Spiral Arm Of M51 Research Articles

Spatially-resolved stellar population properties of the M51–NGC 5195 system from multi-wavelength photometric data

Using multi-band photometric images of M51 and its companion NGC 5195 from ultraviolet to optical and infrared, we investigate spatially resolved stellar population properties of this interacting system with stellar population synthesis models. The observed infrared excess (IRX) is used to constrain dust extinction. Stellar mass is also inferred from the model fitting. By fitting observed spectral energy distributions (SEDs) with synthetical ones, we derive two-dimensional distributions of stellar age, metallicity, dust extinction and stellar mass. In M51, two grand-designed spiral arms extending from the bulge show young age, rich metallicity and abundant dust. The inter-arm regions are filled with older, metal-poorer and less dusty stellar populations. Except for the spiral arm extending from M51 into NGC 5195, the stellar population properties of NGC 5195 are quite featureless. NGC 5195 is much older than M51, and its core is very dusty with AV up to 1.67 mag and dense in stellar mass surface density. The close encounters might drive the dust in the spiral arm of M51 into the center of NGC 5195.

Molecular Gas Density Measured with H2CO and CS toward a Spiral Arm of M51

Abstract Observations of various molecular lines toward a disk region of a nearby galaxy are now feasible, and they are being employed as diagnostic tools to study star formation activities there. However, the spatial resolution attainable for a nearby galaxy with currently available radio telescopes is 10–1000 pc, which is much larger than the scales of individual star-forming regions and molecular-cloud cores. Hence, it is of fundamental importance to elucidate which part of an interstellar medium such spatially unresolved observations are tracing. Here we present sensitive measurements of the H2CO ( ) line at 72 GHz toward giant molecular clouds (GMCs) in the spiral arm of M51 using the NRO 45 m and IRAM 30 m telescopes. In conjunction with the previously observed H2CO (202 − 101) and CS (2 − 1 and 3 − 2) lines, we derive the H2 density of the emitting regions to be (0.6–2.6) × 104 cm−3 and (2.9–12) × 104 cm−3 for H2CO and CS, respectively, by the non-LTE analyses, where we assume the source size of 0.8–1 kpc and the gas kinetic temperature of 10–20 K. The derived H2 density indicates that the emission of H2CO and CS is not localized to star-forming cores, but is likely distributed over an entire region of GMCs. Such widespread distributions of H2CO and CS are also supported by models assuming lognormal density distributions over the 1 kpc region. Thus, contributions from the widespread less dense components should be taken into account for interpretation of the molecular emission observed with a GMC-scale resolution. The different H2 densities derived for H2CO and CS imply their different distributions. We discuss these differences in terms of the formation processes of H2CO and CS.

A 3 mm Spectral Line Survey toward the Barred Spiral Galaxy NGC 3627

Abstract We conduct spectral line survey observations in the 3 mm band toward a spiral arm, a bar-end, and a nuclear region of the nearby barred spiral galaxy NGC 3627 with the IRAM 30 m telescope and the Nobeyama 45 m telescope. Additional observations are performed toward the spiral arm and the bar-end in the 2 mm band. We detect 8, 11, and 9 molecular species in the spiral arm, the bar-end, and the nuclear region, respectively. Star formation activities are different among the three regions, and in particular, the nucleus of NGC 3627 is known as a low-ionization nuclear emission region/Seyfert 2 type nucleus. In spite of these physical differences, the chemical composition shows impressive similarities among the three regions. This result means that the characteristic chemical composition associated with these regions is insensitive to the local physical conditions such as star formation rate, because such local effects are smeared out by extended quiescent molecular gas on scales of 1 kpc. Moreover, the observed chemical compositions are also found to be similar to those of molecular clouds in our Galaxy and the spiral arm of M51, whose elemental abundances are close to those in NGC 3627. Therefore, this study provides us with a standard template of the chemical composition of extended molecular clouds with the solar metallicity in nearby galaxies.

