Joshua J. Adams

The HETDEX pilot survey - II. The evolution of the Lyα escape fraction from the ultraviolet slope and luminosity function of 1.9 < z < 3.8 LAEs

We study the escape of Lyα photons from Lyα emitting galaxies (LAEs) and the overall galaxy population using a sample of 99 LAEs at 1.9 (3-6) × 1042 erg s–1 (0.25-0.5 L*), have a mean E(B – V) = 0.13 ± 0.01, implying an attenuation of ~70% in the UV. They show a median UV uncorrected SFR = 11 M ☉ yr–1, dust-corrected SFR = 34 M ☉ yr–1, and Lyα equivalent widths (EWs) which are consistent with normal stellar populations. We measure a median Lyα escape fraction of 29%, with a large scatter and values ranging from a few percent to 100%. The Lyα escape fraction in LAEs correlates with E(B – V) in a way that is expected if Lyα photons suffer from similar amounts of dust extinction as UV continuum photons. This result implies that a strong enhancement of the Lyα EW with dust, due to a clumpy multi-phase interstellar medium (ISM), is not a common process in LAEs at these redshifts. It also suggests that while in other galaxies Lyα can be preferentially quenched by dust due to its scattering nature, this is not the case in LAEs. We find no evolution in the average dust content and Lyα escape fraction of LAEs from z ~ 4 to 2. We see hints of a drop in the number density of LAEs from z ~ 4 to 2 in the redshift distribution and the Lyα luminosity function, although larger samples are required to confirm this. The mean Lyα escape fraction of the overall galaxy population decreases significantly from z ~ 6 to z ~ 2, in agreement with recent results. Our results point toward a scenario in which star-forming galaxies build up significant amounts of dust in their ISM between z ~ 6 and 2, reducing their Lyα escape fraction, with LAE selection preferentially detecting galaxies which have the highest escape fractions given their dust content. The fact that a large escape of Lyα photons is reached by z ~ 6 implies that better constraints on this quantity at higher redshifts might detect re-ionization in a way that is uncoupled from the effects of dust.

The HETDEX pilot survey. III. The low metallicities of high-redshift lyα galaxies

We present the results of Keck/NIRSPEC spectroscopic observations of three Lyα emitting galaxies (LAEs) at z~ 2.3 discovered with the HETDEX pilot survey. We detect Hα, [O III], and Hβ emission from two galaxies at z= 2.29 and 2.49, designated HPS194 and HPS256, respectively, representing the first detection of multiple rest-frame optical emission lines in galaxies at high redshift selected on the basis of their Lyα emission. We find that the redshifts of the Lyα emission from these galaxies are offset redward of the systemic redshifts (derived from the Hα and [O III] emission) by Δv = 162 ± 37 (photometric) ± 42 (systematic) km s–1 for HPS194 and Δv = 36 ± 35 ± 18 km s–1 for HPS256. An interpretation for HPS194 is that a large-scale outflow may be occurring in its interstellar medium. This outflow is likely powered by star-formation activity, as examining emission line ratios implies that neither LAE hosts an active galactic nucleus. Using the upper limits on the [N II] emission, we place meaningful constraints on the gas-phase metallicities in these two LAEs of Z< 0.17 and < 0.28 Z ☉ (1σ). Measuring the stellar masses of these objects via spectral energy distribution (SED) fitting (~1010 and 6 × 108 M ☉, respectively), we study the nature of LAEs in a mass-metallicity plane. At least one of these two LAEs appears to be more metal poor than continuum-selected star-forming galaxies at the same redshift and stellar mass, implying that objects exhibiting Lyα emission may be systematically less chemically enriched than the general galaxy population. We use the SEDs of these two galaxies to show that neglecting the contribution of the measured emission line fluxes when fitting stellar population models to the observed photometry can result in overestimates of the population age by orders of magnitude and the stellar mass by a factor of ~2. This effect is particularly important at z 7, where similarly strong emission lines may masquerade in the photometry as a 4000 A break.

