Ages and Metallicities of Young Globular Clusters in the Merger Remnant NGC 7252

UV-to-visual spectra of eight young star clusters in the merger remnant and protoelliptical galaxy NGC 7252, obtained with the Blanco 4-m telescope on Cerro Tololo, are presented. These clusters lie at projected distances of 3-15 kpc from the center and move with a velocity dispersion of 140+/-35 km/s in the line of sight. Seven of the clusters show strong Balmer absorption lines in their spectra [EW(H-beta)= 6-13 Angstrom], while the eighth lies in a giant HII region and shows no detectable absorption features. Based on comparisons with model-cluster spectra by Bruzual & Charlot (1996) and Bressan, Chiosi, & Tantalo (1996), six of the absorption-line clusters have ages in the range of 400-600 Myr, indicating that they formed early on during the recent merger. These clusters are globular clusters as judged by their small effective radii and ages corresponding to ~100 core crossing times. The one emission-line object is <10 Myr old and may be a nascent globular cluster or an OB association. The mean metallicities measured for three clusters are solar to within +/-0.15 dex, suggesting that the merger of two likely Sc galaxies in NGC 7252 formed a globular-cluster system with a bimodal metallicity distribution. Since NGC 7252 itself shows the characteristics of a 0.5-1 Gyr old protoelliptical, its second-generation solar-metallicity globulars provide direct evidence that giant ellipticals with bimodal globular-cluster systems can form through major mergers of gas-rich disk galaxies.Comment: 34 pages, incl. 6 figures in EPS format, AAS LaTeX, to be published in AJ, Vol. 116, Nov. 199

Quantitative Estimates of Environmental Effects on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies

A simple model is constructed to evaluate the change of star formation rate of a disk galaxy due to environmental effects in clusters of galaxies. Three effects, (1) tidal force from the potential well of the cluster, (2) increase of external pressure when the galaxy plows into the intracluster medium, (3) high-speed encounters between galaxies, are investigated. General analysis indicates that the star formation rate increases significantly when the pressure of molecular clouds rises above $\sim 3\times 10^5 cm^{-3} K$ in $\sim 10^8$ yr. The tidal force from the potential well of the cluster increases pressures of molecular clouds in a disk galaxy infalling towards the cluster center. Before the galaxy reaches the cluster center, the star formation rate reaches a maximum. The peak is three to four times larger than the initial value. If this is the main mechanism of the Butcher-Oemler effect, blue galaxies are expected to be located within $\sim 300$ kpc from the center of the cluster. However this prediction is inconsistent with the recent observations. The increase of external pressure when the galaxy plows into the intracluster medium does not change star formation rate of a disk galaxy significantly. The velocity perturbation induced by a single high-speed encounter between galaxies is too small to affect star formation rate of a disk galaxy, while successive high-speed encounters (galaxy harassment) trigger star formation activity because of the accumulation of gas in the galaxy center. Therefore, the galaxy harassment remains as the candidate for a mechanism of the Butcher-Oemler effect.Comment: 12 pages, 13 figures. To be published in Ap

Cattle dung can increase CH4 production and influence the methanogen community of rewetted peat soils

201

Above- and belowground CH4 fluxes from boreal forest shrubs and Scots pine

201

Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies

We present long-slit spectroscopy, B and R bandpass imaging, and 21 cm observations of a sample of early-type galaxies in nearby clusters which are known to be either in a star-forming phase or to have had star formation which recently terminated. From the long-slit spectra, obtained with the Blanco 4-m telescope, we find that emission lines in the star-forming cluster galaxies are significantly more centrally concentrated than in a sample of field galaxies. The broadband imaging reveals that two currently star-forming early-type galaxies in the Pegasus I cluster have blue nuclei, again indicating that recent star formation has been concentrated. In contrast, the two galaxies for which star formation has already ended show no central color gradient. The Pegasus I galaxy with the most evident signs of ongoing star formation (NGC7648), exhibits signatures of a tidal encounter. Neutral hydrogen observations of that galaxy with the Arecibo radiotelescope reveal the presence of ~4 x 10^8 solar masses of HI. Arecibo observations of other current or recent star-forming early-type galaxies in Pegasus I indicate smaller amounts of gas in one of them, and only upper limits in others.Comment: to be published in Astronomical Journa

3C 48: Stellar Populations and the Kinematics of Stars and Gas in the Host Galaxy

We present deep Keck LRIS spectroscopy of the host galaxy of 3C 48. Our observations at various slit positions sample the different luminous components near the quasar, including the apparent tidal tail to the NW and several strong emission line regions. By fitting Bruzual & Charlot (1996) population synthesis models to our spectra, we obtain ages for the most recent major episodes of star formation in various parts of the host galaxy covered by our slits. There is vigorous current star formation in regions just NE and SE of the quasar and post-starburst regions with ages up to ~10^8 years in other parts of the host galaxy, but most of the NW tidal tail shows no sign of significant recent star formation. We use these model fits, together with the kinematics of the stars and gas, to outline a plausible evolutionary history for the host galaxy, its recent starburst activity, the triggering of the quasar, and the interaction of the radio jet with the ambient gas. There is strong evidence that the 3C 48 host is an ongoing merger, and that it is probably near the peak of its starburst activity. Nevertheless, the quasar itself seems to suffer little extinction, perhaps because we are viewing it along a particularly favorable line-of-sight.Comment: 27 pages plus 11 figures (7 postscript, 4 gif). Postscript version including figures (1840 kb) available at http://www.ifa.hawaii.edu/~canaguby/preprints.html . Accepted for publication in Ap

Revealing sources of biological methane production in boreal upland forests

201

Revealing sources of CH4 in a boreal upland forest

201

Star cluster formation and star formation: the role of environment and star-formation efficiencies

“The original publication is available at www.springerlink.com”. Copyright Springer. DOI: 10.1007/s10509-009-0088-5By analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to determine whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star-formation rate, burst strength, star-formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few billion years later.Peer reviewe

Detecting metal-rich intermediate-age globular clusters in NGC4570 using K-band photometry

“The original publication is available at www.springerlink.com”. Copyright Springer. DOI: 10.1007/s10509-009-0093-8Globular cluster systems (GCSs) of most early-type galaxies feature two peaks in their optical colour distributions. Blue-peak globular clusters (GCs) are believed to be old and metal-poor, whereas the ages, metallicities, and the origin of the red-peak GCs are still being debated. We obtained deep K-band photometry and combined it with Hubble Space Telescope observations in g and z to yield a full spectral energy distribution from the optical to the near-infrared. This now allows us to break the age–metallicity degeneracy. We used our evolutionary synthesis models galev for star clusters to compute a large grid of models with different metallicities and a wide range of ages. Comparing these models to our observations revealed a large population of intermediate-age (1–3 Gyr) and metal-rich (≈solar-metallicity) GCs, that will give us further insights into the formation history of this galaxy.Peer reviewe