The Low-Redshift Lyman Continuum Survey Saldana-Lopez, Alberto; Schaerer, Daniel; Chisholm, John ...
Astronomy and astrophysics (Berlin),
07/2022, Volume:
663
Journal Article
Peer reviewed
Open access
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 − 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 (
f
esc
abs
). 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
f
esc
abs
(0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC (1
σ
upper limits of
f
esc
abs
≲ 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, Ly
α
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
f
esc
abs
: strong LyC leakers (highest
f
esc
abs
) show weak absorption lines, low UV attenuation, and large Ly
α
equivalent widths. We provide several empirical calibrations to estimate
f
esc
abs
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 − 6 (plus 3 high-
z
galaxy composites), finding that these high-redshift, UV-bright galaxies (
M
UV
≲ −21) may have low escape fractions,
f
esc
abs
≲ 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.
Disentangling the Hercules Stream Bensby, T; Oey, M. S; Feltzing, S ...
Astrophysical journal/The Astrophysical journal,
02/2007, Volume:
655, Issue:
2
Journal Article
Peer reviewed
Open access
Using high-resolution spectra of nearby F and G dwarf stars, we have investigated the detailed abundance and age structure of the Hercules stream. We find that the stars in the stream have a wide ...range of stellar ages, metallicities, and element abundances. By comparing to existing samples of stars in the solar neighborhood with kinematics typical of the Galactic thin and thick disks, we find that the properties of the Hercules stream distinctly separate into the abundance and age trends of the two disks. Hence, we find it unlikely that the Hercules stream is a unique Galactic stellar population but rather is a mixture of thin and thick disk stars. This points toward a dynamical origin for the Hercules stream, probably caused by the Galactic bar.
Aims: Recent observations have challenged our understanding of rotational mixing in massive stars by revealing a population of fast- rotating objects with apparently normal surface nitrogen ...abundances. However, several questions have arisen because of a number of issues, which have rendered a reinvestigation necessary; these issues include the presence of numerous upper limits for the nitrogen abundance, unknown multiplicity status, and a mix of stars with different physical properties, such as their mass and evolutionary state, which are known to control the amount of rotational mixing.
Methods: We have carefully selected a large sample of bright, fast-rotating early-type stars of our Galaxy (40 objects with spectral types between B0.5 and O4). Their high-quality, high-resolution optical spectra were then analysed with the stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the temperature of the target. Several internal and external checks were performed to validate our methods; notably, we compared our results with literature data for some well-known objects, studied the effect of gravity darkening, or confronted the results provided by the two codes for stars amenable to both analyses. Furthermore, we studied the radial velocities of the stars to assess their binarity.
Results: This first part of our study presents our methods and provides the derived stellar parameters, He, CNO abundances, and the multiplicity status of every star of the sample. It is the first time that He and CNO abundances of such a large number of Galactic massive fast rotators are determined in a homogeneous way.
Narrowband HST WFPC2 images reveal a bow-shock-like halo around the H II region N30B toward the Be supergiant Hen S22 located within the larger DEM L106 nebula in the Large Magellanic Cloud. ...High-dispersion spectra of N30B show a narrow Halpha emission component from the ionized gas; the velocity variations indicate a gas flow of - 5 to - 10 km s(-1) in the vicinity of the H II regions, which is resultant from interactions with Hen S22's stellar wind and responsible for the bow-shock morphology. Spectra of N30B's halo show broad Halpha profiles extending over more than 1000 km s(-1), similar to that of Hen S22, indicating that the halo is a reflection nebula of Hen S22. Broadband morphologies of N30B's halo are also consistent with the reflection nebula interpretation. We use dust-scattering properties and the observed brightnesses of the reflection nebula and Hen S22 to constrain the reflection geometry. The reflected stellar Halpha emission and absorption vary across the reflection nebula as a result of viewing S22's anisotropic wind from different angles. This reflection nebula, together with the edge-on orientation of Hen S22's disk, provides an invaluable opportunity to study the disk and polar winds of a Be supergiant.
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![The Wind of the B[e] Superg...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1538-3881_0004-6256/small "The Wind of the B[e] Supergiant Henize S22 Viewed through a Reflection Nebula in DEM L106")