Grand-design Spiral Galaxy M51 Research Articles

A Method to Extract Spatially Resolved Polycyclic Aromatic Hydrocarbon Emission from Spitzer Spectra: Application to M51

Abstract The mid-infrared spectrum contains rich diagnostics to probe the physical properties of galaxies, among which the pervasive emission features from polycyclic aromatic hydrocarbons (PAHs) offer promising means of estimating the star formation rate (SFR) relatively immune from dust extinction. This paper investigates the effectiveness of PAH emission as a SFR indicator on subkiloparsec scales by studying the Spitzer/IRS mapping-mode observations of the nearby grand-design spiral galaxy M51. We present a new approach of analyzing the spatial elements of the spectral data cube that simultaneously maximizes spatial resolution and spatial coverage, while yielding reliable measurements of the total, integrated 5–20 μm PAH emission. We devise a strategy of extracting robust PAH emission using spectra with only partial spectral coverage, complementing missing spectral regions with properly combined mid-infrared photometry. We find that in M51 the PAH emission correlates tightly with the extinction-corrected far-ultraviolet, near-ultraviolet, and Hα emission, from scales of ∼0.4 kpc close to the nucleus to 6 kpc out in the disk of the galaxy, indicating that PAH serves as an excellent tracer of SFR over a wide range of galactic environments. But regional differences exist. Close to the active nucleus of M51 the 6.2 μm feature is weaker, and the overall level of PAH emission is suppressed. The spiral arms and the central star-forming region of the galaxy emit stronger 7.7 and 8.6 μm PAH features than the inter-arm regions.

Stellar and gas mass distributions for understanding the nature of spiral arms

AbstractNumerical simulations of disk galaxies with steady (long-lived) and dynamic (short-lived) spiral arms suggest that offsets between stellar and gas spiral arms depend on their nature or lifetime (Baba et al.2015). Based on this theoretical study, we investigated gas-star offsets in the nearby grand-design spiral galaxy M51, and found that its two spiral arms exhibit different offset dependences against radius. One arm is consistent with a steady arm, while the other is consistent with a dynamic arm. We deduce that this difference is likely due to a tidal interaction with the companion galaxy (Egusa et al.2017). For this study, a stellar mass distribution with a high accuracy at a high spatial resolution is essential, which has come to be available by applying recent SED fitting techniques to multi-wavelength images. We are now working to extend this study to other nearby spiral galaxies.

Clues to the Formation of Spiral Structure in M51 from the Ages and Locations of Star Clusters

Abstract We determine the spatial distributions of star clusters at different ages in the grand-design spiral galaxy M51 using a new catalog based on multi-band images taken with the Hubble Space Telescope (HST). These distributions, when compared with the spiral structure defined by molecular gas, dust, young and old stars, show the following sequence in the inner arms: dense molecular gas (and dust) defines the inner edge of the spiral structure, followed by an overdensity of old stars and then young stellar clusters. The offset between gas and young clusters in the inner arms is consistent with the expectations for a density wave. Clusters as old as a few hundred Myr remain concentrated close to the spiral arms, although the distributions are broader than those for the youngest clusters, which is also consistent with predictions from density wave simulations. The outermost portion of the west arm is different from the rest of the spiral structure in that it contains primarily intermediate-age ( ) clusters; we believe that this is a “material” arm. We have identified four “feathers,” stellar structures beyond the inner arms that have a larger pitch angle than the arms. We do not find age gradients along any of the feathers, but the least coherent feathers appear to have the largest range of cluster ages.

The PdBI Arcsecond Whirlpool Survey (PAWS): The Role of Spiral Arms in Cloud and Star Formation

