No abstract

In this paper we derive ages and masses for 276 clusters in the merger galaxy NGC 3256. This was achieved by taking accurate photometry in four wavebands from archival Hubble Space Telescope images. Photometric measurements are compared to synthetic stellar population (SSP) models to find the most probable age, mass and extinction. The cluster population of NGC 3256 reveals an increase in the star formation rate (SFR) over the last 100 Myr and the initial cluster mass function (ICMF) is best described by a power‐law relation with slope α= 1.85 ± 0.12. Using the observed cluster population for NGC 3256 we calculate the implied mass of clusters younger than 10‐Myr old, and convert this to a cluster formation rate over the last 10 Myr. Comparison of this value with the SFR indicates the fraction of stars found within bound clusters after the embedded phase of cluster formation, Γ, is 22.9 ±7.39.8 per cent for NGC 3256. We carried out an in‐depth analysis into the errors associated with such calculations showing that errors introduced by the SSP fitting must be taken into account and an unconstrained metallicity adds to these uncertainties. Observational biases should also be considered. Using published cluster population data sets we calculate Γ for six other galaxies and examine how Γ varies with environment. We show that Γ increases with the SFR density and can be described as a power‐law type relation of the form Γ (per cent) = (29.0 ± 6.0) Σ0.24±0.04SFR (M ⊙  yr−1  kpc−2).

We have compared far‐ultraviolet (FUV), near‐ultraviolet (NUV) and Hα measurements for star‐forming regions in 21 galaxies, in order to characterize the properties of their discs at radii beyond the main optical radius (R25). In our representative sample of extended and non‐extended ultraviolet (UV) discs, we find that half of the extended UV discs also exhibit extended Hα emission. We find that extended UV discs fall into two categories: those with a sharp truncation in the Hα disc close to the optical edge (R25), and those with extended emission in Hα as well as in the UV. Although most galaxies with strong Hα truncations near R25 show a significant corresponding falloff in UV emission (a factor of 10–100), the transition tends to be much smoother than in Hα, and significant UV emission often extends well beyond this radius, confirming earlier results by Thilker et al. and others. After correcting for dust attenuation the median fraction of total FUV emission from regions outside of R25 is 1.7 per cent, but it can be as high as 35 per cent in the most extreme cases. The corresponding fractions of Hα emission are approximately half as large on average. This difference reflects both a slightly lower ratio of Hα to UV emission in the H ii regions in the outer discs and a lower fraction of star clusters showing H ii regions. Most H ii regions in the extended disc have fluxes consistent with small numbers of ionizing O‐type stars, and this poor sampling of the upper initial mass function (IMF) in small clusters can probably account for the differences in the emission properties, consistent with earlier conclusions by Zaritsky & Christlein, without needing to invoke a significant change in the stellar IMF itself. Consistent Hα/FUV ratios and brightest H ii region to total Hα fluxes in the inner and extended discs across our whole galaxy sample demonstrate no evidence for a change in the cluster luminosity function or the IMF in the low gas density outer disc.

Using deep Subaru/Faint Object Camera And Spectrograph (FOCAS) spectra of 34 H ii regions in both the inner and outer parts of the extended ultraviolet (XUV) disc galaxy NGC 4625 we have measured an abundance gradient out to almost 2.5 times the optical isophotal radius. We applied several strong line abundance calibrations to determine the H ii region abundances, including R23, [N ii]/[O ii], [N ii]/Hα as well as the [O iii] λ4363 auroral line, which we detected in three of the H ii regions. We find that at the transition between the inner and outer disc the abundance gradient becomes flatter. In addition, there appears to be an abundance discontinuity in proximity of this transition. Several of our target H ii regions appear to deviate from the ionization sequence defined in the [N ii]/Hα versus [O iii]/Hβ diagnostic diagram by bright extragalactic H ii regions. Using theoretical models we conclude that the most likely explanations for these deviations are either related to the time evolution of the H ii regions or stochastic variations in the ionizing stellar populations of these low‐mass H ii regions, although we are unable to distinguish between these two effects. Such effects can also impact on the reliability of the strong line abundance determinations.