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Register a new userNew Results on the Ages of Star Clusters in Region B of M82
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Added by
Iraklis S. Konstantopoulos
Publication date
2008
fields
Physics
and research's language is
English
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The post-starburst region B in M82 and its massive star cluster component have been the focus of multiple studies, with reports that there is a large population of coeval clusters of age ~1 Gyr, which were created with a Gaussian initial mass distribution. This is in disagreement with other studies of young star clusters, which invariably find a featureless power-law mass distribution. Here, we present Gemini-North optical spectra of seven star clusters in M82-B and show that their ages are all between 10 and 300 Myr (a factor of 3-100 younger than previous photometric results) and that their extinctions range between near-zero and 4 mag (Av). Using new HST ACS-HRC U-band observations we age date an additional ~30 clusters whose ages/extinctions agree well with those determined from spectroscopy. Completeness tests show that the reported `turn-over in the luminosity/mass distributions is most likely an artefact, due to the resolved nature of the clusters. We also show that the radial velocities of the clusters are inconsistent with them belonging to a bound region.
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Like other starburst galaxies, M82 hosts compact, massive young star clusters that are interesting both in their own right and as benchmarks for population synthesis models. Can spectral synthesis models at resolutions around 1000 adequately reproduce the near-IR spectral features and the energy distribution of these clusters between 0.8 and 2.4 microns? How do the derived cluster properties compare with previous results from optical studies? We analyse the spectra of 5 massive clusters in M82, using data acquired with the spectrograph SpeX on the InfraRed Telescope Facility (NASA/IRTF) and a new population synthesis tool with a highly improved near-IR extension, based on a recent collection of empirical and theoretical spectra of red supergiant stars. We obtain excellent fits across the near-IR with models at quasi-solar metallicity and a solar neighbourhood extinction law. Spectroscopy breaks a strong degeneracy between age and extinction in the near-IR colours in the red supergiant-dominated phase of evolution. The estimated near-IR ages cluster between 9 and 30 Myr, i.e. the ages at which the molecular bands due to luminous red supergiants are strongest in the current models. They do not always agree with optical spectroscopic ages. Adding optical data sometimes leads to the rejection of the solar neighbourhood extinction law. This is not surprising considering small-scale structure around the clusters, but it has no significant effect on the near-IR based spectroscopic ages. [abridged]
We present the structural parameters of 99 Super Star Clusters (SSCs) in the Disk of M82. Moffat-EFF, King and Wilson models were fitted using a chi^2 minimisation method to background-subtracted Surface Brightness Profiles (SBPs) in F435W (B), F555W (V), and F814W (I) bands of the Advanced Camera for Surveys (ACS) of the Hubble Space Telescope (HST). The majority of the SSC profiles is best-fitted by the Moffat-EFF profile. The scale parameter rd and the shape parameter gamma in the three filters are identical within the measurement errors. The analysed sample is big enough to allow characterisation of the distributions of core radii Rc and gamma. The obtained distribution of Rc follows a log-normal form, with center and sigma(log(Rc/pc)) being 1.73 pc and 0.25, respectively. The gamma distribution is also log-normal with center and sigma(log(gamma)) being 2.88 and 0.08, respectively. M82 is well-known for the absence of current star formation in its disk, with all disk SSCs older than 50 Myr and hardly any cluster older than ~300 Myr. The derived distributions compare very well with the distributions for intermediate-age clusters in the Large Magellanic Cloud (LMC), which is also a low-mass late-type galaxy similar to M82. On the other hand, the distributions of Rc in both these galaxies are shifted towards larger values as compared to SSCs of similar age in the giant spiral galaxy M83. M82 and LMC also span a narrower range of gamma values as compared to that in M83.
We report on new aspects of the star-forming region S235AB revealed through high-resolution observations at radio and mid-infrared wavelengths. Using the Very Large Array, we carried out sensitive observations of S235AB in the cm continuum (6, 3.6, 1.3, and 0.7) and in the 22 GHz water maser line. These were complemented with Spitzer Space Telescope Infrared Array Camera archive data to clarify the correspondence between radio and IR sources. We made also use of newly presented data from the Medicina water maser patrol, started in 1987, to study the variability of the water masers found in the region. S235A is a classical HII region whose structure is now well resolved. To the south, no radio continuum emission is detected either from the compact molecular core or from the jet-like structure observed at 3.3 mm, suggesting emission from dust in both cases. We find two new compact radio continuum sources (VLA-1 and VLA-2) and three separate maser spots. VLA-1 coincides with one of the maser spots and with a previously identified IR source (M1). VLA-2 lies towards S235B and represents the first radio detection from this peculiar nebula that may represent an ionized wind from a more evolved star. The two other maser spots coincide with an elongated structure previously observed within the molecular core in the C34S line. This structure is perpendicular to a bipolar molecular outflow observed in HCO+(1-0) and may trace the associated equatorial disk. The Spitzer images reveal a red object towards the molecular core. This is the most viable candidate for the embedded source originating the outflow and maser phenomenology. The picture emerging from these and previous data shows the extreme complexity of a small (< 0.5 pc) star-forming region where widely different stages of stellar evolution are present.
Recent HST/ACS images of M82 covering the entire galaxy have been used to detect star clusters. The galaxy is known to contain a young population (age < 10 Myr) in its starburst nucleus, surrounded by a post-starburst disk of age < 1 Gyr. We detect more than 650 star clusters in this galaxy, nearly 400 of them in the post-starburst disk. These data have been used to derive the luminosity, mass and size functions separately for the young nuclear, and intermediate-age disk clusters. In this contribution, we discuss the evolutionary status of these clusters, especially, on the chances of some of these clusters surviving to become old globular clusters.
There is growing evidence that star clusters can no longer be considered simple stellar populations (SSPs). Intermediate and old age clusters are often found to have extended main sequence turn-offs (eMSTOs) which are difficult to explain with single age isochrones, an effect attributed to rotation. In this paper, we provide the first characterisation of this effect in young (<20Myr) clusters. We determine ages for 4 young massive clusters (2 LMC, 2 Galactic) by three different methods: using the brightest single turn-off (TO) star; using the luminosity function (LF) of the TO; and by using the lowest $L_{rm bol}$ red supergiant (RSG). The age found using the cluster TO is consistently younger than the age found using the lowest RSG $L_{rm bol}$. Under the assumption that the lowest luminosity RSG age is the `true age, we argue that the eMSTOs of these clusters cannot be explained solely by rotation or unresolved binaries. We speculate that the most luminous stars above the TO are massive blue straggler stars formed via binary interaction, either as mass gainers or merger products. Therefore, using the cluster TO method to infer ages and initial masses of post-main sequence stars such as Wolf-Rayet stars, luminous blue variables and RSGs, will result in ages inferred being too young and masses too high.
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