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Register a new userGravitational Infall onto Molecular Filaments II. Externally Pressurized Cylinders
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Fabian Heitsch
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In an extension of Fischera & Martin (2012a) and Heitsch (2013), two aspects of the evolution of externally pressurized, hydrostatic filaments are discussed. (a) The free-fall accretion of gas onto such a filament will lead to filament parameters (specifically, FWHM--column density relations) inconsistent with the observations of Arzoumanian et al. (2011), except for two cases: For low-mass, isothermal filaments, agreement is found as in the analysis by Fischera & Martin (2012b). Magnetized cases, for which the field scales weakly with the density as $Bpropto n^{1/2}$, also reproduce observed parameters. (b) Realistically, the filaments will be embedded not only in gas of non-zero pressure, but also of non-zero density. Thus, the appearance of sheet-embedded filaments is explored. Generating a grid of filament models and comparing the resulting column density ratios and profile shapes with observations suggests that the three-dimensional filament profiles are intrinsically flatter than isothermal, beyond projection and evolution effects.
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Two aspects of filamentary molecular cloud evolution are addressed: (1) Exploring analytically the role of the environment for the evolution of filaments demonstrates that considering them in isolation (i.e. just addressing the fragmentation stability) will result in unphysical conclusions about the filaments properties. Accretion can also explain the observed decorrelation between FWHM and peak column density. (2) Free-fall accretion onto finite filaments can lead to the characteristic fans of infrared-dark clouds around star-forming regions. The fans may form due to tidal forces mostly arising at the ends of the filaments, consistent with numerical models and earlier analytical studies.
We present results of our study on eight dense cores, previously classified as starless, using infrared (3-160 {micron}) imaging observations with textit{AKARI} telescope and molecular line (HCN and N$_2$H$^+$) mapping observations with textit{KVN} telescope. Combining our results with the archival IR to mm continuum data, we examined the starless nature of these eight cores. Two of the eight cores are found to harbor faint protostars having luminosity of $sim0.3-4.4$ L$_{odot}$. The other six cores are found to remain as starless and probably are in a dynamically transitional state. The temperature maps produced using multi-wavelength images show an enhancement of about 3-6 K towards the outer boundary of these cores, suggesting that they are most likely being heated externally by nearby stars and/or interstellar radiation fields. Large virial parameters and an over-dominance of red asymmetric line profiles over the cores may indicate that the cores are set into either an expansion or an oscillatory motion, probably due to the external heating. Most of the starless cores show coreshine effect due to the scattering of light by the micron-size dust grains. This may imply that the age of the cores is of the order of $sim10^{5}$ years, being consistent with the timescale required for the cores to evolve into an oscillatory stage due to the external perturbation. Our observational results support the idea that the external feedback from nearby stars and/or interstellar radiation fields may play an important role in the dynamical evolution of the cores.
Growth of the structure in the Universe manifest as accretion flows of galaxies onto groups and clusters. Thus, the present day properties of groups and their member galaxies are influenced by the characteristics of this continuous infall pattern. Several works both theoretical, in numerical simulations, and in observations, study this process and provide useful steps for a better understanding of galaxy systems and their evolution. We aim at exploring the streaming flow of galaxies onto groups using observational peculiar velocity data. The effects of distance uncertainties are also analyzed as well as the relation between the infall pattern and group and environment properties.This work deals with analysis of peculiar velocity data and their projection on the direction to group centers, to determine the mean galaxy infall flow. We applied this analysis to the galaxies and groups extracted from the Cosmicflows-3 catalog. We also use mock catalogs derived from numerical simulations to explore the effects of distance uncertainties on the derivation of the galaxy velocity flow onto groups. We determine the infalling velocity field onto galaxy groups with cz < 0.033 using peculiar velocity data. We measure the mean infall velocity onto group samples of different mass range, and also explore the impact of the environment where the group reside. Well beyond the group virial radius, the surrounding large-scale galaxy overdensity may impose additional infalling streaming amplitudes in the range 200 to 400 km s$^{-1}$. Also, we find that groups in samples with a well controlled galaxy density environment show an increasing infalling velocity amplitude with group mass, consistent with the predictions of the linear model. These results from observational data are in excellent agreement with those derived from the mock catalogs.
Filamentary structures are common morphological features of the cold, molecular interstellar medium (ISM). Recent studies have discovered massive, hundred-parsec-scale filaments that may be connected to the large-scale, Galactic spiral arm structure. Addressing the nature of these Giant Molecular Filaments (GMFs) requires a census of their occurrence and properties. We perform a systematic search of GMFs in the fourth Galactic quadrant and determine their basic physical properties. We identify GMFs based on their dust extinction signatures in near- and mid-infrared and velocity structure probed by ^{13}CO line emission. We use the ^{13}CO line emission and ATLASGAL dust emission data to estimate the total and dense gas masses of the GMFs. We combine our sample with an earlier sample from literature and study the Galactic environment of the GMFs. We identify nine GMFs in the fourth Galactic quadrant; six are located in the Centaurus spiral arm and three in inter-arm regions. Combining this sample with an earlier study using the same identification criteria in the first Galactic quadrant results in 16 GMFs, nine of which are located within spiral arms. The GMFs have sizes of 80-160 pc and ^{13}CO-derived masses between 5-90 x 10^{4} Msun. Their dense gas mass fractions are between 1.5-37%, being higher in the GMFs connected to spiral arms. We also compare the different GMF-identification methods and find that emission and extinction based techniques overlap only partially, highlighting the need to use both to achieve a complete census.
Gravitational instability plays an important role in driving gas accretion in massive protostellar discs. Particularly strong is the global gravitational instability, which arises when the disc mass is of order 0.1 of the mass of the central star and has a characteristic spatial scale much greater than the discs vertical scale-height. In this paper we use three-dimensional numerical hydrodynamics to study the development of gravitational instabilities in a disc which is embedded in a dense, gaseous envelope. We find that global gravitational instabilities are the dominant mode of angular momentum transport in the disc with infall, in contrast to otherwise identical isolated discs. The accretion torques created by low-order, global modes of the gravitational instability in a disc subject to infall are larger by a factor of several than an isolated disc of the same mass. We show that this global gravitational instability is driven by the strong vertical shear at the interface between the disc and the envelope, and suggest that this process may be an important means of driving accretion on to young stars.
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