Supergiant Shells Research Articles

Stellar Population near NGC 2021: Procession of Star Formation in the South Rim of Supergiant Shell LMC 4

Supergiant shells (SGSs) are the largest interstellar structures where heated and enriched gas flows into the host galaxy’s halo. The SGSs in the Large Magellanic Cloud (LMC) are so close that their stars can be resolved with ground-based telescopes to allow studies of star-formation history. Aiming to study the star formation history and energy budget of LMC 4, we have conducted a pilot study of the cluster NGC 2021 and the OB associations in its vicinity near the south rim of LMC 4. We use the Magellanic Cloud Photometric Survey data of the LMC to establish a methodology to examine the stellar population and assess the massive star formation history. We find a radial procession of massive star formation from the northwest part of the OB association LH79 through NGC 2021 to the OB association LH78 in the south. Using the stellar content of NGC 2021 and the assumption of Salpeter’s initial mass function, we estimate that ∼4 supernovae have occurred in NGC 2021, injecting at least 4 × 1051 erg of kinetic energy into the interior of LMC 4.

Properties of Star Formation of the Large Magellanic Cloud As Probed by Young Stellar Objects

We perform a systematic study of the evolutionary stages and stellar masses of young stellar objects (YSOs) in the Large Magellanic Cloud (LMC) to investigate the properties of star formation in the galaxy. There are 4825 sources in our YSO sample, which are constructed by combining the previous studies identifying YSOs in the LMC. Spectral energy distributions of the YSOs from optical to infrared wavelengths were fitted with a model consisting of stellar, polycyclic aromatic hydrocarbon, and dust emissions. We utilize the stellar-to-dust luminosity ratios thus derived to study the evolutionary stages of the sources; younger YSOs are expected to show lower stellar-to-dust luminosity ratios. We find that most of the YSOs are associated with the interstellar gas across the galaxy, which are younger with more gas, suggesting that more recent star formation is associated with larger amounts of the interstellar medium (ISM). N157 shows a hint of higher stellar-to-dust luminosity ratios between active star-forming regions in the LMC, suggesting that recent star formation in N157 is possibly in later evolutionary stages. We also find that the stellar mass function tends to be bottom-heavy in supergiant shells (SGSs), indicating that gas compression by SGSs may be ineffective in compressing the ISM enough to trigger massive star formation. There is no significant difference in the stellar mass function between YSOs likely associated with the interface between colliding SGSs and those with a single SGS, suggesting that gas compression by collisions between SGSs may also be ineffective for massive star formation.

Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA

NGC 602 is a young, low-metallicity star cluster in the “Wing” of the Small Magellanic Cloud. We reveal the recent evolutionary past of the cluster through analysis of high-resolution (∼0.4 pc) Atacama Large Millimeter/submillimeter Array observations of molecular gas in the associated H ii region N90. We identify 110 molecular clumps (R < 0.8 pc) traced by CO emission, and study the relationship between the clumps and associated young stellar objects (YSOs) and pre-main-sequence (PMS) stars. The clumps have high virial parameters (typical α vir = 4–11) and may retain signatures of a collision in the last ≲8 Myr between H i components of the adjacent supergiant shell SMC-SGS 1. We obtain a CO-bright-to-H2 gas conversion factor of X CO,B = (3.4 ± 0.2) × 1020 cm−2 (K km s−1)−1, and correct observed clump properties for CO-dark H2 gas to derive a total molecular gas mass in N90 of 16,600 ± 2400 M ⊙. We derive a recent (≲1 Myr) star formation rate of 130 ± 30 M ⊙ Myr−1 with an efficiency of 8% ± 3% assessed through comparing total YSO mass to total molecular gas mass. Very few significant radial trends exist between clump properties or PMS star ages and distance from NGC 602. We do not find evidence for a triggered star formation scenario among the youngest (≲2 Myr) stellar generations, and instead conclude that a sequential star formation process in which NGC 602 did not directly cause recent star formation in the region is likely.

