Magellanic Bridge Research Articles

Study of HI and Star Formation Sites in the Magellanic Bridge

Tidal features represent an important process for the recycling of matter across extremely large spatial scales. The Magellanic Bridge is the closest gaseous tidal feature to our own Galaxy, and offers an ideal opportunity to study the active processes in such phenomena at high spatial resolutions otherwise unavailable. The neutral hydrogen of the western Magellanic Bridge, has been observed with both the Parkes 64m telescope and ATCA interferometer and the resulting dataset has revealed an extremely complex and chaotic morphology. We discuss these observations here with reference to a variety of analyses: the spatial correlation of apparently expanding HI shells with OB associations and the outcome of sensitive Hα and CO surveys.

High-resolution H I observations of the Western Magellanic Bridge

The 21-cm line emission from a 7 × 6d eg 2 region east of and adjoining the Small Magellanic Cloud (SMC) has been observed with the Australia Telescope Compact Array and the Parkes telescopes. This region represents the westernmost part of the Magellanic Bridge, a gas-rich tail extending ∼14 ◦ to the Large Magellanic Cloud. A rich and complex neutral hydrogen (H I) structure containing shells, bubbles and filaments is revealed. On the larger scale, the H I of the Bridge is organized into two velocity components. This bimodality, which appears to originate in the SMC, converges to a single velocity component within the observed region. A census of shell-like structures suggests a shell population with characteristics similar to that of the SMC. The mean kinematic age of the shells is ∼6 Myr, in agreement with the SMC shell population, but not with ages of OB clusters populating the Magellanic Bridge, which are approximately a factor of 3 older. In general, the projected spatial correlation of Bridge H I shells with OB associations is poor and as such, there does not appear to be a convincing relationship between the positions of OB associations and that of expanding spherical H I structures. This survey has found only one H I shell that has an identifiable association with a known Hα shell. The origin of the expanding structures is therefore generally still uncertain, although current theories regarding their formation include gravitational and pressure instabilities, high-velocity cloud collisions and ram pressure effects.

A new look at the large-scale H I structure of the Large Magellanic Cloud

We present a Parkes multibeam H I survey of the Large Magellanic Cloud (LMC). This survey, which is sensitive to spatial structure in the range 200 pc to 10 kpc, complements the Australia Telescope Compact survey, which is sensitive to structure in the range 15‐500 pc. With an rms column density sensitivity of 8 × 10 16 cm −2 for narrow lines and 4 × 10 17 cm −2 for typical linewidths of 40 km s −1 , emission is found to be extensive well beyond the main body of the LMC. Arm-like features extend from the LMC to join the Magellanic Bridge and the Leading Arm, a forward counterpart to the Magellanic Stream. These features, whilst not as dramatic as those in the Small Magellanic Cloud, appear to have a common origin in the Galactic tidal field, in agreement with recent 2MASS and DENIS results for the stellar population. The diffuse gas that surrounds the LMC, particularly at PAs of 90 ◦ ‐330 ◦ , appears to be loosely associated with tidal features, but loosening by the ram pressure of tenuous Galactic halo gas against the outer parts of the LMC cannot be discounted. High-velocity clouds, which lie between the Galaxy and the LMC in velocity and that appear in the ultraviolet spectra of some LMC stars, are found to be associated with the LMC if their heliocentric velocity exceeds approximately +100 km s −1 . They are possibly the product of energetic outflows from the LMC disc. The H I mass of the LMC is found to be (4.8 ± 0.2) × 10 8 M� (for an assumed distance of 50 kpc),

Detection of carbon monoxide within the Magellanic Bridge

The Mopra 22-m and SEST 15-m telescopes have been used to detect and partially map a region of 12 CO(1-0) line emission within the Magellanic Bridge, a region lying between the Large and Small Magellanic Clouds (LMC and SMC, respectively). The emission appears to be embedded in a cloud of neutral hydrogen, and is in the vicinity of an IRAS source. The CO emission region is found to have a 60 µm/100 µm flux density ratio typical for 12 CO(1-0) detections within the SMC, although it has a significantly lower 12 CO brightness and velocity width. These suggest that the observed region is of a low metallicity, supporting earlier findings that the Magellanic Bridge is not as evolved as the SMC and Magellanic Stream, which are themselves of a lower metallicity than the Galaxy. Our observations, along with empirical models based on SMC observations, indicate that the radius of the detected CO region has an upper limit of ∼16 pc. This detection is, to our knowledge, the first detection of CO emission from the Magellanic Bridge and is the only direct evidence of star formation through molecular cloud collapse in this region.

