Mode Of Star Formation Research Articles

Bimodality in Damped Lyα Systems

We report evidence for a bimodality in damped Lyα systems (DLAs). Using [C II] 158 μm cooling rates, lc, we find a distribution with peaks at lc = 10−27.4 and 10−26.6 ergs s−1 H−1 separated by a trough at lcritc ≈ 10−27.0 ergs s−1 H−1. We divide the sample into DLAs with lc ≤ lcritc and DLAs with lc > lcritc and find the K-S probabilities that velocity width, metallicity, dust-to-gas ratio, and Si II equivalent width in the two subsamples are drawn from the same parent population are small. These quantities are significantly larger in the high cool population, while the H I column densities are indistinguishable in the two populations. We find the DLA gas is heated by local radiation fields and background radiation, rather than background radiation alone. The rare appearance of faint, extended objects in the Hubble Ultra Deep Field rules out in situ star formation as the dominant star-formation mode for the high cool population, but is compatible with in situ star formation as the dominant mode for the low cool population. Star formation in the high cool DLAs likely arises in Lyman break galaxies. Using Si II equivalent width as a mass indicator, we construct bivariate distributions of metallicity, lc, and areal SFR versus the mass indicators. Tentative evidence is found for correlations and parallel sequences, which suggest similarities with the bimodality found in nearby galaxies. We suggest that the transition-mass model provides a plausible scenario for the bimodality we have found. As a result, the bimodality in current galaxies may have originated in DLAs.

Constrained semi-analytical models of galactic outflows

We present semi-analytic models of galactic outflows that are constrained by available observations on high-redshift star formation and reionization. Galactic outflows are modelled in a manner akin to models of stellar wind-blown bubbles. Large-scale outflows can generically escape from low-mass haloes (M≲ 109M⊙) for a wide range of model parameters while this is not the case in high-mass haloes (M≳ 1011M⊙). The flow generically accelerates within the halo virial radius, then starts to decelerate, and traverses well into the intergalactic medium (IGM), before freezing to the Hubble flow. The acceleration phase can result in shell fragmentation due to the Rayleigh–Taylor instability, although the final outflow radius is not significantly altered. The gas-phase metallicities of the outflow and within the galaxy are computed assuming uniform instantaneous mixing. Ionization states of different metal species are calculated and used to examine the detectability of metal lines from the outflows. The global influence of galactic outflows is also investigated using porosity-weighted averages and probability density functions of various physical quantities. Models with only atomic cooled haloes significantly fill the IGM at z∼ 3 with metals (with −2.5 ≳[Z/Z⊙]≳−3.7), the actual extent depending on the efficiency of winds, the initial mass function and the fractional mass that goes through star formation. The reionization history has a significant effect on the volume filling factor, due to radiative feedback. In these models, a large fraction of outflows at z∼ 3 are supersonic, hot (T≥ 105K) and have low density, making metal lines difficult to detect. They may also result in significant perturbations in the IGM gas on scales probed by the Lyman α forest. On the contrary, models including molecular cooled haloes with a normal mode of star formation can potentially volume fill the universe at z≥ 8 without drastic dynamic effects on the IGM, thereby setting up a possible metallicity floor (−4.0 ≤[Z/Z⊙]≤−3.6). The fluctuations of order unity at z∼ 8 that become the mildly non-linear fluctuations traced by Lyman α forest at z < 4 will then have this metallicity. Interestingly, molecular cooled haloes with a ‘top-heavy’ mode of star formation are not very successful in establishing the metallicity floor because of the additional radiative feedback that they induce.

The effect of magnetic fields on star cluster formation

We examine the effect of magnetic fields on star cluster formation by performing simulations following the self-gravitating collapse of a turbulent molecular cloud to form stars in ideal MHD. The collapse of the cloud is computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is using both weak and strong magnetic fields. Whilst even at very low strengths the magnetic field is able to significantly influence the star formation process, for magnetic fields with plasma beta < 1 the results are substantially different to the hydrodynamic case. In these cases we find large-scale magnetically-supported voids imprinted in the cloud structure; anisotropic turbulent motions and column density structure aligned with the magnetic field lines, both of which have recently been observed in the Taurus molecular cloud. We also find strongly suppressed accretion in the magnetised runs, leading to up to a 75% reduction in the amount of mass converted into stars over the course of the calculations and a more quiescent mode of star formation. There is also some indication that the relative formation efficiency of brown dwarfs is lower in the strongly magnetised runs due to the reduction in the importance of protostellar ejections.