The signature of large-scale turbulence driving on the structure of the interstellar medium

ABSTRACT The mechanisms that maintain turbulence in the interstellar medium (ISM) are still not identified. This work investigates how we can distinguish between two fundamental driving mechanisms: the accumulated effect of stellar feedback versus the energy injection from galactic scales. We perform a series of numerical simulations describing a stratified star-forming ISM subject to self-consistent stellar feedback. Large-scale external turbulent driving, of various intensities, is added to mimic galactic driving mechanisms. We analyse the resulting column density maps with a technique called Multi-scale non-Gaussian segmentation, which separates the coherent structures and the Gaussian background. This effectively discriminates between the various simulations and is a promising method to understand the ISM structure. In particular, the power spectrum of the coherent structures flattens above 60 pc when turbulence is driven only by stellar feedback. When large-scale driving is applied, the turn-over shifts to larger scales. A systematic comparison with the Large Magellanic Cloud (LMC) is then performed. Only 1 out of 25 regions has a coherent power spectrum that is consistent with the feedback-only simulation. A detailed study of the turn-over scale leads us to conclude that regular stellar feedback is not enough to explain the observed ISM structure on scales larger than 60 pc. Extreme feedback in the form of supergiant shells likely plays an important role but cannot explain all the regions of the LMC. If we assume ISM structure is generated by turbulence, another large-scale driving mechanism is needed to explain the entirety of the observations.

Three new high-mass X-ray binaries in the Large Magellanic Cloud

Context. The Magellanic Clouds host a large population of high-mass X-ray binary (HMXB) systems, and although the Large Magellanic Cloud (LMC) is an order of magnitude more massive than the Small Magellanic Cloud, there are significantly fewer known HMXBs in the former. Aims. We conducted a search for new HMXBs in XMM-Newton observations that were performed with the aim of investigating supernova remnant candidates in the supergiant shells LMC5 and LMC7. The three observed fields are located in regions that have not been widely explored in the X-ray band. Methods. We analysed the XMM-Newton data to look for sources with hard X-ray spectrum and their counterparts with optical colours and brightness values that are typical of HMXBs. Results. We report the discovery of three new Be/X-ray binaries, two of them showing pulsations in their X-ray flux. With a luminosity of 6.5 × 1034 erg s−1, we see that XMMU J045315.1−693242 in LMC7 was relatively X-ray faint. The long-term OGLE I-band light curve of the V = 15.5 mag counterpart suggests a 49.6 day or 24.8 day orbital period for the binary system. Then, XMMU J045736.9−692727, which is also located in LMC7, was brighter, with a luminosity of 5.6 × 1035 erg s−1 and hard spectrum with a power-law photon index of 0.63. The X-ray flux revealed clear pulsations with a period of 317.7 s. We obtained optical high resolution spectra from the V = 14.2 mag counterpart using the SALT-HRS spectrograph. Hα and Hβ were observed in emission with complex line profiles and equivalent widths of −8.0 Å and −1.3 Å, respectively. The I-band light curve obtained from OGLE shows a series of four strong outbursts followed by a sudden drop in brightness by more than 1 mag within 73–165 days and a recovery to the level from before the outbursts. RX J0524.2−6620, previously classified as X-ray binary candidate, is located at the eastern part of LMC5. We report the discovery of 360.7 s pulsations. During the XMM-Newton observation the luminosity was at ∼4 × 1035 erg s−1 and the source showed a hard spectrum with a power-law photon index of 0.78. The Hα emission line profile obtained from SALT-HRS is characterised by two broad peaks with a separation corresponding to ∼178 km s−1, along with an equivalent width of −4.2 Å. The long-term OGLE I-band light curve of the V = 14.9 mag counterpart reveals a quasi-periodic flaring activity while the colour evolution during the flares follows a hysteresis loop with redder colour during the rise. Based on the modelling the Hα line profiles measured from XMMU J045736.9−692727 and RX J0524.2−6620, we derived constraints on the size of the Be disks. Conclusions. Our discovery of two pulsars among three new Be/X-ray binaries increases the number of known HMXB pulsars in the LMC to 25.

The Shellless Supernova Remnant B0532–67.5 in the Large Magellanic Cloud

Abstract The supernova remnant (SNR) B0532−67.5 in the Large Magellanic Cloud (LMC) was first diagnosed by its nonthermal radio emission, and its SNR nature was confirmed by the observation of diffuse X-ray emission; however, no optical SNR shell is detected. The OB association LH75, or NGC 2011, is projected within the boundary of this SNR. We have analyzed the massive star population in and around SNR B0532−67.5 using optical photometric data to construct color–magnitude diagrams, using stellar evolutionary tracks to estimate stellar masses, and using isochrones to assess the stellar ages. From these analyses, we find a 20–25 Myr population in LH75 and a younger population less than 10 Myr old to the southwest of LH75. The center of SNR B0532−67.5 is located closer to the core of LH75 than to the massive stars to its southwest. We conclude that the supernova progenitor was probably a member of LH75 with an initial mass of ∼15 M ⊙. The supernova exploded in an H i cavity excavated by the energy feedback of LH75. The low density of the ambient medium prohibits the formation of a visible nebular shell. Despite the low density in the ambient medium, physical properties of the hot gas within the SNR interior do not differ from SNRs with a visible shell by more than a factor of 2–3. The large-scale H i map shows that SNR B0532−67.5 is projected in a cavity that appears to be connected with the much larger cavity of the supergiant shell LMC-4.

