Molecular Spiral Arms in M31

Loinard, Laurent; Dame, T. M.; Koper, E.J.R.; Lequeux, J.; Thaddeus, P.; Young, J. S.

doi:10.1086/310268

Cited by 43 publications

(63 citation statements)

References 9 publications

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“…Each survey was smoothed to a resolution of km s~1 (or 0¡ .3 ] 3 km s~1 for the surveys with sampling), and 0¡ .6 ] 3 0 ¡ .25 integration of the original survey was then taken only over channels for which the intensity in the smoothed survey exceeded 3 times its rms noise (typically D0.05 K). This "" masked moment ÏÏ method was originally developed to analyze 21 cm surveys of galaxies with limited signal-tonoise, and has recently been applied to extragalactic and Galactic CO surveys (e.g., Adler et al 1992 ;Digel et al 1996 ;Loinard et al 1999).…”

Section: Synthesis Of the Datamentioning

confidence: 99%

“…May, D. C. Murphy, M. Sahu, H. Alvarez, & L. Bronfman, 2001, in preparation ;(17) Grabelsky et al 1987 ;(18) Boulanger et al 1998 ;(19) Nyman, Bronfman, & Thaddeus 1989 ;(20) Bronfman et al 1989 ;(21) de Geus, Bronfman, & Thaddeus 1990. standing of how stars form. Since GMCs preferentially form in the arms of spiral galaxies (e.g., Loinard et al 1999 ;Aalto et al 1999), and it is possible to resolve the kinematic distance ambiguity for many of those in the inner Milky Way by appeal to the associated Population I, CO surveys have helped to reÐne our knowledge of the spiral structure of our system (e.g., Dame et al 1986 ;Grabelsky et al 1988). By virtue of the precise kinematic information they provide, CO surveys have also been of great value in the interpretation of Galactic continuum surveys from satellite observatories such as GRO, ROSAT , and COBE.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

The Milky Way in Molecular Clouds: A New Complete CO Survey

Dame

Hartmann

Thaddeus

2001

ApJ

Self Cite

2,143

178

2,713

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New large-scale CO surveys of the Ðrst 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 (1 8 ¡) strip 4¡È10¡ wide in latitude, and beamwidth or sample nearly all large local clouds at higher lati-1 4 ¡ tudes. 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 signiÐcant 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 o b o \ 32¡. The close agreement of the observed and predicted maps implies that only D2% of the total CO emission at o b o \ 32¡ lies outside our current sampling, mainly in the regions of Chamaeleon and the Gum Nebula. Taking into account this small amount of unobserved emission, the mean molecular column density decreases from D3 ] 1020 cm~2 at o b o \ 5¡ to D0.1 ] 1020 cm~2 at o b o \ 30¡ ; this drop is D6 times steeper than would be expected from a planeparallel layer, but is consistent with recent measurements of the mean molecular column density at higher latitudes.The ratio of the predicted molecular column density map to the observed CO intensity map provides a calibration of the mass conversion factor Out of the Galactic plane, 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.

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Section: Synthesis Of the Datamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The Milky Way in Molecular Clouds: A New Complete CO Survey

Dame

Hartmann

Thaddeus

2001

ApJ

Self Cite

2,143

178

2,713

View full text Add to dashboard Cite

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“…Molecular spiral arms were barely resolved in these surveys even in the nearest galaxies (e.g. Koper et al 1991;Garcia-Burillo et al 1993;Loinard et al 1996;Heyer et al 2004). Only in the Magellanic Clouds the single-dish surveys resolved giant molecular clouds ).…”

Section: Introductionmentioning

confidence: 94%

“…More recently, a survey of the southwestern half, made at an angular resolution of 1 ′ with the FCRAO 14-m telescope, was published by Loinard et al (1996Loinard et al ( , 1999. The latter authors (1999, their Table 2) give a nearly complete overview of previous CO observations of M 31.…”

