VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds

Abstract

We discuss the abundances of interstellar CH, CH + , and CN in the Magellanic Clouds, derived from spectra of seven SMC and 13 LMC stars obtained (mostly) with the VLT UVES. CH and/or CH + have now been detected toward three SMC and nine LMC stars; CN is detected toward Sk 143 (SMC) and Sk À67 2 ( LMC). These data represent nearly all the optical detections of these molecular species in interstellar media beyond the Milky Way. In the LMC, the CH /H 2 ratio is comparable to that found for diffuse Galactic molecular clouds in four sight lines but is lower by factors of 2.5-4.0 in two others. In the SMC, the CH / H 2 ratio is comparable to the local Galactic value in one sight line but is lower by factors of 10-15 in two others. The abundance of CH in the Magellanic Clouds thus appears to depend on local physical conditions and not just on metallicity. In both the SMC and the LMC, the observed relationships between the column density of CH and those of CN, CH + , Na i, and K i are generally consistent with the trends observed in our Galaxy.Using existing data for the rotational populations of H 2 in these sight lines, we estimate temperatures, radiation field strengths, and local hydrogen densities for the diffuse molecular gas. The inferred temperatures range from about 45 to 90 K, the radiation fields range from about 1 to 900 times the typical local Galactic field, and the densities (in most cases) lie between 100 and 600 cm À3 . Densities estimated from the observed N(CH ), under the assumption that CH is produced via steady state gas-phase reactions, are considerably higher than those derived from H 2 . Much better agreement is found by assuming that the CH is made via the (still undetermined) process(es) responsible for the observed CH + . A significant fraction of the CH and CH + in diffuse molecular material in the SMC and LMC may be produced in photon-dominated regions. The excitation temperature obtained from the populations of the two lowest CN rotational levels toward Sk À67 2 is quite consistent with the temperature of the cosmic microwave background radiation measured with COBE.Toward most of our targets, the UVES spectra also reveal absorption at velocities corresponding to the Magellanic Clouds ISM from several of the strongest of the diffuse interstellar bands ( DIBs; at 5780, 5797, and 6284 8). On average, the three DIBs are weaker by factors of 7-9 (LMC) and about 20 (SMC), compared to those typically observed in Galactic sight lines with similar N(H i), presumably due to the lower metallicities and stronger radiation fields in the LMC and SMC. The three DIBs are also weaker (on average, but with some exceptions), by factors of order 2-6, relative to E(BÀV ), N(Na i), and N(K i) in the Magellanic Clouds. The detection of several of the socalled C 2 DIBs toward Sk 143 and Sk À67 2 with strengths similar to those in comparable Galactic sight lines, however, indicates that no single, uniform scaling factor (e.g., one related to metallicity) applies to all DIBs (or for all sight lines) in the Magellan...

Towards DIB mapping in galaxies beyond 100 Mpc

“…When examining the ensemble of data globally, the dispersion is larger, pointing at secondary parameters modulating this correlation. In particular, for a given reddening, DIBs in the Magellanic Clouds are a factor 2-6 fainter than Galactic sightlines, as pointed out by Welty et al (2006). All of our measurements in AM 1353-272 B agree well with this general relation.…”

“…The sample is by no means complete, but covers a diversity of environments, methodologies, sampled areas, etc. This includes examples of spatially resolved measurements of the two large spirals in the Local Group (Puspitarini et al 2013;Cordiner et al 2011) as well as for the Magellanic Clouds (Welty et al 2006). Likewise, we included some supernovae (Phillips et al 2013) and nearby galaxies (Heckman & Lehnert 2000).…”

Time series of high-resolution spectra of SN 2014J observed with the TIGRE telescope

“…In our Galaxy, such DIBs are quite commonly observed in high-resolution spectra of massive stars (Herbig 1995). There exist also measurements for the Small and Large Magellanic Clouds (Vladilo et al 1987;Ehrenfreund, P. et al, 2002;Cox et al 2006;Cox, N. L. J. et al, 2007;Welty et al 2006). Some spectral observations of extragalactic SNe have already succeeded in detecting DIBs (D'Odorico et al 1989;Sollerman et al 2005;Cox & Patat 2008.…”

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“…Most measured DIBs have a Galactic origin. Still, they have also been detected in the Magellanic clouds, M31 and M33 (Ehrenfreund et al 2002;Welty et al 2006;Cordiner et al 2008aCordiner et al , 2008bCordiner et al , 2011van Loon et al 2013) and in a few line of sights toward e.g., starburst galaxies or in SNe Ia spectra (Sollerman et al 2005;Cox & Patat 2008;Heckman & Lehnert 2000;Phillips et al 2013). Recently a DIB gradient was established for the first time in a 160 Mpc distant galaxy (Monreal-Ibero et al 2015).…”