Abstract
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.
Abstract
The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at ...these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the S
ii
nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the S
ii
deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. We show that LyC leakers tend to be more S
ii
deficient and that the fraction of their detections increases as S
ii
deficiency becomes more prominent. Correlational studies suggest that S
ii
deficiency complements other LyC diagnostics (such as strong Ly
α
emission and high O
iii
/O
ii
). Our results verify an additional technique by which reionization-era galaxies could be studied.
Abstract
Understanding the production and escape of Ly
α
radiation from star-forming galaxies is a long-standing problem in astrophysics. The ability to predict the Ly
α
luminosity of galaxies would ...open up new ways of exploring the epoch of reionization (EOR) and estimating Ly
α
emission from galaxies in cosmological simulations where radiative transfer calculations cannot be done. We apply multivariate regression methods to the Lyman Alpha Reference Sample data set to obtain a relation between the galaxy properties and the emitted Ly
α
. The derived relation predicts the Ly
α
luminosity of our galaxy sample to good accuracy, regardless of whether we consider only direct observables (rms dispersion around the relation of ∼0.19 dex) or derived physical quantities (rms ∼ 0.27 dex). We confirm the predictive ability on a separate sample of compact star-forming galaxies and find that the prediction works well, but that aperture effects on measured Ly
α
luminosity may be important, depending on the redshift of the galaxy. We apply statistical feature selection techniques to determine an order of importance of the variables in our data set, enabling future observations to be optimized for predictive ability. When using physical variables, we are able to determine that the most important predictive parameters are, in order, star formation rate, dust extinction, compactness, and the gas covering fraction. We discuss the application of our results in terms of studying the EOR and intensity mapping experiments.
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.
Despite strong interest in the starburst phenomenon in extragalactic astronomy, the concept remains ill-defined. Here we use a strict definition of starburst to examine the statistical properties of ...starburst galaxies in the local universe. We also seek to establish links between starburst galaxies, post-starburst (hereafter postburst) galaxies, and active galaxies. Data were selected from the Sloan Digital Sky Survey DR7. We applied a novel method of treating dust attenuation and derive star formation rates, ages, and stellar masses assuming a two-component stellar population model. Dynamical masses are calculated from the width of the H-alpha line. These masses agree excellently with the photometric masses. The mass (gas+stars) range is approximately 10( exp 9) - 10(exp 11.5) solar mass. As a selection criterion for starburst galaxies, we use, the birthrate parameter, b = SFR/SFR, requiring that b is greater than 3. For postburst galaxies, we use, the equivalent width of Hdelta in absorption with the criterion EW (sub Hdelta_abs) is greater than 6 A. Results. We find that only 1% of star-forming galaxies are starburst galaxies. They contribute 3-6% to the stellar production and are therefore unimportant for the local star formation activity. The median starburst age is 70 Myr roughly independent of mass, indicating that star formation is mainly regulated by local feedback processes. The b-parameter strongly depends on burst age. Values close to b = 60 are found at ages approximately 10 Myr, while almost no starbursts are found at ages greater than 1 Gyr. The median baryonic burst mass fraction of sub-L galaxies is 5% and decreases slowly towards high masses. The median mass fraction of the recent burst in the postburst sample is 5-10%. A smaller fraction of the postburst galaxies, however, originates in non-bursting galaxies. The age-mass distribution of the postburst progenitors (with mass fractions is greater than 3%) is bimodal with a break at logM(solar mass ) 10.6, above which the ages are doubled. The starburst and postburst luminosity functions (LFs) follow each other closely until M(sub r ) (is) approximately -21, when active galactic nuclei (AGNs) begin to dominate. The postburst LF continues to follow the AGN LF, while starbursts become less significant. This suggests that the number of luminous starbursts is underestimated by about one dex at high luminosities, because of having large amounts of dust and/or being outshone by an AGN. It also indicates that the starburst phase preceded the AGN phase. Finally, we look at the conditions for global gas outflow caused by stellar feedback and find that massive starburst galaxies are susceptible to such outflows.