MOLECULAR DISTRIBUTION IN THE SPIRAL ARM OF M51

ABSTRACT Molecular line images of 13CO, C18O, CN, CS, CH3OH, and HNCO are obtained toward the spiral arm of M51 at a 7 ″ × 6 ″ ?> resolution with the Combined Array for Research in Millimeter-wave Astronomy. Distributions of the molecules averaged over a 300 pc scale are found to be almost similar to one another and to essentially trace the spiral arm. However, the principal component analysis shows a slight difference of distributions among molecular species, particularly for CH3OH and HNCO. These two species do not correlate well with the star formation rate, implying that they are not enhanced by local star formation activities but by galactic-scale phenomena such as spiral shocks. Furthermore, the distributions of HNCO and CH3OH are found to be slightly different, and their origins deserve further investigation. The present results provide us with an important clue for understanding the 300 pc scale chemical composition in the spiral arm and its relation to galactic-scale dynamics.

SHORT GMC LIFETIMES: AN OBSERVATIONAL ESTIMATE WITH THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS)

We describe and execute a novel approach to observationally estimate the lifetimes of giant molecular clouds (GMCs). We focus on the cloud population between the two main spiral arms in M51 (the inter-arm region) where cloud destruction via shear and star formation feedback dominates over formation processes. By monitoring the change in GMC number densities and properties from one side of the inter-arm to the other, we estimate the lifetime as a fraction of the inter-arm travel time. We find that GMC lifetimes in M51's inter-arm are finite and short, 20 to 30 Myr. Such short lifetimes suggest that cloud evolution is influenced by environment, in which processes can disrupt GMCs after a few free-fall times. Over most of the region under investigation shear appears to regulate the lifetime. As the shear timescale increases with galactocentric radius, we expect cloud destruction to switch primarily to star formation feedback at larger radii. We identify a transition from shear- to feedback-dominated disruption through a change in the behavior of the GMC number density. The signature suggests that shear is more efficient at completely dispersing clouds, whereas feedback transforms the population, e.g. by fragmenting high mass clouds into lower mass pieces. Compared to the characteristic timescale for molecular hydrogen in M51, our short lifetimes suggest that gas can remain molecular while clouds disperse and reassemble. We propose that galaxy dynamics regulates the cycling of molecular material from diffuse to bound (and ultimately star-forming) objects, contributing to long observed molecular depletion times in normal disk galaxies. We also speculate that, in more extreme environments such as elliptical galaxies and concentrated galaxy centers, star formation can be suppressed when the shear timescale becomes so short that some clouds can not survive to collapse and form stars.

SPECTRAL LINE SURVEY TOWARD THE SPIRAL ARM OF M51 IN THE 3 AND 2 mm BANDS

We have conducted a spectral line survey in the 3 mm and 2 mm bands toward two positions in a spiral arm of M51 (NGC 5194) with the IRAM 30 m telescope. In this survey, we have identified 13 molecular species, including CN, CCH, N2H+, HNCO, and CH3OH. Furthermore, 6 isotopologues of the major species have been detected. On the other hand, SiO, HC3N, CH3CN, and the deuterated species such as DCN and DCO+ are not detected. The deuterium fractionation ratios are evaluated to be less than 0.8 % and 1.2 % for DCN/HCN and DCO+/HCO+, respectively. By comparing the results of the two positions with different star formation activities, we have found that the observed chemical compositions do not strongly depend on star formation activities. They seem to reflect a chemical composition averaged over the 1-kpc scale region including many giant molecular clouds. Among the detected molecules CN, CCH, and CH3OH are found to be abundant. High abundances of CN, and CCH are consistent with the above picture of a wide spread distribution of molecules, because they can be produced by photodissociation. On the other hand, it seems likely that CH3OH is liberated into the gas phase by shocks associated with large scale phenomena such as cloud-cloud collisions and/or by non-thermal desorption processes such as photoevaporation due to cosmic-ray induced UV photons. The present result demonstrates a characteristic chemical composition of a giant molecular cloud complex in the spiral arm, which can be used as a standard reference for studying chemistry in AGNs and starbursts.

