Occlusion Effects and the Distribution of Interstellar Cloud Sizes and Masses (original) (raw)
Related papers
Received; accepted Submitted to the Astrophysical Journal – 2 –
1995
The frequency distributions of sizes of “clouds ” and “clumps ” within clouds are significantly flatter for extinction surveys than for CO spectral line surveys, even for comparable size ranges. A possible explanation is the blocking of extinction clouds by larger foreground clouds (occlusion), which should not affect spectral line surveys much because clouds are resolved in velocity space along a given line of sight. We present a simple derivation of the relation between the true and occluded size distributions, assuming clouds are uniformly distributed in space or the distance to a cloud comples is much greater than the size of the complex. Because the occlusion is dominated by the largest clouds, we find that occlusion does not affect the measured size distribution except for sizes comparable to the largest size, implying that occlusion is not responsible for the discrepancy if the range in sizes of the samples is large. However, we find that the range in sizes for many of the pu...
Observing the structure of the interstellar clouds
Journal of Physics: Conference Series, 2005
We discuss some methods for characterizing the mass distribution in the interstellar clouds. This is done in the light of available observational techniques and of appropriate statistical tools capable to highlight the structural properties of the astronomical images. The potentialities of the optical and near-IR imaging of dark clouds as well as of their far-IR and millimetric mapping are also discussed with respect to the analysis in terms of statistical characterization. We briefly address the hypothesis that the origin of the structure observed in interstellar clouds is the turbulence which could be a natural consequence of the physical status of the interstellar medium. We finally discuss how the fractal geometry offers an interesting tool for characterizing the structure of the interstellar clouds introducing the so called multifractal spectrum. The physical interpretation of this spectrum can offer a further tool in discriminating among different possible kind of internal dynamics.
Interstellar extinction from photometric surveys: application to four high-latitude areas
Open Astronomy, 2018
Information on interstellar extinction and dust properties may be obtained from modern large photometric surveys data. Virtual Observatory facilities allow users to make a fast and correct cross-identification of objects from various surveys. It yields a multicolor photometry data on detected objects and makes it possible to estimate stellar parameters and calculate interstellar extinction. A 3D extinction map then can be constructed. The method was applied to 2MASS, SDSS, GALEX and UKIDSS surveys. Results for several high-latitude areas are obtained, compared with independent sources and discussed here.
Bird’s eye view of molecular clouds in the Milky Way
Astronomy and Astrophysics, 2021
Context. Describing how the properties of the interstellar medium are combined across various size scales is crucial for understanding star formation scaling laws and connecting Galactic and extragalactic data of molecular clouds. Aims. We describe how the statistical structure of the clouds and its connection to star formation changes from sub-parsec to kiloparsec scales in a complete region within the Milky Way disk. Methods. We built a census of molecular clouds within 2 kpc from the Sun using data from the literature. We examined the dust-based column density probability distributions (N-PDFs) of the clouds and their relation to star formation as traced by young stellar objects (YSOs). We then examined our survey region from the outside, within apertures of varying sizes, and describe how the N-PDFs and their relation to star formation changes with the size scale. Results. We present a census of the molecular clouds within 2 kpc distance, including 72 clouds and YSO counts for 44 of them. The N-PDFs of the clouds are not well described by any single simple model; use of any single model may bias the interpretation of the N-PDFs. The top-heaviness of the N-PDFs correlates with star formation activity, and the correlation changes with Galactic environment (spiral-and inter-arm regions). We find that the density contrast of clouds may be more intimately linked to star formation than the dense gas mass fraction. The aperture-averaged N-PDFs vary with the size scale and are more top-heavy for larger apertures. The top-heaviness of the aperture N-PDFs correlates with star formation activity up to roughly 0.5 kpc, depending on the environment. Our results suggest that the relations between cloud structure and star formation are environment specific and best captured by relative quantities (e.g. the density contrast). Finally, we show that the density structures of individual clouds give rise to a kiloparsec-scale Kennicutt-Schmidt relation as a combination of sampling effects and blending of different galactic environments.
Context. The kinematics of molecular gas are crucial for setting the stage for star formation. One key question related to the kinematic properties of gas is how they depend on the spatial scale. Aims. We aim to describe the CO spectra, velocity dispersions, and especially the linewidth-size relation of molecular gas from cloud (parsec-) scales to kiloparsec scales in a complete region within the Milky Way disk. Methods. We used the census of molecular clouds within 2 kpc from our earlier work, together with CO emission data for them from the literature. We studied the kinematics and the Larson relations for the sample of individual clouds. We also mimicked a face-on view of the Milky Way and analysed the kinematics of the clouds within apertures of 0.25-2 kpc in size. In this way, we describe the scale-dependence of the CO gas kinematics and Larson’s relations. Results. We describe the spectra of CO gas at cloud scales and in apertures between 0.25-2 kpc in our survey area. The spectra within the apertures are relatively symmetric, but show non-Gaussian high-velocity wings. At cloud scales, our sample shows a linewidthsize relation σv = 1.5 ·R 0.3±0.1 with a large scatter. The mass-size relation in the sample of clouds is MCO = 794 ·R 1.5±0.5 . The relations are also present for the apertures at kiloparsec-scales. The best-fit linewidth-size relation for the apertures is σv = 0.5 · R 0.35±0.01, and the best-fit mass-size relation is MCO = 229 · R 1.4±0.1 . A suggestive dependence on Galactic environment is seen. Apertures closer to the Galactic centre and the Sagittarius spiral arm have slightly higher velocity dispersions. We explore the possible effect of a diffuse component in the survey area and find that such a component would widen the CO spectra and could flatten the linewidth-size relation. Understanding the nature of the possible diffuse CO component and its effects on observations is crucial for connecting Galactic and extragalactic data.