Protostellar collapse: a comparison between smoothed particle hydrodynamics and adaptative mesh refinement calculations

Context. The rapid development of parallel supercomputers is enabling the detailed study of the collapse and the fragmentation of prestellar cores with increasingly accurate numerical simulations. Due to the advances also in sub-millimeter observation technology, we are now able to consider many different modes of low-mass star formation using observations of a range of initials conditions. The challenge for the simulations is to reproduce the observational results.Aims. Two main numerical methods, namely AMR and SPH, are widely used to simulate the collapse and the fragmentation of prestellar cores. We thoroughly compare these two methods within their standard framework.Methods. We use the AMR code RAMSES and the SPH code DRAGON. Our simplified physical model consists of an isothermal sphere rotating about the z -axis. First we study the conservation of angular momentum as a function of the resolution. Then, we explore a wide range of simulation parameters to study the fragmentation of prestellar cores.Results. There appears to be convergence between the two methods, provided numerical resolution in each case is sufficient. We deduced numerical resolution criteria adapted to our physical cases, in terms of resolution per Jeans mass, for an accurate description of the formation of protostellar cores. This convergence is encouraging for future work in simulations of low-mass star formation, providing the aforementioned criteria are fulfilled.

Diffuse UV light associated with the Spiderweb Galaxy: evidence for in situ star formation outside galaxies

We present detailed images of diffuse UV intergalactic light (IGL), situated in a 60kpc halo that surrounds the radio galaxy MRC 1138-262 at z=2. We discuss the nature of the IGL and rule out faint cluster galaxies, nebular continuum emission, synchrotron, inverse Compton emission and scattering of galactic stellar light as possible sources of the IGL. Dust scattered quasar light is an unlikely possibility that cannot be ruled out entirely. We conclude that the source of the IGL is most likely to be a young stellar population distributed in a halo encompassing the radio and satellite galaxies, undergoing star formation at a rate greater than 57 Msun/yr. Within 70kpc of the radio core, approximately 44% of the star formation that is traced by UV light occurs in this diffuse mode. The average UV colour of the IGL is bluer than the average galaxy colour, and there is a trend for the IGL to become bluer with increasing radius from the radio galaxy. Both the galaxies and the IGL show a UV colour--surface brightness relation which can be obtained by variations in either stellar population age or extinction. These observations show a different, but potentially important mode of star formation, that is diffuse in nature. Star formation, as traced by UV light, occurs in two modes in the high redshift universe: one in the usual Lyman break galaxy clump-like mode on kpc scales, and the other in a diffuse mode over a large region surrounding massive growing galaxies. Such a mode of star formation can easily be missed by high angular resolution observations that are well suited for detecting high surface brightness compact galaxies. Extrapolating from these results, it is possible that a significant amount of star formation occurs in large extended regions within the halos of the most massive galaxies forming at high redshift.

Signatures of Dynamical Star Formation in the Ophiuchus Association of Pre–Main‐Sequence Stars

A sample of 58 probable members of the association of pre-main-sequence stars around the filamentary ρ Ophiuchi cloud is investigated. Using astrometric proper motions from the UCAC-2 catalog and the convergent point method, the total heliocentric systemic velocity is estimated at 19 km s^(-1), and the mean distance at 145 pc. A small, statistically insignificant difference between the geometric convergent point and the actual direction of velocity defined by the observed radial velocity implies a small, if any, rate of expansion of the association. The Ophiuchus association appears to have a considerable depth, with half of the members lying within 15 pc of the center. When the moving cluster distances are taken into account, the near-infrared M_(Ks) versus (J - K_s) H-R diagram reveals an apparent large spread of ages between 14 Myr and younger than 1 Myr. Most of the youngest stars are located along a slightly curved strip just south of the densest cloud, extending in the east-west direction roughly aligned with the central streamer of warm dust. The intersection of this strip with a thin segment of shocked dust visible in the IRAS 12 μm map at (α,δ) = (16^h31^m, −24.5°) is marked with a small cluster of probably very young stars around ROXs 43A. The large extent and depth of the association, the moderate rate of expansion, the spread in ages of about 14 Myr, and the alignment of very young stars with the dusty streamer point at a dynamical mode of star formation in this region, scattered far and wide around the main core.

