HST/STIS optical spectroscopy of five super star clusters in the starburst galaxy M82

Abstract

We present optical spectroscopy obtained with the Space Telescope Imaging Spectrograph of five young massive star clusters in the starburst galaxy M82. A detailed analysis is performed for one cluster ‘M82‐A1’ and its immediate environment in the starburst core. From Hubble Space Telescope archive images, we find that it is elliptical with an effective radius of 3.0 ± 0.5 pc and is surrounded by a compact (r= 4.5 ± 0.5 pc) H ii region. We determine the age and reddening of M82‐A1 using synthetic spectra from population synthesis models by fitting both the continuum energy distribution and the depth of the Balmer jump. We find an age of 6.4 ± 0.5 Myr and a photometric mass estimate of M= 7–13 × 105 M⊙. We associate its formation with the most recent starburst event 4–6 Myr ago. We find that the oxygen abundance of the H ii region surrounding M82‐A1 is solar or slightly higher. The H ii region has a high pressure P/k= 1–2 × 107 cm−3 K. The diffuse gas in region A has a slightly lower pressure, which together with the broad Hα emission‐line width, suggests that both the thermal and turbulent pressures in the M82 starburst core are unusually high. We discuss how this environment has affected the evolution of the cluster wind for M82‐A1. We find that the high pressure may have caused the pressure‐driven bubble to stall. We also obtain spectroscopic ages for clusters B1‐2 and B2‐1 in the ‘fossil’ starburst region and for the intermediate age clusters F and L. These are consistent with earlier studies and demonstrate that star formation activity, sufficiently intense to produce super star clusters, has been going on in M82 during the past Gyr, perhaps in discrete and localized episodes.

On the Heating Efficiency Derived From Observations of Young Super Star Clusters in M82

“…However, as noted by Silich et al (2007), this cannot be the whole story. The observed properties of the H ii region associated with the massive (1.3 × 10 6 M ), young (age ∼ 6 Myr) and thus powerful (with a mechanical luminosity, L mech ≈ 2.5×10 40 erg s −1 ) SSC M82-A1 (Smith et al 2006) are 932 SILICH ET AL.…”

Dusty Supernovae Running the Thermodynamics of the Matter Reinserted Within Young and Massive Super Stellar Clusters

“…The large central temperature depressions at the critical line and the slow acceleration of the reinserted matter within the cluster are indicative of the last stationary solution found before crossing the critical line. The presence of dust in the thermalized plasma within young SSCs provides a natural explanation for the observational and theoretical studies that have derived a strongly reduced mechanical energy output from some of the massive clusters in the galaxy M82 in order to account for the size of the compact H ii regions that surround them (see Smith et al 2006;Silich et al 2007Silich et al , 2009. Further evidence for the bimodal solution and the positive star formation feedback may come from the interpretation of the abundance variations from star to star in massive globular clusters (GC), such as ω Cen in the Milky Way where different generations of stars are enriched by different contributors, and among these by core-collapse SNe (Gratton et al 2004) or from observations of the most massive GCs in M31 where the run of α to Fe-peak elements is consistent with a primordial enrichment from stars with masses larger than 10 M (Meylan et al 2001).…”

An Arc of Young Stars in the Halo of M82

“…Subsequent studies of the stellar fossil record in M82 have since found evidence of widespread elevated levels of star-forming activity roughly 0.5-1 Gyr in the past (e.g., de Grijs et al 2001;Mayya et al 2006). More recent episodes of star-forming activity have been restricted to the inner regions of M82 (e.g., Gallagher & Smith 1999;Forster Schreiber et al 2003;Smith et al 2006).…”