmed narrowband selected LAEs at z ≳ 7, and confirms the bright-end bump in the Lyα luminosity function we previously derived based on the photometric sample, supporting a patchy reionization scenario. Two luminous LAEs appear physically linked with a projected distance of 1.1 pMpc and velocity difference of ∼170 km s−1. They likely sit in a common ionized bubble produced by themselves or with close neighbors, which reduces the intergalactic medium attenuation of Lyα. A tentative narrow N v λ1240 line is seen in one source, hinting at activity of a central massive black hole with metal-rich line-emitting gas.

Bayesian Redshift Classification of Emission-line Galaxies with Photometric Equivalent Widths

Abstract We present a Bayesian approach to the redshift classification of emission-line galaxies when only a single emission line is detected spectroscopically. We consider the case of surveys for high-redshift Lyα-emitting galaxies (LAEs), which have traditionally been classified via an inferred rest-frame equivalent width (EW; ) greater than 20 Å. Our Bayesian method relies on known prior probabilities in measured emission-line luminosity functions and EW distributions for the galaxy populations, and returns the probability that an object in question is an LAE given the characteristics observed. This approach will be directly relevant for the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), which seeks to classify ∼106 emission-line galaxies into LAEs and low-redshift emitters. For a simulated HETDEX catalog with realistic measurement noise, our Bayesian method recovers 86% of LAEs missed by the traditional > 20 Å cutoff over 2 < z < 3, outperforming the EW cut in both contamination and incompleteness. This is due to the method’s ability to trade off between the two types of binary classification error by adjusting the stringency of the probability requirement for classifying an observed object as an LAE. In our simulations of HETDEX, this method reduces the uncertainty in cosmological distance measurements by 14% with respect to the EW cut, equivalent to recovering 29% more cosmological information. Rather than using binary object labels, this method enables the use of classification probabilities in large-scale structure analyses. It can be applied to narrowband emission-line surveys as well as upcoming large spectroscopic surveys including Euclid and WFIRST.

Lyα and UV Sizes of Green Pea Galaxies

Abstract Green Peas are nearby analogs of high-redshift Lyα-emitting galaxies (LAEs). To probe their Lyα escape, we study the spatial profiles of Lyα and UV continuum emission of 24 Green Pea galaxies using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. We extract the spatial profiles of Lyα emission from their 2D COS spectra, and of the UV continuum from both 2D spectra and NUV images. The Lyα emission shows more extended spatial profiles than the UV continuum, in most Green Peas. The deconvolved full width at half maximum of the Lyα spatial profile is about 2–4 times that of the UV continuum, in most cases. Because Green Peas are analogs of high z LAEs, our results suggest that most high-z LAEs probably have larger Lyα sizes than UV sizes. We also compare the spatial profiles of Lyα photons at blueshifted and redshifted velocities in eight Green Peas with sufficient data quality, and find that the blue wing of the Lyα line has a larger spatial extent than the red wing in four Green Peas with comparatively weak blue Lyα line wings. We show that Green Peas and MUSE z = 3–6 LAEs have similar Lyα and UV continuum sizes, which probably suggests that starbursts in both low-z and high-z LAEs drive similar gas outflows illuminated by Lyα light. Five Lyman continuum (LyC) leakers in this sample have similar Lyα to UV continuum size ratios (∼1.4–4.3) to the other Green Peas, indicating that their LyC emissions escape through ionized holes in the interstellar medium.

Lyα-emitting galaxies as a probe of reionization: large-scale bubble morphology and small-scale absorbers

The visibility of LyA emitting galaxies during the Epoch of Reionization is controlled by both diffuse HI patches in large-scale bubble morphology and small-scale absorbers. To investigate the impact on LyA photons, we apply a novel combination of analytic and numerical calculations to three scenarios: (i) the `bubble' model, where only diffuse HI outside ionized bubbles is present; (ii) the `web' model, where HI exists only in overdense self-shielded gas; and (iii) the more realistic 'web-bubble' model, which contains both. Our analysis confirms that there is a degeneracy between the ionization structure of the intergalactic medium (IGM) and the HI fraction inferred from LyA surveys, as the three models suppress LyA flux equally with very different HI fractions. We argue that a joint analysis of the LyA luminosity function and the rest-frame equivalent width distribution/LyA fraction can break this degeneracy and provide constraints on the reionization history and its topology. We further show that constraints can improve if we consider the full shape of the M_UV-dependent redshift evolution of the LyA fraction of Lyman break galaxies. Contrary to conventional wisdom, we find that (i) a drop of LyA fraction larger for UV-faint than for UV-bright galaxies can be reproduced with web and web-bubble models and therefore does not provide exclusive evidence of patchy reionization, and (ii) the IGM-transmission PDF is unimodal for bubble models and bimodal in web models. We further highlight the importance of galaxy-absorber cross-correlation. Comparing our models to observations, the neutral fraction at z~7 is likely to be of order of tens of per cent when interpreted with bubble or web-bubble models. Alternatively, we obtain a conservative lower limit ~1% in the web models, if we allow for a drop in the photoionization rate by a factor of ~100 from the post-reionized universe. [abridged]

