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Journal ArticleDOI

Molecular gas in the Andromeda galaxy

01 Jul 2006-Astronomy and Astrophysics (Springer, Berlin, Heidelberg)-Vol. 453, Iss: 2, pp 459-475
TL;DR: M 31, the closest large spiral galaxy to our own, is the best object for studying molecular clouds and their relation to the spiral structure as discussed by the authors, and it is also one of the best places where to estimate molecular clouds masses through the Virial Theorem.
Abstract: M 31, the closest large spiral galaxy to our own, is the best object for studying molecular clouds and their relation to the spiral structure. As one of the astronomical objects with the best known distance (0.78 ± 0.02 Mpc), it is also one of the best places where to estimate molecular clouds masses through the Virial Theorem.

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Book
09 Jan 2011
TL;DR: In this paper, a comprehensive and richly illustrated textbook on the astrophysics of the interstellar and intergalactic medium is presented, including the gas and dust, as well as the electromagnetic radiation, cosmic rays, and magnetic and gravitational fields, present between the stars in a galaxy and also between galaxies themselves.
Abstract: This is a comprehensive and richly illustrated textbook on the astrophysics of the interstellar and intergalactic medium--the gas and dust, as well as the electromagnetic radiation, cosmic rays, and magnetic and gravitational fields, present between the stars in a galaxy and also between galaxies themselves. Topics include radiative processes across the electromagnetic spectrum; radiative transfer; ionization; heating and cooling; astrochemistry; interstellar dust; fluid dynamics, including ionization fronts and shock waves; cosmic rays; distribution and evolution of the interstellar medium; and star formation. While it is assumed that the reader has a background in undergraduate-level physics, including some prior exposure to atomic and molecular physics, statistical mechanics, and electromagnetism, the first six chapters of the book include a review of the basic physics that is used in later chapters. This graduate-level textbook includes references for further reading, and serves as an invaluable resource for working astrophysicists. * Essential textbook on the physics of the interstellar and intergalactic medium * Based on a course taught by the author for more than twenty years at Princeton University * Covers radiative processes, fluid dynamics, cosmic rays, astrochemistry, interstellar dust, and more * Discusses the physical state and distribution of the ionized, atomic, and molecular phases of the interstellar medium * Reviews diagnostics using emission and absorption lines * Features color illustrations and detailed reference materials in appendices * Instructor's manual with problems and solutions (available only to teachers)

1,143 citations

Journal ArticleDOI
TL;DR: In this article, the authors show that the ratio of molecular to atomic gas in galaxies is determined by hydrostatic pressure and that the relation between the two is nearly linear, and propose a modified star formation prescription based on pressure determining the degree to which the ISM is molecular.
Abstract: We show that the ratio of molecular to atomic gas in galaxies is determined by hydrostatic pressure and that the relation between the two is nearly linear. The pressure relation is shown to be good over 3 orders of magnitude for 14 galaxies, including dwarfs, H I-rich, and H2-rich galaxies, as well as the Milky Way. The sample spans a factor of 5 in mean metallicity. The rms scatter of individual points of the relation is only about a factor of 2 for all the galaxies, although some show much more scatter than others. Using these results, we propose a modified star formation prescription based on pressure determining the degree to which the ISM is molecular. The formulation is different in high- and low-pressure regimes, defined by whether the gas is primarily atomic or primarily molecular. This formulation can be implemented in simulations and provides a more appropriate treatment of the outer regions of spiral galaxies and molecule-poor systems, such as dwarf irregulars and damped Lyα systems.

636 citations


Cites background from "Molecular gas in the Andromeda gala..."

  • ...However, throughout the entire disk of the Milky Way, ΣHI ≥ ΣH2 (except for the central 300 pc; Dame 1993), as is also the case in M33 (Heyer et al. 2004) and most of M31 (Nieten et al. 2006)....

