10 research outputs found
The Low-Redshift Lyman Continuum Survey. Unveiling the ISM properties of low- Lyman continuum emitters
Combining 66 ultraviolet (UV) spectra and ancillary data from the
Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier
studies, we form a statistical sample of star-forming galaxies at
to study the role of the cold interstellar medium (ISM) gas in the leakage of
ionizing radiation. We first constrain the massive star content (ages and
metallicities) and UV attenuation, by fitting the stellar continuum with a
combination of simple stellar population models. The models, together with
accurate LyC flux measurements, allow to determine the absolute LyC photon
escape fraction for each galaxy (). We measure the
equivalent widths and residual fluxes of multiple HI and low-ionization state
(LIS) lines, and the geometrical covering fraction adopting the picket-fence
model. The spans a wide range, with a median (0.16,
0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in
the LyC. The HI and LIS line equivalent widths scale with the UV luminosity and
attenuation, and inversely with the residual flux of the lines. The HI and LIS
residual fluxes are correlated, indicating that the neutral gas is spatially
traced by the LIS transitions. We find the observed trends of the absorption
lines and the UV attenuation are primarily driven by the covering fraction. The
non-uniform gas coverage demonstrates that LyC photons escape through
low-column density channels in the ISM. The equivalent widths and residual
fluxes of the UV lines strongly correlate with : strong
LyC leakers show weak absorption lines, low UV attenuation, and large
Ly equivalent widths. We finally show that simultaneous UV absorption
line and dust attenuation measurements can predict, on average, the escape
fraction of galaxies and the method can be applied to galaxies across a wide
redshift range.Comment: 30 pages, 16 figures, 3 tables; accepted for publication in Astronomy
and Astrophysics on December 16, 2021. Tables A1 to A4 are part of the LzLCS
science products and will be publicly available in a dedicated websit
The Low-Redshift Lyman Continuum Survey. Unveiling the ISM properties of low-z Lyman-continuum emitters
Aims: Combining 66 ultraviolet (UV) spectra and ancillary data from the recent Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier studies, we form a statistical sample of star-forming galaxies at z ⌠0.2 â 0.4 with which we study the role of cold interstellar medium (ISM) gas in the leakage of ionizing radiation. We also aim to establish empirical relations between the H I neutral and low-ionization state (LIS) absorption lines with different galaxy properties. Methods: We first constrain the massive star content (stellar ages and metallicities) and UV attenuation by fitting the stellar continuum with a combination of simple stellar population models. The models, together with accurate LyC flux measurements, allow us to determine the absolute LyC photon escape fraction for each galaxy (fescabs). We then measure the equivalent widths and residual fluxes of multiple H I and LIS lines, and the geometrical covering fraction of the UV emission, adopting the picket-fence model. Results: The LyC escape fraction spans a wide range, with a median fescabs (0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC (1Ï upper limits of fescabs âČ 0.01 for non-detections, typically). The H I and LIS line equivalent widths scale with the UV luminosity and attenuation, and inversely with the residual flux of these lines. Additionally, Lyα equivalent widths scale with both the H I and LIS residual fluxes, but anti-correlate with the corresponding H I or LIS equivalent widths. The H I and LIS residual fluxes are correlated, indicating that the neutral gas is spatially traced by the low-ionization transitions. We find that the observed trends of the absorption lines and the UV attenuation are primarily driven by the geometric covering fraction of the gas. The observed nonuniform gas coverage also demonstrates that LyC photons escape through low-column-density channels in the ISM. The equivalent widths and residual fluxes of both the H I and LIS lines strongly correlate with fescabs: strong LyC leakers (highest fescabs) show weak absorption lines, low UV attenuation, and large Lyα equivalent widths. We provide several empirical calibrations to estimate fescabs from UV absorption lines. Finally, we show that simultaneous UV absorption line and dust attenuation measurements can, in general, predict the escape fraction of galaxies. We apply our method to available measurements of UV LIS lines of 15 star-forming galaxies at z ⌠4 â 6 (plus 3 high-z galaxy composites), finding that these high-redshift, UV-bright galaxies (MUV âČ â21) may have low escape fractions, fescabs âČ 0.1. Conclusions: UV absorption lines trace the cold ISM gas of galaxies, which governs the physics of the LyC escape. We show that, with some assumptions, the absolute LyC escape can be statistically predicted using UV absorption lines, and the method can be applied to study galaxies across a wide redshift range, including in the epoch of cosmic reionization
Tracing Lyα and LyC Escape in Galaxies with Mg II Emission
International audienceStar-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 âł 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 λλ2796, 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 2Ï 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 Lyα, 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 Lyα and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization
The Low-redshift Lyman Continuum Survey. I. New, Diverse Local Lyman Continuum Emitters
The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the Epoch of Reionization is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with the Hubble Space Telescope Cosmic Origins Spectrograph for a z = 0.2-0.4 sample of 66 galaxies. After careful processing of the far-UV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape
The Low-redshift Lyman Continuum Survey. II. New Insights into LyC Diagnostics
International audienceThe Lyman continuum (LyC) cannot be observed at the epoch of reionization (z Ⳡ6) 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 Lyα, 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
The Low-Redshift Lyman Continuum Survey I: New, Diverse Local Lyman-Continuum Emitters
The origins of Lyman continuum (LyC) photons responsible for the reionization
of the universe are as of yet unknown and highly contested. Detecting LyC
photons from the epoch of reionization is not possible due to absorption by the
intergalactic medium, which has prompted the development of several indirect
diagnostics to infer the rate at which galaxies contribute LyC photons to
reionize the universe by studying lower-redshift analogs. We present the
Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with
HST/COS for a z=0.2-0.4 sample of 66 galaxies. After careful processing of the
FUV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected
with 97.725% confidence, nearly tripling the number of known local LCEs. We
estimate escape fractions from the detected LyC flux and upper limits on the
undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the
35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling
the number of known local LCEs with cosmologically relevant LyC escape.Comment: ApJS, accepted. 28 pages, 19 figures, 8 tables. Full machine readable
tables will be made available by the publisher at the time of publicatio
Low-Redshift Lyman Continuum Survey
VizieR online Data Catalogue associated with article published in journal Astronomy & Astrophysics with title 'The Low-Redshift Lyman Continuum Survey. Unveiling the ISM properties of low-z Lyman-continuum emitters.' (bibcode: 2022A&A...663A..59S