Magellanic Clouds ISM

• Large Magellanic Cloud
• Small Magellanic Cloud
• Optical and UV Surveys
• List of optical and UV spectra
• Column Densities

The Magellanic Clouds (two smaller satellite galaxies which may be orbiting our Galaxy) are characterized by a number of environmental differences, relative to the Milky Way -- lower metallicities (heavy element abundances), lower dust to gas ratios, generally stronger radiation fields, and differences in UV extinction. Knowledge of the gas-phase abundances, dust depletion patterns, and physical conditions (temperature, density, ionization) in the ISM of the Magellanic Clouds is thus of considerable interest for understanding interstellar processes, for testing theoretical models of interstellar clouds, and for understanding the (generally) even lower metallicity QSO Absorption-Line Systems .

Large Magellanic Cloud

High resolution optical profiles of Na I, Ca II, Ca I, and K I were used together with UV spectra obtained with IUE to determine accurate abundances in the various interstellar component groups observed toward the LMC SN 1987A. For the main LMC components, the gas-phase abundances of a number of species, relative to Zn II, seem consistent with the pattern seen for warm, low density Galactic disk clouds (see figure below). The relative abundances in the components at "intermediate" velocities (100-200 km/s), however, are more similar to those found in the SMC ISM and in warm, diffuse clouds in the Galactic halo.

Here are a few representative spectra illustrating the complex interstellar absorption along the line of sight to the SN. The Ca II and Na I spectra were obtained by Vidal-Madjar et al. (1987) and Magain (1987) using the ESO CAT+CES, at resolutions of 3-5 km/s; at least 44 components are discernible in Ca II. Even higher resolution (0.5 km/s) spectra of Na I and K I, obtained by Pettini & Gillingham at the AAT, reveal even more complex structure and many relatively narrow components (b-values from 0.3-1.0 km/s). The Si II and Fe II spectra were obtained with IUE , at resolutions of 17-25 km/s; special processing techniques have been used to improve the S/N ratios. Component groups 1 through 4 (and perhaps 5) are likely due to gas in the Galactic disk and halo; groups 6 through 10 are probably due to gas in the LMC. Various column density ratios can be used to estimate "average" physical properties for these component groups.

HST STIS echelle spectra were obtained in Cycle 11 for two LMC stars located in regions characterized by different UV extinction. Sk-67 5 is at the northwest end of the main LMC bar, where the UV extinction is typical of the LMC; Sk-70 115 is in the LMC2 region (southeast of 30 Dor), where the UV extinction is more similar to that in the SMC (steeper far-UV rise, weaker 2175 A bump). The main components toward Sk-67 5 exhibit abundance/depletion patterns similar to those found for warm clouds in the Galactic disk. The main component toward Sk-70 115, however, has less severe depletion of Si than would be expected from the depletions of other refractory elements (e.g., Fe, Ni) -- reminiscent of the patterns seen toward Sk 155 in the SMC (see below).

Click here to see a UK Schmidt image of the LMC , obtained by D. Malin. SN 1987A is located slightly below and to the right of the bright 30 Dor H II region (left center).

Welty, D.E., Frisch, P.C., Sonneborn, G., & York, D. G., 1999, ApJ, 512, 636,

Interstellar Abundances in the Magellanic Clouds. II. The Line of Sight to SN 1987A in the LMC.

Wong, T., Hughes, A., Fukui, Y., Kawamura, A., Mizuno, N., Ott, J., Muller, E., Pineda, J. L., Welty, D. E., Kim, S., Mizuno, Y., Murai, M., & Onishi, T. 2009, ApJ, 696, 370,

Molecular and Atomic Gas in the Large Magellanic Cloud - I. Conditions for CO Detection.

Wong, T., Hughes, A., Ott, J., Muller, E., Pineda, J. L., Bernard, J.-P., Chu, Y.-H., Fukui, Y., Gruendl, R., Henkel, C., Kawamura, A., Klein, U., Looney, L. W., Maddison, S., Mizuno, Y., Seale, J., & Welty, D. E. 2011, ApJS, 197, 16,

The Magellanic Mopra Assessment (MAGMA). I. The Molecular Cloud Population of the Large Magellanic Cloud.

