The 60 micron to 20 centimeter infrared-to-radio ratio within spiral galaxies

Bicay, M. D.; Hélou, G.

doi:10.1086/169243

Cited by 70 publications

(85 citation statements)

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“…An exponential kernel was chosen because it was found to work as well as, or better than, Gaussian kernels by M06a. This is consistent with ''leaky box'' type models where CR electron escape is expected to occur on timescales less than or comparable to the diffusion timescales ( Bicay & Helou 1990). The kernel takes the form (r) ¼ e Àr = r 0 , where r 0 is its e-folding length l modified by geometric factors related to the position angle and inclination i of a galaxy such that…”

Section: Phenomenological Image-smearing Modelsupporting

confidence: 87%

“…Coupling the shared origin of a galaxy's FIR and radio emission with the fact that the mean free path of dust-heating photons ($100 pc) is significantly shorter than the expected diffusion length of CR electrons ($1Y2 kpc) led Bicay & Helou (1990) to conjecture that the radio image of a galaxy should resemble a smoothed version of its infrared image. Consequently, it appears that the close spatial correlation between the FIR and radio continuum emission within galaxies can be used to characterize the propagation history of CR electrons.…”

Section: Introductionmentioning

confidence: 99%

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Connecting Far‐Infrared and Radio Morphologies of Disk Galaxies: Cosmic‐Ray Electron Diffusion After Star Formation Episodes

Murphy

Hélou

Kenney

et al. 2008

ApJ

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111

182

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We present results on the ISM properties of 29 galaxies based on a comparison of Spitzer far-infrared and Westerbork Synthesis Radio Telescope radio continuum imagery. Of these 29 galaxies, 18 are close enough to resolve at P1 kpc scales at 70 m and 22 cm. We extend the approach of our earlier work of smoothing infrared images to approximate cosmic-ray (CR) electron spreading and thus largely reproduce the appearance of radio images. Using a wavelet analysis, we decompose each 70 m image into one component containing the star-forming structures and a second one for the diffuse disk. The components are smoothed separately, and their combination compared to a free-free corrected 22 cm radio image; the scale lengths are then varied to best match the radio and smoothed infrared images. We find that late-type spirals having high amounts of ongoing star formation benefit most from the two-component method. We also find that the disk component dominates for galaxies having low star formation activity, whereas the structure component dominates at high star formation activity. We propose that this result arises from an age effect rather than from differences in CR electron diffusion due to varying ISM parameters. The bulk of the CR electron population in actively star-forming galaxies is significantly younger than that in less active galaxies due to recent episodes of enhanced star formation; these galaxies are observed within $10 8 yr since the onset of the most recent star formation episode. The sample irregulars have anomalously low best-fit scale lengths for their surface brightnesses compared to the rest of the sample spirals, which we attribute to enhanced CR electron escape.

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Section: Phenomenological Image-smearing Modelsupporting

confidence: 87%

Section: Introductionmentioning

confidence: 99%

Connecting Far‐Infrared and Radio Morphologies of Disk Galaxies: Cosmic‐Ray Electron Diffusion After Star Formation Episodes

Murphy

Hélou

Kenney

et al. 2008

ApJ

Self Cite

111

182

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“…However, the 21 cm continuum contours clearly show a better correlation to the molecular gas. This correlation underscores the importance of gas density and associated magnetic fields in the extraction of synchrotron emission from cosmic rays that diffuse from their source with a characteristic $1 kpc scale length (Bicay, Helou, & Condon 1989;Bicay & Helou 1990;Helou & Bicay 1993). However, the separation between the major axis of the CO bar and the ridge of H ii regions is less than the CO beamwidth, and the lack of a CO/21 cm offset may be due to the 1 0 resolution data used in the comparison.…”

Section: Co Gas and Star Formation Tracers: Morphology Comparisonsmentioning

confidence: 90%

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

Crosthwaite¹,

Turner²,

Buchholz³

et al. 2002

The Astronomical Journal

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We present '' on-the-fly '' maps of CO (1-0) and CO (2-1) in the barred spiral galaxy M83. The entire optical disk is filled with CO emission in both lines, including interarm regions. For a '' standard '' conversion factor the total molecular mass is 2:5 Â 10 9 M , 4% of the total dynamical mass. Molecular gas constitutes 80% of total gas mass within the inner 10 0 and 30% overall. CO (1-0) emission and 21 cm continuum emission are extremely well correlated spatially in regions where H 2 dominates the gas surface density. The spatial correlation between CO (1-0) and 100 lm emission is not as good. While the kinematics of the outer H i disk is severely nonaxisymmetrically perturbed, the kinematics of the CO disk and inner H i disk is well behaved and circular. The molecular disk appears to have a distinct hard edge at 5 0 . The hard edge is accompanied by a steep decline in the total gas surface density at that radius, although the H i does continue outward at a lower level. This edge is coincident with the onset of strong warping of the outer disk. The mean ratio of CO (2-1)/ CO (1-0) emission is $1, high by Galactic standards. Peaks in this ratio in interarm regions suggest that the CO emission is optically thin there, among other possibilities. Thus, M83 is unusual in that it has a relatively axisymmetric, almost entirely molecular, inner 10 0 gas disk surrounded by a warped, entirely atomic, outer gas disk. A sharp discontinuity separates the inner and outer gas disks. This kind of gas distribution is not seen in other late-type, gas-rich spirals.

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“…Bicay & Helou (1990) smeared IRAS images of spiral galaxies by using parametrized kernels to match the Article published by EDP Sciences et al (2003). Inclination is defined assuming i = 0…”

Section: Introductionmentioning

confidence: 99%

Radio spectral index images of the spiral galaxies NGC 0628, NGC 3627, and NGC 7331

Paladino

Murgia

Orrù

2009

A&A

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Aims. To understand the cosmic-ray propagation mechanism in galaxies and its correlation with the sites of star formation, we compare the spatially resolved radio spectral index of three spiral galaxies with their IR distribution. Methods. We present new low-frequency radio continuum observations of the galaxies NGC 0628, NGC 3627, and NGC 7331, taken at 327 MHz with the Very Large Array. Results. We complemented our data set with sensitive archival observations at 1.4 GHz and studied the variations in the radio spectral index within the disks of these spiral galaxies. We also compared the spectral index distribution and the IR distribution, using 70 μm Spitzer observations. Conclusions. We find that in these galaxies the nonthermal spectral index is anticorrelated with the radio brightness. Bright regions, like the bar in NGC 3627 or the circumnuclear region in NGC 7331, are characterized by a flatter spectrum with respect to the underlying disk. Therefore, a systematic steepening of the spectral index with the increasing distance from the center of these galaxies is observed. Furthermore, by comparing the radio images with the 70 μm images of the Spitzer satellite, we find that a similar anticorrelation exists between the radio spectral index and the infrared brightness, as expected from the local correlation between the radio continuum and the infrared emission. Our results support the idea that the electron diffusion must be efficient in regions of intense star formation. The observed anticorrelation between radio brightness and spectral index may imply that the cosmic ray density and the magnetic field strength are significantly higher in these regions than in their surroundings.

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The 60 micron to 20 centimeter infrared-to-radio ratio within spiral galaxies

Cited by 70 publications

References 0 publications

Connecting Far‐Infrared and Radio Morphologies of Disk Galaxies: Cosmic‐Ray Electron Diffusion After Star Formation Episodes

Connecting Far‐Infrared and Radio Morphologies of Disk Galaxies: Cosmic‐Ray Electron Diffusion After Star Formation Episodes

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

Radio spectral index images of the spiral galaxies NGC 0628, NGC 3627, and NGC 7331

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