IMPACT OF COSMIC VARIANCE ON THE GALAXY–HALO CONNECTION FOR Lyα EMITTERS

ABSTRACT In this paper we study the impact of cosmic variance and observational uncertainties in constraining the mass and occupation fraction, f occ -->?> , of dark matter (DM) halos hosting Lyα-emitting galaxies (LAEs) at high redshift. To this end, we construct mock catalogs from an N-body simulation to match the typical size of observed fields at z = 3.1 ( ∼ 1 deg 2 -->?> ). In our model a DM halo with mass in the range M min < M h < M max -->?> can only host one detectable LAE at most. We proceed to explore the parameter space determined by M min -->?> , M max -->?> , and f occ -->?> with a Markov Chain Monte Carlo algorithm using the angular correlation function and the LAEs’ number density as observational constraints. We find that the preferred minimum and maximum masses in our model span a wide range 10 10.0 h − 1 M ⊙ ≤ M min ≤ 10 11.1 h − 1 M ⊙ -->?> , 10 11.0 h − 1 M ⊙ ≤ M max ≤ 10 13.0 h − 1 M ⊙ , -->?> followed by a wide range in the occupation fraction 0.02 ≤ f occ ≤ 0.30 -->?> . As a consequence, the median mass, M 50, of all the consistent models has a large uncertainty M 50 = 3.16 − 2.37 + 9.34 × 10 10 -->?> h − 1 M ⊙ -->?> . However, we find that the same individual models have a relatively tight 1 &sgr; -->?> scatter around the median mass Δ M 1 &sgr; = 0.55 − 0.31 + 0.11 -->?> dex. We are also able to show that f occ -->?> is uniquely determined by M min -->?> , regardless of M max -->?> . We argue that upcoming large surveys covering at least 25 deg2 should be able to put tighter constraints on M min -->?> and f occ -->?> through the LAE number density distribution width constructed over several fields of ∼1 deg2.

SPECTROSCOPIC CONFIRMATION OF A PROTOCLUSTER AT z ≈ 3.786

ABSTRACT We present new observations of the field containing the z = 3.786 protocluster PC 217.96+32.3. We confirm that it is one of the largest known and most overdense high-redshift structures. Such structures are rare even in the largest cosmological simulations. We used the Mayall/MOSAIC1.1 imaging camera to image a 1.°2 × 0.°6 area (≈150 × 75 comoving Mpc) surrounding the protocluster’s core and discovered 165 candidate Lyα emitting galaxies (LAEs) and 788 candidate Lyman Break galaxies (LBGs). There are at least two overdense regions traced by the LAEs, the largest of which shows an areal overdensity in its core (i.e., within a radius of 2.5 comoving Mpc) of 14 ± 7 relative to the average LAE spatial density ( ) in the imaged field. Further, is twice that derived by other field LAE surveys. Spectroscopy with Keck/DEIMOS yielded redshifts for 164 galaxies (79 LAEs and 85 LBGs); 65 lie at a redshift of 3.785 ± 0.010. The velocity dispersion of galaxies near the core is σ = 350 ± 40 km s−1, a value robust to selection effects. The overdensities are likely to collapse into systems with present-day masses of >1015 M ⊙ and >6 × 1014 M ⊙. The low velocity dispersion may suggest a dynamically young protocluster. We find a weak trend between narrow-band (Lyα) luminosity and environmental density: the Lyα luminosity is enhanced on average by 1.35× within the protocluster core. There is no evidence that the Lyα equivalent width depends on environment. These suggest that star formation and/or active galactic nucleus (AGN) activity is enhanced in the higher-density regions of the structure. PC 217.96+32.3 is a Coma cluster analog, witnessed in the process of formation.