THE HETDEX PILOT SURVEY. I. SURVEY DESIGN, PERFORMANCE, AND CATALOG OF EMISSION-LINE GALAXIES

We present a catalog of emission-line galaxies selected sol ly by their emission-line fluxes using a wide-field integral field spectrograph. This work is partially motivat ed as a pilot survey for the upcoming Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We describe the observations, reductions, detections, redshift classifications, line fluxes, and counterpart information f r 397 emission-line galaxies detected over 169 ⊓⊔ with a 3500-5800̊A bandpass under 5̊ A full-width-half-maximum (FWHM) spectral resolution. Th e survey’s best sensitivity for unresolved objects under photometric conditions is between 4− 20× 10 erg s cm depending on the wavelength, and Ly α luminosities between3− 6× 10 erg s are detectable. This survey method complements narrowband and color-selection techni ques in the search for high redshift galaxies with its different selection properties and large volume probed. Th e four survey fields within the COSMOS, GOODS-N, MUNICS, and XMM-LSS areas are rich with existing, complemen tary data. We find 104 galaxies via their high redshift Lyα emission at1.9 < z < 3.8, and the majority of the remainder objects are low redshift [ OII]3727 emitters atz < 0.56. The classification between low and high redshift objects de pends on rest frame equivalent width, as well as other indicators, where available. Based o n matches to X-ray catalogs, the active galactic nuclei (AGN) fraction amongst the Ly α emitters (LAEs) is 6%. We also analyze the survey’s complete ness and contamination properties through simulations. We find fi ve high-z, highly-significant, resolved objects with full-width-half-maximum sizes> 44 ⊓⊔ which appear to be extended Ly α nebulae. We also find three high-z objects with rest frame Ly α equivalent widths above the level believed to be achievable with normal star formation, EW0 > 240Å. Future papers will investigate the physical properties o f this sample. Subject headings: galaxies: formation — galaxies: evolution —galaxies: high -redshift — cosmology: observations

VIRUS: a massively replicated 33k fiber integral field spectrograph for the upgraded Hobby-Eberly Telescope

The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter, deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 96 units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. The method of industrial replication, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers, offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments. The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX+) using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed in late 2011 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope. VIRUS and HET will open up wide field surveys of the emission-line universe for the first time. We present the design, cost, and current status of VIRUS as it enters production, and review performance results from the VIRUS prototype. We also present lessons learned from our experience designing for volume production and look forward to the application of the VIRUS concept on future extremely large telescopes (ELTs).

The Central Dark Matter Distribution of NGC 2976

We study the mass distribution in the late-type dwarf galaxy NGC 2976 through stellar kinematics obtained with the Visible Integral Field Replicable Unit Spectrograph Prototype and anisotropic Jeans models as a test of cosmological simulations and baryonic processes that putatively alter small-scale structure. Previous measurements of the Hα emission-line kinematics have determined that the dark matter halo of NGC 2976 is most consistent with a cored density profile. We find that the stellar kinematics are best fit with a cuspy halo. Cored dark matter halo fits are only consistent with the stellar kinematics if the stellar mass-to-light ratio is significantly larger than that derived from stellar population synthesis, while the best-fitting cuspy model has no such conflict. The inferred mass distribution from a harmonic decomposition of the gaseous kinematics is inconsistent with that of the stellar kinematics. This difference is likely due to the gas disk not meeting the assumptions that underlie the analysis such as no pressure support, a constant kinematic axis, and planar orbits. By relaxing some of these assumptions, in particular the form of the kinematic axis with radius, the gas-derived solution can be made consistent with the stellar kinematic models. A strong kinematic twist in the gas of NGC 2976s center suggests caution, and we advance the mass model based on the stellar kinematics as more reliable. The analysis of this first galaxy shows promising evidence that dark matter halos in late-type dwarfs may in fact be more consistent with cuspy dark matter distributions than earlier work has claimed.