Tracking down the origin of superbubbles and supergiant shells in the Magellanic Clouds with Minkowski tensor analysis

Aims.We develop an automatic bubble-recognition routine based on Minkowski functionals (MF) and tensors (MT) to detect bubble-like interstellar structures in optical emission line images.Methods.Minkowski functionals and MT are powerful mathematical tools for parameterizing the shapes of bodies. Using the papaya2-library, we created maps of the desired MF or MT of structures at a given window size. We used maps of the irreducible MTψ2, which is sensitive to elongation, to find filamentary regions in Hα, [S II], and [O III] images of the Magellanic Cloud Emission Line Survey. Using the phase ofψ2, we were able to draw lines perpendicular to each filament and thus obtain line-density maps. This allowed us to find the center of a bubble-like structure and to detect structures at different window sizes.Results.The detected bubbles in all bands are spatially correlated to the distribution of massive stars, showing that we indeed detect interstellar bubbles without large spatial bias. Eighteen out of 59 supernova remnants in the Large Magellanic Cloud (LMC) and 13 out of 20 superbubbles are detected in at least one wavelength. The lack of detection is mostly due to surrounding emission that disturbs the detection, a too small size, or the lack of a (circular) counterpart in our emission line images. In line-density maps at larger scales, maxima can be found in regions with high star formation in the past, often inside supergiant shells (SGS). In SGS LMC 2, there is a maximum west of the shell where a collision of large gas clouds is thought to have occurred. In the Small Magellanic Cloud (SMC), bubble detection is impaired by the more complex projected structure of the galaxy. Line maps at large scales show large filaments in the SMC in a north-south direction, especially in the [S II] image. The origin of these filaments is unknown.

Giant molecular cloud formation at the interface of colliding supershells in the large magellanic cloud

ABSTRACT We investigate the H i envelope of the young, massive GMCs in the star-forming regions N48 and N49, which are located within the high column density H i ridge between two kpc-scale supergiant shells, LMC 4 and LMC 5. New long-baseline H i 21 cm line observations with the Australia Telescope Compact Array (ATCA) were combined with archival shorter baseline data and single dish data from the Parkes telescope, for a final synthesized beam size of 24.75 arcsec by 20.48 arcsec, which corresponds to a spatial resolution of ∼6 pc in the LMC. It is newly revealed that the H i gas is highly filamentary and that the molecular clumps are distributed along filamentary H i features. In total 39 filamentary features are identified and their typical width is ∼21 (8–49) [pc]. We propose a scenario in which the GMCs were formed via gravitational instabilities in atomic gas which was initially accumulated by the two shells and then further compressed by their collision. This suggests that GMC formation involves the filamentary nature of the atomic medium.

Ultraviolet Imaging Telescope View of Dwarf Irregular Galaxy IC 2574: Is the Star Formation Triggered Due to Expanding H i Shells?

Abstract Star-forming dwarf galaxies in the local volume are diverse and ideal test beds to understand the details of star formation in a variety of environments. Here we present a deep far-UV (FUV) imaging study of the nearby dwarf irregular galaxy IC 2574 using the Ultraviolet Imaging Telescope. We identified 419 FUV-bright regions with radii between 15 and 285 pc in the galaxy and found that 28.6% of them are located in H i shells, 12.6% inside holes, and 60.1% away from the holes. The H i column density is found to be more than 1021 cm−2 for 82.3% of the identified regions. Thirty of the 48 H i holes show triggered star formation in their shells, while 16 holes do not show any related FUV emission. Cross-matching with Hα emission, we found that 23 holes have both FUV and Hα emission in their shells, signifying a very recent trigger. Therefore, star formation in the galaxy has been partly triggered due to the expanding H i holes, whereas in the majority of the sites, it is driven by other mechanisms. Irrespective of the location, larger star-forming complexes were found to have multiple substructures. We report two resolved components for the remnant cluster of the supergiant shell and estimate their masses. The star formation rate of IC 2574 is found to be 0.57 M ⊙ yr−1, which is slightly higher compared to the average value of other nearby dwarf irregular galaxies.

