Starburst Models Research Articles

INFRARED AND ULTRAVIOLET STAR FORMATION IN BRIGHTEST CLUSTER GALAXIES IN THE ACCEPT SAMPLE

We present IR and UV photometry for a sample of brightest cluster galaxies (BCGs). The BCGs are from a heterogeneous but uniformly characterized sample, the Archive of Chandra Cluster Entropy Profile Tables (ACCEPT), of X-ray galaxy clusters from the Chandra X-ray telescope archive with published gas temperature, density, and entropy profiles. We use archival GALEX, Spitzer, and 2MASS observations to assemble spectral energy distributions (SEDs) and colors for BCGs. We find that while the SEDs of some BCGs follow the expectation of red, dust-free old stellar populations, many exhibit signatures of recent star formation in the form of excess UV or mid-IR emission, or both. We establish a mean near-UV to 2MASS K color of 6.59 \pm 0.34 for quiescent BCGs. We use this mean color to quantify the UV excess associated with star formation in the active BCGs. We use fits to a template of an evolved stellar population and library of starburst models and mid-IR star formation relations to estimate the obscured star formation rates. Many of the BCGs in X-ray clusters with low central gas entropy exhibit enhanced UV (38%) and mid-IR emission (43%), above that expected from an old stellar population. These excesses are consistent with on-going star formation activity in the BCG, star formation that appears to be enabled by the presence of high density, X-ray emitting gas in the the core of the cluster of galaxies. This hot, X-ray emitting gas may provide the enhanced ambient pressure and some of the fuel to trigger the star formation. This result is consistent with previous works that showed that BCGs in clusters with low central gas entropy host H{\alpha} emission-line nebulae and radio sources, while clusters with high central gas entropy exhibit none of these features. UV and mid-IR measurements combined provide a complete picture of unobscured and obscured star formation occurring in these systems.

A new model for the infrared emission of IRAS F10214+4724

AbstractWe present a new model for the infrared emission of the high redshift hyperluminous infrared galaxy IRAS F10214+4724 which takes into account recent photometric data from Spitzer and Herschel that sample the peak of its spectral energy distribution. We first demonstrate that the combination of the AGN tapered disc and starburst models of Efstathiou and coworkers, while able to give an excellent fit to the average spectrum of type 2 AGN measured by Spitzer, fails to match the spectral energy distribution of IRAS F10214+4724. This is mainly due to the fact that the ν Sν distribution of the galaxy falls very steeply with increasing frequency (a characteristic of heavy absorption by dust) but shows a silicate feature in emission. We propose a model that assumes two components of emission: clouds that are associated with the narrow-line region and a highly obscured starburst. The emission from the clouds must suffer significantly stronger gravitational lensing compared to the emission from the torus to explain the observed spectral energy distribution.

THE NATURE OF STARBURSTS. I. THE STAR FORMATION HISTORIES OF EIGHTEEN NEARBY STARBURST DWARF GALAXIES

We use archival HST observations of resolved stellar populations to derive the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In this first paper we present the observations, color-magnitude diagrams, and the SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data reduction, differential extinction, and treatment of photometric completeness. We adopt a star formation rate (SFR) threshold normalized to the average SFR of the individual system as a metric for classifying starbursts in SFHs derived from resolved stellar populations. This choice facilitates finding not only currently bursting galaxies but also "fossil" bursts increasing the sample size of starburst galaxies in the nearby (D<8 Mpc) universe. Thirteen of the eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr with variations on small timescales. A long >100 Myr temporal baseline is thus fundamental to any starburst definition or identification method. The longer lived bursts rule out rapid "self-quenching" of starbursts on global scales. The bursting galaxies' gas consumption timescales are shorter than the Hubble time for all but one galaxy confirming the short-lived nature of starbursts based on fuel limitations. Additionally, we find the strength of the H{\alpha} emission usually correlates with the CMD based SFR during the last 4-10 Myr. However, in four cases, the H{\alpha} emission is significantly less than what is expected for models of starbursts; the discrepancy is due to the SFR changing on timescales of a few Myr. The inherently short timescale of the H{\alpha} emission limits identifying galaxies as starbursts based on the current characteristics which may or may not be representative of the recent SFH of a galaxy.

