Formation and evolution of interstellar filaments; Hints from velocity dispersion measurements

We investigate the gas velocity dispersions of a sample of filaments recently detected as part of the Herschel Gould Belt Survey in the IC5146, Aquila, and Polaris interstellar clouds. To measure these velocity dispersions, we use 13CO, C18O, and N2H+ line observations obtained with the IRAM 30m telescope. Correlating our velocity dispersion measurements with the filament column densities derived from Herschel data, we show that interstellar filaments can be divided into two regimes: thermally subcritical filaments, which have transonic velocity dispersions (c_s ~< \sigma_tot < 2 c_s) independent of column density, and are gravitationally unbound; and thermally supercritical filaments, which have higher velocity dispersions scaling roughly as the square root of column density (\sigma_tot ~ \Sigma^0.5), and are self-gravitating. The higher velocity dispersions of supercritical filaments may not directly arise from supersonic interstellar turbulence but may be driven by gravitational contraction/accretion. Based on our observational results, we propose an evolutionary scenario whereby supercritical filaments undergo gravitational contraction and increase in mass per unit length through accretion of background material while remaining in rough virial balance. We further suggest that this accretion process allows supercritical filaments to keep their approximately constant inner widths (~ 0.1 pc) while contracting.Comment: 16 pages, 8 figures, 1 table, 1 appendix. Accepted for publication in Astronomy and Astrophysic

A high-resolution extinction mapping technique for face-on disc galaxies

We present a new dust extinction technique with which we are able to retrieve parsec-scale gas surface density maps for entire nearby galaxies. The method measures the dust attenuation in optical bands on a pixel-by-pixel basis against a smoothed, reconstructed stellar distribution. The contribution of foreground light along the line-of-sight is calibrated using dust emission observations, assuming that the dust sits in a layer close to the mid-plane of the face-on galaxy. Here, we apply this technique to M51 (NGC 5194) as a proof-of-concept, obtaining a resolution of 0.14" (5 pc). Our dust (and gas) surface density map is consistent with independent dust- and CO-based studies at lower resolution. We find that discrepancies between our estimates of surface density and other studies stem primarily from the choice of dust model (i.e. different dust absorption coefficients). When assuming the same dust opacity law, our technique produces surface densities that are consistent with independent studies. This dust extinction technique provides us with gas surface density maps at an unprecedented resolution for full disc coverage studies of nearby galaxies. The resulting well-resolved spatial information opens the possibility for more in-depth examination of the influence of large-scale dynamics (and also stellar feedback mechanisms) on the interstellar medium at parsec-scales, and consequently star formation in nearby galaxies.Comment: MNRAS accepted. 15 pages, 12 figure

Molecular clouds in M51 from high-resolution extinction mapping

Here we present the cloud population extracted from M51, following the application of our new high-resolution dust extinction technique to the galaxy (Faustino Vieira et al. 2023). With this technique, we are able to image the gas content of the entire disc of M51 down to 5 pc (0.14"), which allows us to perform a statistical characterisation of well-resolved molecular cloud properties across different large-scale dynamical environments and with galactocentric distance. We find that cloud growth is promoted in regions in the galaxy where shear is minimised; i.e. clouds can grow into higher masses (and surface densities) inside the spiral arms and molecular ring. We do not detect any enhancement of high-mass star formation towards regions favourable to cloud growth, indicating that massive and/or dense clouds are not the sole ingredient for high-mass star formation. We find that in the spiral arms there is a significant decline of cloud surface densities with increasing galactocentric radius, whilst in the inter-arm regions they remain relatively constant. We also find that the surface density distribution for spiral arm clouds has two distinct behaviours in the inner and outer galaxy, with average cloud surface densities at larger galactocentric radii becoming similar to inter-arm clouds. We propose that the tidal interaction between M51 and its companion (NGC 5195) - which heavily affects the nature of the spiral structure - might be the main factor behind this.Comment: MNRAS accepted, 20 pages, 16 figure

Star cluster progenitors are dynamically decoupled from their parent molecular clouds

The formation of stellar clusters dictates the pace at which galaxies evolve, and solving the question of their formation will undoubtedly lead to a better understanding of the Universe as a whole. While it is well known that star clusters form within parsec-scale overdensities of interstellar molecular gas called clumps, it is, however, unclear whether these clumps represent the high-density tip of a continuous gaseous flow that gradually leads towards the formation of stars, or a transition within the gas physical properties. Here, we present a unique analysis of a sample of 27 infrared dark clouds embedded within 24 individual molecular clouds that combine a large set of observations, allowing us to compute the mass and velocity dispersion profiles of each, from the scale of tens of parsecs down to the scale of tenths of a parsec. These profiles reveal that the vast majority of the clouds, if not all, are consistent with being self-gravitating on all scales, and that the clumps, on parsec-scale, are often dynamically decoupled from their surrounding molecular clouds, exhibiting steeper density profiles (ρ∝r−2) and flat velocity dispersion profiles (σ∝r0), clearly departing from Larson’s relations. These findings suggest that the formation of star clusters correspond to a transition regime within the properties of the self-gravitating molecular gas. We propose that this transition regime is one that corresponds to the gravitational collapse of parsec-scale clumps within otherwise stable molecular clouds