Testing massive star evolution, star formation history, and feedback at low metallicity

Stars that start their lives with spectral types O and early B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 massive stars of spectral types O (23 stars) and B (297 stars) in the Wing of the Small Magellanic Cloud (SMC). The spectra, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models, and the stellar parameters were determined. We find that the stellar winds of our sample stars are generally much weaker than theoretically expected. The stellar rotation rates show broad, tentatively bimodal distributions. The upper Hertzsprung–Russell diagram (HRD) is well populated by the stars of our sample from a specific field in the SMC Wing. A few very luminous O stars are found close to the main sequence, while all other, slightly evolved stars obey a strict luminosity limit. Considering additional massive stars in evolved stages, with published parameters and located all over the SMC, essentially confirms this picture. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with an initial mass below ∼30 M⊙ seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with initially more than ∼30 M⊙ appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We measured the key parameters of stellar feedback and established the links between the rates of star formation and supernovae. Our study demonstrates that in metal-poor environments stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars. We found indications of the stochastic mode of massive star formation, where the resulting stellar population is fully capable of producing large-scale structures such as the supergiant shell SMC-SGS 1 in the Wing. The low level of feedback in metal-poor stellar populations allows star formation episodes to persist over long timescales.

The Fate of Supernova-heated Gas in Star-forming Regions of the LMC: Lessons for Galaxy Formation?

Abstract Galactic winds and fountains driven by supernova-heated gas play an integral role in redistributing gas in galaxies, depositing metals in the circumgalactic medium, and quenching star formation. The interplay between these outflows and ram-pressure stripping (RPS) due to the galaxy’s motion through an ambient medium may enhance these effects by converting fountain flows into expelled gas. In this paper, we present controlled, 3D simulations of RPS combined with thermally driven, local outflows from clustered supernovae in an isolated disk galaxy modeled on the Large Magellanic Cloud (LMC), a dwarf satellite of the Milky Way on its first infall. Observational evidence of local outflows emanating from supergiant shells in the LMC and a trailing filament of H i gas originating from these regions—with no obvious Leading Arm counterpart—may represent a perfect example of this process. Our simulations present a proof of concept that ram pressure can convert fountain flows into expelled gas. We find that fountains launched near the peak star formation time of the LMC can comprise part of the LMC filament in the Trailing Stream but with lower column densities than observed. Larger, more numerous outflows from the LMC may be possible and may contribute more mass, but higher-inertia gas will lengthen the timescale for this gas to be swept away by ram pressure. Given the high-resolution observations, increased knowledge of star formation histories, and growing evidence of multiphase ionized outflows, the LMC is an ideal test bed for future wind models.

Star formation complexes in the ‘galaxy-sized’ supergiant shell of the galaxy HolmbergI

We present the results of observations of the galaxy Holmberg I carried out at the Russian 6-m telescope in the narrow-band imaging, long-slit spectroscopy, and scanning Fabry-Perot interferometer modes. A detailed analysis of gas kinematics, ionization conditions, and metallicity of star-forming regions in the galaxy is presented. The aim of the paper is to analyse the propagation of star formation in the galaxy and to understand the role of the ongoing star formation in the evolution of the central 'galaxy-sized' supergiant HI shell (SGS), where all regions of star formation are observed. We show that star formation in the galaxy occurs in large unified complexes rather than in individual giant HII regions. Evidence of the triggered star formation is observed both on scales of individual complexes and of the whole galaxy. We identified two supernova-remnant candidates and one late-type WN star and analysed their spectrum and surrounding-gas kinematics. We provide arguments indicating that the SGS in Holmberg I is destructing by the influence of star formation occurring on its rims.

Statistical Analysis of Supernova Remnants in the Large Magellanic Cloud

Abstract We construct the most complete sample of supernova remnants (SNRs) in any galaxy—the Large Magellanic Cloud (LMC) SNR sample. We study their various properties such as spectral index (α), size, and surface brightness. We suggest an association between the spatial distribution and environment density of LMC SNRs, and their tendency to be located around supergiant shells. We find evidence that the 16 known type Ia LMC SNRs are expanding in a lower density environment compared to the Core-Collapse (CC) type. The mean diameter of our entire population (74) is 41 pc, which is comparable to nearby galaxies. We did not find any correlation between the type of SN explosion, ovality, or age. The relationship with slope a = 0.96 implies that the randomized diameters are readily mimicking such an exponent. The rate of SNe occurring in the LMC is estimated to be ∼1 per 200 yr. The mean α of the entire LMC SNR population is −0.52, which is typical of most SNRs. However, our estimates show a clear flattening of the synchrotron α as the remnants age. As predicted, the CC SNRs in our sample are significantly brighter radio emitters than type Ia remnants. We also estimate the relation for the LMC to have a slope ∼3.8, which is comparable with other nearby galaxies. We also find the residency time of electrons in the galaxy (4.0–14.3 Myr), implying that SNRs should be the dominant mechanism for the production and acceleration of CRs.