Far‐Ultraviolet Spectroscopic ExplorerObservations of the Magellanic Bridge Gas toward Two Early‐Type Stars: Molecules, Physical Conditions, and Relative Abundances

We discuss Far Ultraviolet Spectroscopic Explorer (FUSE) observations of two early-type stars, DI 1388 (l = 2913, b = -411) and DGIK 975 (l = 2873, b = -360), in the low-density and low-metallicity (Z ~ 0.08 Z☉) gas of the Magellanic Bridge (MB). The data have a spectral resolution of about 15,000, and signal-to-noise ratios range between 10 and 30 per resolution element in the spectra of DI 1388 and between 7 and 11 in the spectra of DGIK 975. DI 1388 is situated near the SMC, while DGIK 975 is closer to the LMC, allowing us to probe the MB gas in a widely different locations. Toward DI 1388, the FUSE observations show molecular hydrogen, O VI, and numerous other atomic or ionic transitions in absorption, implying the presence of multiple gas phases in a complex arrangement. The relative abundance (with respect to S II) pattern in the MB along the DI 1388 sight line is attributed to varying degrees of depletion onto dust similar to that of halo clouds. The N/O ratio is near solar, much higher than N/O in damped Lyα systems, implying subsequent stellar processing to explain the origin of nitrogen in the MB. The diffuse molecular cloud in this direction has a low column density and low molecular fraction [log N(H2) ≈ 15.43 dex; fH2 ~ 10-5 to 10-4], yet two excitation temperatures (T01 = 94 K and T23 = 341 K) are needed to fit the distribution of the different rotational levels. Although this is not typically seen in the Galaxy, we show that this is not uncommon in the Magellanic Clouds. H2 is observed in both the Magellanic Stream and the MB, yet massive stars form only in the MB, implying significantly different physical processes between them. In the MB some of the H2 could have been pulled out from the SMC via tidal interaction, but some also could have formed in situ in dense clouds where star formation might have taken place. Toward DGIK 975, the presence of neutral and weakly and highly ionized species suggests that this sight line has also several complex gas phases. The highly ionized species of O VI, C IV, and Si IV toward both stars have very broad features, indicating that multiple components of hot gas at different velocities are present. C IV/O VI varies within the MB, but C IV/Si IV is relatively constant for both sight lines. Several sources (a combination of turbulent mixing layer, conductive heating, and cooling flows) may be contributing to the production of the highly ionized gas in the MB. Finally, this study has confirmed previous results that the high-velocity cloud HVC 291.5-41.2+80 is mainly ionized, composed of weakly and highly ions. The high ion ratios are consistent with a radiatively cooling gas in a fountain flow model.

The expansion of the giant filamentary shell, DEM 171, in the Magellanic Bridge

The expansion of the giant filamentary shell, DEM 171, in the Magellanic Bridge

HST/STIS observations of the high-velocity interstellar cloud HVC 291.2-41.2+80: a warm, mainly ionized high-velocity cloud

We present intermediate-resolution HST/STIS spectra of a high-velocity interstellar cloud (vLSR = +80 km s−1) towards DI 1388, a young star in the Magellanic Bridge located between the Small and Large Magellanic Clouds. The STIS data have a signal-to-noise ratio (S/N) of 20–45 and a spectral resolution of about 6.5 km s−1 (FWHM). The high-velocity cloud absorption is observed in the lines of C ii, O i, Si ii, Si iii, Si iv and S iii. Limits can be placed on the amount of S ii and Fe ii absorption that is present. An analysis of the relative abundances derived from the observed species, particularly C ii and O i, suggests that this high-velocity gas is warm (Tk∼103–104 K) and predominantly ionized. This hypothesis is supported by the presence of absorption produced by highly ionized species, such as Si iv. This sightline also intercepts two other high-velocity clouds that produce weak absorption features at vLSR = +113 and +130 km s−1 in the STIS spectra.

