Gas In Spiral Galaxies Research Articles

Molecular clouds as hubs in spiral galaxies: gas inflow and evolutionary sequence

ABSTRACT We decomposed the molecular gas in the spiral galaxy NGC 628 (M74) into multiscale hub-filament structures using the CO (2$-$1) line by the dendrogram algorithm. All leaf structures as potential hubs were classified into three categories, i.e. leaf-HFs-A, leaf-HFs-B and leaf-HFs-C. Leaf-HFs-A exhibit the best hub-filament morphology, which also have the highest density contrast, the largest mass and the lowest virial ratio. We employed the filfinder algorithm to identify and characterize filaments within 185 leaf-HFs-A structures, and fitted the velocity gradients around the intensity peaks. Measurements of velocity gradients provide evidence for gas inflow within these structures, which can serve as a kinematic evidence that these structures are hub-filament structures. The numbers of the associated 21 μm and H α structures and the peak intensities of 7.7 μm, 21 μm, and H α emissions decrease from leaf-HFs-A to leaf-HFs-C. The spatial separations between the intensity peaks of CO and 21 μm structures of leaf-HFs-A are larger than those of leaf-HFs-C. These evidence indicate that leaf-HFs-A are more evolved than leaf-HFs-C. There may be an evolutionary sequence from leaf-HFs-C to leaf-HFs-A. Currently, leaf-HFs-C lack a distinct gravitational collapse process that would result in a significant density contrast. The density contrast can effectively measure the extent of the gravitational collapse and the depth of the gravitational potential of the structure which, in turn, shapes the hub-filament morphology. Combined with the kinematic analysis presented in previous studies, a picture emerges that molecular gas in spiral galaxies is organized into network structures through the gravitational coupling of multiscale hub-filament structures. Molecular clouds, acting as knots within these networks, serve as hubs, which are local gravitational centres and the main sites of star formation.

Asymmetries in random motions of neutral hydrogen gas in spiral galaxies

Context. The velocity dispersion ellipsoid of gas in galactic discs is usually assumed to be isotropic. Under this approximation, no projection effect occurs in the random motions of gas, as traced by the line-of-sight velocity dispersion. However, it has been recently shown that random motions of the neutral hydrogen gas of the Triangulum galaxy (M 33) exhibit a bisymmetric perturbation which is aligned with the minor axis of the galaxy, suggesting a projection effect. Aims. To investigate if perturbations in the velocity dispersion of nearby discs are comparable to those of M 33, the sample is extended to 32 galaxies from The H I Nearby Galaxy Survey (THINGS) and the Westerbork H I Survey of Spiral and Irregular Galaxies (WHISP). Methods. We studied velocity asymmetries in the disc planes by performing Fourier transforms of high-resolution H I velocity dispersion maps corrected for beam-smearing effects, and we measured the amplitudes and phase angles of the Fourier harmonics. Results. In all velocity dispersion maps, we find strong perturbations of first, second, and fourth orders. The strongest asymmetry is the bisymmetry, which is predominantly associated with the presence of spiral arms. The first order asymmetry is generally orientated close to the disc major axis, and the second and fourth order asymmetries are preferentially orientated along intermediate directions between the major and minor axes of the discs. These results are evidence that strong projection effects shape the H I velocity dispersion maps. The most likely source of systematic orientations is the anisotropy of velocities, through the projection of streaming motions that are stronger along one of the planar directions in the discs. Moreover, systematic phase angles of asymmetries in the H I velocity dispersion could arise from tilted velocity ellipsoids, that is when the velocities are correlated. We expect a larger incidence of correlation between the radial and tangential velocities of H I gas with |ρRθ|∼0.6, which could be tested against the kinematics of the youngest stellar populations of the Milky Way. Conclusions. H I velocity dispersions cannot be considered devoid of projection effects. The systematic orientations of asymmetries can be explained by the projection of unresolved streaming motions mainly arising from spiral arms. Our methodology is a powerful tool to constrain the dominant direction of streaming motions and thus the shape of the velocity ellipsoid of H I gas, which is de facto anisotropic at the angular scales probed by the observations. The next step is to study the shape of the velocity ellipsoids of molecular and ionised gas and their link with galaxy mass and/or morphology, in addition to extending the sample size.

Cosmicflows-4: the baryonic Tully–Fisher relation providing ∼10 000 distances

ABSTRACT The interstellar gas in spiral galaxies can constitute a significant fraction of the baryon mass and it has been demonstrated that the sum of stellar and gas components correlates well with the kinematic signature of the total mass content, the widths of H i line profiles. The correlation of baryonic mass with H i linewidths is used here to obtain distances for 9984 galaxies extending to ∼0.05c. The sample is H i flux limited and a correction is required to account for an H i selection bias. The absolute scale is established by 64 galaxies with known distances from studies of Cepheid variables and/or the magnitudes of stars at the tip of the red giant branch. The calibration of the baryonic relationship results in a determination of the Hubble constant of H0 = 75.5 ± 2.5 km s−1 Mpc−1. The error estimate is statistical. This material will be combined with contributions from other methodologies in a subsequent paper where systematic uncertainties will be investigated.

