We present a new algorithm for detecting filamentary structure FilFinder. The algorithm uses the techniques of mathematical morphology for filament identification, presenting a complementary approach ...to current algorithms which use matched filtering or critical manifolds. Unlike other methods, FilFinder identifies filaments over a wide dynamic range in brightness. We apply the new algorithm to far-infrared imaging data of dust emission released by the Herschel Gould Belt Survey team. Our preliminary analysis characterizes both filaments and fainter striations. We find a typical filament width of 0.09 pc across the sample, but the brightness varies from cloud to cloud. Several regions show a bimodal filament brightness distribution, with the bright mode (filaments) being an order of magnitude brighter than the faint mode (striations). Using the Rolling Hough Transform, we characterize the orientations of the striations in the data, finding preferred directions that agree with magnetic field direction where data are available. There is a suggestive but noisy correlation between typical filament brightness and literature values of the star formation rates for clouds in the Gould Belt.
Abstract
The work of Adamo et al. showed that the mass distributions of young massive stellar clusters were truncated above a maximum-mass scale in the nearby galaxy M83 and that this truncation mass ...varies with the galactocentric radius. Here, we present a cloud-based analysis of Atacama Large Millimeter/submillimeter Array CO(1 → 0) observations of M83 to search for such a truncation mass in the molecular cloud population. We identify a population of 873 molecular clouds in M83 that is largely similar to those found in the Milky Way and Local Group galaxies, though clouds in the centre of the galaxy show high surface densities and enhanced turbulence, as is common for clouds in high-density nuclear environments. Like the young massive clusters, we find a maximum-mass scale for the molecular clouds which decreases radially in the galaxy. We find that the most young massive cluster tracks the most massive molecular cloud with the cluster mass being 10−2 times that of the most massive molecular cloud. Outside the nuclear region of M83 (Rg > 0.5 kpc), there is no evidence for changing internal conditions in the population of molecular clouds, with the average internal pressures, densities and free-fall times remaining constant for the cloud population over the galaxy. This result is consistent with the bound cluster formation efficiency depending only on the large-scale properties of the interstellar medium rather than the internal conditions of individual clouds.
Kinematics of the atomic ISM in M33 on 80 pc scales Koch, Eric W; Rosolowsky, Erik W; Lockman, Felix J ...
Monthly notices of the Royal Astronomical Society,
09/2018, Letnik:
479, Številka:
2
Journal Article
ABSTRACT We sort 4683 molecular clouds between 10° < < 65° from the Bolocam Galactic Plane Survey based on observational diagnostics of star formation activity: compact 70 m sources, mid-IR ...color-selected YSOs, H2O and CH3OH masers, and UCH ii regions. We also present a combined NH3-derived gas kinetic temperature and H2O maser catalog for 1788 clumps from our own GBT 100 m observations and from the literature. We identify a subsample of 2223 (47.5%) starless clump candidates (SCCs), the largest and most robust sample identified from a blind survey to date. Distributions of flux density, flux concentration, solid angle, kinetic temperature, column density, radius, and mass show strong (>1 dex) progressions when sorted by star formation indicator. The median SCC is marginally subvirial ( ∼ 0.7) with >75% of clumps with known distance being gravitationally bound ( < 2). These samples show a statistically significant increase in the median clump mass of ΔM ∼ 170-370 M from the starless candidates to clumps associated with protostars. This trend could be due to (i) mass growth of the clumps at M ˙ ∼ 200 - 440 M Myr−1 for an average freefall 0.8 Myr timescale, (ii) a systematic factor of two increase in dust opacity from starless to protostellar phases, and/or (iii) a variation in the ratio of starless to protostellar clump lifetime that scales as ∼M−0.4. By comparing to the observed number of CH3OH maser containing clumps, we estimate the phase lifetime of massive (M > 103 M ) starless clumps to be 0.37 0.08 Myr (M/103 M )−1; the majority (M < 450 M ) have phase lifetimes longer than their average freefall time.
ABSTRACT
We investigate how the spectral properties of atomic (H i) and molecular (H2) gas, traced by CO(2−1) , are related in M33 on 80 pc scales. We find the H i and CO(2−1) velocity at peak ...intensity to be highly correlated, consistent with previous studies. By stacking spectra aligned to the velocity of H i peak intensity, we find that the CO line width (σHWHM = 4.6 ± 0.9 ${\rm km\, s^{-1}}$ ; σHWHM is the effective Gaussian width) is consistently smaller than the H i line width (σHWHM = 6.6 ± 0.1 ${\rm km\, s^{-1}}$), with a ratio of ∼0.7, in agreement with Druard et al. The ratio of the line widths remains less than unity when the data are smoothed to a coarser spatial resolution. In other nearby galaxies, this line width ratio is close to unity which has been used as evidence for a thick, diffuse molecular disc that is distinct from the thin molecular disc dominated by molecular clouds. The smaller line width ratio found here suggests that M33 has a marginal thick molecular disc. From modelling individual lines of sight, we recover a strong correlation between H i and CO line widths when only the H i located closest to the CO component is considered. The median line width ratio of the line-of-sight line widths is 0.56 ± 0.01. There is substantial scatter in the H i –CO(2−1) line width relation, larger than the uncertainties, that results from regional variations on <500 pc scales, and there is no significant trend in the line widths, or their ratios, with galactocentric radius. These regional line width variations may be a useful probe of changes in the local cloud environment or the evolutionary state of molecular clouds.