Observations of the Interstellar Medium in the Magellanic Bridge

We present ultraviolet and optical spectra of DI1388, a young star in the Magellanic Bridge, a region of gas between the Small and Large Magellanic Clouds. The data have signal-to-noise ratios of 20–45 and a spectral resolution of 6.5 kms −1 . Interstellar absorption by the Magellanic Bridge at vLSR ≈ 200 kms −1 is visible in the lines of C I, C II, C II*, C IV, N I, O I, Al II, Si II, Si III, Si IV, S II, Ca II, Fe II, and Ni II. The relative gas-phase abundances of C II, N I, O I, Al II, Si II, Fe II, and Ni II with respect to S II are similar to those found in Galactic halo clouds, despite a significantly lower metallicity in the Magellanic Bridge. The higher ionization species in the cloud have a column density ratio N(C +3 )/N(Si +3 ) ∼ 1.9, similar to that inferred for collisionally ionized Galactic cloud interfaces at temperatures ∼ 10 5 K. We identify sub-structure in the stronger interstellar lines, with a broad component (FWHM ∼ 20 kms −1 ) at ∼ 179 kms −1 and a sharp component (FWHM ∼ 11 kms −1 ) at 198 kms −1 . The abundance analysis for these clouds indicates that the feature at 198 kms −1 consists of a low electron density, mainly neutral gas that may be associated with an interface responsible for the highly ionized gas. The 179 kms −1 cloud consists of warmer, lower density gas that is partially ionized. Subject headings: Galaxies: abundances — ISM: abundances — Magellanic Bridge — ISM: structure

The Magellanic System's Interactive Formations

AbstractThe interaction between the Galaxy and the Magellanic Clouds has resulted in several high-velocity complexes which are connected to the Clouds. The complexes are known as the Magellanic Bridge, an HI connection between the Large and Small Magellanic Clouds, the Magellanic Stream, a 10° × 100° HI filament which trails the Clouds, and the Leading Arm, a diffuse HI filament which leads the Clouds. The mechanism responsible for these features formation remains under some debate, with the lack of detailed HI observations being one of the limiting factors in resolving the issue. Here I present several large mosaics of HI Parkes All-Sky Survey (HIPASS) data which show the full extent of the three Magellanic complexes at almost twice the resolution of previous observations. These interactive features are connected, but unique in their spatial and velocity distribution. The differences may shed light on their origin and present environment. Dense clumps of HI along the sightline to the Sculptor Group, which may or may not be associated with the Magellanic complexes, are also discussed.

Detection of Cold Atomic Clouds in the Magellanic Bridge

We report a direct detection of cold atomic hydrogen in the Magellanic Bridge using 21 cm absorption spectroscopy toward the radio source B0312-770. With a maximum absorption optical depth of τ = 0.10 and a maximum 21 cm emission brightness temperature of 1.4 K (NH I = 1.2 × 1020 cm-2), this line of sight yields a spin temperature, Ts, between 20 and 40 K. H I 21 cm absorption and emission spectroscopy toward seven other low H I column density sight lines on the periphery of the LMC and SMC reveal absorption toward one additional background radio source behind the SMC with τ = 0.03. The data have typical sensitivities of στ = 0.005 to 0.070 in absorption and σTB = 0.03 K in emission. These data demonstrate the presence of a cold atomic phase, which is probably accompanied by molecular condensations, in the tenuous interstellar medium of the Bridge region. Young OB stars observed in the Magellanic Bridge could form in situ from these cold condensations rather than migrate from regions of active star formation in the main body of the SMC. The existence of cold condensations and star formation in the Magellanic Bridge might be understood as a small-scale version of the mechanism that produces star formation in the tidal tails of interacting galaxies.

The Interstellar Medium in the Magellanic Bridge

HST ultra-violet and ground-based optical spectroscopy are presented for DI 1388, an early-type star in the Magellanic Bridge. These data have been supplemented with H I radio mapping of its environment. The spectroscopy shows interstellar components at LSR velocities of 0, 90 and 200 km s−1 – corresponding to the local ISM, a high velocity cloud and the Magellanic Bridge ISM, respectively. The last is the first absorption line detection of this material and shows the presence of both low (e.g., N I) and high (Si IV) ionization material. Further optical and HST UV observations of DI 1338 and another Bridge star, DGIK 975, are scheduled to investigate the chemical homogeneity and star formation history of the Bridge.