Section: Introductionmentioning

confidence: 99%

Molecular Gas in the Andromeda Galaxy

Guèlin

Müller

Nieten

et al. 1997

Springer Proceedings in Physics

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Abstract. We present a new 12 CO(J=1-0)-line survey of the Andromeda galaxy, M 31, with the highest resolution to date (23 ′′ , or 85 pc along the major axis), observed On-the-Fly with the IRAM 30-m telescope. We mapped an area of about 2 • × 0.• 5 which was tightly sampled on a grid of 9′′ with a velocity resolution of 2.6 km s −1 . The r.m.s. noise in the velocity-integrated map is around 0.35 K km s −1 on the T mb -scale. Emission from the 12 CO(1-0) line is detected from galactocentric radius R = 3 kpc to R = 16 kpc, but peaks in intensity at R ∼ 10 kpc. Some clouds are visible beyond R = 16 kpc, the farthest of them at R = 19.4 kpc. The molecular gas traced by the (1-0) line is concentrated in narrow arm-like filaments, which often coincide with the dark dust lanes visible at optical wavelengths. The H  arms are broader and smoother than the molecular arms. Between R = 4 kpc and R = 12 kpc the brightest CO filaments and the darkest dust lanes define a two-armed spiral pattern that is well described by two logarithmic spirals with a constant pitch angle of 7• -8• . Except for some bridge-like structures between the arms, the interarm regions and the central bulge are free of emission at our sensitivity. The arm-interarm brightness ratio averaged over a length of 15 kpc along the western arms reaches about 20 compared to 4 for H  at an angular resolution of 45 ′′ . In several selected regions we also observed the 12 CO(2-1)-line on a finer grid. Towards the bright CO emission in our survey we find normal ratios of the (2-1)-to-(1-0) line intensities which are consistent with optically thick lines and thermal excitation of CO. We compare the (velocity-integrated) intensity distribution of CO with those of H , FIR at 175 µm and radio continuum, and interpret the CO data in terms of molecular gas column densities. For a constant conversion factor X CO , the molecular fraction of the neutral gas is enhanced in the spiral arms and decreases radially from 0.6 on the inner arms to 0.3 on the arms at R ≃ 10 kpc. We also compare the distributions of H , H 2 and total gas with that of the cold (16 K) dust traced at λ175 µm. The ratios N(H i)/I 175 and (N(H i) + 2N(H 2 ))/I 175 increase by a factor of ∼ 20 between the centre and R ≃ 14 kpc, whereas the ratio 2N(H 2 )/I 175 only increases by a factor of 4. For a constant value of X CO , this means that either the atomic and total gas-to-dust ratios increase by a factor of ∼ 20 or that the dust becomes colder towards larger radii. A strong variation of X CO with radius seems unlikely. The observed gradients affect the cross-correlations between gas and dust. In the radial range R = 8-14 kpc total gas and cold dust are well correlated; molecular gas is better correlated with cold dust than atomic gas. At smaller radii no significant correlations between gas and dust are found. The mass of the molecular gas in M 31 within a radius of 18 kpc is M(H 2 ) = 3.6 × 10 8 M ⊙ at the adopted distance of 780 kpc. This is 12% of the total neutral gas mass within this radius and 7%...

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“…Previous masking methods used masks created from spatially smoothed versions of the original CO data cubes to filter out regions dominated by noise (Adler et al 1992;Digel et al 1996;Loinard et al 1999). We use the 21 cm atomic hydrogen data at 17 ×17 ×1.27 km s −1 resolution presented in Sect.…”

Section: Introductionmentioning

confidence: 99%

Molecular and atomic gas in the Local Group galaxy M 33

Gratier

Braine

Rodríguez-Fernández

et al. 2010

A&A

145

154

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We present high-resolution large-scale observations of the molecular and atomic gas in the Local Group galaxy M 33. The observations were carried out using the HEterodyne Receiver Array (HERA) at the 30 m IRAM telescope in the CO(2-1) line, achieving a resolution of 12 × 2.6 km s −1 , enabling individual giant molecular clouds (GMCs) to be resolved. The observed region is 650 square arcminutes mainly along the major axis and out to a radius of 8.5 kpc, and covers entirely the 2 × 40 radial strip observed with the HIFI and PACS Spectrometers as part of the HERM33ES Herschel key program. The achieved sensitivity in main-beam temperature is 20-50 mK at 2.6 km s −1 velocity resolution. The CO(2-1) luminosity of the observed region is 1.7 ± 0.1 × 10 7 K km s −1 pc 2 and is estimated to be 2.8 ± 0.3 × 10 7 K km s −1 pc 2 for the entire galaxy, corresponding to H 2 masses of 1.9 × 10 8 M and 3.3 × 10 8 M respectively (including He), calculated with N(H 2 )/I CO(1−0) twice the Galactic value due to the half-solar metallicity of M 33. The H i 21 cm VLA archive observations were reduced, and the mosaic was imaged and cleaned using the multi-scale task in the CASA software package, yielding a series of datacubes with resolutions ranging from 5 to 25 . The H i mass within a radius of 8.5 kpc is estimated to be 1.4 × 10 9 M . The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9 ± 0.1 kpc whereas the atomic gas surface density is constant at Σ H i = 6± 2 M pc −2 deprojected to face-on. For an N(H 2 )/I CO(1−0) conversion factor twice that of the Milky Way, the central kiloparsec H 2 surface density is Σ H 2 = 8.5 ± 0.2 M pc −2 . The star formation rate per unit molecular gas (SF efficiency, the rate of transformation of molecular gas into stars), as traced by the ratio of CO to H α and FIR brightness, is constant with radius. The SFE, with a N(H 2 )/I CO(1−0) factor twice galactic, appears 2-4 times greater than for large spiral galaxies. A morphological comparison of molecular and atomic gas with tracers of star formation is presented showing good agreement between these maps both in terms of peaks and holes. A few exceptions are noted. Several spectra, including those of a molecular cloud situated more than 8 kpc from the galaxy center, are presented.

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Molecular Spiral Arms in M31

Cited by 43 publications

References 9 publications

The Milky Way in Molecular Clouds: A New Complete CO Survey

The Milky Way in Molecular Clouds: A New Complete CO Survey

Molecular Gas in the Andromeda Galaxy

Molecular and atomic gas in the Local Group galaxy M 33

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