Probing the Hot X-Ray Corona around the Massive Spiral Galaxy, NGC 6753, Using Deep XMM-Newton Observations

Abstract X-ray emitting gaseous coronae around massive galaxies are a basic prediction of galaxy formation models. Although the coronae around spiral galaxies offer a fundamental test of these models, observational constraints on their characteristics are still scarce. While the presence of extended hot coronae has been established around a handful of massive spiral galaxies, the short X-ray observations only allowed for measurements of the basic characteristics of the coronae. In this work, we utilize deep XMM-Newton observations of NGC 6753 to explore its extended X-ray corona in unprecedented detail. Specifically, we establish the isotropic morphology of the hot gas, suggesting that it resides in hydrostatic equilibrium. The temperature profile of the gas shows a decrease with an increasing radius: it drops from in the innermost parts to at a 50 kpc radius. The temperature map reveals the complex temperature structure of the gas. We study the metallicity distribution of the gas, which is uniform at Solar. This value is about an order of magnitude lower than that obtained for elliptical galaxies with similar dark matter halo mass, hinting that the hot gas in spiral galaxies predominantly originates from external gas inflows rather than from internal sources. By extrapolating the density profile of the hot gas out to the virial radius, we estimate the total gas mass and derive the total baryon mass of NGC 6753. We conclude that the baryon mass fraction is , implying that about half of the baryons are missing.

MOLECULAR GAS VELOCITY DISPERSIONS IN THE ANDROMEDA GALAXY

ABSTRACT In order to characterize the distribution of molecular gas in spiral galaxies, we study the line profiles of CO (1 – 0) emission in Andromeda, our nearest massive spiral galaxy. We compare observations performed with the IRAM 30 m single-dish telescope and with the CARMA interferometer at a common resolution of 23 arcsec ≈ 85 pc × 350 pc and 2.5 km s−1. When fitting a single Gaussian component to individual spectra, the line profile of the single dish data is a factor of 1.5 ± 0.4 larger than the interferometric data one. This ratio in line widths is surprisingly similar to the ratios previously observed in two other nearby spirals, NGC 4736 and NGC 5055, but measured at ∼0.5–1 kpc spatial scale. In order to study the origin of the different line widths, we stack the individual spectra in five bins of increasing peak intensity and fit two Gaussian components to the stacked spectra. We find a unique narrow component of FWHM = 7.5 ± 0.4 km s−1 visible in both the single dish and the interferometric data. In addition, a broad component with FWHM = 14.4 ± 1.5 km s−1 is present in the single-dish data, but cannot be identified in the interferometric data. We interpret this additional broad line width component detected by the single dish as a low brightness molecular gas component that is extended on spatial scales >0.5 kpc, and thus filtered out by the interferometer. We search for evidence of line broadening by stellar feedback across a range of star formation rates but find no such evidence on ∼100 pc spatial scale when characterizing the line profile by a single Gaussian component.

THE WEAK CARBON MONOXIDE EMISSION IN AN EXTREMELY METAL-POOR GALAXY, SEXTANS A

Carbon monoxide (CO) is one of the primary coolants of gas and an easily accessible tracer of molecular gas in spiral galaxies but it is unclear if CO plays a similar role in metal poor dwarfs. We carried out a deep observation with IRAM 30 m to search for CO emission by targeting the brightest far-IR peak in a nearby extremely metal poor galaxy, Sextans A, with 7% Solar metallicity. A weak CO J=1-0 emission is seen, which is already faint enough to place a strong constraint on the conversion factor (a_CO) from the CO luminosity to the molecular gas mass that is derived from the spatially resolved dust mass map. The a_CO is at least seven hundred times the Milky Way value. This indicates that CO emission is exceedingly weak in extremely metal poor galaxies, challenging its role as a coolant in these galaxies.

Halo Gas Velocities of NGC 4244 and NGC 891 Using Multi-long-slit Spectroscopy

Extra-planar gas in spiral galaxies can originate from disk-halo cycling driven by star formation in the disk or from infalling gas clouds. Studying the kinematics of halo gas can provide evidence for which scenario is more plausible. Edge-on galaxies provide a direct view of the extra-planar gas. We measure gas velocities from Hα emission as a function of height above the galactic disk for NGC 4244, which has a low star formation rate, and NGC 891, which has a high star formation rate. Using a multi-slit setup on the ARC 3.5 m telescope, we are able to measure velocities along 16 slits simultaneously. We compare our preliminary results to studies of HI velocities in these galaxies.