Welty, D. E., Lauroesch, J. T., Hobbs, L. M., & York, D. G., in preparation,

Interstellar Abundances in the Magellanic Clouds. IV. The Sight Lines toward the LMC Stars Sk-67 5 and Sk-70 115.

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Small Magellanic Cloud

HST GHRS echelle and grating spectra of a number of species were obtained in Cycle 4 toward one star in the SMC in order to determine the abundance/depletion pattern(s) present in the various interstellar components (the first such study of the SMC ISM based on high-resolution UV spectra). This figure shows some of the interstellar absorption features along the line-of-sight to Sk 108 seen in the GHRS echelle-B data (FWHM = 4.2 km/s). Vertical dotted lines separate the five component groups (G1, G2, S1, S2, S3) identified via similar relative abundances and/or velocities in fits to the profiles. The smooth lines are the fits to the data (denoted by points); the individual components are noted by tick marks. The G1 and G2 components, at v < 70 km/s, arise in the Galactic halo and disk; the S1, S2, and S3 components, at v < 70 km/s, are due to gas in the SMC. Note that the Cr II and Zn II 2062 components are interleaved, with Cr II at -62.5 km/s with respect to Zn II.

Click here to see a UK Schmidt picture of the SMC , obtained by D. Malin. Sk 108 is located in the northeastern (upper left) part of the main "bar" of the SMC.

(Top) Here we compare the relative abundances found for the SMC and LMC ISM with the average Galactic warm, low density cloud (W), cold, dense cloud (C), and halo cloud (H) gas-phase abundance patterns (for Al, Si, Ti, Cr, Mn, Fe, and Ni), expressed as ratios with respect to Zn, relative to Solar ratios (i.e., [X/Zn]). These patterns reflect both the nucleosynthetic history of the gas and any selective depletion. The [Zn/H] ratio is shown at the far right; the points for the LMC and SMC reflect primarily the lower than Solar metallicity of those systems. The pattern seen for the main LMC (L) component (blend) is quite similar to the Galactic warm cloud pattern; the SMC (S) components resemble more the (less depleted) halo cloud pattern. Since the stellar abundance ratios are not much different from Solar for these elements in the LMC and SMC, the depletion patterns may also be similar to those found in the ISM of our Galaxy -- despite the lower metallicities and dust-to-gas ratios in the LMC and SMC. (Bottom) The pattern seen for a small number of QSO Absorption-Line Systems (0.7 < z < 3.4; from current literature) is somewhat similar to the halo and SMC patterns (though a large range is evident). Note also the wide range in [Zn/H] -- from -0.3 to <-1.7. Both nucleosynthetic and depletion effects (comparable in magnitude) may be present. The similarities between the depletion patterns in the LMC and SMC and those seen in low-density environments in the Galactic disk and halo suggest that we may infer similar patterns in the QSOALS -- which should allow the underlying total abundances in the QSOALS to be determined more accurately.

HST STIS echelle spectra of the star Sk 155, in the SMC ``wing'' region, however, revealed several striking differences in the relative abundances/depletions in the main SMC components (Welty et al. 2001). The depletions of Fe and Ni range from mild ([Fe,Ni/Zn] ~ -0.3 to -0.8 dex) to severe ([Fe,Ni/Zn] ~ -1.7 dex); Mg and Si, however, are at most very mildly depleted ([Mg,Si/Zn] ~ -0.2 to +0.1) throughout. The combination of severe depletion for Cr, Mn, Fe, and Ni and very mild depletion for Mg and Si has not been seen in any Galactic sightline. The apparent mild Si depletion --- if characteristic of the SMC ISM --- would have significant implications for models of IS dust (which currently rely heavily on silicates) and for the interpretation of the gas-phase abundances observed for QSO absorption-line systems (if the dust in QSOALS resembles that in the SMC). In addition, detection of C I and other trace neutral species has enabled estimates for the pressures (n_H T) and electron densities (n_e) in some of the SMC clouds. The observed somewhat elevated thermal pressures are consistent with predictions of theoretical models for lower metallicity systems with enhanced radiation fields.