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Journal ArticleDOI
TL;DR: In this paper, the authors estimate the conversion factor relating CO emission to H2 mass, αCO, in five Local Group galaxies that span approximately an order of magnitude in metallicity.
Abstract: We estimate the conversion factor relating CO emission to H2 mass, αCO, in five Local Group galaxies that span approximately an order of magnitude in metallicity—M 31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the αCO that best allows a single gas-to-dust ratio (δGDR) to describe each system. This approach remains sensitive to CO-dark envelopes H2 surrounding molecular clouds. In M 31, M 33, and the LMC we find αCO 3-9 M ☉ pc–2 (K km s–1)–1, consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log (O/H) 8.2 and 8.0), exhibit a much higher αCO. Our best estimates are αNGC6822 CO 30 M ☉ pc–2 (K km s–1)–1 and αSMC CO 70 M ☉ pc–2 (K km s–1)–1. These results are consistent with the conversion factor becoming a strong function of metallicity around 12 + log (O/H) ~ 8.4-8.2. We favor an interpretation where decreased dust shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity.

545 citations

Journal ArticleDOI
TL;DR: In this paper, the authors modeled the dust mass along the line of sight from infrared (IR) emission and then solved for the alpha-CO that best allows a single gas-to-dust ratio (delta_GDR) to describe each system.
Abstract: We estimate the conversion factor relating CO emission to H2 mass, alpha_CO, in five Local Group galaxies that span approximately an order of magnitude in metallicity - M31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the alpha_CO that best allows a single gas-to-dust ratio (delta_GDR) to describe each system. This approach remains sensitive to CO-dark envelopes of H2 surrounding molecular clouds. In M 31, M 33, and the LMC we find alpha_CO \approx 3-9 M_sun pc^-2 (K km s^-1)^-1, consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log(O/H) \approx 8.2 and 8.0), exhibit a much higher alpha_CO. Our best estimates are \alpha_NGC6822 \approx 30 M_sun/pc^-2 (K km s^-1)^-1 and \alpha_SMC \approx 70 M_sun/pc^-2 (K km s-1)-1. These results are consistent with the conversion factor becoming CO a strong function of metallicity around 12 + log(O/H) \sim 8.4 - 8.2. We favor an interpretation where decreased dust-shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity.

377 citations


Cites background or methods from "Molecular gas in the Andromeda gala..."

  • ...Especially in M 31 and M 33, much of the H I is at large radius and appears to have a different δGDR from the inner galaxy (Nieten et al. 2006)....

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  • ...For M 31, we use the CO map taken by Nieten et al. (2006) using the IRAM 30m telescope....

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  • ...M 31 is inclined and does show a gradient in δGDR (Nieten et al. 2006)....

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  • ...This map has already been masked by Nieten et al. (2006) and so contains only positive signal....

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  • ...Nieten et al. (2006) observed δGDR to be higher in the inner part of M 31 than the 10 kpc ring containing most of the CO....

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Journal ArticleDOI
TL;DR: In this article, the authors present a theoretical framework in which bound stellar clusters arise naturally at the high-density end of the hierarchy of the interstellar medium (ISM) and due to short free-fall times, these high density regions achieve high local star formation efficiencies, enabling them to form bound clusters.
Abstract: We present a theoretical framework in which bound stellar clusters arise naturally at the high-density end of the hierarchy of the interstellar medium (ISM). Due to short free-fall times, these high-density regions achieve high local star formation efficiencies, enabling them to form bound clusters. Star-forming regions of lower density remain substructured and gas-rich, ending up unbound when the residual gas is expelled. Additionally, the tidal perturbation of star-forming regions by nearby, dense giant molecular clouds imposes a minimum density contrast required for the collapse to a bound cluster. The fraction of all star formation that occurs in bound stellar clusters (the cluster formation efficiency, hereafter CFE) follows by integration of these local clustering and survival properties over the full density spectrum of the ISM, and hence is set by galaxy-scale physics. We derive the CFE as a function of observable galaxy properties, and find that it increases with the gas surface density, from Γ ∼ 1 per cent in low-density galaxies to a peak value of Γ ∼ 70 per cent at densities of Σg ∼ 103 M⊙ pc−2. This explains the observation that the CFE increases with the star formation rate density in nearby dwarf, spiral and starburst galaxies. Indeed, comparing our model results with observed galaxies yields excellent agreement. The model is applied further by calculating the spatial variation of the CFE within single galaxies. We also consider the variation of the CFE with cosmic time and show that it increases with redshift, peaking in high-redshift, gas-rich disc galaxies. It is estimated that up to 30–35 per cent of all stars in the Universe once formed in bound stellar clusters. We discuss how our theory can be verified with Gaia and ALMA, and provide possible implementations for theoretical work and for simulations of galaxy formation and evolution.