LIGHT-ELEMENT ABUNDANCE VARIATIONS AT LOW METALLICITY: THE GLOBULAR CLUSTER NGC 5466*

We present low-resolution (R � 850) spectra for 67 asymptotic giant branch (AGB), horizontal branch, and red giant branch (RGB) stars in the low-metallicity globular cluster NGC 5466, taken with the VIRUS-P integral-field spectrograph at the 2.7 m Harlan J. Smith telescope at McDonald Observatory. Sixty-six stars are confirmed, and one rejected, as cluster members based on radial velocity, which we measure to an accuracy of 16 km s −1 via template-matching techniques. CN and CH band strengths have been measured for 29 RGB and AGB stars in NGC 5466, and the band-strength indices measured from VIRUS-P data show close agreement with those measured from Keck/LRIS spectra previously taken for five of our target stars. We also determine carbon abundances from comparisons with synthetic spectra. The RGB stars in our data set cover a range in absolute V magnitude from +2 to −3, which permits us to study the rate of carbon depletion on the giant branch as well as the point of its onset. The data show a clear decline in carbon abundance with rising luminosity above the luminosity function “bump” on the giant branch, and also a subdued range in CN band strength, suggesting ongoing internal mixing in individual stars but minor or no primordial star-to-star variation in light-element abundances.

B2 0902+34: A collapsing protogiant elliptical galaxy at z = 3.4

We have used the visible integral-field replicable unit spectrograph prototype, a new integral field spectrograph, to study the spatially and spectrally resolved Lyman-α emission line structure in the radio galaxy B2 0902+34 at z = 3.4. We observe a halo of Lyman-α emission with a velocity dispersion of ≈250 km s −1 extending to a radius of 50 kpc. A second feature is revealed in a spatially resolved region where the line profile shows blueshifted structure. This may be viewed as either H i absorption at ≈−450 km s −1 or secondary emission at ≈−900 km s −1 from the primary peak. B2 0902+34 is also the only high-redshift radio galaxy with a detection of 21 cm absorption. Our new data, in combination with the 21 cm absorption, suggest two important and unexplained discrepancies. First, nowhere in the line profiles of the Lyman-α halo is the 21 cm absorber population evident. Second, the 21 cm absorption redshift is higher than the Lyman-α emission redshift. In an effort to explain these two traits, we have undertaken the first three-dimensional Monte Carlo simulations of resonant scattering in radio galaxies. We have created a simple model with two photoionized cones embedded in a halo of neutral hydrogen. Lyman-α photons propagate from these cones through the optically thick H i halo until reaching the virial radius. Though simple, the model produces the features in the Lyman-α data and predicts the 21 cm properties. To reach agreement between this model and the data, global infall of the H i is strictly necessary. The amount of gas necessary to match the model and data is surprisingly high, 10 12 M� , an order of magnitude larger than the stellar mass. The collapsing structure and large gas mass lead us to interpret B2 0902+34 as a protogiant elliptical galaxy. This interpretation is a falsifiable alternative to the presence of extended H i shells ejected through feedback events such as starburst superwinds. An understanding of these gas features and a classification of this system’s evolutionary state give unique observational evidence of the formation events in massive galaxies.

Volume phase holographic grating performance on the VIRUS-P instrument

The Visible Integral-field Replicable Unit Spectrograph Prototype (VIRUS-P) has been in operation on the Harlan J Smith 2.7m Telescope at McDonald Observatory since October of 2006. The prototype was created to test the design and science capabilities of the full VIRUS instrument, wherein 150 copies of the spectrograph will be installed on the Hobby Eberly Telescope (HET). We here discuss the specialized test bench built to assess the blue optimized Volume Phase Holographic (VPH) grating performance. We also give lab and on-telescope efficiency measurements for three such gratings in the wavelength range 3400-6800Å. Two sources of stray light relevant to most spectrograph designs are also discussed.