Abstract
The Lyman continuum (LyC) cannot be observed at the epoch of reionization (
z
≳ 6) owing to intergalactic H
i
absorption. To identify LyC emitters (LCEs) and infer the fraction of escaping ...LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Ly
α
, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the O
iii
/O
ii
flux ratio and star formation rate surface density, predict whether a galaxy is an LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H
i
column density, ionization parameter, and stellar feedback. We find that LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.
Abstract
Star-forming galaxies are considered the likeliest source of the H
i
ionizing Lyman continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above
z
≳ 6, ...the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of
indirect
indicators for LyC that can be calibrated at lower redshifts and applied in the epoch of reionization. Emission from the Mg
ii
λλ
2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for eight LyC emitter candidates, selected to have strong Mg
ii
emission lines. We securely detect LyC emission in 50% (4/8) of the galaxies with 2
σ
significance. This high detection rate suggests that strong Mg
ii
emitters might be more likely to leak LyC than similar galaxies without strong Mg
ii
. Using photoionization models, we constrain the escape fraction of Mg
ii
as ∼15%–60%. We confirm that the escape fraction of Mg
ii
correlates tightly with that of Ly
α
, which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg
ii
emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg
ii
emission can be adopted to estimate the escape fraction of Ly
α
and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization.
The abundance of interstellar molecular nitrogen (N2) is of considerable importance: models of steady-state gas-phase interstellar chemistry, together with millimetre-wavelength observations of ...interstellar N2H+ in dense molecular clouds predict that N2 should be the most abundant nitrogen-bearing molecule in the interstellar medium. Previous attempts to detect N2 absorption in the far-ultraviolet or infrared (ice features) have hitherto been unsuccessful. Here we report the detection of interstellar N2 at far-ultraviolet wavelengths towards the moderately reddened star HD 124314 in the constellation of Centaurus. The N2 column density is larger than expected from models of diffuse clouds and significantly smaller than expected for dense molecular clouds. Moreover, the N2 abundance does not explain the observed variations in the abundance of atomic nitrogen (N I) towards high-column-density sightlines, implying that the models of nitrogen chemistry in the interstellar medium are incomplete.
ABSTRACT We have used the unique far-UV imaging capability offered by a sounding-rocket-borne instrument to acquire observations of C/2012 S1 (ISON) when its angular separation with respect to the ...Sun was 26 3 on 2013 November 20.49. At the time of observation, the comet's heliocentric distance and velocity relative to the Sun were rh = 0.43 au and = −62.7 km s−1. Images dominated by C i λ1657 and H i λ1216 were acquired over a 106 × 106 km2 region. The water production rate implied by the Ly observations is constrained to be 8 × 1029 s−1 while the neutral carbon production rate was 4 ×1028 s−1. The radial profile of C i was consistent with it being a dissociation product of a parent molecule with a lifetime τ ∼ 5 × 104 s, favoring a parent other than CO. We constrain the QCO production rate to 1028 s−1 with 1 errors derived from photon statistics. The upper limit on the QCO/ is 6%.
We present new infrared observations of the emission/reflection nebula IC 405 obtained with the Spitzer Space Telescope. Infrared images in the four IRAC bands (3.6,4.5, 5.8, and 8.0 km) and two MIPS ...bands (24 and 70 km) are complemented by IRS spectroscopy (5-30 km) of two nebular filaments. The IRAC (8.0 km) and MIPS imaging shows evidence of a bow shock associated with the runaway 09.5 V star, HD 34078, created by the interaction between the star and nebular material. The ratio of emission at 24 to 70 km is higher in the immediate vicinity of HD 34078 than in the outer filaments, providing evidence for elevated dust temperatures (T sub(d) 90 K) in the shock region. The nebular imaging reveals that the morphology is band dependent, with varying contributions from aromatic emission features, H sub(2), and dust emission. Nebular spectroscopy is used to quantify these contributions, showing several aromatic emission bands between 6-14 km, the S(5), S(3), S(2), and S(1) pure rotational emission lines of H sub(2), and atomic fine-structure lines of Ne, S, and Ar. The low-dispersion spectra provide constraints on the ionization state of the large molecules responsible for the aromatic infrared features. H sub(2) rotational temperatures of the two bright nebular filaments are determined from the observed line strengths. An average T(H sub(2)) 6 400 K is inferred, with evidence for additional nonuniform excitation by UV photons in the intense radiation field of HD 34078. The photoexcitation hypothesis is supported by direct measurement of the far-UV H sub(2) fluorescence spectrum, obtained with FUSE.
We demonstrate that the G140L Segment B channel of the Cosmic Origins Spectrograph recently installed on the Hubble Space Telescope (HST) has an effective area consistent with ~10 cm2 in the bandpass ...between the Lyman edge at 912 A and Lyman Delta *b, rising to a peak in excess of 1000 cm2 longward of 1130 A. This is a new wavelength regime for HST and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale length of the hardness variation in the metagalactic ionizing background over the redshift range 2 3 can now be carried out with unprecedented sensitivity.
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