MULTI-TRANSITION STUDY OF M51'S MOLECULAR GAS SPIRAL ARMS

Two selected regions in the molecular gas spiral arms in M51 were mapped with the Owens Valley Radio Observatory (OVRO) mm-interferometer in the 12CO(2–1), 13CO(1–0), C18O(1–0), HCN(1–0), and HCO+(1–0) emission lines. The CO data have been combined with the 12CO(1–0) data from Aalto et al. covering the central 3.5 kpc to study the physical properties of the molecular gas. All CO data cubes were short spacing corrected using IRAM 30 m (12CO(1–0): NRO 45 m) single-dish data. A large velocity gradient analysis finds that the giant molecular clouds (GMCs) are similar to Galactic GMCs when studied at 180 pc (120 pc) resolution with an average kinetic temperature of Tkin = 20(16) K and H2 density of n(H2) = 120(240) cm−3 when assuming virialized clouds (a constant velocity gradient ). The associated conversion factor between H2 mass and CO luminosity is close to the Galactic value for most regions analyzed. Our findings suggest that the GMC population in the spiral arms of M51 is similar to those of the Milky Way and therefore the strong star formation occurring in the spiral arms has no strong impact on the molecular gas in the spiral arms. Extinction inferred from the derived H2 column density is very high (AV about 15–30 mag), about a factor of 5–10 higher than the average value derived toward H ii regions. Thus, a significant fraction of the ongoing star formation could be hidden inside the dust lanes of the spiral arms. A comparison of MIPS 24 μm and Hα data, however, suggests that this is not the case and most of the GMCs studied here are not (yet) forming stars. We also present low (45) resolution OVRO maps of the HCN(1–0) and HCO+(1–0) emission at the location of the brightest 12CO(1–0) peak.

THE GALAXY-WIDE DISTRIBUTIONS OF MEAN ELECTRON DENSITY IN THE H II REGIONS OF M51 AND NGC 4449

Using ACS-HST images to yield continuum subtracted photometric maps in H\alpha of the Sbc galaxy M51 and the dwarf irregular galaxy NGC 4449, we produced extensive (over 2000 regions for M51, over 200 regions for NGC4449) catalogues of parameters of their HII regions: their H\alpha luminosities, equivalent radii and coordinates with respect to the galaxy centers. From these data we derived, for each region, its mean luminosity weighted electron density, <n_e>, determined from the H\alpha luminosity and the radius, R, of the region. Plotting these densities against the radii of the regions we find excellent fits for <n_e> varying as R^{-1/2}. This relatively simple relation has not, as far as we know, been predicted from models of HII region structure, and should be useful in constraining future models. Plotting the densities against the galactocentric radii, r, of the regions we find good exponential fits, with scale lengths of close to 10 kpc for both galaxies. These values are comparable to the scale lengths of the HI column densities for both galaxies, although their optical structures, related to their stellar components are very different. This result indicates that to a first approximation the HII regions can be considered in pressure equilibrium with their surroundings. We also plot the electron density of the HII regions across the spiral arms of M51, showing an envelope which peaks along the ridge lines of the arms.

Star Formation and Asymmetry in the Spiral Arms of M51: Variable Star Formation Caused by More than One Spiral Density Wave

In the inner 3 kpc of M51, we find that logarithmic spirals provide good fits to the peak intensities in molecular gas observed by the Berkeley-Illinois-Maryland Association array in the CO (J = 1–0) emission line along the spiral arms. However, we measure significant asymmetries between the location and density of the arms observed on one side of the galaxy compared with those on the opposite side. Between a radius of 1 and 2.2 kpc, the gas distribution traced in CO is lopsided with densities twice as large in one arm than the opposite one. In the same region, the spiral arms are offset by 20° from the position of the arm on the opposite side of the galaxy after a rotation of 180°. We use the ratio of CO emission to the emission in the Paα hydrogen recombination line to estimate the efficiency of star formation along each arm. Except for a narrow region at about 2 kpc in which star formation is surprisingly inefficient, the gas depletion time is approximately 200 million years despite large variations in the molecular gas density in the arms. We account for the deviations from two-armed bisymmetric structure with a simple model that includes an additional three-armed spiral density wave. This model accounts for the angular offset between the arm on one side compared with its opposite, the lopsided distribution in molecular gas, and interarm star formation to the northeast of the galaxy nucleus. Since the star formation efficiency is unaffected by the variations in gas density, and the variations in gas density can be accounted for by the presence of an additional three-armed density wave, we suggest that the star formation rate is variable and is highest where and when the maxima of the two spiral density waves coincide or constructively add. The inner region of M51 provides good evidence for the presence of more than one spiral density wave and a resulting variable rate of star formation.