Cold dust and young starbursts: spectral energy distributions of Herschel SPIRE sources from the HerMES survey★

We present spectral energy distributions (SEDs) for 68 Herschel sources detected at 5-sigma at 250, 350 and 500 mu in the HerMES SWIRE-Lockman field. We explore whether existing models for starbursts, quiescent star-forming galaxies and for AGN dust tori are able to model the full range of SEDs measured with Herschel. We find that while many galaxies (~ 56 %) are well fitted with the templates used to fit IRAS, ISO and Spitzer sources, for about half the galaxies two new templates are required: quiescent ('cirrus') models with colder (10-20 K) dust, and a young starburst model with higher optical depth than Arp 220. Predictions of submillimetre fluxes based on model fits to 4.5-24 mu data agree rather poorly with the observed fluxes, but the agreement is better for fits to 4.5-70 mu data. Herschel galaxies detected at 500 mu tend to be those with the very highest dust masses.

Massive star formation in Wolf-Rayet galaxies

(Abridged) We perform a comprehensive multiwavelength analysis of a sample of 20 starburst galaxies that show the presence of a substantial population of Wolf-Rayet (WR) stars. In this paper we present the analysis of the O and WR star populations. We study the spatial localization of the WR-rich clusters via the detection of the blue WR bump (broad He II 4686) and the red WR bump (broad C IV 5808). We perform a detailed fitting of the nebular and broad emission lines within these broad features and derive the numbers of WN, WC and O stars using (i) the standard assumption of constant WR luminosities and (ii) considering metallicity-dependent WR luminosities. We then compare our results with the predictions given by evolutionary synthesis models and with previous empirical results. Aperture effects and the exact positioning of the slit onto the WR-rich bursts play a fundamental role in their detection. As expected, the total number of WR stars increases with increasing metallicity, but objects with 12+log(O/H)<8.2 show a rather constant WR/(WR+O) ratio. The computed WCE/WNL ratios are different than those empirically found in nearby star-forming galaxies, indicating that the observed galaxies are experiencing a strong and very short burst. Considering metallicity-dependent WR luminosities, our data agree with a Salpeter-like IMF in all regimes. We consider that the contribution of the WCE stars is not negligible at low metallicities. Although available models reproduce fairly well the WR properties at high metallicities, new evolutionary synthesis models for young starbursts including all involved parameters (age, metallicity, star-formation history, IMF and WR stars properties such as metallicity-dependent WR luminosities, stellar rotation and the WR binnary channel) are absolutely needed to perform an appropriate comparison with the observational data.

Polycyclic aromatic hydrocarbon (PAH) luminous galaxies atz ~ 1

<i>Aims. <i/>The NEP-deep survey, an extragalactic AKARI survey towards the north ecliptic pole (NEP), provides a comprehensive wavelength coverage from 2 to 24 <i>μ<i/>m using all 9 photometric bands of the infrared camera (IRC). It allows us to photometrically identify galaxies whose mid-IR emission is clearly dominated by PAHs. <i>Methods. <i/> We propose a single-colour selection method to identify such galaxies, using two mid-IR flux ratios at 11-to-7 <i>μ<i/>m and 15-to-9 <i>μ<i/>m (PAH-to-continuum flux ratio in the rest frame), which are useful for identifying starburst galaxies at <i>z<i/> ~ 0.5 and 1, respectively. We perform a fitting of the spectral energy distributions (SEDs) from optical to mid-IR wavelengths, using an evolutionary starburst model with a proper treatment of radiative transfer (SBURT), in order to investigate their nature. <i>Results. <i/> The SBURT model reproduces observed optical-to-mid-IR SEDs of more than a half of the PAH-selected galaxies. Based on the 8 <i>μ<i/>m luminosity, we find ultra luminous infrared galaxies (ULIRGs) among PAH-selected galaxies. Their PAH luminosity is higher than local ULIRGs with a similar luminosity, and the PAH-to-total IR luminosity ratio is consistent with that of less luminous starburst galaxies. They are a unique galaxy population at high redshifts, and we call these PAH-selected ULIRGs “PAH-luminous” galaxies. Although they are not as massive as submillimetre galaxies at <i>z<i/> ~ 2, they have the stellar mass of <i>><i/> 3 × 10<sup>10<sup/> and therefore are moderately massive.