Abstract The process that leads to the formation of the bright star-forming sites observed along prominent spiral arms remains elusive. We present results of a multi-wavelength study of a spiral arm segment in the nearby grand-design spiral galaxy M51 that belongs to a spiral density wave and exhibits nine gas spurs. The combined observations of the (ionized, atomic, molecular, dusty) interstellar medium with star formation tracers (H ii regions, young <10 Myr stellar clusters) suggest (1) no variation in giant molecular cloud (GMC) properties between arm and gas spurs, (2) gas spurs and extinction feathers arising from the same structure with a close spatial relation between gas spurs and ongoing/recent star formation (despite higher gas surface densities in the spiral arm), (3) no trend in star formation age either along the arm or along a spur, (4) evidence for strong star formation feedback in gas spurs, (5) tentative evidence for star formation triggered by stellar feedback for one spur, and (6) GMC associations being not special entities but the result of blending of gas arm/spur cross sections in lower resolution observations. We conclude that there is no evidence for a coherent star formation onset mechanism that can be solely associated with the presence of the spiral density wave. This suggests that other (more localized) mechanisms are important to delay star formation such that it occurs in spurs. The evidence of star formation proceeding over several million years within individual spurs implies that the mechanism that leads to star formation acts or is sustained over a longer timescale.

Gas and stellar spiral arms and their offsets in the grand-design spiral galaxy M51

Theoretical studies on the response of interstellar gas to a gravitational potential disc with a quasi-stationary spiral arm pattern suggest that the gas experiences a sudden compression due to standing shock waves at spiral arms. This mechanism, called a galactic shock wave, predicts that gas spiral arms move from downstream to upstream of stellar arms with increasing radius inside a corotation radius. In order to investigate if this mechanism is at work in the grand-design spiral galaxy M51, we have measured azimuthal offsets between the peaks of stellar mass and gas mass distributions in its two spiral arms. The stellar mass distribution is created by the spatially resolved spectral energy distribution fitting to optical and near infrared images, while the gas mass distribution is obtained by high-resolution CO and HI data. For the inner region (r < 150"), we find that one arm is consistent with the galactic shock while the other is not. For the outer region, results are less certain due to the narrower range of offset values, the weakness of stellar arms, and the smaller number of successful offset measurements. The results suggest that the nature of two inner spiral arms are different, which is likely due to an interaction with the companion galaxy.

THE AGE, MASS, AND SIZE DISTRIBUTIONS OF STAR CLUSTERS IN M51

ABSTRACT We present a new catalog of 3816 compact star clusters in the grand design spiral galaxy M51 based on observations taken with the Hubble Space Telescope. The age distribution of the clusters declines starting at very young ages, and can be represented by a power law, , with . No significant changes in the shape of the age distribution at different masses is observed. The mass function of the clusters younger than can also be described by a power law, , with . We compare these distributions with the predictions from various cluster disruption models, and find that they are consistent with models where clusters disrupt approximately independent of their initial mass, but not with models where lower mass clusters are disrupted earlier than their higher mass counterparts. We find that the half-light radii of clusters more massive than and with ages between 100 and 400 are larger by a factor of ≈3–4 than their counterparts that are younger than 107 years old, suggesting that the clusters physically expand during their early life.

High-resolution, 3D radiative transfer modeling

Context: Dust reprocesses about half of the stellar radiation in galaxies. The thermal re-emission by dust of absorbed energy is considered driven merely by young stars and, consequently, often applied to trace the star formation rate in galaxies. Recent studies have argued that the old stellar population might anticipate a non-negligible fraction of the radiative dust heating. Aims: In this work, we aim to analyze the contribution of young (< 100 Myr) and old (~ 10 Gyr) stellar populations to radiative dust heating processes in the nearby grand-design spiral galaxy M51 using radiative transfer modeling. High-resolution 3D radiative transfer (RT) models are required to describe the complex morphologies of asymmetric spiral arms and clumpy star-forming regions and model the propagation of light through a dusty medium. Methods: In this paper, we present a new technique developed to model the radiative transfer effects in nearby face-on galaxies. We construct a high-resolution 3D radiative transfer model with the Monte-Carlo code SKIRT accounting for the absorption, scattering and non-local thermal equilibrium (NLTE) emission of dust in M51. The 3D distribution of stars is derived from the 2D morphology observed in the IRAC 3.6 {\mu}m, GALEX FUV, H{\alpha} and MIPS 24 {\mu}m wavebands, assuming an exponential vertical distribution with an appropriate scale height. The dust geometry is constrained through the far-ultraviolet (FUV) attenuation, which is derived from the observed total-infrared-to-far-ultraviolet luminosity ratio. The stellar luminosity, star formation rate and dust mass have been scaled to reproduce the observed stellar spectral energy distribution (SED), FUV attenuation and infrared SED. (abridged)

