It has been suggested that the escape fraction of hydrogen ionizing photons (fsubesc) from galaxies is evolving with time, but the picture is far from clear. The lack of local detections could in ...principle be a combined effect of an evolving escape fraction, the low number galaxies observed, the selection criteria of these targets, and technical problems associated with the instrument best adapted for low redshift targets, the Far Ultraviolet Spectroscopic Explorer (FUSE). In an attempt to tackle the last of these effects, we apply a new background routine developed for FUSE data to study galaxies from the archive in search of possible Lyman continuum (LyC) leakage. With the small sample, we suggest a possible trend toward higher fsubesc with lower mass and with enhanced SSFR. None of the galaxies with high values of E(B-V)subI were found to show any sign of leakage.
We study the stellar cluster population in two adjacent fields in the nearby, face-on spiral galaxy M83 using multiwavelength Wide Field Camera 3/Hubble Space Telescope imaging. After automatic ...detection procedures, the clusters are selected through visual inspection to be centrally concentrated, symmetric, and resolved on the images, which allows us to differentiate between clusters and likely unbound associations. We compare our sample with previous studies and show that the differences between the catalogues are largely due to the inclusion of a large numbers of diffuse associations within previous catalogues as well as the inclusion of the central starburst region, where the completeness limit is significantly worse than in the surrounding regions. We derive the size distribution of the clusters, which is well described by a lognormal distribution with a peak at ∼2.5 pc, and find evidence for an expansion in the half-light radius of clusters with age. The luminosity function of the clusters is well approximated by a power law with an index of −2 over most of the observed range; however, a steepening is seen at M
V
=−9.3 and −8.8 in the inner and outer fields, respectively. Additionally, we show that the cluster population is inconsistent with a pure power-law mass distribution, but instead exhibits a truncation at the high-mass end. If described as a Schechter function, the characteristic mass is 1.6 × 105 and 0.5 × 105 M⊙ for the inner and outer fields, respectively, in agreement with previous estimates of other cluster populations in spiral galaxies. Comparing the predictions of the mass-independent disruption (MID) and mass-dependent disruption (MDD) scenarios with the observed distributions, we find that both models can accurately fit the data. However, for the MID case, the fraction of clusters destroyed (or mass lost) per decade in age is dependent on the environment; hence, the age and mass distributions of clusters are not universal. In the MDD case, the disruption time-scale scales with galactocentric distance (being longer in the outer regions of the galaxy) in agreement with analytic and numerical predictions. Finally, we discuss the implications of our results on other extragalactic surveys, focusing on the fraction of stars that form in clusters and the need (or lack thereof) for infant mortality.
The age distribution of stellar clusters in M83 Silva-Villa, E; Adamo, A; Bastian, N ...
Monthly Notices of the Royal Astronomical Society Letters,
2014, Letnik:
440, Številka:
1
Journal Article
Recenzirano
Odprti dostop
In order to empirically determine the time-scale and environmental dependence of stellar cluster disruption, we have undertaken an analysis of the unprecedented multipointing (seven), multiwavelength ...(U, B, V, Hα, and I) Hubble Space Telescope imaging survey of the nearby, face-on spiral galaxy M83. The images are used to locate stellar clusters and stellar associations throughout the galaxy. Estimation of cluster properties (age, mass, and extinction) was done through a comparison of their spectral energy distributions with simple stellar population models. We constructed the largest catalogue of stellar clusters and associations in this galaxy to-date, with ∼1800 sources with masses above ∼5000 M and ages younger than ∼300 Myr. In this Letter, we focus on the age distribution of the resulting clusters and associations. In particular, we explicitly test whether the age distributions are related with the ambient environment. Our results are in excellent agreement with previous studies of age distributions in the centre of the galaxy, which gives us confidence to expand out to search for similarities or differences in the other fields which sample different environments. We find that the age distribution of the clusters inside M83 varies strongly as a function of position within the galaxy, indicating a strong correlation with the galactic environment. If the age distributions are approximated as a power law of the form
, we find ζ values between 0 and −0.62 (ζ ∼ −0.40 for the whole galaxy), in good agreement with previous results and theoretical predictions.