THE IMPORTANCE OF NEBULAR CONTINUUM AND LINE EMISSION IN OBSERVATIONS OF YOUNG MASSIVE STAR CLUSTERS

In this spectroscopic study of infant massive star clusters, we find that continuum emission from ionized gas rivals the stellar luminosity at optical wavelengths. In addition, we find that nebular line emission is significant in many commonly used broad-band HST filters including the F814W I-band, the F555W V-band and the F435W B-band. Two young massive clusters (YMCs) in NGC 4449 were targeted for spectroscopic observations after Reines et al. (2008a) discovered an F814W I-band excess in their photometric study of radio-detected clusters in the galaxy. The spectra were obtained with the Dual Imaging Spectrograph on the 3.5 m APO telescope. We supplement these data with HST and SDSS photometry. By comparing our data to the Starburst99 and GALEV models, we find that nebular continuum emission competes with the stellar light in our observations and that the relative contribution is largest in the U- and I-bands, where the Balmer and Paschen jumps are located. The spectra also exhibit strong line emission including the [SIII] 9069,9532 lines in the HST F814W I-band. We find that the combination of nebular continuum and line emission can account for the F814W I-band excess found by Reines et al. (2008a). In an effort to provide a benchmark for estimating the impact of ionized gas emission on photometric observations of YMCs, we compute the relative contributions of the stellar continuum, nebular continuum, and emission lines to the total flux of a 3 Myr-old cluster through various HST filter/instrument combinations, including filters in the WFC3. We urge caution when comparing observations of YMCs to evolutionary synthesis models since nebular emission can have a large impact on magnitudes and colors of young (< 5 Myr) clusters, significantly affecting inferred properties such as ages, masses and extinctions. (Abridged)

EVOLVING STARBURST MODELING OF FAR-INFRARED/SUBMILLIMETER/MILLIMETER LINE EMISSION. II. APPLICATION TO M 82

We present starburst models for far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) line emission of molecular and atomic gas in an evolving starburst region, which is treated as an ensemble of non-interacting hot bubbles which drive spherical shells of swept-up gas into a surrounding uniform gas medium. These bubbles and shells are driven by stellar winds and supernovae within massive star clusters formed during an instantaneous starburst. The underlying stellar radiation from the evolving clusters affects the properties and structure of photodissociation regions (PDRs) in the shells, and hence the spectral energy distributions (SEDs) of the molecular and atomic line emission from these swept-up shells and the associated parent giant molecular clouds (GMCs) contains a signature of the stage of evolution of the starburst. The physical and chemical properties of the shells and their structure are computed using a a simple well known similarity solution for the shell expansion, a stellar population synthesis code, and a time-dependent PDR chemistry model. The SEDs for several molecular and atomic lines ($^{12}$CO and its isotope $^{13}$CO, HCN, HCO$^+$, C, O, and C$^+$) are computed using a non-local thermodynamic equilibrium (non-LTE) line radiative transfer model. By comparing our models with the available observed data of nearby infrared bright galaxies, especially M 82, we constrain the models and in the case of M 82, we provide estimates for the ages (5 - 6 Myr, 10 Myr) of recent starburst activity. We also derive a total H$_2$ gas mass of $\sim$ 2 - 3.4 $\times$ 10$^8$ M$_{\odot}$ for the observed regions of the central 1 kpc starburst disk of M 82.

Modelling the Spoon IRS diagnostic diagram

We explore whether our models for starbursts, quiescent star-forming galaxies and for AGN dust tori are able to model the full range of IRS spectra measured with Spitzer. The diagnostic plot of 9.7 mu silicate optical depth versus 6.2 mu PAH equivalent width, introduced by Spoon and coworkers in 2007, gives a good indication of the age and optical depth of a starburst, and of the contribution of an AGN dust torus. However there is aliasing between age and optical depth at later times in the evolution of a starburst, and between age and the presence of an AGN dust torus. Modeling the full IRS spectra and using broad-band 25-850 mu fluxes can help to resolve these aliases. The observed spectral energy distributions require starbursts of a range of ages with initial dust optical depth ranging from 50-200, optically thin dust emission ('cirrus') illuminated by a range of surface brightnesses of the interstellar radiation field, and AGN dust tori with a range of viewing angles.