FEATHERING INSTABILITY OF SPIRAL ARMS. II. PARAMETER STUDY

We report the results of a parameter study of the feathering stability in the galactic spiral arms. A two-dimensional razor-thin magnetized self-gravitating gas disk with an imposed two-armed stellar spiral structure is considered. Using the formulation developed previously by Lee and Shu, a linear stability analysis of the spiral shock is performed in a localized Cartesian geometry. Results of the parameter study of the base state with a spiral shock are also presented. The single-mode feathering instability that leads to growing perturbations may explain the feathering phenomenon found in nearby spiral galaxies. The self-gravity of the gas, characterized by its average surface density, is an important parameter which 1) shifts the spiral shock further downstream; 2) increases the growth rate and decreases the characteristic spacing of the feathering structure due to the instability. On the other hand, while the magnetic field suppresses the velocity fluctuation associated with the feathers, it does not strongly affect their growth rate. Using a set of typical parameters of the grand-design spiral galaxy M51 at 2 kpc from the center, the spacing of the feathers with the maximum growth rate is found to be 530 pc, which agrees with the previous observational studies.

Depolarization of synchrotron radiation in a multilayer magneto-ionic medium

Depolarization of diffuse radio synchrotron emission is classified in terms of wavelength-independent and wavelength-dependent depolarization in the context of regular magnetic fields and of both isotropic and anisotropic turbulent magnetic fields. Previous analytical formulas for depolarization due to differential Faraday rotation are extended to include internal Faraday dispersion concomitantly, for a multilayer synchrotron emitting and Faraday rotating magneto-ionic medium. In particular, depolarization equations for a two- and three-layer system (disk-halo, halo-disk-halo) are explicitly derived. To both serve as a `user's guide' to the theoretical machinery and as an approach for disentangling line-of-sight depolarization contributions in face-on galaxies, the analytical framework is applied to data from a small region in the face-on grand-design spiral galaxy M51. The effectiveness of the multiwavelength observations in constraining the pool of physical depolarization scenarios is illustrated for a two- and three-layer model along with a Faraday screen system for an observationally motivated magnetic field configuration.

THE PHYSICAL CHARACTERISTICS OF THE GAS IN THE DISK OF CENTAURUS A USING THEHERSCHEL SPACE OBSERVATORY

We search for variations in the disk of Centaurus A of the emission from atomic fine structure lines using Herschel PACS and SPIRE spectroscopy. In particular we observe the [C II](158 $\mu$m), [N II](122 and 205 $\mu$m), [O I](63 and 145 $\mu$m) and [O III](88 $\mu$m) lines, which all play an important role in cooling the gas in photo-ionized and photodissociation regions. We determine that the ([C II]+[O I]$_{63}$)/$F_{TIR}$ line ratio, a proxy for the heating efficiency of the gas, shows no significant radial trend across the observed region, in contrast to observations of other nearby galaxies. We determine that 10 - 20% of the observed [C II] emission originates in ionized gas. Comparison between our observations and a PDR model shows that the strength of the far-ultraviolet radiation field, $G_0$, varies between $10^{1.75}$ and $10^{2.75}$ and the hydrogen nucleus density varies between $10^{2.75}$ and $10^{3.75}$ cm$^{-3}$, with no significant radial trend in either property. In the context of the emission line properties of the grand-design spiral galaxy M51 and the elliptical galaxy NGC 4125, the gas in Cen A appears more characteristic of that in typical disk galaxies rather than elliptical galaxies.