Profiles of 4 UV lines toward Sk 155, observed with STIS/E230H. Only the SMC absorption, at 60 km/s < v < 240 km/s, is shown; tick marks indicate components found in fits to the line profiles. Main component groups A, B, and C are noted. Striking differences between the profiles of undepleted Si II and Zn II (left) versus those of more heavily depleted Fe II (right) indicate significant component-to-component differences in relative gas-phase abundances. The stronger Fe II 2586 line reveals additional lower column density components. Dotted line for Si II shows the profile predicted from Fe/Zn if Galactic depletions are assumed.

Gas-phase abundance ratios [X/Zn] (relative to solar ratios) for three strongest SMC component groups toward Sk 155. Solid lines show typical values found for Galactic cold, dense clouds (C), warm, diffuse clouds (W), and halo clouds (H). Since Zn is typically only mildly depleted, these [X/Zn] reflect primarily the depletions of elements X. Toward Sk 155, significant component to component variations are seen for the more severely depleted elements Cr, Mn, Fe, and Ni. The elements Mg, Si, and S are at most very mildly depleted in all three Sk 155 component groups --- even when Fe and Ni are severely depleted. Such depletion patterns have not been seen in the Galactic ISM --- where [Mg,Si/Zn] ~ -0.8 dex for [Fe,Ni/Zn] ~ -1.7 dex.

Welty, D. E., Lauroesch, J.T., Blades, J.C., Hobbs, L.M., & York, D.G., 1997, ApJ, 489, 672,

Interstellar Abundances in the Magellanic Clouds. I. GHRS Observations of the SMC Star Sk 108.

Welty, D. E., Lauroesch, J. T., Blades, J. C., Hobbs, L. M., & York, D. G. 2001, ApJ, 554, L75,

Unusual Depletions toward the SMC Star Sk 155 --- Differences in Dust Composition in the SMC Interstellar Medium?

Mallouris, C., Welty, D. E., York, D. G., Moos, H. W., Sembach, K. R., Friedman, S. D., Jenkins, E. B., Lemoine, M., Oegerle, W. R., Savage, B. D., Shull, J. M., Sonneborn, G., & Vidal-Madjar, A. 2001, ApJ, 558, 133,

FUSE Observations of Interstellar Gas toward the SMC Star Sk 108.

Welty, D. E., Howk, J. C., Lehner, N., & Black, J. H., 2013, MNRAS, 428, 1107,

Detection of Interstellar C2 and C3 in the Small Magellanic Cloud.

Welty, D. E., Lauroesch, J. T., Blades, J. C., Hobbs, L. M., & York, D. G., in preparation,

Interstellar Abundances in the Magellanic Clouds. III. STIS and FUSE Observations of the SMC Star Sk 155.

Welty, D. E., Blades, J. C., Frisch, P. C., Hobbs, L. M., Lauroesch, J. T., Sonneborn, G., & York, D. G., 1998, poster at IAU Symposium 190 --- New Views of the Magellanic Clouds (Victoria, BC; 1998 July),

Interstellar Abundances in the Magellanic Clouds.

Welty, D. E. 1999, invited talk at Session 63 (Evolution of Chemical Abundances over Cosmic Time) of AAS meeting 194 (1999 June),

Relative Abundances in the Milky Way and the Magellanic Clouds.

Welty, D. E. 2001, invited talk at IAP XVIIth Colloquium --- Gaseous Matter in Galaxies and Intergalactic Space (Paris; 2001 June),

Interstellar Abundances in the Magellanic Clouds.

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Optical and UV Surveys

Moderate and/or high resolution optical spectra have been obtained (at the AAT and ESO) for Na I, K I, Ca I, Ca II, Ti II, Fe I, CH, CH+, CN, and some of the diffuse interstellar bands (DIBs) toward a number of Magellanic Clouds stars. Analyses of those spectra are yielding the first general, quantitative surveys of those atomic and molecular species in the ISM of the Magellanic Clouds. The high resolution spectra of Na I and K I will also be used to help interpret lower resolution H_2 spectra of those sight lines obtained with FUSE. Some results of these surveys:

• Na I and K I are much weaker, relative to H, in the Magellanic Clouds -- due to lower metallicities, stronger radiation fields, and (perhaps) reduced grain-assisted recombination there;
• depletions, as indicated by Na I/Ca II and Ti II/H, seem to be generally less severe in the Magellanic Clouds than in the local Galactic ISM;
• the abundance of CH appears to depend on local physical conditions -- not just on metallicity;
• densities inferred from CH, assuming steady-state gas-phase reactions, are much higher than those inferred from H_2; much better agreement is found if most of the CH is produced via the (still undetermined) process(es) responsible for the observed CH+; and
• while some of the typically stronger DIBs generally are weaker in the LMC and SMC [by factors of about 10 and 20, respectively, relative to N(H)], no single, uniform scaling factor (e.g., one related to metallicity) applies to all DIBs (or all sight lines) in the Magellanic Clouds.

