arXiv subjects

Mimi Song

Publications and source records attributed to Mimi Song.

25 records · Page 2Linked to original sources

arXiv2014

Rapid Decline of Lyman-alpha Emission Toward the Reionization Era

The observed deficit of strongly Lyman-alpha emitting galaxies at z>6.5 is attributed to either increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this, we have performed very deep near-IR spectroscopy of z>7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lyman-alpha fraction at z~8 (combined photometric redshift peak at z=7.7) using two methods. First, we derived NLyα/Ntot directly using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al. 2012) that compares the z>7 galaxy spectra to models of the Lyman-alpha equivalent width (WLyα) distribution at z~6. We explored two simple evolutionary scenarios: smooth evolution where Lyman-alpha is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity), and patchy evolution where Lyman-alpha is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature we find that at z~8 the Lyman-alpha fraction has dropped by a factor >3(84% confidence interval) using both the smooth and patchy scenarios compared to the z~6 values. Furthermore, we find a tentative evidence that the data favor the patchy scenario over smooth (with "positive" Bayesian evidence), extending trends observed at z~7 to higher redshift. If this decrease is a result of reionization as predicted by theory, then our data imply the volume averaged neutral hydrogen fraction in the IGM to be >0.3 suggesting that the reionization of the universe is in progress at z~8.

astro-ph.CO↗

arXiv2014

The HETDEX Pilot Survey V: The Physical Origin of Lyman-alpha Emitters Probed by Near-infrared Spectroscopy

We present the results from a VLT/SINFONI and Keck/NIRSPEC near-infrared spectroscopic survey of 16 Lyman-alpha emitters (LAEs) at $z$ = 2.1 - 2.5 in the COSMOS and GOODS-N fields discovered from the HETDEX Pilot Survey. We detect rest-frame optical nebular lines (H$α$ and/or [OIII]$λ$5007) for 10 of the LAEs and measure physical properties, including the star formation rate (SFR), gas-phase metallicity, gas-mass fraction, and Ly$α$ velocity offset. We find that LAEs may lie below the mass-metallicity relation for continuum-selected star-forming galaxies at the same redshift. The LAEs all show velocity shifts of Ly$α$ relative to the systemic redshift ranging between +85 and +296 km s$^{-1}$ with a mean of +180 km s$^{-1}$. This value is smaller than measured for continuum-selected star-forming galaxies at similar redshifts. The Ly$α$ velocity offsets show a moderate correlation with the measured star formation rate (2.5$σ$), but no significant correlations are seen with the SFR surface density, specific SFR, stellar mass, or dynamical mass ($\lesssim$ 1.5$σ$). Exploring the role of dust, kinematics of the interstellar medium (ISM), and geometry on the escape of Ly$α$ photons, we find no signature of selective quenching of resonantly scattered Ly$α$ photons. However, we also find no evidence that a clumpy ISM is enhancing the Ly$α$ equivalent width. Our results suggest that the low metallicity in LAEs may be responsible for yielding an environment with a low neutral hydrogen column density as well as less dust, easing the escape of Ly$α$ photons over that in continuum-selected star-forming galaxies.

astro-ph.GA↗

arXiv2014

Spectral Energy Distribution Fitting of HETDEX Pilot Survey Lyman-alpha Emitters in COSMOS and GOODS-N

We use broadband photometry extending from the rest-frame UV to the near-IR to fit the individual spectral energy distributions (SEDs) of 63 bright (L(Ly-alpha) > 10^43 ergs/s) Ly-alpha emitting galaxies (LAEs) in the redshift range 1.9 < z < 3.6. We find that these LAEs are quite heterogeneous, with stellar masses that span over three orders of magnitude, from 7.5 < log M < 10.5. Moreover, although most LAEs have small amounts of extinction, some high-mass objects have stellar reddenings as large as E(B-V) ~0.4. Interestingly, in dusty objects the optical depths for Ly-alpha and the UV continuum are always similar, indicating that Ly-alpha photons are not undergoing many scatters before escaping their galaxy. In contrast, the ratio of optical depths in low-reddening systems can vary widely, illustrating the diverse nature of the systems. Finally, we show that in the star formation rate (SFR)-log mass diagram, our LAEs fall above the "main-sequence" defined by z ~ 3 continuum selected star-forming galaxies. In this respect, they are similar to sub-mm-selected galaxies, although most LAEs have much lower mass.

astro-ph.GA↗

arXiv2013

The Spectrally Resolved Lyman-alpha Emission of Three Lyman-alpha Selected Field Galaxies at z~2.4 from the HETDEX Pilot Survey

