Jeremy D. Murphy

Two ten-billion-solar-mass black holes at the centres of giant elliptical galaxies

Observational work conducted over the past few decades indicates that all massive galaxies have supermassive black holes at their centres. Although the luminosities and brightness fluctuations of quasars in the early Universe suggest that some were powered by black holes with masses greater than 10 billion solar masses, the remnants of these objects have not been found in the nearby Universe. The giant elliptical galaxy Messier 87 hosts the hitherto most massive known black hole, which has a mass of 6.3 billion solar masses. Here we report that NGC 3842, the brightest galaxy in a cluster at a distance from Earth of 98 megaparsecs, has a central black hole with a mass of 9.7 billion solar masses, and that a black hole of comparable or greater mass is present in NGC 4889, the brightest galaxy in the Coma cluster (at a distance of 103 megaparsecs). These two black holes are significantly more massive than predicted by linearly extrapolating the widely used correlations between black-hole mass and the stellar velocity dispersion or bulge luminosity of the host galaxy. Although these correlations remain useful for predicting black-hole masses in less massive elliptical galaxies, our measurements suggest that different evolutionary processes influence the growth of the largest galaxies and their black holes.

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. 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

DWARF GALAXY DARK MATTER DENSITY PROFILES INFERRED FROM STELLAR AND GAS KINEMATICS

We present new constraints on the density profiles of dark matter (DM) halos in seven nearby dwarf galaxies from measurements of their integrated stellar light and gas kinematics. The gas kinematics of low-mass galaxies frequently suggest that they contain constant density DM cores, while N-body simulations instead predict a cuspy profile. We present a data set of high-resolution integral-field spectroscopy on seven galaxies and measure the stellar and gas kinematics simultaneously. Using Jeans modeling on our full sample, we examine whether gas kinematics in general produce shallower density profiles than are derived from the stars. Although two of the seven galaxies show some localized differences in their rotation curves between the two tracers, estimates of the central logarithmic slope of the DM density profile, γ, are generally robust. The mean and standard deviation of the logarithmic slope for the population are γ = 0.67 ± 0.10 when measured in the stars and γ = 0.58 ± 0.24 when measured in the gas. We also find that the halos are not under-concentrated at the radii of half their maximum velocities. Finally, we search for correlations of the DM density profile with stellar velocity anisotropy and other baryonic properties. Two popular mechanisms to explain cored DM halos are an exotic DM component or feedback models that strongly couple the energy of supernovae into repeatedly driving out gas and dynamically heating the DM halos. While such models do not yet have falsifiable predictions that we can measure, we investigate correlations that may eventually be used to test models. We do not find a secondary parameter that strongly correlates with the central DM density slope, but we do find some weak correlations. The central DM density slope weakly correlates with the abundance of α elements in the stellar population, anti-correlates with H I fraction, and anti-correlates with vertical orbital anisotropy. We expect, if anything, the opposite of these three trends for feedback models. Determining the importance of these correlations will require further model developments and larger observational samples.

The MASSIVE survey. I. A volume-limited integral-field spectroscopic study of the most massive early-type galaxies within 108 Mpc

Massive early-type galaxies represent the modern-day remnants of the earliest major star formation episodes in the history of the universe. These galaxies are central to our understanding of the evolution of cosmic structure, stellar populations, and supermassive black holes, but the details of their complex formation histories remain uncertain. To address this situation, we have initiated the MASSIVE Survey, a volume-limited, multi-wavelength, integral-field spectroscopic (IFS) and photometric survey of the structure and dynamics of the ~100 most massive early-type galaxies within a distance of 108 Mpc. This survey probes a stellar mass range M* > 10^{11.5} Msun and diverse galaxy environments that have not been systematically studied to date. Our wide-field IFS data cover about two effective radii of individual galaxies, and for a subset of them, we are acquiring additional IFS observations on sub-arcsecond scales with adaptive optics. We are also acquiring deep K-band imaging to trace the extended halos of the galaxies and measure accurate total magnitudes. Dynamical orbit modeling of the combined data will allow us to simultaneously determine the stellar, black hole, and dark matter halo masses. The primary goals of the project are to constrain the black hole scaling relations at high masses, investigate systematically the stellar initial mass function and dark matter distribution in massive galaxies, and probe the late-time assembly of ellipticals through stellar population and kinematical gradients. In this paper, we describe the MASSIVE sample selection, discuss the distinct demographics and structural and environmental properties of the selected galaxies, and provide an overview of our basic observational program, science goals and early survey results.