Multiwavelength Study of Massive Galaxies atz∼2. I. Star Formation and Galaxy Growth

Examining a sample of massive galaxies at 1.4<z<2.5 with K_{Vega}<22 from the Great Observatories Origins Deep Survey, we compare photometry from Spitzer at mid- and far-IR, to submillimeter, radio and rest-frame ultraviolet wavelengths, to test the agreement between different tracers of star formation rates (SFRs) and to explore the implications for galaxy assembly. For z~2 galaxies with moderate luminosities(L_{8um}<10^{11}L_sun), we find that the SFR can be estimated consistently from the multiwavelength data based on local luminosity correlations. However,20--30% of massive galaxies, and nearly all those with L_{8um}>10^{11}L_sun, show a mid-IR excess which is likely due to the presence of obscured active nuclei, as shown in a companion paper. There is a tight and roughly linear correlation between stellar mass and SFR for 24um-detected galaxies. For a given mass, the SFR at z=2 was larger by a factor of ~4 and ~30 relative to that in star forming galaxies at z=1 and z=0, respectively. Typical ultraluminous infrared galaxies (ULIRGs) at z=2 are relatively 'transparent' to ultraviolet light, and their activity is long lived (~400 Myr), unlike that in local ULIRGs and high redshift submillimeter-selected galaxies. ULIRGs are the common mode of star formation in massive galaxies at z=2, and the high duty cycle suggests that major mergers are not the dominant trigger for this activity.Current galaxy formation models underpredict the normalization of the mass-SFR correlation by about a factor of 4, and the space density of ULIRGs by an orderof magnitude, but give better agreement for z>1.4 quiescent galaxies.

A Dense Gas Trigger for OH Megamasers

HCN and CO line diagnostics provide new insight into the OH megamaser (OHM) phenomenon, suggesting a dense gas trigger for OHMs. We identify three physical properties that differentiate OHM hosts from other starburst galaxies: (1) OHMs have the highest mean molecular gas densities among starburst galaxies; nearly all OHM hosts have (H2) = 103-104 cm-3 (OH line-emitting clouds likely have n(H2) > 104 cm-3). (2) OHM hosts are a distinct population in the nonlinear part of the IR-CO relation. (3) OHM hosts have exceptionally high dense molecular gas fractions, LHCN/LCO > 0.07, and comprise roughly half of this unusual population. OH absorbers and kilomasers generally follow the linear IR-CO relation and are uniformly distributed in dense gas fraction and LHCN, demonstrating that OHMs are independent of OH abundance. The fraction of non-OHMs with high mean densities and high dense gas fractions constrains beaming to be a minor effect: OHM emission solid angle must exceed 2π steradians. Contrary to conventional wisdom, IR luminosity does not dictate OHM formation; both star formation and OHM activity are consequences of tidal density enhancements accompanying galaxy interactions. The OHM fraction in starbursts is likely due to the fraction of mergers experiencing a temporal spike in tidally driven density enhancement. OHMs are thus signposts marking the most intense, compact, and unusual modes of star formation in the local universe. Future high-redshift OHM surveys can now be interpreted in a star formation and galaxy evolution context, indicating both the merging rate of galaxies and the burst contribution to star formation.

On the Rapid Collapse and Evolution of Molecular Clouds

Stars generally form faster than the ambipolar diffusion time, suggesting that several processes short-circuit the delay and promote a rapid collapse. These processes are considered here, including turbulence compression in the outer parts of giant molecular cloud (GMC) cores and GMC envelopes, GMC core formation in an initially supercritical state, and compression-induced triggering in dispersing GMC envelopes. The classical issues related to star formation timescales are addressed: high molecular fractions, low efficiencies, long consumption times for CO and HCN, rapid GMC core disruption and the lack of a stable core, long absolute but short relative timescales with accelerated star formation, and the slow motions of protostars. We consider stimuli to collapse from changes in the density dependence of the ionization fraction, the cosmic ray ionization rate, and various dust properties at densities above ~105 cm-3. We favor the standard model of subcritical GMC envelops and suggest they would be long-lived if not for disruption by rapid star formation in GMC cores. The lifecycle of GMCs is illustrated by a spiral arm section in the Hubble Heritage image of M51, showing GMC formation, star formation, GMC disruption with lingering triggered star formation, and envelope dispersal. There is no delay between spiral arm dust lanes and star formation; the classical notion results from heavy extinction in the dust lane and triggered star formation during cloud dispersal. Differences in the IMF for the different modes of star formation are considered.

