Mapping Lyman Continuum escape in Tololo 1247-232

Low redshift, spatially resolved Lyman continuum (LyC) emitters allow us to clarify the processes for LyC escape from these starburst galaxies. We use Hubble Space Telescope (HST) WFC3 and ACS imaging of the confirmed low-redshift LyC emitter Tol 1247-232 to study the ionization structure of the gas and its relation to the ionizing star clusters. We perform ionization parameter mapping (IPM) using [O III]4959, 5007 and [O II]3727 imaging as the high- and low-ionization tracers, revealing broad, large-scale, optically thin regions originating from the center, and reaching the outskirts of the galaxy, consistent with LyC escape. We carry out stellar population synthesis modeling of the 26 brightest clusters using our HST photometry. Combining these data with the nebular photometry, we find a global LyC escape fraction of f_esc = 0.12, with uncertainties also consistent with zero escape and with all measured f_esc values for this galaxy. Our analysis suggests that, similar to other candidate LyC emitters, a two-stage starburst has taken place in this galaxy, with a 12 Myr old, massive, central cluster likely having pre-cleared regions in and around the center, and the second generation of 2 - 4 Myr old clusters dominating the current ionization, including some escape from the galaxy.Comment: Accepted for publication in Ap

The effect of diffuse background on the spatially-resolved Schmidt relation in nearby spiral galaxies

The global Schmidt law of star formation provides a power-law relation between the surface densities of star-formation rate (SFR) and gas, and successfully explains plausible scenarios of galaxy formation and evolution. However, star formation being a multi-scale process, requires spatially-resolved analysis for a better understanding of the physics of star formation. It has been shown that the removal of a diffuse background from SFR tracers, such as H$\alpha$, far-ultraviolet (FUV), infrared, leads to an increase in the slope of the sub-galactic Schmidt relation. We reinvestigate the local Schmidt relations in nine nearby spiral galaxies taking into account the effect of inclusion and removal of diffuse background in SFR tracers as well as in the atomic gas.We used multiwavelength data obtained as part of the surveys such as SINGS, KINGFISH, THINGS, and HERACLES. Making use of a novel split of the overall light distribution as a function of spatial scale, we subtracted the diffuse background in the SFR tracers as well as the atomic gas. Using aperture photometry, we study the Schmidt relations on background subtracted and unsubtracted data at physical scales varying between 0.5--2 kpc. The fraction of diffuse background varies from galaxy to galaxy and accounts to $\sim$34 % in H$\alpha$, $\sim$43 % in FUV, $\sim$37 % in 24 $\mu$m, and $\sim$75\% in H I on average. We find that the inclusion of diffuse background in SFR tracers leads to a linear molecular gas Schmidt relation and a bimodal total gas Schmidt relation. However, the removal of diffuse background in SFR tracers leads to a super-linear molecular gas Schmidt relation. A further removal of the diffuse background from atomic gas results in a slope $\sim$1.4 $\pm$ 0.1, which agrees with dynamical models of star formation accounting for flaring effects in the outer regions of galaxies.Comment: 33 pages, 25 figures, 6 tables, accepted for publication in A&

Testing metallicity indicators at z~1.4 with the gravitationally lensed galaxy CASSOWARY 20

We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming (SFR ~6 Msol/yr) galaxy at z=1.433, magnified by a factor of 11.5 by the gravitational lensing produced by a massive foreground galaxy at z=0.741. We analysed the integrated physical properties of the HII regions of CSWA 20 using temperature- and density-sensitive emission lines. We find the abundance of oxygen to be ~1/7 of solar, while carbon is ~50 times less abundant than in the Sun. The unusually low C/O ratio may be an indication of a particularly rapid timescale of chemical enrichment. The wide wavelength coverage of X-shooter gives us access to five different methods for determining the metallicity of CSWA 20, three based on emission lines from HII regions and two on absorption features formed in the atmospheres of massive stars. All five estimates are in agreement, within the factor of ~2 uncertainty of each method. The interstellar medium of CSWA 20 only partially covers the star-forming region as viewed from our direction; in particular, absorption lines from neutrals and first ions are exceptionally weak. We find evidence for large-scale outflows of the interstellar medium (ISM) with speeds of up 750 km/s, similar to the values measured in other high-z galaxies sustaining much higher rates of star formation.Comment: 18 pages, 11 figures, accepted for publication in MNRA