Topology of Neutral Hydrogen within the Small Magellanic Cloud

International Astronomical Union Colloquium, 1995

We present the results of a kinematic survey of the Small Magellanic Cloud, performed with a scanning FP interferometer mainly at Hα. These data allow us to study the overall dynamics of the ionized hydrogen of this galaxy as well as the kinematics of classes of objects. In this presentation we exemplify how the availability of data in several wavelengths can be very useful in establishing the nature of large nebular complexes of the Small Magellanic Cloud. Our kinematic study allows us to discriminate faint shocked nebulae (such as supernova remnants) inside bright HII regions. This is confirmed by recently available radio and X-ray data.

Astronomy and Astrophysics, 2004

The objective of this series of two papers is to investigate small-scale molecular structures in the Magellanic Clouds (hereafter MCs). We report on the FUSE detections of the HD and CO molecules on the lines of sight towards three Large Magellanic stars: Sk −67D05, Sk −68D135, and Sk −69D246. HD is also detected for the first time on the lines of sight towards two Small Magellanic Cloud stars: AV 95 and Sk 159. While the HD and CO abundances are expected to be lower in the Large Magellanic Cloud where molecular fractions are a third of the Galactic value and where the photodissociation flux is up to thousands times larger, we report an average HD/H2 ratio of 1.4±0.5 ppm and CO/H2 ratio ranging from 0.8 to 2.7 ppm similar to the Galactic ones. We tentatively identify a deuterium reservoir (hereafter D-reservoir) towards the Small Magellanic Cloud, along the light path to AV 95. We derive a D/H ratio ranging from 1. 10 −6 to 1.1 10 −5. Combining FUSE and HST/STIS data we also analyzed the H2,

This paper describes the first results from a 20 deg 2 mosaic of the Small Magellanic Cloud (SMC) in the A21-cm line of neutral hydrogen. The mosaic consists of 320 separate pointings with the 375-m array of the Australia Telescope Compact Array. The angular resolution is 1' • 5 (26 pc, for a distance of 60 kpc) and the velocity resolution is l -6 k m s _ 1 . The images reveal a structure of remarkable complexity, with much of the spatial power contained in high-brightness temperature compact knots and filaments. Numerous wind-blown 'bubbles' and 'supershells' are evident in the data, both inside and outside the stellar confines of the SMC. Some high-density Hi regions are seen to correlate with Ha regions, indicating sites of current star formation. However, many high-column-density HI regions are devoid of optical emission and may represent regions of future star formation. These regions may be under-abundant in diffuse molecular gas due to the high radiation field and low metallicity of the SMC.

arXiv (Cornell University), 2015

Morphology of the complex HI gas distribution can be quantified by statistics like the Minkowski functionals, and can provide a way to statistically study the large scale structure in the HI maps both at low redshifts, and during the epoch of reionization (EoR). At low redshifts, the 21cm emission traces the underlying matter distribution. Topology of the HI gas distribution, as measured by the genus, could be used as a "standard ruler". This enables the determination of distance-redshift relation and also the discrimination of various models of dark energy and of modified gravity. The topological analysis is also sensitive to certain primordial non-Gaussian features. Compared with two-point statistics, the topological statistics are more robust against the nonlinear gravitational evolution, bias, and redshift-space distortion. The HI intensity map observation naturally avoids the sparse sampling distortion, which is an important systematic in optical galaxy survey. The large cosmic volume accessible to SKA would provide unprecedented accuracy using such a measurement. During the EoR, topology can be a powerful and intuitive tool to distinguish among the different evolutionary stages of reionization, where the ionized regions make up a significant fraction of the volume. Furthermore, it can also discriminate among various reionization models. The genus curves evolve during cosmological reionization, and for different reionization scenarios, the topology of the HI gas distribution can be significantly different even if the global ionization fractions are the same. It can provide clear and intuitive diagnostics for how the reionization takes place, and indirectly probes the properties of radiation-sources. In this brief chapter we will describe the scientific background of the topology study, and forecast the potential of the SKA for measuring cosmological parameters and constraining structure formation mechanism through the study of topology of HI gas distribution.

Proceedings of Advancing Astrophysics with the Square Kilometre Array — PoS(AASKA14), 2015

Morphology of the complex HI gas distribution can be quantified by statistics like the Minkowski functionals, and can provide a way to statistically study the large scale structure in the HI maps both at low redshifts, and during the epoch of reionization (EoR). At low redshifts, the 21cm emission traces the underlying matter distribution. Topology of the HI gas distribution, as measured by the genus, could be used as a "standard ruler". This enables the determination of distance-redshift relation and also the discrimination of various models of dark energy and of modified gravity. The topological analysis is also sensitive to certain primordial non-Gaussian features. Compared with two-point statistics, the topological statistics are more robust against the nonlinear gravitational evolution, bias, and redshift-space distortion. The HI intensity map observation naturally avoids the sparse sampling distortion, which is an important systematic in optical galaxy survey. The large cosmic volume accessible to SKA would provide unprecedented accuracy using such a measurement. During the EoR, topology can be a powerful and intuitive tool to distinguish among the different evolutionary stages of reionization, where the ionized regions make up a significant fraction of the volume. Furthermore, it can also discriminate among various reionization models. The genus curves evolve during cosmological reionization, and for different reionization scenarios, the topology of the HI gas distribution can be significantly different even if the global ionization fractions are the same. It can provide clear and intuitive diagnostics for how the reionization takes place, and indirectly probes the properties of radiation-sources. In this brief chapter we will describe the scientific background of the topology study, and forecast the potential of the SKA for measuring cosmological parameters and constraining structure formation mechanism through the study of topology of HI gas distribution.