A CO J = 3–2 map of M51 with HARP-B: radial properties of the spiral structure

We present the first complete CO J=3-2 map of the nearby grand-design spiral galaxy M51 (NGC 5194), at a spatial resolution of ~600 pc, obtained with the HARP-B instrument on the James Clerk Maxwell Telescope. The map covers the entire optical galaxy disk and out to the companion NGC 5195, with CO J=3-2 emission detected over an area of ~9'x6' (~21x14 kpc). We describe the CO J=3-2 integrated intensity map and combine our results with maps of CO J=2-1, CO J=1-0 and other data from the literature to investigate the variation of the molecular gas, atomic gas and polycyclic aromatic hydrocarbon (PAH) properties of M51 as a function of distance along the spiral structure on sub-kpc scales. We find that for the CO J=3-2 and CO J=2-1 transitions there is a clear difference between the variation of arm and inter-arm emission with galactocentric radius, with the inter-arm emission relatively constant with radius and the contrast between arm and inter-arm emission decreasing with radius. For CO J=1-0 and HI the variation with radius shows a similar trend for the arm and inter-arm regions, and the arm-inter-arm contrast appears relatively constant with radius. We investigate the variation of CO line ratios (J=3-2/2-1, J=2-1/1-0 and J=3-2/1-0) as a function of distance along the spiral structure. Line ratios are consistent with the range of typical values for other nearby galaxies in the literature. The highest CO J=3-2/2-1 line ratios are found in the central ~1 kpc and in the spiral arms and the lowest line ratios in the inter-arm regions.We find no clear evidence of a trend with radius for the spiral arms but for the inter-arm regions there appears to be a trend for all CO line ratios to increase with radius. We find a strong relationship between the ratio of CO J=3-2 intensity to stellar continuum-subtracted 8mu PAH surface brightness and the CO J=3-2 intensity that appears to vary with radius.

GEOMETRIC OFFSETS ACROSS SPIRAL ARMS IN M51: NATURE OF GAS AND STAR FORMATION TRACERS

We report measurements of geometric offsets between gas spiral arms and associated star forming regions in the grand-design spiral galaxy M51. These offsets are a suggested measure of the star formation timescale after the compression of gas at spiral arm entry. A surprising discrepancy, by an order of magnitude, has been reported in recent offset measurements in nearby spiral galaxies. Measurements using CO and H-alpha emission find large and ordered offsets in M51. On the contrary, small or non-ordered offsets have been found using the HI 21cm and 24 micron emissions, possible evidence against gas flow through spiral arms, and thus against the conventional density-wave theory with a stationary spiral pattern. The goal of this paper is to understand the cause of this discrepancy. We investigate potential causes by repeating those previous measurements using equivalent data, methods, and parameters. We find offsets consistent with the previous measurements and conclude that the difference of gas tracers, i.e., HI versus CO, is the primary cause. The HI emission is contaminated significantly by the gas photo-dissociated by recently-formed stars and does not necessarily trace the compressed gas, the precursor of star formation. The HI gas and star forming regions coincide spatially and tend to show small offsets. We find mostly positive offsets with substantial scatter between CO and H-alpha, suggesting gas flow through spiral arms (i.e., density-wave) though the spiral pattern may not necessarily be stationary.

STACKING STAR CLUSTERS IN M51: SEARCHING FOR FAINT X-RAY BINARIES

The population of low-luminosity (< 10^35 erg/s) X-Ray Binaries (XRBs) has been investigated in our Galaxy and M31 but not further. To address this problem, we have used data from the Chandra X-Ray Observatory and the Hubble Space Telescope to investigate the faint population of XRBs in the grand-design spiral galaxy M51. A matching analysis found 25 star clusters coincident with 20 X-ray point sources within 1.5" (60 pc). From X-ray and optical color-color plots we determine that this population is dominated by high-mass XRBs. A stacking analysis of the X-ray data at the positions of optically-identified star clusters was completed to probe low-luminosity X-ray sources. No cluster type had a significant detection in any X-ray energy band. An average globular cluster had the largest upper limit, 9.23 x 10^34 erg/s, in the full-band (0.3 - 8 keV) while on average the complete sample of clusters had the lowest upper limit, 6.46 x 10^33 erg/s in the hard-band (2 - 8 keV). We determined average luminosities of the young and old star cluster populations and compared the results to those from the Milky Way. We conclude that deeper X-ray data is required to identify faint sources with a stacking analysis.

