We describe the Sloan

Stephan's Quintet (SQ) is a compact group that we find in an atypical moment when a high velocity intruder is passing through it. The intrusion is particularly interesting because a previous intruder had stripped most of the gas from the group members. This debris field was shocked in the ongoing collision with the new intruder. This evolutionary history agrees well with observations and explains how a strongly interacting system can show low levels of star formation. We present new multiwavelength data including previously unpublished ROSAT X-ray, Hα interference filter/Fabry-Perot, ISO MIR/FIR and radio line and continuum images. These observations and previously published data provide new insights as well as support for some previous hypotheses. 1) Fabry-Perot and HI velocities allow us to unambiguously distinguish between gas associated with SQ and the new intruder. 2) Most detected emission regions are found in the remnant ISM of the NI which allows us to infer its size and present physical state.3) The few emission regions associated with the stripped ISM of SQ include the best candidate tidal dwarf galaxy. 4) Multiwavelength data suggest that strong MIR/FIR emission from the Seyfert 2 nucleus of NGC7319 comes from dust heated directly by a power-law continuum rather than a starburst. 5) The correspondance between extended X-ray/radio continuum/forbidden optical emission confirms the existence of a -2large scale shock in SQ. 6) We confirm the presence of two stripped spiral members in the process of transformation into E/S0 morphology. Finally, 7) Observations are consistent with the idea that the collision in SQ is ongoing with possible detection of HII region ablation and Rayleigh-Taylor instabilities.

We report the first APOGEE metallicities and α-element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds, the largest Milky Way (MW) dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from [Fe/H] = −0.2 to very metal-poor stars with [Fe/H] ≈ −2.5, the most metal-poor Magellanic Cloud (MC) stars detected to date. The LMC [α/Fe]–[Fe/H] distribution is very flat over a large metallicity range but rises by ∼0.1 dex at −1.0 < [Fe/H] ≲ −0.5. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent “starburst.” At the metal-poor end, we capture the increase of [α/Fe] with decreasing [Fe/H] and constrain the “α-knee” to [Fe/H] ≲ −2.2 in both MCs, implying a low star formation efficiency of ∼0.01 Gyr−1. The MC knees are more metal-poor than those of less massive MW dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW’s gravitational potential only recently.

The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf Galaxy (Sgr), Fornax (Fnx), and the now fully disrupted Gaia Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [α/Fe]–[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the Magellanic Clouds (MCs) observed by Nidever et al. in the α-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3–4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier (∼5–7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.

Abstract. We present optical spectroscopy of H ii regions in the Local Group galaxy IC 10 and UBV R photometry of foreground stars in three fields towards this galaxy. From these data, we find that the foreground reddening due to the Milky Way is E(B − V ) = 0.77 ± 0.07 mag. We find that IC 10 contains considerable internal dust, which qualitatively explains the variety of reddening values found by studies of its different stellar populations. Based upon our foreground reddening, IC 10 has intrinsic photometric properties like those of a blue compact dwarf galaxy, and not those of a dwarf irregular. This result is consistent with much evidence that IC 10 is in the throes of a starburst that began at least 10 Myr ago. We also report the discovery of a new WR star in the H ii region HL111c.Key words. galaxies: individual: IC 10 -galaxies: local group -galaxies: fundamental parameters -galaxies: starburst -galaxies: dwarf IntroductionIC 10 has long been recognized as a peculiar object (Hubble 1936), and is now considered the Local Group's representative of a starburst galaxy (van den Bergh 2000). As we will show, IC 10 may very well be the nearest example of a blue compact dwarf galaxy (BCD). Certainly, its surface brightness is similar to that seen in BCDs once we account for the foreground reddening. Among Local Group dwarf galaxies, IC 10 has the highest surface density of WR stars and the highest current rate of star formation (Mateo 1998). The presence of so many WR stars and the high Hα luminosity emphasize that IC 10 is undergoing a strong burst of star formation that began at Send offprint requests to: M. G. Richer, e-mail: richer@astrosen.unam.mx least 10 Myr ago. Observations of 21-cm emission from H i reveal that IC 10 consists of an inner disk embedded in an extended, complex, counter-rotating envelope (Shostak & Skillman 1989), and leads Wilcots & Miller (1998) to conclude that IC 10 is still in its formative stage. These H i observations emphasize the youth of the current star formation episode, for there is a notable lack of interstellar medium structures that are attributable to supernovae (Wilcots & Miller 1998). Several studies of the stellar populations in IC 10 have revealed the existence of young, intermediate-age, and old stellar populations (Massey & Armandroff 1995;Sakai et al. 1999;Borissova et al. 2000). However, with the exception of the very recent star formation, very little is known of the history of star formation in IC 10, and nothing is known of the star formation history outside of the star-forming region.Article published by EDP Sciences and available at http://www.aanda.org or http://dx