The extremely metal-poor galaxy DDO 68: the luminous blue variable, Hα shells and the most luminous stars

The paper presents new results of the ongoing study of the unusual Lynx-Cancer void galaxy DDO 68 with record-low-metallicity regions (12+log(O/H) ~7.14) of the current star formation (SF). They include: a) a new spectrum and photometry with the 6-m SAO RAS telescope (BTA) for the Luminous Blue Variable (LBV = DDO68-V1). Photometric data sets are complemented with those based on the Sloan Digital Sky Survey (SDSS) and the Hubble Space Telescope (HST) archive images; b) the analysis of the DDO~68 supergiant shell (SGS) and the prominent smaller H-alpha arcs/shells visible at the HST image coupled with kinematics maps in H-alpha obtained with the Fabry-Perot interferometer (FPI) at the BTA; c) the list of identified at the HST images of about 50 most luminous stars (-9.1 < M_V < -6.0 mag) related to star-forming regions with the known extremely low O/H. This is intended to pave the path for the actual science with the next generation of giant telescopes. We confirm the earlier hints on significant variations of the LBV optical light deriving its amplitude of dV > 3.7~mag for the first time. New data suggest that in 2008--2010 the LBV reached M_V = --10.5 and probably underwent a giant eruption. We argue that the structure of star-forming complexes along the SGS (`Northern Ring') perimeter provides evidence for the sequential induced SF episodes caused by the shell gas instabilities and gravitational collapse. The variability of some DDO~68 luminous extremely metal-poor stars can be monitored with medium-size telescopes at sites with superb seeing.

Complexes of triggered star formation in supergiant shell of Holmberg II

We report a detailed analysis of all regions of current star formation in the walls of the supergiant HI shell (SGS) in the galaxy Holmberg II based on observations with a scanning Fabry-Perot interferometer at the 6-m SAO RAS telescope. We compare the structure and kinematics of ionized gas with that of atomic hydrogen and with the stellar population of the SGS. Our deep H$\alpha$ images and archival images taken by the HST demonstrate that current star formation episodes are larger and more complicated than previously thought: they represent unified star-forming complexes with sizes of several hundred pc rather than 'chains' of separate bright nebulae in the walls of the SGS. The fact that we are dealing with unified complexes is evidenced by identified faint shell-like structures of ionized and neutral gas which connect several distinct bright HII regions. Formation of such complexes is due to the feedback of stars with very inhomogeneous ambient gas in the walls of the SGS. The arguments supporting an idea about the triggering of star formation in SGS by the HI supershells collision are presented. We also found a faint ionized supershell inside the HI SGS expanding with a velocity of no greater than 10-15 km/s. Five OB stars located inside the inner supershell are sufficient to account for its radiation, although a possibility of leakage of ionizing photons from bright HII regions is not ruled out as well.

DENSE MOLECULAR CLUMPS ASSOCIATED WITH THE LARGE MAGELLANIC CLOUD SUPERGIANT SHELLS LMC 4 AND LMC 5

We investigate the effects of Supergiant Shells (SGSs) and their interaction on dense molecular clumps by observing the Large Magellanic Cloud (LMC) star forming regions N48 and N49, which are located between two SGSs, LMC 4 and LMC 5. $^{12}$CO ($J$=3-2, 1-0) and $^{13}$CO ($J$=1-0) observations with the ASTE and Mopra telescopes have been carried out towards these regions. A clumpy distribution of dense molecular clumps is revealed with 7 pc spatial resolution. Large velocity gradient analysis shows that the molecular hydrogen densities ($n({\rm H}_2)$) of the clumps are distributed from low to high density ($10^3$-$10^5$ cm$^{-3}$) and their kinetic temperatures ($T_{\rm kin}$) are typically high (greater than $50$ K). These clumps seem to be in the early stages of star formation, as also indicated from the distribution of H$\alpha$, young stellar object candidates, and IR emission. We found that the N48 region is located in the high column density HI envelope at the interface of the two SGSs and the star formation is relatively evolved, whereas the N49 region is associated with LMC 5 alone and the star formation is quiet. The clumps in the N48 region typically show high $n({\rm H}_2)$ and $T_{\rm kin}$, which are as dense and warm as the clumps in LMC massive cluster-forming areas (30 Dor, N159). These results suggest that the large-scale structure of the SGSs, especially the interaction of two SGSs, works efficiently on the formation of dense molecular clumps and stars.