An Updated View of Giant Molecular Clouds, Gas Flows and Star Formation in M51 with PAWS

AbstractWe present an overview of the latest results from the PdBI Arcsecond Whirlpool Survey (PAWS, PI: E. Schinnerer), which has mapped CO(1-0) emission in the nearby grand-design spiral galaxy M51 at 40pc resolution. Our data are sensitive to GMCs above 105 M⊙, allowing the construction of the largest GMC catalog to date – containing over 1500 objects – using the CPROPS algorithm (Rosolowsky &amp; Leroy 2006). In the inner disk of M51, the properties of the CO emission show significant variation that can be linked to the dynamical environment in which the molecular gas is located. We find that dynamically distinct regions host clouds with different properties and exhibit different GMC mass spectra, as well as distinct patterns of star formation. To understand how this sensitivity to environment emerges, we consider the role of pressure on GMC stabilization (including shear and star formation feedback-driven turbulence). We suggest that, in the presence of significant external pressure, streaming motions driven by the spiral arm can act to reduce the surface pressure on clouds. The resulting stabilization impacts the global pattern of star formation and can account for the observed non-monotonic radial dependence of the gas depletion time. Our findings have implications for the observed scatter in the standard GMC relations and extragalactic star formation laws.

DYNAMICALLY DRIVEN EVOLUTION OF THE INTERSTELLAR MEDIUM IN M51

Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H_2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage.

Radial Dependence of the Pattern Speed of M51

The grand-design spiral galaxy M51 has long been a crucial target for theories of spiral structure. Studies of this iconic spiral can address the question of whether strong spiral structure is transient (e.g. interaction-driven) or long-lasting. As a clue to the origin of the structure in M51, we investigate evidence for radial variation in the spiral pattern speed using the radial Tremaine-Weinberg (TWR) method. We implement the method on CO observations tracing the ISM-dominant molecular component. Results from the method's numerical implementation--combined with regularization, which smooths intrinsically noisy solutions--indicate two distinct patterns speeds inside 4 kpc at our derived major axis PA=170 deg., both ending at corotation and both significantly higher than the conventionally adopted global value. Inspection of the rotation curve suggests that the pattern speed interior to 2 kpc lacks an ILR, consistent with the leading structure seen in HST near-IR observations. We also find tentative evidence for a lower pattern speed between 4 and 5.3 kpc measured by extending the regularized zone. As with the original TW method, uncertainty in major axis position angle (PA) is the largest source of error in the calculation; in this study, where \delta PA=+/-5 deg. a ~20% error is introduced to the parameters of the speeds at PA=170 deg. Accessory to this standard uncertainty, solutions with PA=175 deg. (also admitted by the data) exhibit only one pattern speed inside 4 kpc, and we consider this circumstance under the semblance of a radially varying PA.

A New High‐Resolution CO Map of the Inner 2.́5 of M51. I. Streaming Motions and Spiral Structure

The Owens Valley millimeter array has been used to map the CO 1-0 emission in the inner 25 of the grand-design spiral galaxy M51 at 2''-3'' resolution. These new images reveal the molecular spiral arms with unprecedented clarity; the emission in the two major arms (northeast and southwest) originates from supermassive cloud complexes, giant molecular associations (GMAs), which are for the first time resolved both along and perpendicular to the arms. The overall morphology of the CO emission is symmetric in reflection about the nucleus, with major complexes occurring opposite each other in the two major arms. On the other hand, the CO line flux in the area of the southwest arm closest to the nucleus is approximately twice as bright as that from the analogous location in the northeast arm. Streaming motions can be studied in detail and appear with great clarity along the major and minor axes of M51. The streaming velocities are very large, 60-150 km s-1. Our maps offer, for the first time, sufficient resolution to resolve the structure in the molecular streaming motions. Both the radial and tangential velocity components show steep gradients, in qualitative accordance with predictions of the density-wave models of Roberts & Stewart. Our data thus support the presence of galactic shocks in the arms of M51. In general, velocity gradients across arms are higher by a factor of 2-10 than previously found. They vary in steepness along the spiral arms, becoming particularly steep in between GMAs. The steep gradients cause conditions of strong reverse shear in several regions in the arms, and thus the notion that shear is generally reduced by streaming motions in spiral arms will have to be modified. Of the three GMAs studied on the southwest arm, only one shows reduced shear. We find an unusual structure, an expansion in the northeast molecular arm at 25'' radius (1.2 kpc) southeast of the center. This broadening occurs right after the end of the northeast arm at the inner Lindblad resonance (ILR). Multiple-peak spectra, velocity twists, and structures with apparently high velocity dispersion are associated with this feature. Bifurcations in the molecular spiral arm structure, at a radius of ≈3.3 kpc (73''), may be evidence of a secondary compression of the gas caused by, and occurring near, the 4/1 ultraharmonic resonance. Several molecular spurs protrude from the main spiral arms, in particular on the western side of the map. Inside the radius of the ILR, we detect narrow (≈5'') molecular spiral arms, possibly related to the K-band arms found in the same region. We find evidence of noncircular motions in the inner 20'' of M51. The magnitude of these deviations is 20-30 km s-1, and they are consistent with gas on elliptical orbits in a bar.