358 citations

References
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Journal ArticleDOI
TL;DR: The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars as discussed by the authors, and the value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag).
Abstract: The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars. The distance to 10 stars exceeds 2 kpc and 34 stars lie beyond 1 kpc. Stars with color excess E(B-V) up to 0.5 mag are observed. The value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag). For stars with accurate E(B-V), the deviations from this mean are generally less than a factor of 1.5. A notable exception is the dark cloud star, rho Oph. A reduction in visual reddening efficiency for the grains that are larger than normal in the rho Oph dark cloud probably explains this result. The conversion of atomic hydrogen into molecular form in dense clouds was observed in the gas to E(B-V) correlation plots. The best estimate for the mean total gas density for clouds and the intercloud medium, as a whole, in the solar neighborhood and in the plane of the galaxy is 1.15 atoms per cu. cm; those for the atomic gas and molecular gas alone are 0.86 atoms per cu cm and 0.143 molecules per cu cm respectively. For the intercloud medium, where molecular hydrogen is a negligible fraction of the total gas, atomic gas density was found to equal 0.16 atoms per cu cm with a Gaussian scale height perpendicular to the plane of about 350 pc, as derived from high latitude stars.

2,625 citations


"Molecular gas in the Andromeda gala..." refers background in this paper

  • ...In the Milky Way, gas and dust were found to correlate in detail (Bohlin et al. 1978; Boulanger & Pérault 1988; Boulanger et al. 1996)....

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Journal ArticleDOI
TL;DR: In this article, a large-scale CO survey of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, was combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey.
Abstract: New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth ( °) sample the entire Galactic plane over a strip 4°-10° wide in latitude, and beamwidth or ° sample nearly all large local clouds at higher latitudes. Compared with the previous composite CO survey of Dame et al. (1987), the new survey has 16 times more spectra, up to 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle. Each of the component surveys was integrated individually using clipping or moment masking to produce composite spatial and longitude-velocity maps of the Galaxy that display nearly all of the statistically significant emission in each survey but little noise. The composite maps provide detailed information on individual molecular clouds, suggest relationships between clouds and regions widely separated on the sky, and clearly display the main structural features of the molecular Galaxy. In addition, since the gas, dust, and Population I objects associated with molecular clouds contribute to the Galactic emission in every major wavelength band, the precise kinematic information provided by the present survey will form the foundation for many large-scale Galactic studies. A map of molecular column density predicted from complete and unbiased far-infrared and 21 cm surveys of the Galaxy was used both to determine the completeness of the present survey and to extrapolate it to the entire sky at |b| 5°), X shows little systematic variation with latitude from a mean value of (1.8 ± 0.3) × 1020 cm-2 K-1 km-1 s. Given the large sky area and large quantity of CO data analyzed, we conclude that this is the most reliable measurement to date of the mean X value in the solar neighborhood.

2,266 citations

Journal ArticleDOI
TL;DR: A wide variety of least-squares linear regression procedures used in observational astronomy, particularly investigations of the cosmic distance scale, are presented and discussed in this article, where a formula for the intercept offset between two parallel data sets, which propagates slope errors from one regression to the other, and a generalization of the Working-Hotelling confidence bands to nonstandard least squares lines.
Abstract: A wide variety of least-squares linear regression procedures used in observational astronomy, particularly investigations of the cosmic distance scale, are presented and discussed. The classes of linear models considered are (1) unweighted regression lines, with bootstrap and jackknife resampling; (2) regression solutions when measurement error, in one or both variables, dominates the scatter; (3) methods to apply a calibration line to new data; (4) truncated regression models, which apply to flux-limited data sets; and (5) censored regression models, which apply when nondetections are present. For the calibration problem we develop two new procedures: a formula for the intercept offset between two parallel data sets, which propagates slope errors from one regression to the other; and a generalization of the Working-Hotelling confidence bands to nonstandard least-squares lines. They can provide improved error analysis for Faber-Jackson, Tully-Fisher, and similar cosmic distance scale relations.