REST-UV ABSORPTION LINES AS METALLICITY ESTIMATOR: THE METAL CONTENT OF STAR-FORMING GALAXIES AT z ∼ 5

ABSTRACT We measure a relation between the depth of four prominent rest-UV absorption complexes and metallicity for local galaxies and verify it up to . We then apply this relation to a sample of 224 galaxies at ( ) in the Cosmic Evolution Survey (COSMOS), for which unique UV spectra from the Deep Imaging Multi-object Spectrograph (DEIMOS) and accurate stellar masses from the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) are available. The average galaxy population at and is characterized by 0.3–0.4 dex (in units of ) lower metallicities than at z ∼ 2, but comparable to . We find galaxies with weak or no Lyα emission to have metallicities comparable to z ∼ 2 galaxies and therefore may represent an evolved subpopulation of galaxies. We find a correlation between metallicity and dust in good agreement with local galaxies and an inverse trend between metallicity and star-formation rate consistent with observations at z ∼ 2. The relation between stellar mass and metallicity (MZ relation) is similar to , but there are indications of it being slightly shallower, in particular for the young, Lyα-emitting galaxies. We show that, within a “bathtub” approach, a shallower MZ relation is expected in the case of a fast (exponential) build-up of stellar mass with an e-folding time of 100–200 Myr. Because of this fast evolution, the process of dust production and metal enrichment as a function of mass could be more stochastic in the first billion years of galaxy formation compared to later times.

HST EMISSION LINE GALAXIES AT z ∼ 2: COMPARING PHYSICAL PROPERTIES OF LYMAN ALPHA AND OPTICAL EMISSION LINE SELECTED GALAXIES

ABSTRACT We compare the physical and morphological properties of z ∼ 2 Lyα emitting galaxies (LAEs) identified in the HETDEX Pilot Survey and narrow band studies with those of z ∼ 2 optical emission line selected galaxies (oELGs) identified via HST WFC3 infrared grism spectroscopy. Both sets of galaxies extend over the same range in stellar mass ( 7.5 < log M / M ⊙ < 10.5 ?> ), size (0.5 < R < 3.0 kpc), and star formation rate ( ∼ 1 < SFR < 100 M ⊙ ?> yr−1). Remarkably, a comparison of the most commonly used physical and morphological parameters—stellar mass, half-light radius, UV slope, SFR, ellipticity, nearest neighbor distance, star formation surface density, specific SFR, [O iii] luminosity, and [O iii] equivalent width—reveals no statistically significant differences between the populations. This suggests that the processes and conditions which regulate the escape of Lyα from a z ∼ 2 star-forming galaxy do not depend on these quantities. In particular, the lack of dependence on the UV slope suggests that Lyα emission is not being significantly modulated by diffuse dust in the interstellar medium. We develop a simple model of Lyα emission that connects LAEs to all high-redshift star-forming galaxies where the escape of Lyα depends on the sightline through the galaxy. Using this model, we find that mean solid angle for Lyα escape is &OHgr; Ly &agr; = 2.4 ± 0.8 ?> steradians; this value is consistent with those calculated from other studies.

Blue wings and bumps via Fermi-like acceleration of Ly α photons across shocks

We explore the impact of Fermi-like acceleration of Lyman α (Ly α) photons across shock fronts on the observed Ly α spectral line shape. We first confirm the result of Neufeld & McKee (1988) that this mechanism gives rise to extended blue wings which may have been observed in some radio galaxies. Our Monte Carlo radiative transfer calculations further show that in a minor modification of the shell model, in which we add an additional static shell of hydrogen, this process can naturally explain the small blue bumps observed in a subset of Ly α-emitting galaxies, which have been difficult to explain with conventional shell models. Blue bumps can be produced with an additional column density of static hydrogen as small as |$N^{\rm static}_{\rm H\,\small {I}} \ll N^{\rm shell}_{\rm H\,\small {I}}$|⁠, and typically occur at roughly the outflow velocity of the shell. In our model the spectra of so-called blue-bump objects might reflect an evolutionary stage in which the outflows regulating the escape of Ly α photons are still engulfed within a static interstellar medium.