Starburst and cirrus models for submillimeter galaxies

We present radiative transfer models for submillimeter galaxies with spectroscopic redshifts and mid-infrared spectroscopy from Spitzer/IRS and analyze available Spitzer/MIPS 24, 70 and 160 <i>μ<i/>m data. We use two types of starburst models, a cirrus model and a model for the emission of an AGN torus in order to investigate the nature of these objects. We find that for three of the objects (25%) cirrus emission alone can account for the mid-infrared spectrum and the MIPS and submillimeter data. For the remaining objects we find that we need a combination of starburst and cirrus in order to fit simultaneously the multi-wavelength data. We find that the typical submillimeter galaxy has comparable luminosity in the starburst (median ) and cirrus (median ) components. This could arise if the galaxies have been forming stars continuously for the last 250 Myr with the star formation occurring in the last 5 Myr being shrouded by high-optical-depth molecular cloud dust, whereas the rest of the starlight is attenuated by diffuse dust or cirrus with an <i>A<i/><sub>V<sub/> of about 1 mag.

Dissecting the star formation history of starburst galaxies: the case of NGC 7673

We have collected archival data on NGC 7673 to constrain the star formation history that produced the young star clusters and the field stellar population in this galaxy during the last 2 Gyr. We have considered the sample of 50 star clusters detected by Hubble Space Telescope/WFPC2 in the UV, V and I bands and estimated their age, intrinsic reddening and mass via comparison of their colours with STARBURST99 models. We have found two prominent epochs of cluster formation occurred about 20 and 2 Myr ago, with somewhat minor events between 3 and 6 Myr ago. The star clusters are characterized by an intrinsic reddening E(B-V) < 0.4 mag and a mass lower than 2 x 10 6 M ⊙ . Out of the 50 star clusters, we have selected 31 located within the boundaries of the (IUE) large slit that was employed to obtain the spectrum of NGC 7673 between 1150 and 3350 A. For each cluster, we have built a synthetic spectrum corresponding to the age, mass and intrinsic reddening derived from the cluster colours, properly redshifted to NGC 7673. The spectra have then been added together in a final, clusters integrated spectrum. This and the IUE, FUSE spectra of NGC 7673 have allowed us to describe the star formation history of the unresolved stars in the field as either exponentially decaying or multiburst. In the first case, we have derived an e-folding time of 700 (900) Myr and an initial star formation rate of 16 (13) M ⊙ yr -1 when the Fitzpatrick's (Calzetti's) extinction law is used. In the case of a multiburst star formation history, we have assumed that the same bursts which produced the star clusters built up also the field population. In this way, the field population turns out to be composed by a young (<40 Myr) component three (two) times brighter than the star clusters, and a component as old as ∼850 (450) Myr, about 200 (100) times more massive than the star clusters together. These star formation histories fit equally well the observed UV spectrum of NGC 7673.

THE INITIAL CLUSTER MASS FUNCTION OF SUPER STAR CLUSTERS IN IRREGULAR AND SPIRAL GALAXIES

The initial cluster mass function (ICMF) is a fundamental property of star formation in galaxies. To gauge its universality, we measure and compare the ICMFs in irregular and spiral galaxies. Our sample of irregular galaxies is based on 13 nearby galaxies selected from a volume-limited sample from the fifth data release of the Sloan Digital Sky Survey (SDSS). The extinctions, ages, and masses were determined by comparing their u'g'i'z' magnitudes to those generated from starburst models. Completeness corrections were performed using Monte Carlo simulations in which artificial clusters were inserted into each galaxy. We analyzed three nearby spiral galaxies with SDSS data in exactly the same way to derive their ICMF based on a similar number of young, massive clusters as the irregular galaxy ICMF. We find that the ICMFs of irregular and spiral galaxies for masses >3x10^4 M_sun are statistically indistinguishable. For clusters more massive than 3x10^4 M_sun, the ICMF of the irregular galaxies is reasonably well fit by a power law dN(M)/dM ~ M^-a_M with a_M = 1.88 +/- 0.09. Similar results were obtained for the ICMF of the spiral galaxy sample but with a_M = 1.75 +/- 0.06. We discuss the implications of our result for theories of star cluster formation: the shape of the ICMF appears to be independent of metallicity and galactic shear rate.

