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Register a new userWhat is the Physical Origin of Strong Lya Emission? I. Demographics of Lya Emitter Structures
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We present the results of structure analyses for a large sample of 426 Lya emitters (LAEs) at z~2.2 that are observed with HST/ACS and WFC3-IR by deep extra-galactic legacy surveys. We confirm that the merger fraction and the average ellipticity of LAEs stellar component are 10-30 % and 0.4-0.6, respectively, that are comparable with previous study results. We successfully identify that some LAEs have a spatial offset between Lya and stellar-continuum emission peaks, d_Lya, by ~2.5-4 kpc beyond our statistical errors. To uncover the physical origin of strong Lya emission found in LAEs, we investigate Lya equivalent width (EW) dependences of these three structural parameters, merger fraction, d_Lya, and ellipticity of stellar distribution in the range of EW(Lya)=20-250A. Contrary to expectations, we find that merger fraction does not significantly increase with Lya EW. We reveal an anti-correlation between d_Lya and EW(Lya) by Kolmogorov-Smirnov (KS) test. There is a trend that the LAEs with a large Lya EW have a small ellipticity. This is consistent with the recent theoretical claims that Lya photons can more easily escape from face-on disks having a small ellipticity, due to less inter-stellar gas along the line of sight, although our KS test indicates that this trend is not statistically significant. Our results of Lya-EW dependence generally support the idea that an HI column density is a key quantity determining Lya emissivity.
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We present a statistical study of velocities of Lya, interstellar (IS) absorption, and nebular lines and gas covering fraction for Lya emitters (LAEs) at z~2. We make a sample of 22 LAEs with a large Lya equivalent width (EW) of > 50A based on our deep Keck/LRIS observations, in conjunction with spectroscopic data from the Subaru/FMOS program and the literature. We estimate the average velocity offset of Lya from a systemic redshift determined with nebular lines to be dv_Lya=234+-9 km s-1. Using a Kolmogorv-Smirnov test, we confirm the previous claim of Hashimoto et al. (2013) that the average dv_Lya of LAEs is smaller than that of LBGs. Our LRIS data successfully identify blue-shifted multiple IS absorption lines in the UV continua of four LAEs on an individual basis. The average velocity offset of IS absorption lines from a systemic redshift is dv_IS=204+-27 km s-1, indicating LAEs gas outflow with a velocity comparable to typical LBGs. Thus, the ratio, R^Lya_ IS = dv_Lya/dv_IS of LAEs, is around unity, suggestive of low impacts on Lya transmission by resonant scattering of neutral hydrogen in the IS medium. We find an anti-correlation between Lya EW and the covering fraction, f_c, estimated from the depth of absorption lines, where f_c is an indicator of average neutral hydrogen column density, N_HI. The results of our study support the idea that N_HI is a key quantity determining Lya emissivity.
We construct a flux-limited sample of 135 candidate z~1 Lya emitters (LAEs) from Galaxy Evolution Explorer (GALEX) grism data using a new data cube search method. These LAEs have luminosities comparable to those at high redshifts and lie within a 7 Gyr gap present in existing LAE samples. We use archival and newly obtained optical spectra to verify the UV redshifts of these LAEs. We use the combination of the GALEX UV spectra, optical spectra, and X-ray imaging data to estimate the active galactic nucleus (AGN) fraction and its dependence on Lya luminosity. We remove the AGNs and compute the luminosity function (LF) from 60 z~1 LAE galaxies. We find that the best fit LF implies a luminosity density increase by a factor of ~1.5 from z~0.3 to z~1 and ~20 from z~1 to z~2. We find a z~1 volumetric Lya escape fraction of 0.7+/-0.4%.