(left) CH vs. H_2 (Welty et al. 2006). Green circles and red triangles denote LMC and SMC sight lines, respectively; open symbols indicate limits for one (or both) of the species. All other (black) symbols denote Galactic sight lines; length of crosses indicate 1-sigma uncertainties (for all sight lines). The solid lines show weighted and unweighted fits to the Galactic data. CH/H_2 is generally consistent with Galactic values in the LMC, but is significantly lower in 2 of the 3 SMC sight lines. (right) Depletion of Ti vs. total hydrogen column density (Welty & Crowther 2010). The depletions have in each case been calculated with respect to the solar/stellar Ti abundances for the Milky Way and Magellanic Clouds. Evidently, Ti is less severely depleted in the Magellanic Clouds, for the sight lines sampled thus far.

Numerous UV and far-UV spectra of Magellanic Clouds stars, obtained with HST and FUSE (mostly) for stellar studies, exhibit absorption lines from H I, H_2, CO, HD, and many neutral and ionized atomic species present in the intervening interstellar clouds. Analyses of those interstellar absorption features can provide much useful information on the abundances, depletions, and physical conditions (temperatures, densities, radiation fields, molecular fractions) in the ISM at many locations within the Magellanic Clouds. We have undertaken several surveys using these archival UV and far-UV spectra, in order to better understand both the interstellar material (gas and dust) in lower metallicity systems and the chemical and dynamical evolution of the Magellanic system (LMC, SMC, Magellanic Bridge, Magellanic Stream):

• H I and H_2 -- for the relationship between atomic and molecular gas, comparisons between H I emission (21 cm) and absorption, and constraints on the temperature, density, and radiation field:
  • dust-to-gas ratios, as estimated via E(B-V)/N(H), are lower by factors of about 3 in the LMC and 4.5 in the SMC than in our Galaxy -- similar to the differences in metallicity;
  • the E(B-V)/N(H2) ratios are more similar in the three galaxies, however;
  • while there is qualitative agreement with recent models of the atomic-to-molecular transition, the data suggest that the transition occurs at lower column densities than predicted by the models; and
  • in at least one case, the observed N(H2) is an order of magnitude lower than the value predicted via 21cm and dust emission.
• CO -- for interstellar chemistry and the relationship between CO and H_2;
• C I -- for constraints on interstellar pressures and densities (as noted above, they appear to be higher in lower metallicity gas); and
• various dominant atomic species -- for overall metallicities and depletions (are they uniform within each galaxy? are the depletion patterns generally different from those seen in our Galaxy?).

Optical and UV spectra of the SMC star Sk 13. Absorption at v < 50 km/s is from the Milky Way (disk and halo); absorption at v > 50 km/s is from the SMC (multiple components); Mg II is a doublet (with separation 114 km/s). Component structures derived from the optical spectra (with higher spectral resolution and S/N ratio) can be used to model the profiles of similarly distributed species observed with HST STIS (FWHM ~ 6-10 km/s). Note the (first) detection of the S-process product Ge II (at v ~ 150 km/s) in the SMC ISM. Zn is typically very mildly depleted into dust; Mg and Ge are typically moderately depleted; Ti, Fe, and Ni can be severely depleted.

Spectra of CO C-X (0-0) (1087), Na I (5895), and H_2 Lyman 1-0 R5 (1089) toward the LMC star Sk-68 135. The CO and H_2 spectra are from FUSE; the Na I spectrum is from UVES; only the LMC absorption is shown. Tick marks above the CO band denote the individual R- and P-branch lines in the band; the right-most (longer) tick mark denotes the 1088 line of Cl I, which is blended with the P-branch lines. For the three LMC sight lines with reported CO absorption (so far), CO/H_2 is slightly higher than for Galactic sight lines with comparable N(H_2) -- surprising, given the lower metallicity of the LMC -- is the CO formed differently there?