The sources that drove cosmological reionization left clues regarding their identity in the slope and inhomogeneity of the ultraviolet ionizing background (UVB): bright quasars (QSOs) generate a hard ...UVB with predominantly large-scale fluctuations while Population II stars generate a softer one with smaller scale fluctuations. Metal absorbers probe the UVB's slope because different ions are sensitive to different energies. Likewise, they probe spatial fluctuations because they originate in regions where a galaxy-driven UVB is harder and more intense. We take a first step towards studying the reionization-epoch UVB's slope and inhomogeneity by comparing observations of 12 metal absorbers at z ∼ 6 versus predictions from a cosmological hydrodynamic simulation using three different UVBs: a soft, spatially inhomogeneous ‘galaxies+QSOs’ UVB; a homogeneous ‘galaxies+QSOs’ UVB, and a ‘QSOs-only’ model. All UVBs reproduce the observed column density distributions of
$\mathrm{C\,\small {ii}}$
,
$\mathrm{Si\,\small {iv}}$
, and
$\mathrm{C\,\small {iv}}$
reasonably well although high-column, high-ionization absorbers are underproduced, reflecting numerical limitations. With upper limits treated as detections, only a soft, fluctuating UVB reproduces both the observed
$\mathrm{Si\,\small {iv}}/\mathrm{C\,\small {iv}}$
and
$\mathrm{C\,\small {ii}}/\mathrm{C\,\small {iv}}$
distributions. The QSOs-only UVB overpredicts both
$\mathrm{C\,\small {iv}}/\mathrm{C\,\small {ii}}$
and
$\mathrm{C\,\small {iv}}/\mathrm{Si\,\small {iv}}$
, indicating that it is too hard. The Haardt & Madau (2012) UVB underpredicts
$\mathrm{C\,\small {iv}}/\mathrm{Si\,\small {iv}}$
, suggesting that it lacks amplifications near galaxies. Hence current observations prefer a soft, fluctuating UVB as expected from a predominantly Population II background although they cannot rule out a harder one. Future observations probing a factor of 2 deeper in metal column density will distinguish between the soft, fluctuating and QSOs-only UVBs.
The reionization of carbon Finlator, Kristian; Thompson, Robert; Huang, Shuiyao ...
Monthly Notices of the Royal Astronomical Society,
03/2015, Letnik:
447, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Observations suggest that C ii was more abundant than C iv in the intergalactic medium towards the end of the hydrogen reionization epoch (z ~ 6). This transition provides a unique opportunity to ...study the enrichment history of intergalactic gas and the growth of the ionizing ultraviolet background (UVB) at early times. We study how carbon absorption evolves from z = 10 to 5 using a cosmological hydrodynamic simulation that includes a self-consistent multifrequency UVB as well as a well-constrained model for galactic outflows to disperse metals. Our predicted UVB is within ~2-4 times of that from Haardt & Madau, which is fair agreement given the uncertainties. Nonetheless, we use a calibration in post-processing to account for Lyman ... forest measurements while preserving the predicted spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way that it always exceeds the volume average in regions where metals are found. This implies both that a spatially uniform UVB is a poor approximation and that metal absorption is not sensitive to the epoch when H ii regions overlap globally even at column densities of 10... cm... We find, consistent with observations, that the C ii mass fraction drops to low redshift while C iv rises owing the combined effects of a growing UVB and continued addition of carbon in low-density regions. This is mimicked in absorption statistics, which broadly agree with observations at z = 6-3 while predicting that the absorber column density distributions rise steeply to the lowest observable columns. Our model reproduces the large observed scatter in the number of low-ionization absorbers per sightline, implying that the scatter does not indicate a partially neutral Universe at z ~ 6. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
Low- and intermediate-mass galaxies are widely discussed as cause of reionization at redshift z ∼ 10–6. However, observational proof of galaxies that are leaking ionizing radiation (Lyman ...continuum; LyC) is a currently ongoing challenge and the list of LyC emitting candidates is still short. Tololo 1247−232 is among those very few galaxies with recently reported leakage. We performed intermediate resolution ultraviolet spectroscopy with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope and confirm ionizing radiation emerging from Tololo 1247−232. Adopting an improved data reduction procedure, we find that LyC escapes from the central stellar clusters, with an escape fraction of 1.5 ± 0.5 per cent only, i.e. the lowest value reported for the galaxy so far. We further make use of far-ultraviolet absorption lines of Si ii and Si iv as a probe of the neutral and ionized interstellar medium (ISM). We find that most of the ISM gas is ionized, likely facilitating LyC escape from density bounded regions. Neutral gas covering as a function of line-of-sight velocity is derived using the apparent optical depth method. The ISM is found to be sufficiently clumpy, supporting the direct escape of LyC photons. We further report on broad-band UV and optical continuum imaging as well as narrow-band imaging of Lyα, Hα and Hβ. Using stellar population synthesis, an Lyα escape fraction of 8 per cent was derived. We also performed Very Large Array 21cm imaging. The hydrogen hyperfine transition was not detected, but a deep upper limit atomic gas mass of ≲109 M⊙ could be derived. The upper limit gas fraction defined as
${M_{\rm H\,\small {I}}}/{M_*}$
is only 20 per cent. Evidence is found that the H i gas halo is relatively small compared to the Lyman Alpha Reference Sample (Hayes et al. 2013, 2014; Östlin et al. 2014).
