Search

Super star cluster (SSC) A1 (⁠|$3.1\times 10^5\, \mathrm{M}_\odot$|⁠) in NGC 3125 has one of the strongest (⁠|$EW=4.6\pm 0.5\,$|  Å) broad (⁠|$FWHM=1131\pm 40\,$|  km s−1) He  ii λ1640 emission lines in the nearby universe and constitutes an important template for interpreting observations of extreme He  ii emitters out to redshifts of z ∼ 2 – 3. We use Cosmic Origins Spectrograph (COS) observations of A1 to show that there is no significant contamination of the He  ii line with nebular emission and that the line is redshifted by 121 ± 17 km s−1 relative to ISM lines. We compare the COS G130M + G160M observations of A1 to recent binary BPASS and single-star Charlot & Bruzual (C&B) simple stellar population (SSP) models with very massive stars (VMS) of up to |$300\, \mathrm{M}_\odot$|⁠. On the other hand, we suggest why BPASS models fail to reproduce A1's He  ii emission. On the other hand, a C&B model with Z  = 0.008, age = 2.2 Myr, and VMS approaching the Eddington limit provides an excellent fit to the He  ii emission and fits reasonably well C  iii λ1175, N  v λλ1238, 1241, and C  iv λλ1548, 1551. We present O  v λ1371 line-profile predictions showing that this line constitutes an important tracer of youth and VMS in galaxies. Finally, we discuss the presence of VMS in CDFS131717, a highly star-forming low-metallicity galaxy located at z  = 3.071, which has a tentative detection of O  v absorption and strong broad He  ii emission. These features are rare and hint to the presence of short-lived VMS in the galaxy. Our results show the effect of the latest developments of stellar wind theory and the importance of accounting for VMS in models. [ABSTRACT FROM AUTHOR]

We report on recent progress in the modelling of the near-IR spectra of young stellar populations, i.e. populations in which red supergiants (RSGs) are dominant. First, we discuss the determination of fundamental parameters of RSGs from Phoenix model fits to their near-IR spectra; RSG-specific surface abundances are accounted for and effects of the microturbulence parameter are explored. New population synthesis predictions are then described and, as an example, it is shown that the spectra of young star clusters in M 82 can be reproduced very well from 0.5 to 2.4 μm. We warn of remaining uncertainties in cluster ages. [ABSTRACT FROM PUBLISHER]

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 thestarburst99 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 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 usingstarburst99 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 thestarburst99 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 . A total mass of is inferred for the ionizing sources within Tol 89-B. [ABSTRACT FROM AUTHOR]

We present high-dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) ‘F’ in M82, obtained with the 4.2-m William Herschel Telescope, for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster. In addition to the stellar lines, the spectrum contains complex Na iabsorption and broad emission lines from the ionized gas. We measure a stellar velocity dispersion of 13.4±0.7 km s[sup -1] and a projected half-light radius of 2.8±0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2±0.1×10[sup 6] M[sub ⊙], demonstrating that M82-F is a very massive, compact cluster. We determine that the current luminosity-to-mass ratio (L[sub V]/M)[sub ⊙] for M82-F is 45±13. Comparison with spectral synthesis models shows that (L[sub V]/M)[sub ⊙] is a factor of ∼ 5 higher than that predicted for a standard Kroupa initial mass function (IMF) at the well-determined age for M82-F of 60±20 Myr. This high value of (L[sub V]/M)[sub ⊙] indicates a deficit of low-mass stars in M82-F; the current mass function (MF) evidently is ‘top-heavy’. We find that a lower mass cut-off of 2–3 M[sub ⊙] is required to match the observations for a MF with a slope α=2.3. Since the cluster apparently lacks long-lived low-mass stars, it will not become an old globular cluster, but probably will dissolve at an age of ≤ 2 Gyr. We also derive updated luminosity-to-mass ratios for the younger SSCs NGC 1569A and 1705-1. For the first object, the observations are consistent with a slightly steeper MF (α=2.5), whereas for NGC 1705-1 the observed ratio requires the MF to be truncated near 2 M[sub ⊙] for a slope of α=2.3. We discuss the implications of our findings in the context of large-scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster. M82-F probably formed in a dense molecular cloud; however, its high radial velocity with respect to the centre of M82 (∼-175 km s[sup -1]) suggests that it is on an eccentric orbit and now far from its birthplace, so the environment of its formation is unknown. [ABSTRACT FROM AUTHOR]

ABSTRACTWe present high signal-to-noise ratio optical spectra of two luminous super star clusters in the starburst galaxy M82. The data for cluster F and the nearby, highly reddened, cluster L were obtained with the William Herschel Telescope (WHT) at a resolution of 1.6 Å. The blue spectrum (3250--5540 Å) of cluster F shows features typical of mid-B stars. The red spectra (5730--8790 Å) of clusters F and L show the Ca II triplet and numerous F- and G-type absorption features. Strong Ca II and Na i interstellar absorption lines arising in M82 are also detected, and the λ 6283 diffuse interstellar band appears to be present. The quality of the WHT spectra allows us to improve previous age estimates considerably for cluster F. By comparing the blue spectrum with theoretical model cluster spectra using the pegase spectral synthesis code, we derive an age of 60 ± 20 Myr. The strength of the Ca II triplet is also in accord with this age. Cluster L appears to have a similar age, although this is much less certain. The measured radial velocities for the two clusters differ substantially, indicating that they are located in different regions of the M82 disc. Cluster F appears to be deep in M82, slightly beyond the main starburst region, while the highly obscured cluster L lies near the outer edges of the disc. We derive an absolute magnitude MV = -16.5 for F, indicating that it is an extremely massive cluster. The presence of such a luminous super star cluster suggests that the M82 starburst experienced an episode of intense star formation ≈ 60 Myr ago. [ABSTRACT FROM AUTHOR]