THE STELLAR HALOS OF MASSIVE ELLIPTICAL GALAXIES. II. DETAILED ABUNDANCE RATIOS AT LARGE RADIUS

We study the radial dependence in stellar populations of 33 nearby early-type galaxies with central stellar velocity dispersions σ{sub *} ∼> 150 km s{sup –1}. We measure stellar population properties in composite spectra, and use ratios of these composites to highlight the largest spectral changes as a function of radius. Based on stellar population modeling, the typical star at 2R{sub e} is old (∼10 Gyr), relatively metal-poor ([Fe/H] ≈ –0.5), and α-enhanced ([Mg/Fe] ≈ 0.3). The stars were made rapidly at z ≈ 1.5-2 in shallow potential wells. Declining radial gradients in [C/Fe], which follow [Fe/H], also arise from rapid star formation timescales due to declining carbon yields from low-metallicity massive stars. In contrast, [N/Fe] remains high at large radius. Stars at large radius have different abundance ratio patterns from stars in the center of any present-day galaxy, but are similar to average Milky Way thick disk stars. Our observations are thus consistent with a picture in which the stellar outskirts are built up through minor mergers with disky galaxies whose star formation is truncated early (z ≈ 1.5-2)

The MASSIVE Survey. II. Stellar Population Trends Out to Large Radius in Massive Early-type Galaxies

We examine stellar population gradients in ~100 massive early type galaxies spanning 180 < sigma* < 370 km/s and M_K of -22.5 to -26.5 mag, observed as part of the MASSIVE survey (Ma et al. 2014). Using integral-field spectroscopy from the Mitchell Spectrograph on the 2.7m telescope at McDonald Observatory, we create stacked spectra as a function of radius for galaxies binned by their stellar velocity dispersion, stellar mass, and group richness. With excellent sampling at the highest stellar mass, we examine radial trends in stellar population properties extending to beyond twice the effective radius (~2.5 R_e). Specifically, we examine trends in age, metallicity, and abundance ratios of Mg, C, N, and Ca, and discuss the implications for star formation histories and elemental yields. At a fixed physical radius of 3-6 kpc (the likely size of the galaxy cores formed at high redshift) stellar age and [alpha/Fe] increase with increasing sigma* and depend only weakly on stellar mass, as we might expect if denser galaxies form their central cores earlier and faster. If we instead focus on 1-1.5 R_e, the trends in abundance and abundance ratio are washed out, as might be expected if the stars at large radius were accreted by smaller galaxies. Finally, we show that when controlling for \sigmastar, there are only very subtle differences in stellar population properties or gradients as a function of group richness; even at large radius internal properties matter more than environment in determining star formation history.

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).

Focal ratio degradation and transmission in VIRUS-P optical fibers

We have conducted extensive tests of both transmission and focal ratio degradation (FRD) on two integral field units currently in use on the VIRUS-P integral field spectrograph. VIRUS-P is a prototype for the VIRUS instrument proposed for the Hobby-Eberly Telescope at McDonald Observatory. All tests have been conducted at an input f-ratio of F/3.65 and with an 18% central obscuration in order to simulate optical conditions on the HET. Transmission measurements were conducted with narrow-band interference filters (FWHM: 10 nm) at 10 discrete wavelengths (337 to 600 nm), while FRD tests were made at 365 nm, 400 nm and 600 nm. The influence of wavelength, end immersion, fiber type and length on both FRD and transmission is explored. Most notably, we find no wavelength dependence on FRD down to 365 nm. All fibers tested are within the VIRUS instrument specifications for both FRD and transmission. We present the details of our differential FRD testing method and explain a simple and robust technique of aligning the test bench and optical fiber axes to within ±0.1 degrees.

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.