H-ALPHA FABRY-PEROT OBSERVATIONS OF THE DENSITY WAVE PATTERN IN M51

ABSTRACT The Maryland-Caltech wide-field Fabry-Perot camera has been used to image H-alpha emission in the grand design spiral galaxy M51. The velocity field of the ionized gas has been determined over most of the optical disk at an angular resolution of 2-8" and a realtive velocity accuracy of 1-2 km s-1 rms. The well-known kinematic perturbations due to density wave streaming are clearly resolved in both arm and interarm regions. Determination of the radius at which the gas and spiral pattern corotate using reversals in the streaming direction, frequently proposed as the preferred method of locating the corotation resonance, is hampered by the observed complexity of the density wave response. Nonetheless, new evidence is presented that supports previous determinations using other methods that corotation of the inner spiral is located near a glactocentric radius of 6 kpc, and that the outer optical arms cannot be part of the same spiral pattern as the inner arms.

M51 - Molecular spiral arms, giant molecular associations, and superclouds

An aperture synthesis image of the prototypical grand-design spiral galaxy M51 in the CO 1 to 0 line made with the Owens Valley Millimeter Interferometer is presented. The image is a mosaic of 30 1-arcmin fields. Narrow (10-20 arcsec) spiral arms are seen which are coincident with the optical dust lanes and nonthermal radio emission, but are offset from the ridges of H-alpha and thermal radio emission. There are many dense concentrations of emission, termed giant molecular associations (GMAs), both along and between the arms. The GMAs have masses of 10 million to 60 million solar masses, and most of the on-arm ones appear to be bound. They consist of several spectral components, termed molecular superclouds, which also appear to be bound. The interarm GMAs are unbound and are probably due to a secondary compression of the density wave. Streaming motions in the GMAs are consistent with density wave theory. 20 refs.

Spiral structure of M51 - Displacement of the H I from the nonthermal radio arms

view Abstract Citations (67) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spiral Structure of M51: Displacement of the H i from the Nonthermal Radio Arms Tilanus, R. P. J. ; Allen, R. J. Abstract The neutral atomic hydrogen (H I) in the grand design spiral galaxy M51 has been observed with the Westerbork Synthesis Radio Telescope at a resolution of 12" x 18" and compared with other tracers of spiral structure: the dust lanes, the H II regions, and especially the nonthermal radio continuum emission. Over major sections of the inner spiral arms, the H I ridge is parallel to the ridge of nonthermal radio continuum but not coincident with it. The H I ridge is displaced to larger radii by about 200 pc in a segment of the northern arm of 450 pc over a large segment of the southern arm. We have earlier observed this phenomenon in portions of the spiral arms of M83. The present results confirm that the displacement of the H I arms from the region of maximum compression of the interstellar gas in not an isolated peculiarity of one particular galaxy. We present a simple picture which describes the observations in the framework of density-wave streaming of the interstellar gas. This picture was proposed earlier to account for the same phenomenon which we observed in M83. Presuming that the nonthermal continuum ridge traces the locus of maximum density of the interstellar gas, the observations are consistent with the view that most of the H I in the inner spiral arms of M51 is a product of the dissociation of predominantly molecular gas by the star-formation process. Publication: The Astrophysical Journal Pub Date: April 1989 DOI: 10.1086/185419 Bibcode: 1989ApJ...339L..57T Keywords: Extragalactic Radio Sources; Galactic Structure; H I Regions; Interstellar Matter; Spiral Galaxies; Continuous Radiation; H Ii Regions; Radio Emission; Radio Telescopes; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M51; GALAXIES: INTERSTELLAR MATTER; GALAXIES: STRUCTURE; RADIO SOURCES: GALAXIES full text sources ADS | data products SIMBAD (2) NED (2)