The supergiant shell with triggered star formation in the dwarf irregular galaxy IC 2574: neutral and ionized gas kinematics

We analyse the ionized gas kinematics in the star formation regions of the supergiant shell (SGS) of the IC 2574 galaxy using observations with the Fabry-Perot interferometer at the 6-m telescope of SAO RAS; the data of the THINGS survey are used to analyze the neutral gas kinematics in the area. We perform the 'derotation' of the H-alpha and HI data cubes and show its efficiency in kinematics analysis. We confirm the SGS expansion velocity 25 km/s obtained by Walter & Brinks (1999) and conclude that the SGS is located at the far side of the galactic disc plane. We determine the expansion velocities, kinematic ages, and the required mechanical energy input rates for four star formation complexes in the walls of the SGS; for the remaining ones we give the limiting values of the above parameters. A comparison with the age and energy input of the complexes' stellar population shows that sufficient energy is fed to all HII regions except one. We discuss in detail the possible nature of this region and that of another one, which was believed to be an SNR according to radio observations. We measured the expansion velocity of the latter and confirm its identification as an old SNR. Our observations allowed us to identify a faint diffuse H-alpha emission inside the SGS which was never observed before.

SUPERGIANT SHELLS AND MOLECULAR CLOUD FORMATION IN THE LARGE MAGELLANIC CLOUD

We investigate the influence of large-scale stellar feedback on the formation of molecular clouds in the Large Magellanic Cloud (LMC). Examining the relationship between HI and 12CO(J=1-0) in supergiant shells (SGSs), we find that the molecular fraction in the total volume occupied by SGSs is not enhanced with respect to the rest of the LMC disk. However, the majority of objects (~70% by mass) are more molecular than their local surroundings, implying that the presence of a supergiant shell does on average have a positive effect on the molecular gas fraction. Averaged over the full SGS sample, our results suggest that ~12-25% of the molecular mass in supergiant shell systems was formed as a direct result of the stellar feedback that created the shells. This corresponds to ~4-11% of the total molecular mass of the galaxy. These figures are an approximate lower limit to the total contribution of stellar feedback to molecular cloud formation in the LMC, and constitute one of the first quantitative measurements of feedback-triggered molecular cloud formation in a galactic system.

The Supershell-Molecular Cloud Connection in the Milky Way and Beyond

AbstractThe role of large-scale stellar feedback in the formation of molecular clouds has been investigated observationally by examining the relationship between Hi and 12CO(J = 1−0) in supershells. Detailed parsec-resolution case studies of two Milky Way supershells demonstrate an enhanced level of molecularisation over both objects, and hence provide the first quantitative observational evidence of increased molecular cloud production in volumes of space affected by supershell activity. Recent results on supergiant shells in the LMC suggest that while they do indeed help to organise the ISM into over-dense structures, their global contribution to molecular cloud formation is of the order of only ∼ 10%.

X-RAY EMISSION FROM THE SUPERGIANT SHELL IN IC 2574

The M81 group member dwarf galaxy IC 2574 hosts a supergiant shell of current and recent star-formation activity surrounding a 1000 x 500 pc hole in the ambient Hi gas distribution. Chandra X-ray Observatory imaging observations reveal a luminous, L_x ~ 6.5 x 10^{38} erg/s in the 0.3 - 8.0 keV band, point-like source within the hole but offset from its center and fainter diffuse emission extending throughout and beyond the hole. The star formation history at the location of the point source indicates a burst of star formation beginning ~25 Myr ago and currently weakening and there is a young nearby star cluster, at least 5 Myr old, bracketing the likely age of the X-ray source at between 5 and ~25 Myr. The source is thus likely a bright high-mass X-ray binary --- either a neutron star or black hole accreting from an early B star undergoing thermal-timescale mass transfer through Roche lobe overflow. The properties of the residual diffuse X-ray emission are consistent with those expected from hot gas associated with the recent star-formation activity in the region.