With the Blue Channel Spectrograph (BCS) on the MMT telescope, we have obtained spectra to the atmospheric cutoff of quasars previously known to show at least one absorption system at z>1.6 with very strong metal lines (candidate metal-strong damped Lya systems; cMSDLAs). The BCS/MMT spectra yield precise estimates of the HI column densities (NHI) of the systems through Voigt profile analysis of their Lya transitions. Nearly all of the cMSDLAs (41/43) satisfy the NHI criterion of DLAs, 10^20.3. As a population, these systems have systematically higher NHI values than DLAs chosen randomly from quasar sightlines. Combining our NHI measurements with previously measured metal column densities, we estimate metallicities for the MSDLAs. These systems have significantly higher values than randomly selected DLAs; at z~2, the MSDLAs show a median metallicity [M/H] ~ -0.67 that is 0.6dex higher than a corresponding control sample. This establishes MSDLAs as having amongst the most metal-rich gas in the high z universe. Our measurements extend the observed correlation between SiII 1526 equivalent width and the gas metallicity to higher values. If interpreted as a mass-metallicity relation, this implies the MSDLAs are the high mass subset of the DLA population. We demonstrate that dust in the MSDLAs reddens their background quasars, with a median shift in the spectral slope of Da = 0.29. Assuming an SMC extinction law, this implies a median reddening E(B-V)=0.025mag and visual extinction A_V=0.076mag. Future studies of MSDLAs offer the opportunity to study the extinction, nucleosynthesis, and kinematics of the most chemically evolved, gas-rich galaxies at high z. [abridged]
We report the discovery of a giant Lya emitter (LAE) with a Spitzer/IRAC counterpart near the reionization epoch at z=6.595. The giant LAE is found from the extensive 1 deg^2 Subaru narrow-band survey for z=6.6 LAEs in the Subaru/XMM-Newton Deep Survey (SXDS) field, and subsequently identified by deep spectroscopy of Keck/DEIMOS and Magellan/IMACS. Among our 207 LAE candidates, this LAE is not only the brightest narrow-band object with L(Lya) = 3.9+/-0.2 x 10^43 erg/s in our survey volume of 10^6 Mpc^3, but also a spatially extended Lya nebula with the largest isophotal area whose major axis is at least ~3. This object is more likely to be a large Lya nebula with a size of >~17 kpc than to be a strongly-lensed galaxy by a foreground object. Our Keck spectrum with medium-high spectral and spatial resolutions suggests that the velocity width is v(FWHM)=251+/-21 km/s, and that the line-center velocity changes by ~60 km/s in a 10-kpc range. The stellar mass and star-formation rate are estimated to be 0.9-5.0 x 10^10 Mo and >34 Mo/yr, respectively, from the combination of deep optical to infrared images of Subaru, UKIDSS-Ultra Deep Survey, and Spitzer/IRAC. Although the nature of this object is not yet clearly understood, this could be an important object for studying cooling clouds accreting onto a massive halo, or forming-massive galaxies with significant outflows contributing to cosmic reionization and metal enrichment of inter-galactic medium.
We have taken a deep, moderate-resolution Keck/Deimos spectra of QSO, CFHQS2329, at z=6.4. At the wavelength of Lya, the spectrum shows a spatially-extended component, which is significantly more extended than a stellar spectrum, and also a continuum part of the spectrum. The restframe line width of the extended component is 21+-7 A, and thus smaller than that of QSO (52+-4 A), where they should be identical if the light is incomplete subtraction of the QSO component. Therefore, these comparisons argue for the detection of a spatially extended Lya nebulae around this QSO. This is the first z>6 QSO that an extended Lya halo has been observed around. Careful subtraction of the central QSO spectrum reveals a lower limit to the Lya luminosity of (1.7+-0.1)x 10^43 erg s^-1. This emission may be from the theoretically predicted infalling gas in the process of forming a primordial galaxy that is ionized by a central QSO. On the other hand, if it is photoionized by the host galaxy, an estimated star-formation rate of >3.0 Msun yr^-1 is required. If we assume the gas is virialized, we obtain dynamical mass estimate of Mdyn=1.2x10^12 Msun. The derived MBH/Mhost is 2.1x10^-4, which is two orders smaller than those from more massive z~6 QSOs, and places this galaxy in accordance with the local M-sigma relation, in contrast to a previous claim on the evolution of M-sigma relation at z~6. We do not claim evolution or non-evolution of the M-sigma relation based on a single object, but our result highlights the importance of investigating fainter QSOs at z~6.
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