A completely sampled aperture synthesis map of the CO emission in M51

view Abstract Citations (55) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Completely Sampled Aperture Synthesis Map of the CO Emission in M51 Adler, David S. ; Lo, K. Y. ; Wright, Melvyn C. H. ; Rydbeck, Gustav ; Plante, Raymond L. ; Allen, Ronald J. Abstract Interferometric observations from the Berkeley-Illinois-Maryland Association millimeter interferometer are combined with single-dish observations from the Onsala Space Observatory to create high-resolution fully sampled maps of CO (J = 1-0) emission from two 2' fields in the spiral galaxy M51. In previous interferometer CO maps of M51, >= 65% of the CO flux density in each field is missing but assumed to be uniformly distributed through the field of view. In contrast, our maps recover all of the flux density and show directly that 75% of the CO flux density from the inner few kiloparsecs of M51 come from the spiral arms. The CO arms are wider (<~1.3 kpc) and more extended than the CO arms presented in previous interferometer maps. The CO arm is clumpy along the arm with a linear scale ~1 kpc. The average arm-to-interarm intensity contrast over the two fields is >=4.6:1 peak arm-to-interarm CO integrated intensity contrasts of >= 20:1 are seen in the inner arms. Some large "clumps" of interarm CO emission are seen. Streaming motions of up to 70 km s^-1^ (in the plane of the galaxy) are seen in the arms; these motions are consistent with those expected from the spiral density wave theory. Both the intensity contrast and streaming motions are consistent with numerical simulations which show that orbit crowding is the dominant physical mechanism for gathering the molecular clouds into the arms. Dissipative cloud-cloud collisions and gaseous self-gravity must be included in the numerical models to account for the observed clumpiness along the spiral arms. Adopting the Galactic conversion ratio of H_2_ to CO integrated intensity, we find that the inferred gas mass of the molecular cloud complexes in the spiral arms of M51 is systematically larger than the virial mass. An H_2_-to-CO conversion ratio in the inner spiral arms of M51 2.5 times smaller than the Galactic value will correct the discrepancy. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171450 Bibcode: 1992ApJ...392..497A Keywords: Carbon Monoxide; Emission Spectra; Interstellar Matter; Spiral Galaxies; Astronomical Models; H Ii Regions; Interferometry; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M51; GALAXIES: INTERSTELLAR MATTER; TECHNIQUES: INTERFEROMETRIC full text sources ADS | data products SIMBAD (2) NED (1)

Star formation efficiency patterns in the spiral arms of M51

We have studied the distribution of ionized, molecular, and atomic hydrogen in the disk and especially along the spiral arms of the Whirlpool galaxy, M51, by combining recently published maps of the H-alpha, H-2, and H I surface densities, and we report a twofold result. First, we note the existence of a symmetric pattern of peaks and dips in the arm/interarm ratio of the massive star formation efficiency [measured using L(H-alpha)/sigma(H-2 + H I), where sigma is the surface density] along the two main arms, similar to the patterns previously found in NGC 3992 and NGC 628. The pattern is due to enhanced on-arm star formation at specific radii in the disk. These radii correspond to antiresonances of a density wave system, as identified by Elmegreen et al., suggesting that the resonance pattern locally affects the process of star formation. Second, we find that the high peaks in the H-alpha distribution along the arms are not reproduced in H-2, which is by far the most abundant form of neutral hydrogen in the inner disk. The peaks do seem to show up, although less strongly in the H I surface density along the arms.