Dark clouds in the vicinity of the emission nebula Sh2-205: interstellar extinction and distances
Astronomy & Astrophysics, 2016
Results of CCD photometry in the seven-colour Vilnius system for 922 stars down to V = 16-17 mag and for 302 stars down to 19.5 mag are used to investigate the interstellar extinction in an area of 1.5 square degrees in the direction of the P7 and P8 clumps of the dark cloud TGU H942, which lies in the vicinity of the emission nebula Sh2-205. In addition, we used 662 red clump giants that were identified by combining the 2MASS and WISE infrared surveys. The resulting plots of extinction versus distance were compared with previous results of the distribution and radial velocities of CO clouds and with dust maps in different passbands of the IRAS and WISE orbiting observatories. A possible distance of the front edge of the nearest cloud layer at 130 ± 10 pc was found. This dust layer probably covers all the investigated area, which results in extinction of up to 1.8 mag in some directions. A second rise of the extinction seems to be present at 500-600 pc. Within this layer, the clumps P7 and P8 of the dust cloud TGU H942, the Sh2-205 emission nebula, and the infrared cluster FSR 655 are probably located. In the direction of these clouds, we identified 88 young stellar objects and a new infrared cluster.
Highlights of Astronomy, 1992
Diffuse clouds are interstellar objects through which starlight is not greatly extinguished. As a result, many studies rely on ultraviolet and visible measurements. The focus of this review is on spectroscopic work involving atomic and molecular lines. From these measurements, the physical and chemical makeup of the clouds is derived. Here we stress that technological advances have influenced our perception of diffuse clouds.
Structure of interstellar clouds including the effects of cosmic rays and waves
2018
The growth of supermassive black holes across cosmic time leaves a radiative imprint recorded in the X-ray background (XRB). The XRB spectral shape suggests that a large population of distant, hidden nuclei must exist, which are now being revealed at higher and higher redshifts by the deepest surveys performed by Chandra and XMM. Our current understanding of the XRB emission in terms of AGN population synthesis models is here reviewed, and the evolutionary path of nuclear accretion and obscuration, as emerging from the major X-ray surveys, is investigated. The role of galaxy merging versus secular processes in triggering nuclear activity is also discussed in the framework of recent galaxy/black hole co-evolutionary scenarios. Finally, the limits of current instrumentation in the detection of the most obscured and distant black holes are discussed and some possible directions to overcome these limits are presented.
On signatures of clouds in exoplanetary transit spectra
Monthly Notices of the Royal Astronomical Society, 2017
Transmission spectra of exoplanetary atmospheres have been used to infer the presence of clouds/hazes. Such inferences are typically based on spectral slopes in the optical deviant from gaseous Rayleigh scattering or low-amplitude spectral features in the infrared. We investigate three observable metrics that could allow constraints on cloud properties from transmission spectra, namely the optical slope, the uniformity of this slope and condensate features in the infrared. We derive these metrics using model transmission spectra considering Mie extinction from a wide range of condensate species, particle sizes and scaleheights. First, we investigate possible degeneracies among the cloud properties for an observed slope. We find, for example, that spectra with very steep optical slopes suggest sulphide clouds (e.g. MnS, ZnS, Na 2 S) in the atmospheres. Secondly, (non)uniformities in optical slopes provide additional constraints on cloud properties, e.g. MnS, ZnS, TiO 2 and Fe 2 O 3 have significantly non-uniform slopes. Thirdly, infrared spectra provide an additional powerful probe into cloud properties, with SiO 2 , Fe 2 O 3 , Mg 2 SiO 4 and MgSiO 3 bearing strong infrared features observable with James Webb Space Telescope. We investigate observed spectra of eight hot Jupiters and discuss their implications. In particular, no single or composite condensate species considered here conforms to the steep and non-uniform optical slope observed for HD 189733b. Our work highlights the importance of the three above metrics to investigate cloud properties in exoplanetary atmospheres using high-precision transmission spectra and detailed cloud models. We make our Mie scattering data for condensates publicly available to the community.
A Comparative Study of Giant Molecular Clouds in M51, M33, and the Large Magellanic Cloud
The Astrophysical Journal, 2013
We compare the properties of giant molecular clouds (GMCs) in M51 identified by the Plateau de Bure Interferometer Whirlpool Arcsecond Survey (PAWS) with GMCs identified in wide-field, high resolution surveys of CO emission in M33 and the Large Magellanic Cloud (LMC). We find that GMCs in M51 are larger, brighter and have higher velocity dispersions relative to their size than equivalent structures in M33 and the LMC. These differences imply that there are genuine variations in the average mass surface density Σ H2 of the different GMC populations. To explain this, we propose that the pressure in the interstellar medium surrounding the GMCs plays a role in regulating their density and velocity dispersion. We find no evidence for a correlation between size and linewidth in any of M51, M33 or the LMC when the CO emission is decomposed into GMCs, although moderately robust correlations are apparent when regions of contiguous CO emission (with no size limitation) are used. Our work demonstrates that observational bias remains an important obstacle to the identification and study of extragalactic GMC populations using CO emission, especially in molecule-rich galactic environments.