Probing the star formation history using the redshift evolution of luminosity functions

We present a self-consistent, semi-analyticalCDM model of star formation and reioniza- tion. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the electron scattering optical depth to reioniz ation, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 ≤ z ≤ 6 for a reasonable range of model parameters. While simple photoionization feed- back produces the correct shape of LF at z = 6, for z = 3 we need additional feedback that suppresses star formation activities in halos with 10 10 . (M/M⊙) . 10 11 . Models with prolonged continuous star formation activities are prefer red over those with short bursts as they are consistent with the existence of a Balmer break in considerable fraction of observed galaxies even at z ∼ 6. The halo number density evolution from the standardCDM structure formation model that fits LF up to z = 6 is consistent with the upper limits on z ≃ 7 LF and source counts at 8 ≤ z ≤ 12 obtained from the Hubble Ultra Deep Field (HUDF) observations without requiring any dramatic change in the nature of star formation. However, to reproduce the observed LF at 6 ≤ z ≤ 10, obtained from the near-IR observations around strong lensing clusters, we need a strong evolution in the initial mass func tion, reddening correction and the mode of star formation at z & 8. We show that low mass molecular cooled halos, which may be important for reionizing the universe, are not detectabl e in the present deep field obser- vations even if a considerable fraction of its baryonic mass goes through a star burst phase. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the luminosity function in the redshi ft range 6 ≤ z ≤ 12. In our model calculations, the contribution of low mass halos to global SFR density prior to reionization reveals itself in the form of second peak at z ≥ 6. However this peak will not be visible in the observed SFR density as a function of z as most of these galaxies have luminosity below the detection threshold of various ongoing deep field survey s. Accurately measuring the LF at high redshifts can be used to understand the nature of star formation in the dark ages and probe the history of reionization.

The Transition from the First Stars to the Second Stars in the Early Universe

We observe a sharp transition from a singular, high-mass mode of star formation to a low-mass-dominated mode, in numerical simulations, at a metallicity of 10-3 Z☉. We incorporate a new method for including the radiative cooling from metals into adaptive mesh refinement hydrodynamic simulations. Our results illustrate how metals, produced by the first stars, led to a transition from the high-mass star formation mode of Population III stars to the low-mass mode that dominates today. We ran hydrodynamic simulations with cosmological initial conditions in the standard ΛCDM model, with metallicities, from zero to 10-2 Z☉, beginning at redshift z = 99. The simulations were run until a dense core forms at the center of a 5 × 105 M☉ dark matter halo, at z ~ 18. Analysis of the central 1 M☉ core reveals that the two simulations with the lowest metallicities, Z = 0 and 10-4 Z☉, contain one clump with 99% of the mass, while the two with metallicities Z = 10-3 and 10-2 Z☉ each contain two clumps that share most of the mass. The Z = 10-3 Z☉ simulation also produced two low-mass protostellar objects with masses between 10-2 and 10-1 M☉. Gas with Z ≥ 10-3 Z☉ is able to cool to the temperature of the cosmic microwave background (CMB), which sets a lower limit to the minimum fragmentation mass. This suggests that the second-generation stars produced a spectrum of lower mass stars but were still more massive, on average, than stars formed in the local universe.