Context. Mrk 71/NGC 2366 is the closest green pea (GP) analog and candidate Lyman Continuum (LyC) emitter. Recently, 11 LyC-leaking GPs have been detected through direct observations of the ionizing ...continuum, making this the most abundant class of confirmed LyC-emitters at any redshift. High resolution, multiwavelength studies of GPs can lead to an understanding of the method(s), through which LyC escapes from these galaxies. Aims. The proximity of Mrk 71/NCG 2366 offers unprecedented detail on the inner workings of a GP analog, and enables us to identify the mechanisms of LyC escape. Methods. We used 5825–7650 Å integral field unit PMAS observations to study the kinematics and physical conditions in Mrk 71. An electron density map was obtained from the S II ratio. A fortuitous second order contamination by the O II λ3727 doublet enabled the construction of an electron temperature map. Resolved maps of sound speed, thermal broadening, “true” velocity dispersion, and Mach number were obtained and compared to the high resolution magnetohydrodynamic SImulating the LifeCycle of molecular Clouds (SILCC) simulations. Results. Two regions of increased velocity dispersion indicative of outflows are detected to the north and south of the super star cluster, knot B, with redshifted and blueshifted velocities, respectively. We confirm the presence of a faint broad kinematical component, which is seemingly decoupled from the outflow regions, and is fainter and narrower than previously reported in the literature. Within uncertainties, the low- and high-ionization gasses move together. Outside of the core of Mrk 71, an increase in Mach numbers is detected, implying a decrease in gas density. Simulations suggest this drop in density can be as high as ∼4 dex, down to almost optically thin levels, which would imply a nonzero LyC escape fraction along the outflows even when assuming all of the detected H I gas is located in front of Mrk 71 in the line of sight. Conclusions. Our results strongly indicate that kinematical feedback is an important ingredient for LyC leakage in GPs.
We study young star-forming clumps on physical scales of 10-500 pc in the Lyman-Alpha Reference Sample (LARS), a collection of low-redshift (z = 0.03-0.2) UV-selected star forming galaxies. In each ...of the 14 galaxies of the sample, we detect clumps for which we derive sizes and magnitudes in five UV-optical filters, The final sample includes 1400 clumps, of which 600 have magnitude uncertainties below 0.3 in all filters. The UV luminosity function for the total sample of clumps is described by a power law with slope 0, = 2.031:111. Clumps in the LARS galaxies have on average L'SFR values higher than what is observed in II 11 regions of local galaxies and comparable to typical star formation rate (SFR) densities of clumps in = 1-3 galaxies. We derive the dumpiness as the relative contribution from clumps to the UV emission of each galaxy, and study it as a function of galactic-scale properties, i.e. E SFR and the ratio between rotational and dispersion velocities of the gas ( vsio-0). We find that in galaxies with higher EsFR or lower vskr 0, clumps dominate the UV emission of their host systems. All LARS galaxies with Ly cy escape fractions larger than 10% have more than 50% of the UV luminosity from clumps. We tested the robustness of these results against the effect of different physical resolutions, At low resolution, the measured dumpiness appears more elevated than if we could resolve clumps down to single clusters. This effect is small in the redshift range covered by LARS; thus, our results are not driven by the physical resolution.
Context.
Studies of nearby galaxies reveal that around 50% of the total H
α
luminosity in late-type spirals originates from diffuse ionised gas (DIG), which is a warm, diffuse component of the ...interstellar medium that can be associated with various mechanisms, the most important ones being “leaking” HII regions, evolved field stars, and shocks.
Aims.
Using MUSE Wide Field Mode adaptive optics-assisted data, we study the condition of the ionised medium in the nearby (
D
= 3.4 Mpc) flocculent spiral galaxy NGC 7793 at a spatial resolution of ∼10 pc. We construct a sample of HII regions and investigate the properties and origin of the DIG component.
Methods.
We obtained stellar and gas kinematics by modelling the stellar continuum and fitting the H
α
emission line. We identified the boundaries of resolved HII regions based on their H
α
surface brightness. As a way of comparison, we also selected regions according to the H
α
/SII line ratio; this results in more conservative boundaries. Using characteristic line ratios and the gas velocity dispersion, we excluded potential contaminants, such as supernova remnants (SNRs) and planetary nebulae (PNe). The continuum subtracted HeII map was used to spectroscopically identify Wolf Rayet stars (WR) in our field of view. Finally, we computed electron densities and temperatures using the line ratio SII6716/6731 and SIII6312/9069, respectively. We studied the properties of the ionised gas through “BPT” emission line diagrams combined with velocity dispersion of the gas.
Results.