TurbuStat: Turbulence Statistics in Python Koch, Eric W.; Rosolowsky, Erik W.; Boyden, Ryan D. ...
The Astronomical journal,
07/2019, Letnik:
158, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present TurbuStat (v1.0): a python package for computing turbulence statistics in spectral-line data cubes. TurbuStat includes implementations of 14 methods for recovering turbulent properties ...from observational data. Additional features of the software include: distance metrics for comparing two data sets; a segmented linear model for fitting lines with a break point; a two-dimensional elliptical power-law model; multicore fast-Fourier-transform support; a suite for producing simulated observations of fractional Brownian Motion fields, including two-dimensional images and optically thin H i data cubes; and functions for creating realistic world coordinate system information for synthetic observations. This paper summarizes the TurbuStat package and provides representative examples using several different methods. TurbuStat is an open-source package and we welcome community feedback and contributions.
We measure the velocity dispersion, , and surface density, , of the molecular gas in nearby galaxies from CO spectral line cubes with spatial resolution 45-120 pc, matched to the size of individual ...giant molecular clouds. Combining 11 galaxies from the PHANGS-ALMA survey with four targets from the literature, we characterize ∼30,000 independent sightlines where CO is detected at good significance. and show a strong positive correlation, with the best-fit power-law slope close to the expected value for resolved, self-gravitating clouds. This indicates only a weak variation in the virial parameter vir ∝ 2/ , which is ∼1.5-3.0 for most galaxies. We do, however, observe enormous variation in the internal turbulent pressure Pturb ∝ 2, which spans ∼5 dex across our sample. We find , , and Pturb to be systematically larger in more massive galaxies. The same quantities appear enhanced in the central kiloparsec of strongly barred galaxies relative to their disks. Based on sensitive maps of M31 and M33, the slope of the - relation flattens at 10 M pc−2, leading to high for a given and high apparent vir. This echoes results found in the Milky Way and likely originates from a combination of lower beam-filling factors and a stronger influence of local environment on the dynamical state of molecular gas in the low-density regime.
We present a new survey of HCN(1-0) emission, a tracer of dense molecular gas, focused on the little-explored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) ...emission, we investigate the role of dense gas in star formation, finding systematic variations in both the apparent dense gas fraction (traced by the HCN-to-CO ratio) and the apparent star formation efficiency of dense gas. The latter may be unexpected, given the recent popularity of gas density threshold models to explain star formation scaling relations. Our survey used the IRAM 30 m telescope to observe HCN(1-0), CO(1-0), and several other emission lines across 29 nearby disk galaxies whose CO(2-1) emission has previously been mapped by the HERACLES survey. We detected HCN in 48 out of 62 observed positions. We explore one such model in which variations in the Mach number drive many of the trends within galaxy disks, while density contrasts drive the differences between disk and merging galaxies.
We use ALMA observations to derive mass, length, and time scales associated with NGC 253's nuclear starburst. This region forms ∼2 M {sub ☉} yr{sup –1} of stars and resembles other starbursts in ...ratios of gas, dense gas, and star formation tracers, with star formation consuming the gas reservoir at a normalized rate 10 times higher than in normal galaxy disks. We present new ∼35 pc resolution observations of bulk gas tracers (CO), high critical density transitions (HCN, HCO{sup +}, and CS), and their isotopologues. The starburst is fueled by a highly inclined distribution of dense gas with vertical extent <100 pc and radius ∼250 pc. Within this region, we identify 10 starburst giant molecular clouds (GMCs) that appear as both peaks in the dense gas tracer cubes and the HCN-to-CO ratio map. These are massive (∼10{sup 7} M {sub ☉}) structures with sizes (∼30 pc) similar to GMCs in other systems, but compared to GMCs in normal galaxy disks, they have high line widths (σ ∼ 20-40 km s{sup –1}, Mach number M∼90) and high surface and volume densities (Σ{sub mol} ∼ 6000 M {sub ☉} pc{sup –2}, n {sub H2} ∼ 2000 cm{sup –3}). The self gravity from such high densities can explain the high line widths and the short free fall time τ{sub ff} ∼ 0.7 Myr in the clouds helps explain the more efficient star formation in NGC 253. Though the high inclination obscures the geometry somewhat, we show that simple models suggest a compact, clumpy region of high gas density embedded in a more extended, non-axisymmetric, bar-like distribution. Over the starburst, the surface density still exceeds that of a typical disk galaxy GMC and, as in the clouds, timescales in the disk as a whole are short compared to those in normal galaxy disks. The orbital time (∼10 Myr), disk free fall time (≲ 3 Myr), and disk crossing time (≲ 3 Myr) are each an order of magnitude shorter than in a normal galaxy disk. Finally, the CO-to-H{sub 2} conversion factor implied by our cloud calculations is approximately Galactic, contrasting with results showing a low value for the whole starburst region. The contrast provides resolved support for the idea of mixed molecular ISM phases in starburst galaxies.
PDF
