Galactic porosity and a star formation threshold for the escape of ionising radiation from galaxies

The spatial distribution of star formation within galaxies strongly affects the resulting feedback processes. Previous work has considered the case of a single, concentrated nuclear starburst, and also that of distributed single supernovae (SNe). Here, we consider ISM structuring by SNe originating in spatially distributed clusters having a cluster membership spectrum given by the observed HII region luminosity function. We show that in this case, the volume of HI cleared per SN is considerably greater than in either of the two cases considered hitherto. We derive a simple relationship between the ``porosity'' of the ISM and the star formation rate (SFR), and deduce a critical SFR_crit, at which the ISM porosity is unity. This critical value describes the case in which the SN mechanical energy output over a timescale t_e is comparable with the ISM ``thermal'' energy contained in random motions; t_e is the duration of SN mechanical input per superbubble. This condition also defines a critical gas consumption timescale t_exh, which for a Salpeter IMF and random velocities of \simeq 10 km s-1 is roughly 10e10 years. We draw a link between porosity and the escape of ionising radiation from galaxies, arguing that high escape fractions are expected if SFR >~ SFR_crit. The Lyman Break Galaxies, which are presumably subject to infall on a timescale < t_exh, meet this criterion, as is consistent with the significant leakage of ionising photons inferred in these systems. We suggest the utility of this simple parameterisation of escape fraction in terms of SFR for semi-empirical models of galaxy formation and evolution and for modeling mechanical and chemical feedback effects.Comment: Accepted to MNRAS. 11 pages, 1 figure; uses mn2e.cls (included

The Superbubble Size Distribution in the Interstellar Medium of Galaxies

We use the standard, adiabatic shell evolution to predict the size distribution N(R) for populations of OB superbubbles in a uniform ISM. We derive N(R) for simple cases of superbubble creation rate and mechanical luminosity function (MLF). For R < the characteristic radius R_e, N(R) is dominated by stalled objects, while for R>R_e it is dominated by growing objects. We also briefly investigate N(R) resulting from momentum-conserving shell evolution. We predict a peak in N(R) corresponding to individual SNRs. To estimate the MLF, we also examine evolutionary effects on the HII region luminosity function (HII LF), finding that for nebular luminosity fading as a power law in time, there is a minimum observed slope for the HII LFs. Comparison with the largely complete HI hole catalog for the SMC shows surprising agreement in the predicted and observed slope of N(R), suggesting that no other fundamental process is needed to explain the size distribution of shells in the SMC. Further comparison with largely incomplete HI data for M31, M33, and Holmberg II is also encouraging. We present expressions for the ISM porosity parameters, and estimate that they are substantially <1 for all of the galaxies except Holmberg II. Most of these galaxies therefore may not be strongly dominated by a hot interstellar component. However, porosity results for the Galaxy remain inconclusive.Comment: 25 pages, MN latex, 4 figures. MNRAS accepted. Complete abstract and preprint also available at http://ast.cam.ac.uk/~oey/oeypubs.htm

The Sparsest Clusters With O Stars

There is much debate on how high-mass star formation varies with environment, and whether the sparsest star-forming environments are capable of forming massive stars. To address this issue, we have observed eight apparently isolated OB stars in the SMC using HST's Advanced Camera for Surveys. Five of these objects appear as isolated stars, two of which are confirmed to be runaways. The remaining three objects are found to exist in sparse clusters, with <10 companion stars revealed, having masses of 1-4 solar mass. Stochastic effects dominate in these sparse clusters, so we perform Monte Carlo simulations to explore how our observations fit within the framework of empirical, galactic cluster properties. We generate clusters using a simplistic -2 power-law distribution for either the number of stars per cluster (N_*) or cluster mass (M_cl). These clusters are then populated with stars randomly chosen from a Kroupa IMF. We find that simulations with cluster lower-mass limits of M_cl,lo >20 solar mass and N_*,lo >40 match best with observations of SMC and Galactic OB star populations. We examine the mass ratio of the second-most massive and most massive stars (m_max,2/m_max), finding that our observations all exist below the 20th percentile of our simulated clusters. However, all of our observed clusters lie within the parameter space spanned by the simulated clusters, although some are in the lowest 5th percentile frequency. These results suggest that clusters are built stochastically by randomly sampling stars from a universal IMF with a fixed stellar upper-mass limit. In particular, we see no evidence to suggest a m_max - M_cl relation. Our results may be more consistent with core accretion models of star formation than with competitive accretion models, and they are inconsistent with the proposed steepening of the integrated galaxy IMF (IGIMF).Comment: 19 pages, 12 figures, accepted for publication in Ap

Do O-stars form in isolation?

Around 4% of O-stars are observed in apparent isolation, with no associated cluster, and no indication of having been ejected from a nearby cluster. We define an isolated O-star as a star > 17.5 M_\odot in a cluster with total mass 10 M_\odot) stars. We show that the fraction of apparently isolated O-stars is reproduced when stars are sampled (randomly) from a standard initial mass function and a standard cluster mass function of the form N(M) \propto M^-2. This result is difficult to reconcile with the idea that there is a fundamental relationship between the mass of a cluster and the mass of the most massive star in that cluster. We suggest that such a relationship is a typical result of star formation in clusters, and that `isolated O-stars' are low-mass clusters in which massive stars have been able to form.Comment: 6 pages, 5 figures, MNRAS in pres

Statistical Confirmation of a Stellar Upper Mass Limit

We derive the expectation value for the maximum stellar mass (m_max) in an ensemble of N stars, as a function of the IMF upper-mass cutoff (m_up) and N. We statistically demonstrate that the upper IMF of the local massive star census observed thus far in the Milky Way and Magellanic Clouds clearly exhibits a universal upper mass cutoff around 120 - 200 M_sun for a Salpeter IMF, although the result is more ambiguous for a steeper IMF.Comment: PDF, 5 pages, 4 figures. Accepted to the Astrophysical Journal Letter

The star formation process in the Magellanic Clouds

The Magellanic Clouds offer unique opportunities to study star formation both on the global scales of an interacting system of gas-rich galaxies, as well as on the scales of individual star-forming clouds. The interstellar media of the Small and Large Magellanic Clouds and their connecting bridge, span a range in (low) metallicities and gas density. This allows us to study star formation near the critical density and gain an understanding of how tidal dwarfs might form; the low metallicity of the SMC in particular is typical of galaxies during the early phases of their assembly, and studies of star formation in the SMC provide a stepping stone to understand star formation at high redshift where these processes can not be directly observed. In this review, I introduce the different environments encountered in the Magellanic System and compare these with the Schmidt-Kennicutt law and the predicted efficiencies of various chemo-physical processes. I then concentrate on three aspects that are of particular importance: the chemistry of the embedded stages of star formation, the Initial Mass Function, and feedback effects from massive stars and its ability to trigger further star formation.Comment: 12pages, 5figures, invited review at the IAUS 256, The Magellanic System: Stars, Gas, and Galaxies, eds. Jacco van Loon, Joana Oliveir