Study of star formation in NGC 1084

ABSTRACT We present UBVRI broad band, Hα narrow band photometry of the star formingcomplexes in the infra-red bright galaxy NGC 1084. Results of medium resolutionspectroscopy of some of the brighter complexes are also discussed. Spectroscopic datais used to better estimate the internal reddening within the galaxy which is found to behighly variable and to calculate metallicity which is close to the solar value. Diagnosticdiagramidentifies the shockedregionswithin this galaxy.The narrowbandHα fluxandits equivalent width are used to determine the star formation rates of the complexesand the distribution of ages. Star formation rates for a few of the complexes are foundto be as high as 0.5 M ⊙ yr −1 . The star forming complexes lie in the age range 3 Myrto 6.5 Myr. U − B vs V − I colour-colour mixed population model created usingthe Starburst99 model colours is used to estimate the ages of the stellar populationspresent within these regions. Using this technique, it is found that the star formationin NGC 1084 has taken place in a series of short bursts over the last 40 Myr or so. Itis proposed that the likely trigger for enhanced star formation is merger with a gasrich dwarf galaxy.Key words: star forming regions; general - galaxies: individual: NGC 1084

The host galaxy of GRB 011121: morphology and spectral energy distribution

(Abridged) We present a detailed study of the host galaxy of GRB 011121 (at z = 0.36) based on high-resolution imaging in 5 broad-band, optical and near-infrared filters with HST and VLT/ISAAC. The surface brightness profile of this galaxy is best fitted by a Sersic law with index ~ 2 - 2.5 and a rather large effective radius (~ 7.5 kpc). Both the morphological analysis and the F450W - F702W colour image suggest that the host galaxy of GRB 011121 is either a disk-system with a rather small bulge, or one hosting a central, dust-enshrouded starburst. Hence, we modeled the integrated spectral energy distribution of this galaxy by combining stellar population and radiative transfer models, assuming properties representative of nearby starburst or normal star-forming, Sbc-like galaxies. A range of plausible fitting solutions indicates that the host galaxy of GRB 011121 has a stellar mass of 3.1 - 6.9 x10^9 Msun, stellar populations with a maximum age ranging from 0.4 to 2 Gyr, and a metallicity ranging from 1 to 29 per cent of the solar value. Starburst models suggest this galaxy to be nearly as opaque as local starbursts (with an A_V = 0.27 - 0.76 mag). Alternatively, normal star-forming Sbc-like models suggest a high central opacity whereas A_V = 0.12$ -- 0.57 mag along the line of sight. For this subluminous galaxy (with L_B/Lstar_B = 0.26), we determine a model-dependent SFR of 2.4 - 9.4 Msun/yr. The SFR per unit luminosity (9.2 - 36.1 Msun/yr/(L_B/Lstar_B)) is high compared to those of most GRB host galaxies, but consistent with those of most of the hosts at similar low redshift. Our results suggest that the host galaxy of GRB 011121 is a rather large disk-system in a relatively early phase of its star formation history.

Physical conditions of ionized gas and stellar populations in circumnuclear starbursts

We present an emission-line diagnostic analysis of integral-field spectroscopic observations that cover the circumnuclear ring-like regions in a small number of spiral galaxies. We concentrate on the specific case of the Sa galaxy NGC 7742, which hosts a spectacular circumnuclear starburst ring and nuclear regions characterised by low-ionisation emission. The gas in the ring rotates in the opposite sense to the stars in the galaxy, suggesting a recent merging or acquisition event. The combination of integral-field measurements for the Hα+N[ ii ] emission lines from DensePak and the Hβ and [O iii ] emission from SAURON allows the construction of diagnostic diagrams that highlight the transition from star formation in the ring to excitation by high-velocity shocks or by an AGN near the centre. DensePak measurements for the [S ii ] line ratio reveal very low gas densities in the ring, N e −3 , characteristic of massive H ii regions. Comparison with MAPPINGS III models for starbursts with low gas densities shows that the ring is of roughly solar metallicity. We suggest that NGC 7742 cannibalised a smaller galaxy rich in metal-poor gas, and that star formation episodes in the ring have since increased the metallicity to its present value.