1,059 citations

Journal ArticleDOI
TL;DR: In this paper, an automatic, objective routine for analyzing the clumpy structure in a spectral line position-position-velocity data cube is described, which works by first contouring the data at a multiple of the rms noise of the observations, then searching for peaks of emission which locate the clumps, and then following them down to lower intensities.
Abstract: We descibe an automatic, objective routine for analyzing the clumpy structure in a spectral line position-position-velocity data cube. The algorithm works by first contouring the data at a multiple of the rms noise of the observations, then searches for peaks of emission which locate the clumps, and then follows them down to lower intensities. No a proiri clump profile is assumed. By creating simulated data, we test the performance of the algorithm and show that a contour map most accurately depicts internal structure at a contouring interval equal to twice the rms noise of the map. Blending of clump emission leads to small errors in mass and size determinations and in severe cases can result in a number of clumps being misidentified as a single unit, flattening the measured clump mass spectrum. The algorithm is applied to two real data sets as an example of its use. The Rosette molecular cloud is a 'typical' star-forming cloud, but in the Maddalena molecular cloud high-mass star formation is completely absent. Comparison of the two clump lists generated by the algorithm show that on a one-to-one basis the clumps in the star-forming cloud have higher peak temperatures, higher average densities, and are more gravitationally bound than in the non-star-forming cloud. Collective properties of the clumps, such as temperature-size-line-width-mass relations appear very similar, however. Contrary to the initial results reported in a previous paper (Williams & Blitz 1993), we find that the current, more thoroughly tes ted analysis finds no significant difference in the clump mass spectrum of the two clouds.

1,007 citations


"Molecular gas in the Andromeda gala..." refers methods in this paper

  • ...In order to determine the spiral pattern in an objective way, we decomposed the CO map into 170 individual “clouds” using the CLUMPFIND analysis program (Williams et al. 1994)....

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Journal ArticleDOI
TL;DR: In this article, the authors describe the galaxy sample, present the data, and determine global CO fluxes and radial distributions for the galaxies in the FCRAO Extragalactic CO Survey.
Abstract: Emission from the CO molecule at lambda = 2.6 mm has been observed at 1412 positions in 300 galaxies using the 14 m telescope of the Five College Radio Astronomy Observatory (HPBW = 45"); these data comprise the FCRAO Extragalactic CO Survey. In this paper we describe the galaxy sample, present the data, and determine global CO fluxes and radial distributions for the galaxies in the Survey. Future papers will deal with the data analysis, both with regard to the global properties of galaxies and the radial distributions within them. CO emission was detected in 236 of the 300 Survey galaxies for an overall detection rate of 79%; among the 52 Sc galaxies in the Survey, the detection rate was as high as 96%. most of the 193 galaxies observed in multiple positions exhibit CO distributions which peak at the center. However, a small number (10-primarily Sb galaxies) exhibit CO rings at 45" resolution, and a similar number (18-primarily Sc galaxies) have CO distributions which peak on one side of the center. We derive CO isophotal diameters for 151 galaxies and find the mean ratio of CO to optical isophotal diameters to be 0.5. We also find a trend along the Hubble sequence such that the mean ratio of CO to optical isophotal diameters is smallest among the early-type spirals (SO/a, Sa, and Sab) and the mean ratio increases for Sb, Sbc, and Sc galaxies, finally decreasing among the later types. Comparison of the global fluxes we derive for the Survey galaxies with independent measurements from the literature indicates that the global fluxes we derive are accurate to ~40%.

510 citations


"Molecular gas in the Andromeda gala..." refers methods in this paper

  • ...Singledish telescopes were used to survey CO in galaxies (e.g. Nakano et al. 1987; Braine et al. 1993; Young et al. 1995) but with limited angular resolution....

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