The Millimeter Astronomy Legacy Team 90 GHz (MALT90) Pilot Survey

We describe a pilot survey conducted with the Mopra 22-m radio telescope in preparation for the Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90). We identified 182 candidate dense molecular clumps using six different selection criteria and mapped each source simultaneously in 16 different lines near 90 GHz. We present a summary of the data and describe how the results of the pilot survey shaped the design of the larger MALT90 survey. We motivate our selection of target sources for the main survey based on the pilot detection rates and demonstrate the value of mapping in multiple lines simultaneously at high spectral resolution.Comment: Accepted to ApJS. 23 pages and 16 figures. Full resolution version with an appendix showing all the data (12.1 MB) is available at http://malt90.bu.edu/publications/Foster_2011_Malt90Pilot.pd

Magnetic fields from filaments to cores

How important is the magnetic (B-) field when compared to gravity and turbulence in the star-formation process? Does its importance depend on scale and location? We summarize submm dust polarization observations towards the large filamentary infrared dark cloud G34 and towards a dense core in the high-mass star-forming region W51. We detect B-field orientations that are either perpendicular or parallel to the G34 filament axis. These B-field orientations further correlate with local velocity gradients. Towards three cores in G34 we find a varying importance between B-field, gravity, and turbulence that seems to dictate varying types of fragmentation. At highest resolution towards the gravity-dominated collapsing core W51 e2 we resolve new B-field features, such as converging B-field lines and possibly magnetic channels

Molecular clouds in M51 from high-resolution extinction mapping

Here, we present the cloud population extracted from M51, following the application of our new high-resolution dust extinction technique to the galaxy. With this technique, we are able to image the gas content of the entire disc of M51 down to 5 pc (0.14 arcsec), which allows us to perform a statistical characterization of well-resolved molecular cloud properties across different large-scale dynamical environments and with galactocentric distance. We find that cloud growth is promoted in regions in the galaxy where shear is minimized; i.e. clouds can grow into higher masses (and surface densities) inside the spiral arms and molecular ring. We do not detect any enhancement of high-mass star formation towards regions favourable to cloud growth, indicating that massive and/or dense clouds are not the sole ingredient for high-mass star formation. We find that in the spiral arms there is a significant decline of cloud surface densities with increasing galactocentric radius, whilst in the inter-arm regions they remain relatively constant. We also find that the surface density distribution for spiral arm clouds has two distinct behaviours in the inner and outer galaxy, with average cloud surface densities at larger galactocentric radii becoming similar to inter-arm clouds. We propose that the tidal interaction between M51 and its companion (NGC 5195) – which heavily affects the nature of the spiral structure – might be the main factor behind this

Gravity, magnetic field, and turbulence: relative importance and impact on fragmentation in the infrared dark cloud G34.43+00.24

We investigate the interplay between magnetic (B) field, gravity, and turbulence in the fragmentation process of cores within the filamentary infrared dark cloud G34.43+00.24. We observe the magnetic field morphology across G34.43, traced with thermal dust polarization at 350 μm with an angular resolution of 10'' (0.18 pc), and compare with the kinematics obtained from N2H+ across the filament. We derive local velocity gradients from N2H+, tracing motion in the plane of sky, and compare with the observed local B field orientations in the plane of sky. The B field orientations are found to be perpendicular to the long axis of the filament toward the MM1 and MM2 ridge, suggesting that the B field can guide material toward the filament. Toward MM3, the B field orientations appear more parallel to the filament and aligned with the elongated core of MM3, indicating a different role of the B field. In addition to a large-scale east–west velocity gradient, we find a close alignment between local B field orientations and local velocity gradients toward the MM1/MM2 ridge. This local correlation in alignment suggests that gas motions are influenced by the B field morphology or vice versa. Additionally, this alignment seems to become even closer with increasing integrated emission in N2H+, possibly indicating that a growing gravitational pull alignes the B field and gas motion more and more. We analyze and quantify B field, gravity, turbulence, and their relative importance toward the MM1, MM2, and MM3 regions with various techniques over two scales, a larger clump area at 2 pc scale and the smaller core area at 0.6 pc scale. While gravitational energy, B field, and turbulent pressure all grow systematically from large to small scale, the ratios among the three constituents clearly develop differently over scale. We propose that this varying relative importance between B field, gravity, and turbulence over scale drives and explains the different fragmentation types seen at subparsec scale (no fragmentation in MM1; aligned fragmentation in MM2; clustered fragmentation in MM3). We discuss uncertainties, subtleties, and the robustness of our conclusion, and we stress that a multiscale joint analysis is required to understand the dynamics in these systems