We spectroscopically confirm two previously detected WR and SNR candidates and report the discovery of the other seven WR candidates, one SNR, and two PNe within our field of view. The resulting DIG fraction is between ∼27 and 42% depending on the method used to define the boundaries of the HII regions (flux brightness cut in H
α
= 6.7 × 10
−18
erg s
−1
cm
−2
or H
α
/SII = 2.1, respectively). In agreement with previous studies, we find that the DIG exhibits enhanced SII/H
α
and NII/H
α
ratios and a median temperature that is ∼3000 K higher than in HII regions. We also observe an apparent inverse correlation between temperature and H
α
surface brightness. In the majority of our field of view, the observed SII6716/6731 ratio is consistent within 1
σ
with
n
e
< 30 cm
−3
, with an almost identical distribution for the DIG and HII regions. The velocity dispersion of the ionised gas indicates that the DIG has a higher degree of turbulence than the HII regions. Comparison with photoionisation and shock models reveals that, overall, the diffuse component can only partially be explained via shocks and that it is most likely consistent with photons leaking from density bounded HII regions or with radiation from evolved field stars. Further investigation will be conducted in a follow-up paper.
Abstract
The Mg
ii
λλ
2796, 2803 doublet has been suggested as a useful indirect indicator for the escape of Ly
α
and Lyman continuum (LyC) photons in local star-forming galaxies. However, studies to ...date have focused on small samples of galaxies with strong Mg
ii
or strong LyC emission. Here, we present the first study of Mg
ii
to probe a large dynamic range of galaxy properties, using newly obtained high-signal-to-noise, moderate-resolution spectra of Mg
ii
, for a sample of 34 galaxies selected from the Low-redshift Lyman Continuum Survey. We show that the galaxies in our sample have Mg
ii
profiles ranging from strong emission to P-Cygni profiles to pure absorption. We find that there is a significant trend (with a possibility of spurious correlations of ∼2%) that galaxies that are detected as strong LyC emitters (LCEs) show larger equivalent widths of Mg
ii
emission, while non-LCEs tend to show evidence of more scattering and absorption features in Mg
ii
. We then find that Mg
ii
strongly correlates with Ly
α
in both equivalent width and escape fraction, regardless of whether emission or absorption dominates the Mg
ii
profiles. Furthermore, we show that, for galaxies that are categorized as Mg
ii
emitters, one can use the information of Mg
ii
, metallicity, and dust to estimate the escape fraction of LyC within a factor of ∼3. These findings confirm that Mg
ii
lines can be used as a tool for selecting galaxies as LCEs and thus serve as an indirect indicator for the escape of Ly
α
and LyC.
We report results from a new Hubble Space Telescope campaign that targets the O vi lambdalambda1032.1038 A doublet in emission around intensely star-forming galaxies. The program aims to characterize ...the energy balance in starburst galaxies and gas cooling in the difficult-to-map coronal temperature regime of 2-5 x 10 super(5) K. We present the first resolved image of gas emission in the O vi line. Our target, SDSS J115630.63+500822.1, is very compact in the continuum but displays O vi emission to radii of 23 kpc. The surface brightness profile is well fit by an exponential with a scale length of 7.5 kpc. This is 10 times the size of the photoionized gas, and we estimate that about 1/6 the total O vi luminosity comes from resonantly scattered continuum radiation. Spectroscopy-which closely resembles a stacked sample of archival spectra-confirms the O vi emission, and determines the column density and outflow velocity from blueshifted absorption. The combination of measurements enables a large number of calculations with few assumptions. The O vi regions fill only ~10 super(-3)... of the volume. By comparing the cooling time with the cloud sound-crossing time, the cooling distance with the size, and the pressure in the O vi and nebular gas, we conclude that the O vi-bearing gas cannot have been lifted to the scale height at this temperature, and must be cooling in situ through this coronal temperature regime. The coronal phase contains ~1% of the ionized mass, and its kinetic energy at a given instant is ~1% of the budget set by supernova feedback. However, a much larger amount of the gas must have cooled through this phase during the star formation episode. The outflow exceeds the escape velocity and the gas may become unbound, but it will recombine before it escapes and become visible to Lyman (and O i) spectroscopy. The mapping of this gas represents a crucial step in further constraining galaxy formation scenarios and guiding the development of future astronomical satellites.
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