Massive Stars Spectroscopy in the Magellanic Clouds

Recent results from spectroscopic studies of Magellanic Cloud stars are discussed. Emphasis is placed on how to interpret CNO abundances to examine stellar evolution scenarios. Observed CNO abundances are best explained by the new rotating stellar evolution models (see Langer & Heger, these proceedings). In addition, the Rolleston et al. (these proceedings) abundances for B stars in the Magellanic Bridge are discussed.

Star‐forming Processes Far from the Galactic Disk: Inoperative or Indolent Where Operative

Highly supersonic collisions between gaseous clouds may effectively trigger star formation in the disk of our Galaxy, but not in the diffuse environment of the Galactic halo. This is because the observed high-velocity clouds (HVCs) are not dominated by collisions: the characteristic time between cloudlet collisions inside an HVC at an assumed distance of 10 kpc is at least 1 Gyr for collective encounters and at least 10 Gyr if a particular cloudlet is considered. In agreement with this result, we also estimate that the observed cloudlets contain smaller masses than the nonmagnetic Jeans mass that signals favorable conditions for gravitational collapse and further fragmentation in the isothermal regime. The diffuse environment observed around the Magellanic Clouds (MCs) is more difficult to understand than HVCs. Six sparse blue associations and two young B-type stars have been observed in the H I bridge between the MCs, while no stars exist in the H I cloud complexes that make up the Magellanic Stream. We discuss the conditions under which spatially sporadic star formation took place in the Magellanic Bridge during the past 16-25 Myr and the reasons for the complete absence of star formation in the Stream during its entire lifetime. We also estimate the angular resolutions that need to be achieved by follow-up radio observations of these regions that could detect cold cloudlets embedded in the gas.

Under the shadow of the Magellanic Bridge: A measurement of the extragalactic background at ∼ 0.7 keV

We measured the extragalactic 0.7 keV X-ray background by observing the X-ray shadow of a neutral gas cloud in the Magellanic Bridge region. Two ROSAT PSPC observations of total 104 ks were complemented by a detailed H I mapping of the cloud with both the Parkes 64 m telescope and the Australia Telescope Compact Array. From the detected anti-correlation between the observed background intensity and the H I column density of the cloud, we derived the unabsorbed extragalactic background intensity as ∼ 28 keV s−1 cm−2 keV−1 sr−1 at ∼ 0.7 keV. The 95% confidence lower limit 18 keV s−1 cm−2 keV−1 sr−1 is greater than the expected point-like source contribution ≲ 14 keV s−1 cm−2 keV−1 sr−1, constrained by the mean source spectrum together with the total background intensity in the 1-2 keV band. A significant fraction of the 0.7 keV background likely arises in a diffuse hot intergalactic medium of a few million degrees, as has been predicted in hydrodynamic simulations of cosmological structure formation.Richard McCray

Young stars between the Magellanic Clouds. III - Overall properties of associations: A shallow mass function

We present CCD B, V photometry of three more groups of blue stars located in the bridge of the Magellanic Clouds (MC). Two associations near the SMC Wing tip are members of large complexes of blue stars. The other one, located at equal angular distance between the SMC and the LMC, is isolated. ZAMS fittings yield distances to the Sun that are intermediate between the ones of the LMC and the SMC. We find that the stellar bridge has a large line-of-sight depth near the eastern tip of the SMC Wing. A difference in distance modulus of 12 +/- 8 kpc is observed for two associations separated by only 17 arcmin. Isochrone fittings revealed that the inter-Cloud associations are as young as 16 million years. The global mass function of the associations is much flatter than the one obtained by Mateo [ApJ, 331, 261(1988)] for MC's clusters. These associations are characterized by an IMF slope x = 1.3+/-0.1 [Ξ(M) is proportional to (M)^-(1+x)^] for masses in the 1.5-12 M_sun_ range. We also confirm that there is no halo population in the Magellanic Bridge, eastwards of the SMC Wing tip; the halos of the Magellanic Clouds do not touch each other.

Optical surveys of the Magellanic Bridge, Stream and outer halos

The HI envelope of the Magellanic System extends well beyond the obvious visible regions of each of the Clouds. Modern optical surveys based on UK Schmidt telescope plates and fast measuring machines are now starting to reveal the complex optical counterparts of some of these HI features.