Spiral arm amplitude variations and pattern speeds in the grand design galaxies M51, M81, and M100

view Abstract Citations (157) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spiral-Arm Amplitude Variations and Pattern Speeds in the Grand Design Galaxies M51, M81, and M100 Elmegreen, Bruce G. ; Elmegreen, Debra Meloy ; Seiden, Philip E. Abstract The amplitudes of the spiral arms in M51, M81, and M100 have been found to oscillate smoothly around their mean radial variations with a characteristic wavelength of ~5 kpc and a relative amplitude of 100%. The phases of the spirals and the relative amplitudes of the m = 4 and m = 2 Fourier components oscillate slightly too. The absence of corresponding color variations and the symmetry of the amplitude peaks with respect to the galaxy centers suggest that the oscillations are an intrinsic part of the stellar spiral density wave. ThIs observation confirms predictions of both the modal and stellar dynamical theories of spiral structure. The radial positions of the prominent gaps in the spiral arm amplitudes are compared to the expected positions of various wave resonances. Excellent fits are obtained for M81 and M100 if the gaps are at the 4:1 resonances; then the spirals end at the outer Lindblad resonances, the inner Lindblad resonances are well shielded, and the primary star formation ridges and dust lanes are between the 4:1 and corotation resonances. Prominent interarm features and spurs also occur at the 4:1 and corotation resonances in these galaxies. The situation is similar but more complicated in M51, which requires two spiral systems to cover the disk. The best fit is for an inner spiral mode that has an outer Lindblad resonance at the position of a prominent arm intensity gap, and an outer material spiral that corotates with the companion. Then the average position of the inner Lindblad resonances of the outer material arms occurs at the same radius as the corotation resonances of the inner mode. This conjunction of resonances may explain how the companion to M51 triggered a strong spiral throughout most of the disk. Publication: The Astrophysical Journal Pub Date: August 1989 DOI: 10.1086/167733 Bibcode: 1989ApJ...343..602E Keywords: Andromeda Galaxy; Galactic Structure; Spiral Galaxies; Interstellar Gas; Orbital Resonances (Celestial Mechanics); Star Formation; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M51; GALAXIES: INDIVIDUAL MESSIER NUMBER: M81; GALAXIES: INDIVIDUAL MESSIER NUMBER: M100; GALAXIES: INTERNAL MOTIONS; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (4) NED (4)

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)

Spiral structure of M51 - Thermal and nonthermal emission

New observations of the distribution of thermal and nonthermal radio-continuum emission in M51 are presented and compared with two optical tracers of spiral structure: the H-alpha emission from giant H II complexes and the dust lanes. The presence of an underlying smooth nonthermal base-disk component of the radio-continuum emission in M51 is confirmed; this component has a morphology similar to the old disk stars in the galaxy. The thermal radio emission in M51 correlates well with the optical H-alpha emission from the giant H II complexes. There is no evidence that similar complexes are present but hidden in the dust. It is confirmed that the ridges in the nonthermal radio emission correspond closely with the dust lanes along major portions of the spiral arms in M51. A number of features not previously recognized are described. Near the dust lane there is evidence that the cool-cloud component appears to separate kinematically from the component in which the nonthermal radio emission is embedded. 24 references.

The molecular content of interacting and isolated galaxies The effect of environment on the efficiency of star formation

Molecular gas observations of merging/interacting and isolated galaxies are presented in order to study the relationship between environment and the efficiency of star formation. The two galaxy samples differ primarily in their IR properties and are quite similar in their molecular gas contents. The ratios of IR luminosity to H2 mass have a mean value of 78 and 12 solar luminosity/solar mass for interacting and isolated galaxies, respectively. The highest star formation efficiencies (SFEs) appear to occur in the merging and interacting pairs. The SFE in merging/interacting galaxies is greater than that found in the spiral arms of M51 and may be roughly proportional to the rate of cloud-cloud collisions in the interacting systems.

Isophotometry of the nuclei of M31 and M51

view Abstract Citations (8) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Isophotometry of the nuclei of M31 and M51. Feibelman, W. A. Abstract Isophotic contour maps derived from blue plates confirm the change in position angle as a function of distance from the center of M31 in the red region reported by Peterson, Ford, and Rubin (1977). The techniques used for M31 were then applied to a study of M51, for which similar results were obtained for the nuclear region. The spiral arms of M51 were traced for an additional half turn near the center of the galaxy. Publication: The Astronomical Journal Pub Date: April 1979 DOI: 10.1086/112441 Bibcode: 1979AJ.....84..497F Keywords: Andromeda Galaxy; Astronomical Photometry; Galactic Nuclei; Spiral Galaxies; Astronomical Maps; Astronomy; Galactic Nuclei:Contour Maps; Galactic Nuclei:Photometry full text sources ADS | data products NED (2)