We present new results on the spectrally resolved Lyman-alpha (LyA) emission of three LyA emitting field galaxies at z~2.4 with high LyA equivalent width (>100 Angstroms) and LyA luminosity (~10^43 erg/s). At 120 km/s (FWHM) spectral resolution, the prominent double-peaked LyA profile straddles the systemic velocity, where the velocity zero-point is determined from spectroscopy of the galaxies' rest-frame optical nebular emission lines. The average velocity offset from systemic of the stronger redshifted emission component for our sample is 176 km/s while the average total separation between the redshifted and main blueshifted emission components is 380 km/s. These measurements are a factor of ~2 smaller than for UV continuum-selected galaxies that show LyA in emission with lower LyA equivalent width. We compare our LyA spectra to the predicted line profiles of a spherical "expanding shell" LyA radiative transfer grid that models large-scale galaxy outflows. Specifically blueward of the systemic velocity where two galaxies show a weak, highly blueshifted (by ~1000 km/s) tertiary emission peak, the model line profiles are a relatively poor representation of the observed spectra. Since the neutral gas column density has a dominant influence over the shape of the LyA line profile, we caution against equating the observed LyA velocity offset with a physical outflow velocity, especially at lower spectral resolution where the unresolved LyA velocity offset is a convoluted function of several degenerate parameters. Referring to rest-frame ultraviolet and optical Hubble Space Telescope imaging, we find that galaxy-galaxy interactions may play an important role in inducing a starburst that results in copious LyA emission, as well as perturbing the gas distribution and velocity field which have strong influence over the LyA emission line profile.

astro-ph.CO↗

arXiv2013

The VIRUS-P Exploration of Nearby Galaxies (VENGA): Survey Design, Data Processing, and Spectral Analysis Methods

We present the survey design, data reduction, and spectral fitting pipeline for the VIRUS-P Exploration of Nearby Galaxies (VENGA). VENGA is an integral field spectroscopic survey, which maps the disks of 30 nearby spiral galaxies. Targets span a wide range in Hubble type, star formation activity, morphology, and inclination. The VENGA data-cubes have 5.6'' FWHM spatial resolution, ~5A FWHM spectral resolution, sample the 3600A-6800A range, and cover large areas typically sampling galaxies out to ~0.7 R_25. These data-cubes can be used to produce 2D maps of the star formation rate, dust extinction, electron density, stellar population parameters, the kinematics and chemical abundances of both stars and ionized gas, and other physical quantities derived from the fitting of the stellar spectrum and the measurement of nebular emission lines. To exemplify our methods and the quality of the data, we present the VENGA data-cube on the face-on Sc galaxy NGC 628 (a.k.a. M 74). The VENGA observations of NGC 628 are described, as well as the construction of the data-cube, our spectral fitting method, and the fitting of the stellar and ionized gas velocity fields. We also propose a new method to measure the inclination of nearly face-on systems based on the matching of the stellar and gas rotation curves using asymmetric drift corrections. VENGA will measure relevant physical parameters across different environments within these galaxies, allowing a series of studies on star formation, structure assembly, stellar populations, chemical evolution, galactic feedback, nuclear activity, and the properties of the interstellar medium in massive disk galaxies.

astro-ph.CO↗

arXiv2012

The VIRUS-P Exploration of Nearby Galaxies (VENGA): The Xco Gradient in NGC 628

We measure the radial profile of the 12CO(1-0) to H_2 conversion factor (Xco) in NGC 628. The Hαemission from the VENGA integral field spectroscopy is used to map the star formation rate surface density (Σ_{SFR}). We estimate the molecular gas surface density (Σ_{H2}) from Σ_{SFR} by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure Xco. We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (Hαvs. far-UV plus 24μm), and CO maps from different telescopes (single-dish and interferometers). The observed Xco profile is robust against these systematics, drops by a factor of 2 from R~7 kpc to the center of the galaxy, and is well fit by a gradient Δlog(Xco)=0.06\pm0.02 dex kpc^-1. We study how changes in Xco follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in Xco can be explained by a combination of decreasing metallicity, and decreasing Σ_{H2} with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of Xco in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single Xco value when large changes in gas surface density or metallicity are present.

astro-ph.GA↗

arXiv2008

Seoul National University Bright Quasar Survey in Optical (SNUQSO). II. Discovery of 40 Bright Quasars Near the Galactic Plane

We report the discovery of 40 bright quasars and active galactic nuclei (AGNs) at low Galactic latitude (b<20deg). The low Galactic latitude region has been considered a place to avoid when searching for extragalactic sources, because of the high Galactic extinction, as well as a large number of stars contaminating the sample selection. Bright quasars (R<~17) suffer more from such difficulties because they look like bright stars, which are numerous at low b, yet their surface number density is very low. In order to find quasars in this region of the sky less explored for extragalactic sources, we have started a survey of low Galactic latitude bright quasars as a part of the Seoul National University Quasar Survey in Optical (SNUQSO). Quasar candidates have been selected from radio and near-infrared (NIR) data. Out of 88 targets, we identify 29 bright quasars/AGNs around the antigalactic center, and 11 bright quasars/AGNs in the outskirts of the Galactic center, from two observing runs in 2006 at the Bohyunsan Optical Astronomical Observatory (BOAO) in Korea. Our finding demonstrates that quasars/AGNs can be discovered effectively, even at low Galactic latitude, using multiwavelength data.

astro-ph↗