Minor Merger Origin for the Circumnuclear Starburst in NGC 7742

We present an emission-line diagnostic analysis of integral-field spectroscopic observations that cover the central kiloparsec of NGC 7742. This Sa galaxy hosts a spectacular nuclear starburst ring and nuclear regions characterized by low-ionization emission. The gas in the ring rotates in the opposite sense to the stars in the galaxy, suggesting a recent merging or acquisition event. The combination of integral-field measurements for the H-alpha + [NII] emission lines from DensePak and the H-beta and [OIII] emission from Sauron allow the construction of diagnostic diagrams that highlight the transition from star formation in the nuclear ring to excitation by high-velocity shocks or by a central AGN towards the center. DensePak measurements for the [SII] line ratio reveal very low gas densities in the nuclear ring, Ne < 100 per cm^3, characteristic of massive HII regions. Comparison with MAPPINGS III models for starbursts with low gas densities show that the ring is of roughly solar metallicity. This suggests that the gas in the nuclear ring originated in a stellar system capable of substantially enriching the gas metallicity through sustained star formation. We suggest that NGC 7742 cannibalised a smaller galaxy rich in metal-poor gas, and that star formation episodes in the ring have since increased the metallicity to its present value.

SINFONI adaptive optics integral field spectroscopy of the Circinus Galaxy

Aims. We investigate the star formation activity and the gas and stellar dynamics on scales of a few parsecs in the nucleus of the Circinus Galaxy. Methods. Using the adaptive optics near infrared integral field spectrometer SINFONI on the VLT, we have obtained observations of the Circinus galaxy on scales of a few parsecs and at a spectral resolution of 70 km s −1 FWHM. The physical properties of the nucleus are analyzed by means of line and velocity maps extracted from the SINFONI datacube. Starburst models are constrained using the Brγ flux, stellar continuum (as traced via the CO absorption bandheads longward of 2.3 µm), and radio continuum. Results. The similarity of the morphologies of the H2 1−0S (1) 2.12 µ ma nd Brγ 2.17 µm lines to the stellar continuum and also their kinematics, suggest a common origin in star formation. Within 8 pc of the AGN we find there has been a recent starburst in the last 100 Myr, which currently accounts for 1.4% of the galaxy’s bolometric luminosity. The similarity of the spatial scales over which the stars and gas exist indicates that this star formation is occuring within the torus; and comparison of the gas column density through the torus to the maximum possible optical depth to the stars implies the torus is a clumpy medium. The coronal lines show asymmetric profiles with a spatially compact narrow component and a spatially extended blue wing. These characteristics are consistent with some of the emission arising in clouds gravitationally bound to the AGN, and some outflowing in cloudlets which have been eroded away from the bound clouds.