Lyman Alpha Emitting Galaxies in the Nearby Universe

AbstractThe Lyman alpha emission line (Lyα) of neutral hydrogen (Hi) is intrinsically the brightest emission feature in the spectrum of astrophysical nebulae, making it a very attractive observational feature with which to survey galaxies. Moreover as an ultraviolet resonance line, Lyα possesses several unique characteristics that make it useful to study the properties of the interstellar medium and ionising stellar population at all cosmic epochs. In this review, I present a summary of Lyα observations of galaxies in the nearby universe. By ultraviolet continuum selection, at the magnitudes reachable with current facilities, only ≈ 5% of the local galaxy population shows a Lyα equivalent width (WLyα) that exceeds 20 Å. This fraction increases dramatically at higher redshifts, but only in the local universe can we study galaxies in detail and assemble unprecedented multi-wavelength datasets. I discuss many local Lyα observations, showing that when galaxies show net Lyα emission, they ubiquitously also produce large-scale halos of scattered Lyα, that dominate the integrated luminosity. Concerning global measurements, we discuss how WLyα and the Lyα escape fraction (fLyαesc) are higher (WLyα ≳ 20 Å and fLyαesc ≳ 10%) in galaxies that represent the less massive and younger end of the distribution for local objects. This is connected with various properties, such that Lyα-emitting galaxies have lower metal abundances (median value of 12 + log(O/H) ~ 8.1) and dust reddening. However, the presence of galactic outflows/winds is also vital to Doppler shift the Lyα line out of resonance with the atomic gas, and high WLyα is found only among galaxies with winds faster than ~ 50 km s−1. The empirical evidence is then assembled into a coherent picture, and the requirement for star-formation-driven feedback is discussed in the context of an evolutionary sequence where the interstellar medium is accelerated and/or subject to hydrodynamical instabilities, which reduce the scattering of Lyα. Concluding remarks take the form of perspectives upon future developments, and the most pressing questions that can be answered by observation.

SPECTROSCOPIC CONFIRMATION OF THREEz-DROPOUT GALAXIES ATz= 6.844-7.213: DEMOGRAPHICS OF Lyα EMISSION INz∼ 7 GALAXIES

We present the results of our ultra-deep Keck/DEIMOS spectroscopy of z-dropout galaxies in the SDF and GOODS-N. For 3 out of 11 objects, we detect an emission line at ~ 1um with a signal-to-noise ratio of ~ 10. The lines show asymmetric profiles with high weighted skewness values, consistent with being Lya, yielding redshifts of z=7.213, 6.965, and 6.844. Specifically, we confirm the z=7.213 object in two independent DEIMOS runs with different spectroscopic configurations. The z=6.965 object is a known Lya emitter, IOK-1, for which our improved spectrum at a higher resolution yields a robust skewness measurement. The three z-dropouts have Lya fluxes of 3 x 10^-17 erg s^-1 cm^-2 and rest-frame equivalent widths EW_0^Lya = 33-43A. Based on the largest spectroscopic sample of 43 z-dropouts that is the combination of our and previous data, we find that the fraction of Lya-emitting galaxies (EW_0^Lya > 25A) is low at z ~ 7; 17 +- 10% and 24 +- 12% for bright (Muv ~= -21) and faint (Muv ~= -19.5) galaxies, respectively. The fractions of Lya-emitting galaxies drop from z ~ 6 to 7 and the amplitude of the drop is larger for faint galaxies than for bright galaxies. These two pieces of evidence would indicate that the neutral hydrogen fraction of the IGM increases from z ~ 6 to 7, and that the reionization proceeds from high- to low-density environments, as suggested by an inside-out reionization model.

The Lyman α emission of high-z damped Lyman α systems

Abstract Using a spectral stacking technique we searched for the average Lyman α (Lyα) emission from high-z damped Lyα (DLA) galaxies detected in the Sloan Digital Sky Survey quasi-stellar object (QSO) spectra. We used a sample of 341 DLA galaxies of mean redshift 〈z〉= 2.86 and log N(H i) ≥ 20.62 to place a 3σ upper limit of 3.0 × 10−18 erg s−1 cm−2 on the Lyα flux emitted within ∼1.5 arcsec (or 12 kpc) from the QSO line of sight. This corresponds to an average Lyα luminosity of ≤2 × 1041 erg s−1 or 0.03L★(Lyα). This limit is deeper than the limit of most surveys for faint Lyα emitters. The lack of Lyα emission in DLA galaxies is consistent with the in situ star formation, for a given N(H i), being less efficient than what is seen in local galaxies. Thus, the overall DLA population seems to originate from the low-luminosity end of the high-redshift Lyα-emitting galaxies and/or to be located far away from the star-forming regions. The latter may well be true since we detect strong O vi absorption in the stacked spectrum, indicating that DLA galaxies are associated with a highly ionized phase, possibly the relics of galactic winds and/or originating from cold accretion flows. We find the contribution of DLA galaxies to the global star formation rate density to be comparatively lower than that of Lyman break galaxies.