We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO 350−IG038) by means of its stellar cluster ...population. The central starburst region has been observed in eight different high-resolution Hubble Space Telescope (HST) wavebands, sampling the stellar and gas components from UV to near-infrared. Photometric imaging of the galaxy was also carried out at 2.16 μm by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star-forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases (<5 Myr). A fraction of low-mass (below 104 M⊙), very young (1–3 Myr) clusters is missing, either because they are embedded in the parental molecular cloud and heavily extinguished, or because of blending with neighbouring clusters. The range of the cluster masses is wide; we observe that more than 30 per cent of the clusters have masses above 105 M⊙, qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000 Å which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40 Myr; we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ∼38 per cent of the galactic star formation rate.
The numerous and massive young star clusters in blue compact galaxies (BCGs) are used to investigate the properties of their hosts. We test whether BCGs follow claimed relations between cluster ...populations and their hosts, such as the fraction of the total luminosity contributed by the clusters as function of the mean star formation rate (SFR) density, the V-band luminosity of the brightest youngest cluster as related to the mean host SFR and the cluster formation efficiency (i.e. the fraction of star formation happening in star clusters) versus the density of the SFR. We find that BCGs follow the trends, supporting a scenario where cluster formation and environmental properties of the host are correlated. They occupy, in all the diagrams, the regions of higher SFRs, as expected by the extreme nature of the starbursts operating in these systems. We find that the star clusters contribute almost to the 20 per cent of the UV luminosity of the hosts. We suggest that the BCG starburst environment has most likely favoured the compression and collapse of the giant molecular clouds, enhancing the local star formation efficiency, so that massive clusters have been formed. The estimated cluster formation efficiency supports this scenario. BCGs have a cluster formation efficiency comparable to luminous IR galaxies and spiral starburst nuclei (the averaged value is ∼35 per cent) which is much higher than the 8-10 per cent reported for quiescent spirals and dwarf star-forming galaxies.
We test the predictions of spectral synthesis models based on seven different massive-star prescriptions against Legacy ExtraGalactic UV Survey (LEGUS) observations of eight young massive clusters in ...two local galaxies, NGC 1566 and NGC 5253, chosen because predictions of all seven models are available at the published galactic metallicities. The high angular resolution, extensive cluster inventory, and full near-ultraviolet to near-infrared photometric coverage make the LEGUS data set excellent for this study. We account for both stellar and nebular emission in the models and try two different prescriptions for attenuation by dust. From Bayesian fits of model libraries to the observations, we find remarkably low dispersion in the median E(B − V) (∼0.03 mag), stellar masses (∼104 M⊙), and ages (∼1 Myr) derived for individual clusters using different models, although maximum discrepancies in these quantities can reach 0.09 mag and factors of 2.8 and 2.5, respectively. This is for ranges in median properties of 0.05–0.54 mag, 1.8–10 × 104 M⊙, and 1.6–40 Myr spanned by the clusters in our sample. In terms of best fit, the observations are slightly better reproduced by models with interacting binaries and least well reproduced by models with single rotating stars. Our study provides a first quantitative estimate of the accuracies and uncertainties of the most recent spectral synthesis models of young stellar populations, demonstrates the good progress of models in fitting high-quality observations, and highlights the needs for a larger cluster sample and more extensive tests of the model parameter space.
The first
James Webb
Space Telescope (JWST) data on the massive colliding cluster
El Gordo
allow for 23 known families of multiply lensed images to be confirmed and for eight new members of these ...families to be identified. Based on these families, which have been confirmed spectroscopically by MUSE, we derived an initial lens model. This model guided the identification of 37 additional families of multiply lensed galaxies, among which 28 are entirely new systems, and nine were previously known. The initial lens model determined geometric redshifts for the 37 new systems. The geometric redshifts agree reasonably well with spectroscopic or photometric redshifts when those are available. The geometric redshifts enable two additional models that include all 60 families of multiply lensed galaxies spanning a redshift range 2 <
z
< 6. The derived dark-matter distribution confirms the double-peak configuration of mass found by earlier work with the southern and northern clumps having similar masses. We confirm that El Gordo is the most massive known cluster at
z
> 0.8 and has an estimated virial mass close the maximum mass allowed by standard cosmological models. The JWST images also reveal the presence of small-mass perturbers that produce small lensing distortions. The smallest of these is consistent with being a dwarf galaxy at
z
= 0.87 and has an estimated mass of 3.8 × 10
9
M
⊙
, making it the smallest substructure found at
z
> 0.5. The JWST images also show several candidate caustic-crossing events. One of them is detected at high significance at the expected position of the critical curve and is likely a red supergiant star at
z
= 2.1878. This would be the first red supergiant found at cosmological distances. The cluster lensing should magnify background objects at
z
> 6, making more of them visible than in blank fields of a similar size, but there appears to be a deficiency of such objects.
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