The Astrophysical Journal Supplement Series, 2009

We studied star formation activities in the molecular clouds in the Large Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds obtained by NANTEN to compare the cloud distribution with signatures of massive star formation including stellar clusters, and optical and radio HII regions. We find that the molecular clouds are classified into three types according to the activities of massive star formation; Type I shows no signature of massive star formation, Type II is associated with relatively small HII region(s) and Type III with both HII region(s) and young stellar cluster(s). The radio continuum sources were used to confirm that Type I GMCs do not host optically hidden HII regions. These signatures of massive star formation show a good spatial correlation with the molecular clouds in a sense they are located within ∼100 pc of the molecular clouds. Among possible ideas to explain the GMC Types, we favor that the Types indicate an evolutionary sequence; i.e., the youngest phase is Type I, followed by Type II and the last phase is Type III, where the most active star formation takes place leading to cloud dispersal. The number of the three types of GMCs should be proportional to the time scale of each evolutionary stage if a steady state of massive star and cluster formation is a good approximation. By adopting the time scale of the youngest stellar clusters, 10 Myrs, we roughly estimate the timescales of Types I, II and III to be 6 Myrs, 13 Myrs and 7 Myrs, respectively, corresponding to a lifetime of 20-30 Myrs for the GMCs with a mass above the completeness limit, 5 × 10 4 M ⊙ .

The Astrophysical Journal Supplement Series, 2003

Recent H i surveys of the Large Magellanic Cloud (LMC) with the Australia Telescope Compact Array (Kim et al. 1998) and the Parkes multibeam receiver (Staveley-Smith et al. 2003) have focussed, respectively, on the small-scale (< 20 ′) structure of the interstellar medium (ISM) and the large-scale (> 1 •) structure of the galaxy. Using a Fourier-plane technique, we have merged both data sets providing an accurate set of images of the LMC sensitive to structure on scales of 15 pc (for an LMC distance of 50 kpc) upwards. The spatial dynamic range (2.8 orders of magnitude), velocity resolution (1.649 km s −1), brightness temperature sensitivity (2.4 K) and column density sensitivity (8.9 × 10 18 cm −2 per 1.649 km s −1 channel) allow for the studies of phenomena ranging from the galaxy-wide interaction of the LMC with its close neighbours to the small-scale injection of energy from supernovae and stellar associations into the ISM of the LMC. This paper presents the merged data and the size spectrum of H i clouds which is similar to the typical size spectrum of the holes and shells in the H i distribution.

2022

We present the first unbiased survey of neutral hydrogen (HI) absorption in the Small Magellanic Cloud (SMC). The survey utilises pilot HI observations with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope as part of the Galactic ASKAP HI (GASKAP-HI) project whose dataset has been processed with the GASKAP-HI absorption pipeline, also described here. This dataset provides absorption spectra towards 229 continuum sources, a 275% increase in the number of continuum sources previously published in the SMC region, as well as an improvement in the quality of absorption spectra over previous surveys of the SMC. Our unbiased view, combined with the closely matched beam size between emission and absorption, reveals a lower cold gas faction (11%) than the 2019 ATCA survey of the SMC and is more representative of the SMC as a whole. We also find that the optical depth varies greatly between the SMC's bar and wing regions. In the bar we find that the optical depth is generally low (correction factor to the optically thin column density assumption of R HI ∼ 1.04) but increases linearly with column density. In the wing however, there is a wide scatter in optical depth despite a tighter range of column densities.

The Astrophysical …, 2010

Proceedings of the International Astronomical Union, 2012

We have mapped an extensive sample of molecular clouds in the Large Magellanic Cloud (LMC) at 11 pc resolution in the CO(1-0) line as part of the Magellanic Mopra Assessment (MAGMA). We identify clouds as regions of connected CO emission and determine their sizes, line widths, and fluxes. We find that GMCs are not preferentially located in regions of high H i line width or velocity gradient, and that there is no clear H i column density threshold for CO detection. The luminosity function of CO clouds is steeper than dN/dL ∝ L −2 , suggesting a substantial fraction of mass in low-mass clouds. The correlation between size and linewidth, while apparent for the largest emission structures, breaks down when those structures are decomposed into smaller structures. The virial parameter (the ratio of a cloud's kinetic to gravitational energy) shows a wide range of values and exhibits no clear trends with the likelihood of hosting young stellar object (YSO) candidates, suggesting that this parameter is a poor reflection of the evolutionary state of a cloud. More massive GMCs are more likely to harbor a YSO candidate, and more luminous YSOs are more likely to be coincident with detectable CO emission, confirming GMCs as the principal sites of massive star formation.