The role of HD cooling in primordial star formation

The role of HD cooling in the formation of primordial objects is examined by means of a great number of 1-D models of the collapse of halos, exploring a wide range of masses and virialization redshifts. We find that HD has very little effect upon the critical mass separating the objects which are likely to form stars from those which are not. We also find that, once the proto-stellar collapse has started, HD effects are quite negligible. Instead, HD effects can be important during the intermediate stage of gas fragmentation: objects below a certain mass scale (� 3×10 5 M⊙ at zvir = 20 in our “fiducial” case) can be cooled by HD down to T � 50 100 K, whereas H2 cooling never takes the gas below T � 200 K. The lower temperature implies a reduction of a factor � 10 in the Jeans mass of the fragmenting gas, and stars forming in such low-mass halos are probably less massive than their counterparts in larger halos. We estimate the importance of this mode of star formation through a variation of the Press-Schechter formalism, and find that it never exceeds the contribution of halos which are cooled by H2 only. Halos where HD is important account at best for a fraction �0.25 of the total primordial star formation. However, HD cooling might provide a channel through which long-lived low mass stars could be formed in primordial conditions.

Comparison of U BV R photometry of giant HII regions in the galaxy NGC 628 with a detailed grid of evolutionary models for star clusters

We present the results of U BV RI CCD photometry of giant HII regions in the spiral galaxy NGC 628, acquired with the 1.5 m telescope of the Mt. Maidanak Observatory (Uzbekistan) with an angular resolution better than 1″. We estimate the ages and interstellar extinctions of these regions and identify the acting star-formation mode by comparing the observed color indices with a detailed grid of evolutionary models covering the entire range of parameters of the initial mass function and of ages of the young star-formation complexes, and taking into account two star-formation modes. We find a radial gradient of the interstellar extinction in NGC 628, which is consistent with the radial abundance gradient found earlier by other authors from independent spectrophotometry. Our age estimates agree with abundance estimates from independent observations.

Near-IR imaging of Galactic massive clusters: Westerlund 2

Context. Most stars in the Galaxy were formed in massive clusters. To understand nature's favorite mode of star formation and the initial stages of the life of most stars one needs to characterize the youngest and resolved massive clusters in the Milky Way. Unfortunately young massive clusters are challenging observational targets as they are rare, hence found at large distances, are still embedded in their parental molecular cloud, and are swamped by relatively bright nebulae. Aims. In this paper we propose to use deep subarcsec resolution NIR data to derive the basic parameters of the unstudied population of massive cluster Westerlund 2. Methods. We present deep JHK s images (∼0.6 seeing) and photometry of Westerlund 2. This is the most complete photometric census of the cluster's population to date. Results. We detect a total of 4701, 5724, and 5397 sources in the J, H, and K s bands respectively. By comparison with main-sequence and pre-main-sequence model tracks, we determine an average visual extinction toward the cluster of 5.8 mag, a likely distance of 2.8 kpc, and an age of 2.0 ± 0.3 Myr for the core of the cluster. Although we have the sensitivity to reach beyond the hydrogen burning limit in the cluster, we are only complete to about 1 M ⊙ due to source confusion. We find no evidence for a top-heavy MF, and the slope of the derived mass function is -1.20 ± 0.16. Based on the extrapolation of a field IMF, we roughly estimate the total mass of the cluster to be about 10 4 M ⊙ . We find compelling evidence for mass segregation in this cluster.

GALEX-derived Residual Star Formation History of Elliptical Galaxies

Bright early-type galaxies are still forming stars at low-level residual star formation (RSF) mode. The GALEX UV satellite has detected excessive UV flux in >30% of our volume-limited sample of ~ 10 3 galaxies. The RSF galaxy fraction is higher among the dynamically low-mass sample (RSF- σ relation). We postulate that this is a result of a “critical” M BH − σ relation that regulates residual star formation activities in early type galaxies. Implications are discussed.

Metal Enrichment in Galactic Winds

Observations give evidences of the presence of metals in the intergalactic medium (IGM). The stars responsible for transforming hydrogen and helium into more complex atoms do not form outside the galaxies in the standard scenario of galaxy formation. Supernovae–driven winds and their associated feedback was proposed as a possible solution to explain such enrichment of the IGM. It turned out that a proper modelling of supernovae explosions within a turbulent interstellar medium (ISM) is a difficult task. Recent advances have been obtained using a multiphase approach to solve for the thermal state of the ISM, plus some additional recipes to account for the kinetic effect of supernovae on the galactic gas. We briefly describe here our implementation of supernovae feedback within the RAMSES code, and apply it to the formation and evolution of isolated galaxies of various masses and angular momenta. We have explored under what conditions a galactic wind can develop, if one considers only a quiescent mode of star formation. We have also characterized the distribution and evolution of metallicity in the gas outflow spreading in the IGM.

