Received; accepted Submitted to the Astrophysical Journal – 2 – (original) (raw)
Related papers
Occlusion Effects and the Distribution of Interstellar Cloud Sizes and Masses
The Astrophysical Journal, 1996
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 published observed samples is actually quite small, which suggests that occlusion does affect the extinction sample and/or that the discrepancy could arise from the different operational definitions and selection effects involved in the two samples. Size and mass spectra from an IRAS survey (Wood et al. 1994) suggest that selection effects play a major role in all the surveys. We conclude that a reliable determination of the "true" size and mass spectra of clouds will require spectral line surveys with very high signal-to-noise and sufficient resolution and sampling to cover a larger range of linear sizes, as well as careful attention to selection effects.
The size and geometry of the Lya clouds
1998
Spectra of the QSO pair Q0307-195A,B have been obtained in the Lyα forest (3660-3930Å) and C iv (4720-4850 A) regions with a FWHM resolution between 0.7 and 0.5Å. 46 lines have been detected in the spectrum of object A while 36 in the spectrum of object B, of them 29 and 20 were identified as Lyα absorptions respectively. The present observations have been supplemented with data of comparable quality on other 7 QSO pairs available in the literature to give an enlarged sample of 217 Lyα lines with rest equivalent width W o ≥ 0.3Å. The analysis of the hits (i.e. when an absorption line appears in both QSO spectra) and misses (i.e. when a line is seen in any of the QSO spectra, but no line is seen in the other), carried out with an improved statistical approach, indicates that the absorbers have typically a large size 1 : R = 362 h −1 kpc, with 95% confidence limits 298 < R < 426 h −1 kpc and R = 412 h −1 kpc, with 95% confidence limits 333 < R < 514 h −1 kpc for the radius of idealized spherical and disc geometries, respectively. The present data do not allow to establish any correlation of the typical inferred size with the proper separation or with the redshift of the pairs. The correlation between the observed equivalent widths of the absorbers in the adjacent lines of sight becomes poorer and poorer with increasing proper separation. A disc geometry with a column density profile N (r) ∝ (r/R 0) −γ , γ = 4, is found to reasonably reproduce the data with R 0 100 − 200 h −1 kpc, but also spherical clouds with the same column density profile and a power-law distribution of radii may give a satisfactory representation of the observations.
Gaps in the Cloud Cover? Comparing Extinction Measures in Spiral Disks
The Astronomical Journal, 2007
Dust in galaxies can be mapped by either the FIR/sub-mm emission, the optical or infrared reddening of starlight, or the extinction of a known background source. We compare two dust extinction measurements for a set of fifteen sections in thirteen nearby galaxies, to determine the scale of the dusty ISM responsible for disk opacity: one using stellar reddening and the other a known background source. In our earlier papers, we presented extinction measurements of 29 galaxies, based on calibrated counts of distant background objects identified though foreground disks in HST/WFPC2 images. For the 13 galaxies that overlap with the Spitzer Infrared Nearby Galaxies Survey (SINGS), we now compare these results with those obtained from an I − L color map. Our goal is to determine whether or not a detected distant galaxy indicates a gap in the dusty ISM, and hence to better understand the nature and geometry of the disk extinction.
The size and geometry of the Ly_alpha clouds
Arxiv preprint astro-ph/ …, 1998
Spectra of the QSO pair Q0307-195A,B have been obtained in the Lyα forest (3660-3930Å) and C iv (4720-4850Å) regions with a FWHM resolution between 0.7 and 0.5Å. 46 lines have been detected in the spectrum of object A while 36 in the spectrum of object B, of them 29 and 20 were identified as Lyα absorptions respectively. The present observations have been supplemented with data of comparable quality on other 7 QSO pairs available in the literature to give an enlarged sample of 217 Lyα lines with rest equivalent width W o ≥ 0.3Å. The analysis of the hits (i.e. when an absorption line appears in both QSO spectra) and misses (i.e. when a line is seen in any of the QSO spectra, but no line is seen in the other), carried out with an improved statistical approach, indicates that the absorbers have typically a large size 1 : R = 362 h −1 kpc, with 95% confidence limits 298 < R < 426 h −1 kpc and R = 412 h −1 kpc, with 95% confidence limits 333 < R < 514 h −1 kpc for the radius of idealized spherical and disc geometries, respectively. The present data do not allow to establish any correlation of the typical inferred size with the proper separation or with the redshift of the pairs. The correlation between the observed equivalent widths of the absorbers in the adjacent lines of sight becomes poorer and poorer with increasing proper separation. A disc geometry with a column density profile N (r) ∝ (r/R 0) −γ , γ = 4, is found to reasonably reproduce the data with R 0 ≃ 100 − 200 h −1 kpc, but also spherical clouds with the same column density profile and a power-law distribution of radii may give a satisfactory representation of the observations.
A Simple Model of Spectral-Line Profiles from Contracting Clouds
Astrophysical Journal, 1996
A simple analytic model of radiative transfer in two parts of a contracting cloud matches a wide range of line profiles in candidate infall regions and provides a sensitive estimate of V in , the characteristic inward speed of the gas forming the line. The model assumes two uniform regions of equal temperature and velocity dispersion , whose density and velocity are attenuation-weighted means over the front and rear halves of a centrally condensed, contracting cloud. The model predicts two-peak profiles for "slow" infall, V in Ͻ Ͻ , and red-shoulder profiles for "fast" infall, V in 1 . A simple formula expresses V in solely in terms of and of observable parameters of a two-peak line. We apply the model to fit profiles of high and low optical depth lines observed in a dense core with no star (L1544, V in ϭ 0.006 km s Ϫ1 ), with an isolated protostar (L1527, 0.025 km s Ϫ1 ), and with a small group of stars (L1251B, 0.35 km s Ϫ1 ). The mass infall rate obtained from V in and the map size varies from (2-40) ϫ 10 Ϫ6 M J yr Ϫ1 and agrees within a factor 12 in each core with the independently determined rate 1 3 G Ϫ1 for a gravitationally collapsing isothermal sphere. This agreement suggests that the inward motions derived from the line profiles are gravitational in origin.
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.
The Structure of the Small Dark Cloud CB 107
The Astrophysical Journal, 2004
This paper presents the near-IR imaging observations of CB 107, a small dark globule projected against a rich stellar background. By means of accurate photometry, the near-IR two-color diagram J À H versus H À K was obtained for the stellar background. This information was used to estimate the color excesses of the detected stars so that, given the reddening curve, it was possible to derive the extinction map of the cloud. The structural properties of the dark globule were investigated by plotting the extinction dispersion A V , obtained in a given spatial box, as a function of the mean extinction A V . This relationship has shown quite a definite linear behavior, with the slope increasing with the box size. The results of the present analysis, compared with those obtained by other authors on larger dark clouds, suggest that for a given spatial scale the slope of the A V versus A V relation is greater in CB 107 than in larger clouds. The so-called Á-variance method was also used to investigate the structure of the dark globule by evaluating the drift behavior of its extinction map. In this way, we have found that the power spectrum of the extinction map is characterized by a power law with exponent $ 2:7. This value is lower than expected, for the same range of spatial scales, on the basis of previous work on large molecular clouds.
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.
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.
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.