Star formation indicators and line equivalent width in Lyα galaxies

The equivalent width (EW) of the Lyman Alpha (Lya) line is directly related to the ratio of star formation rates determined from Lya flux and UV flux density [SFR(Lya)/SFR(UV)]. We use published data --in the literature EW and SFR(Lya)/SFR(UV) are treated as independent quantities-- to show that the predicted relation holds for the vast majority of observed Lya emitting galaxies (LAEs). We show that the relation between EW and SFR(Lya)/SFR(UV) applies irrespective of a galaxy's `true' underlying star formation rate, and that its only source of scatter is the variation in the spectral slope of the UV continuum between individual galaxies. The derived relation, when combined with the observed EW distribution, implies that the ratio SFR(UV)/SFR(Lya) is described well by a log-normal distribution with a standard deviation of ~0.3-0.35. This result is useful when modelling the statistical properties of LAEs. We further discuss why the relation between EW and SFR(Lya)/SFR(UV) may help identifying galaxies with unusual stellar populations.

Lyα-EMITTING GALAXIES AT REDSHIFTz∼ 4.5 IN THE LALA CETUS FIELD

We present a large sample of Lyα-emitting galaxies (LAEs) spectroscopically confirmed at redshift z ≈ 4.5, based on Inamori-Magellan Areal Camera and Spectrograph spectroscopic observations of candidate z≈ 4.5 Lyα-emitting galaxies in the large area Lyman alpha (LALA) narrowband imaging survey Cetus field. We identify 110 of them as z ≈ 4.5 Lyα emitters based on single-line detections with no continuum emission blueward of the line. Six foreground galaxies are identified, either based on multiple lines or blueward continuum emission. The Lyα confirmation rate varies from <50% to 76% for candidates selected in different narrowband filters at slightly different redshifts. We find a drop in the LAE density at redshift 4.50 ± 0.03 from redshift 4.39 to 4.47 by a factor of 66%, which could be a large-scale void in the distribution of star-forming galaxies (~18 Mpc along the line of sight and ~80 Mpc across). The sample includes many objects with equivalent widths (EWs) 200 A. These large EW candidates are spectroscopically confirmed at the same rate as candidates with more modest EWs. A composite spectrum of all 110 confirmed LAEs shows the characteristic asymmetry of the Lyα line. It also places new and stringent upper limits on the C IV 1549 A/Lyα and He II 1640 A/Lyα line ratios, providing a new upper limit on the fraction of active galactic nuclei in Lyα selected galaxy samples, and on the contribution of Pop III populations. Finally, we calculate the Lyα luminosity function for our z ≈ 4.5 sample, which is consistent with those at other redshifts, showing that there is no evolution in Lyα luminosity function from z = 3.1 to 6.6.

A PLETHORA OF ACTIVE GALACTIC NUCLEI AMONG Lyα GALAXIES AT LOW REDSHIFT

We investigate the fraction of z∼ 0.3 Lyα emitting galaxies (LAEs), which host active galactic nucleus (AGN) activity, which is typically from 1% to 5% at 2 ≲z≲ 6. Using optical spectroscopy of 23 LAEs at 0.2 ⩽z⩽ 0.45 selected with Galaxy Evolution Explorer (GALEX) UV data, we probe for AGNs with a variety of methods, including line widths, diagnostic line ratios, high-ionization emission, infrared activity, and X-ray luminosity. We found that our sample of low-redshift LAEs has an AGN fraction of 43+18−26%, significantly higher than at high redshift. While previous results have shown that low-redshift LAEs have a lower space density than their high-redshift analogs, these results show that star-forming LAEs at low redshift are rarer still. Accounting for differences in available AGN classification methods, we conclude that rest-frame optical spectroscopy is necessary to identify low-luminosity AGNs in LAEs at high redshift, and that limits on the X-ray flux are necessary to determine the extent to which the AGN contaminates the Lyα fluxes.

EVOLUTION OF Lyα GALAXIES: STELLAR POPULATIONS ATz∼ 0.3

We present the results of a stellar population analysis of 30 Lyα emitting galaxies (LAEs) at z ~ 0.3, previously discovered with the Galaxy Evolution Explorer. With a few exceptions, we can accurately fit model spectral energy distributions to these objects, representing the first time this has been done for a large sample of LAEs at z < 3, a gap of ~8 Gyr in the history of the universe. From the 26/30 LAEs which we can fit, we find an age and stellar mass range of 200 Myr-10 Gyr and 109-1011 M ☉, respectively. These objects thus appear to be significantly older and more massive than LAEs at high redshift. We also find that these LAEs show a mild trend toward higher metallicity than those at high redshift, as well as a tighter range of dust attenuation and interstellar medium geometry. These results suggest that low-redshift LAEs have evolved significantly from those at high redshift.