3D Simulations of the 180pc Molecular Ring

Understanding the kinematics and morphology of molecular clouds in the Central Molecular Zone (CMZ) is important for studying the mode of star formation in the central region of the Galaxy. A quasi-continuous ring structure with a radius of 180pc, so-called ‘‘180-pc molecular ring’’, is interpreted as the transition region between the two stable orbits in the bar potential, x1 & x2 [2]. We present, for the first time, 3-D high resolution hydrodynamic simulations of the gas flows in this transition region, and compare the results with previous 2-D studies. We will also see if an m = l perturbation, which is revealed as a displacement of the CMZ, is responsible for some of the discrepancies between the previous simulations and observations.

The XMM-Newton extended survey of the Taurus molecular cloud (XEST)

The Taurus Molecular Cloud (TMC) is the nearest large star-forming region, prototypical for the distributed mode of low-mass star formation. Pre-main sequence stars are luminous X-ray sources, probably mostly owing to magnetic energy release. Aims. The XMM-Newton Extended Survey of the Taurus Molecular Cloud (EST) presented in this paper surveys the most populated =5 square degrees of the TMC, using the XMM-Newton X-ray observatory to study the thermal structure, variability, and long-term evolution of hot plasma, to investigate the magnetic dynamo, and to search for new potential members of the association. Many targets are also studied in the optical, and high-resolution X-ray grating spectroscopy has been obtained for selected bright sources. Methods. The X-ray spectra have been coherently analyzed with two different thermal models (2-component thermal model, and a continuous emission measure distribution model). We present overall correlations with fundamental stellar parameters that were derived from the previous literature. A few detections from Chandra observations have been added. Results. The present overview paper introduces the project and provides the basic results from the X-ray analysis of all sources detected in the XEST survey. Comprehensive tables summarize the stellar properties of all targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus by about an order of magnitude and for the first time systematically accesses very faint and strongly absorbed TMC objects. We find a detection rate of 85% and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169 surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios, and show evidence for lower X-ray luminosities in CTTS compared to WTTS. Detailed analysis (e.g., variability, rotation-activity relations, influence of accretion on X-rays) will be discussed in a series of accompanying papers.

From nuclear clusters to halo globulars: Star clusters as basic galactic building blocks

AbstractI assess the similarities and differences between the star-formation modes in quiescent spiral galaxies versus those in violent starburst regions, including galactic nuclei. As opposed to the quiescent star-formation mode, current empirical evidence on the star-formation processes in the extreme, high-pressure environments induced by galaxy encounters strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in such starburst events. This implies that by using star clusters as unique diagnostic probes, we can trace a galaxy's most violent star formation history very well, at least for the past few Gyr. The sizes, luminosities, and mass estimates of the young massive star clusters are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). Recent evidence lends support to the scenario that GCs, which were once thought to be the oldest building blocks of galaxies, are still forming today.

HST ACS/HRC imaging of the intergalactic HII regions in NGC 1533

AbstractIntergalactic HII regions, far from the confines of a galactic disk, represent a mode of star formation in low-density gas outside of galaxies. The figure below (left) shows an R-band continuum image of NGC 1533 from the SINGG Hα survey (Meurer et al. 2006) overlaid with HI contours and the location of three intergalactic HII regions discovered by Ryan-Weber et al. (2004). The HI contours are 1.6, 2.0, 2.4, 2.8, 3.2 and 4.0 ×1020 cm−2 and have a resolution of ~1′. ACS/HRC images of the intergalactic HII regions (right) are composites of UV, V, and I bands. The half-light radii of the clusters associated with regions 1, 2, and 5 are 24.7, 21.7, and 17.0 pc, respectively, at the distance to NGC 1533 (21 Mpc; Tonry et al. 2001). Assuming a Salpeter IMF with Mup = 100, Hα/UV ratios indicate a small number of ionizing O stars relative to the total number of UV-emitting O and B stars. These young (4-6 Myr), intergalactic stellar populations lend valuable insight to our understanding of the methods by which star formation is triggered and may even represent the first episodes of star formation in emerging galaxies.