The massive star population in the giant H II region Tol 89 in NGC 5398

We present new high spectral resolution Very Large Telescope (VLT)/UV–Visual Echelle Spectrograph (UVES) spectroscopy and archival Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) imaging and spectroscopy of the giant H ii region Tol 89 in NGC 5398. From optical and ultraviolet (UV) HST images, we find that the star-forming complex as a whole contains at least seven young compact massive clusters. We resolve the two brightest optical knots, A and B, into five individual young massive clusters along our slit, A1–4 and B1, respectively. From UV spectral modelling using the starburst99 code of Leitherer et al., and nebular Hβ equivalent widths in the optical, we derive ages that are consistent with the formation of two separate burst events, of ∼4 ± 1 Myr and <3 Myr for knots A (A1–4) and B (B1), respectively. A Large Magellanic Cloud (LMC) metallicity is measured for both knots from a nebular line analysis, while nebular He ii 4686 is observed in knot B and perhaps in knot A. We detect underlying broad wings on the strongest nebular emission lines indicating velocities up to 600 km s−1. From UV and optical spectroscopy, we estimate that there are ∼95 early WN stars and ∼35 early WC stars in Tol 89-A, using empirical template spectra of LMC Wolf–Rayet (WR) stars from Crowther & Hadfield, with the WC population confined to cluster A2. Remarkably, we also detect a small number of approximately three mid WNs in the smallest (mass) cluster in Tol 89-A, A4, whose spectral energy output in the UV is entirely dominated by the WN stars. From the strength of nebular Hβ, we obtain N(O) ∼ 690 and 2800 for knots A and B, respectively, which implies N(WR)/N(O) ∼ 0.2 for knot A. We also employ a complementary approach using starburst99 models, in which the O star content is inferred from the stellar continuum, and the WR population is obtained from spectral synthesis of optical WR features using the grids from Smith et al. We find reasonable agreement between the two methods for the O star content and the N(WR)/N(O) ratio but find that the WR subtype distribution is in error in the starburst99 models, with far too few WN stars being predicted. We attribute this failure to the neglect of rotational mixing in evolutionary models. Our various modelling approaches allow us to measure the cluster masses. We identify A1 as a super star cluster (SSC) candidate with a mass of ∼1–2 × 105 M⊙. A total mass of ∼6 × 105 M⊙ is inferred for the ionizing sources within Tol 89-B.

Star Formation Rates in Cooling Flow Clusters: A UV Pilot Study with Archival XMM-Newton Optical Monitor Data

We have analyzed XMM-Newton Optical Monitor UV (180-400 nm) data for a sample of 33 galaxies. Thirty are cluster member galaxies, and nine are central cluster galaxies (CCGs) in cooling flow clusters having mass deposition rates between 8 and 525 M☉ yr-1. By comparing the ratio of UV to 2MASS J-band fluxes, we find a significant UV excess in many, but not all, cooling flow CCGs, consistent with several previous studies based on optical imaging data (McNamara & O'Connell; Cardiel et al.; Crawford et al.). This UV excess is a direct indication of the presence of young massive stars and, therefore, recent star formation. Using the Starburst99 model of continuous star formation over a 900 Myr period, we derive star formation rates of 0.2-219 M☉ yr-1 for the cooling flow sample. For two-thirds of this sample, it is possible to equate Chandra/XMM cooling flow mass deposition rates with UV-inferred star formation rates, for a combination of starburst lifetime and IMF slope. This is a pilot study of the well-populated XMM UV cluster archive, and a more extensive follow-up study is currently underway.

Dynamical virial masses of Lyman-break galaxy haloes at z = 3

Abstract We improve on our earlier dynamical estimate of the virial masses of the haloes of Lyman-break galaxies (LBGs) at redshift z= 3 by accounting for the effects of seeing, slit width and observational uncertainties. From an analysis of the small number of available rotation curves for LBGs we determine a relation Vc7= (1.9 ± 0.2)σ between circular velocity at a radius of 7 kpc, Vc7, and central line velocity width, σ. We use this relation to transform the measured velocity widths of 32 LBGs to the distribution of circular velocities, Vc7, for the population of LBGs brighter than . We compare this distribution against the predicted distribution for the ‘massive-halo’ model in which LBGs pinpoint all of the highest mass dark matter haloes at that epoch. The observed LBG circular velocities are smaller than the predicted circular velocities by a factor of &amp;gt;1.4 ± 0.15. This is a lower limit, as we have ignored any increase of circular velocity caused by baryonic dissipation. The massive-halo model predicts a median halo virial mass of 1012.3 M⊙, and a small spread of circular velocities, Vc7. Our median estimated dynamical mass is &amp;lt;1011.6±0.3 M⊙, which is significantly smaller; furthermore, the spread of our derived circular velocities is much larger than the massive-halo prediction. These results are consistent with a picture which leaves some of the most massive haloes available for occupation by other populations which do not meet the LBG selection criteria. Our new dynamical mass limit is a factor of 3 larger than our earlier estimate which neglected the effects of seeing and slit width. The median halo mass recently estimated by Adelberger et al. from the measured clustering of LBGs is 1011.86±0.3 M⊙. Our dynamical analysis appears to favour lower masses and to be more in line with the median mass predicted by the collisional starburst model of Somerville et al., which is 1011.3 M⊙.