Spectral Energy Distribution fitting: Application to Ly α-emitting galaxies

Spectral Energy Distribution (SED) fitting is a well-developed astrophysical tool that has recently been applied to high-redshift Ly α-emitting galaxies. If rest-frame ultraviolet through near-infrared photometry is available, it allows the simultaneous determination of the star formation history and dust extinction of a galaxy. Ly α-emitter SED fitting results from the literature find star formation rates ∼3 M ⊙ yr −1, stellar masses ∼10 9 M ⊙ for the general population but ∼10 10 M ⊙ for the subset detected by IRAC, and very low dust extinction, A V ⩽ 0.3, although a couple of outlying analyses prefer significantly more dust and higher intrinsic star formation rates. A checklist of 14 critical choices that must be made when performing SED fitting is discussed.

A Magellan IMACS Spectroscopic Search for Lyα‐emitting Galaxies at Redshift 5.7

We present results from a blind, spectroscopic survey for -->z ~ 5.7 Lyα-emitting galaxies (LAEs) using the Inamori Magellan Areal Camera and Spectrograph. A total of ~200 arcmin2 were observed in the COSMOS and LCIRS fields using a narrowband filter, which transmits between atmospheric emission lines at 8190 A, and a mask with 100 long slits. This observing technique provides higher emission-line sensitivity than narrowband imaging and probes larger volumes than strong lensing. We find 170 emission-line galaxies and identify their redshifts spectroscopically. We confirm three LAEs, the first discovered using multislit narrowband spectroscopy. Their line profiles are narrow, but fitted models suggest intrinsic, unattenuated widths ~400 km s−1 FWHM. The red wing of the line profiles presents features consistent with galactic winds. The star formation rates of these galaxies are at least 5-7 M☉ yr−1 and likely a factor of 2 higher. We estimate that the number density of -->L ≥ 5 × 1042 erg s−1 LAEs is -->9.0−4+12 × 10−5 -->h703 Mpc−3 at redshift 5.7 and constrain the Schechter function parameters describing this population. Galaxies fainter than our detection limit may well be the primary source of ionizing photons at -->z ~ 6. We argue, however, that the break luminosity -->L*,Ly α is not yet well constrained. If this break luminosity is near our detection limit, and somewhat lower than previous estimates, then the detected LAE population could be responsible for ionizing the intergalactic gas at redshift -->z ~ 6. We discuss the potential of multislit narrowband spectroscopy for deeper emission-line surveys.

Mapping Extremely Low Metallicity Galaxies to Redshift One

We describe the results of a narrowband search for ultrastrong emission line galaxies (USELs) with EW(Hβ) ≥ 30 Å. A total of 542 candidate galaxies are found in a one-half square degree survey using two ~120 Å filters centered at 8150 and 9140 Å with Subaru SuprimeCam. Follow-up spectroscopy for randomly selected objects in the candidate sample with Keck II DEIMOS shows that they consist of [O III] λ5007-, [O II] λ3727-, and Hα-selected strong emission line galaxies at intermediate redshift (z < 1) and Lyα-emitting galaxies at high redshift (z ≫ 5). We determine the Hβ luminosity functions and the star formation density of the USELs, which is 5%-10% of the value found from ultraviolet continuum objects at z = 0-1, suggesting that they correspond to a major epoch in the galaxy formation process at these redshifts. Many of the USELs show the temperature-sensitive [O III] λ4363 auroral lines, and about a dozen have oxygen abundances satisfying the criteria of extremely metal-poor galaxies (XMPGs). These XMPGs are the most distant known today. Our high yield rate of XMPGs suggests that the narrowband method is a powerful way to find such populations. Moreover, the lowest metallicity measured in our sample is close to the minimum metallicity found in local galaxies [12 + log(O/H) ~ 7.1], although we need deeper spectra to minimize the errors. HST ACS images of several USELs exhibit widespread morphologies from relatively compact high surface brightness objects to very diffuse low surface brightness ones. The luminosities, metallicities, and star formation rates of USELs are consistent with the strong emitters being start-up intermediate-mass galaxies that will evolve into more normal galaxies and suggest that galaxies are still forming in relatively chemically pristine sites at z < 1.