Star-forming galaxies in SDSS: signs of metallicity evolution

Context. Evolution of galaxies through cosmic time has been widely studied at high redshift, but there are a few studies in this field at lower redshifts. However, low-redshifts studies will provide important clues to the evolution of galaxies, furnishing the required link between local and high-redshift universe. Aims. In this work we focus on the metallicity of the gas in spiral galaxies at low redshift looking for signs of chemical evolution. We analyze the metallicity contents of star forming galaxies of similar luminosities at different redshifts, we studied the metallicity of star forming galaxies from SDSS-DR5 (Sloan Digital Sky Survey-Data Release 5), using different redshift intervals from 0.1 to 0.4. Methods. We used the public data of SDSS-DR5 processed with the STARLIGHT spectral synthesis code, correcting the fluxes for dust extinction, estimating metallicities using the R23 method, and analyzing the samples with respect to the [N ii] λ6583/[O ii] λ3727 line ratio. Results. From a final sample of 207 galaxies, we find a decrement in 12+log(O/H) corresponding to the redshift interval 0.3 < z < 0.4 of ∼0.1 dex with respect to the rest of the sample, which can be interpreted as evidence of the metallicity evolution in low-z galaxies.

Nobeyama CO Atlas of Nearby Spiral Galaxies: Distribution of Molecular Gas in Barred and Nonbarred Spiral Galaxies

Abstract The data from a CO ($J$ = 1–0) mapping survey of 40 nearby spiral galaxies, performed with the Nobeyama 45 m telescope, are presented. The criteria of the sample selection were: (1) RC3 morphological type ranging from Sa to Scd, (2) distance of less than 25 Mpc, (3) inclination angle less than 79$^{\circ}$ (RC3), (4) flux at 100$\mu$m higher than $\sim$ 10 Jy, and (5) spiral structure not destroyed by any interaction. The maps of CO cover most of the optical disk of the galaxies. We investigated the influence of bars on the distribution of molecular gas in spiral galaxies using these data. We confirmed that the degree of the central concentration is higher in barred spirals than in nonbarred spirals, as shown by previous studies. Furthermore, we present observational evidence that bars are efficient in driving molecular gas that lies within the bar length toward the center, while the role in bringing gas in from the outer parts of the disks is small. The transported gas accounts for about half of the molecular gas within the central region in barred spiral galaxies. We found a correlation between the degree of central concentration and the bar strength. Galaxies with stronger bars tend to have a higher central concentration. The correlation implies that stronger bars accumulate molecular gas toward the center more efficiently. These results are consistent with long-lived bars.

Results From VLA Observations of Five Spiral Galaxies In The Virgo Cluster Region

AbstractNew HI synthesis observations have been made for the five spiral galaxies NGC 4307, NGC 4356, NGC 4411a/b and NGC 4492, located in the Virgo Cluster Region, with the Very Large Array in its CS configuration. All these objects, but NGC 4411a, belong to a sample of spirals identified in previous single-dish studies of the HI distribution within the Virgo Cluster and its outskirts. The thirteen members of that sample were selected for showing neutral gas deficiencies as strong as those of the inner galaxies of Virgo and other rich clusters, but apparently lying on the infall region.The goal of the present observations is to progress further on the identification of the external mechanisms that may be responsible for the reduction of the interstellar gas in spiral galaxies and thus contribute to the study of the evolution of galaxies in high-density environments. In particular, the results of this work are relevant for the ongoing debate on the pre-processing of galaxies prior to entering the cluster environment.

Hydrostatic models for the rotation of extra-planar gas in disk galaxies

We show that fluid stationary models are able to reproduce the observed, negative vertical gradient of the rotation velocity of the extra-planar gas in spiral galaxies. We have constructed models based on the simple condition that the pressure of the medium does not depend on density alone (baroclinic instead of barotropic solutions: isodensity and isothermal surfaces do not coincide). As an illustration, we have successfully applied our method to reproduce the observed velocity gradient of the lagging gaseous halo of NGC 891. The fluid stationary models discussed here can describe a hot homogeneous medium as well as a "gas" made of discrete, cold HI clouds with an isotropic velocity dispersion distribution. Although the method presented here generates a density and velocity field consistent with observational constraints, the stability of these configurations remains an open question.

A dynamical model for the extraplanar gas in spiral galaxies

Recent H i observations reveal that the discs of spiral galaxies are surrounded by extended gaseous haloes. This extraplanar gas reaches large distances (several kiloparsecs) from the disc and shows peculiar kinematics (low rotation and inflow). We have modelled the extraplanar gas as a continuous flow of material from the disc of a spiral galaxy into its halo region. The output of our models is pseudo data cubes that can be directly compared to the H i data. We have applied these models to two spiral galaxies (NGC 891 and NGC 2403) known to have a substantial amount of extraplanar gas. Our models are able to reproduce accurately the vertical distribution of extraplanar gas for an energy input corresponding to a small fraction (<4 per cent) of the energy released by supernovae. However, they fail in two important aspects: (1) they do not reproduce the right gradient in rotation velocity; (2) they predict a general outflow of the extraplanar gas, contrary to what is observed. We show that neither of these difficulties can be removed if clouds are ionized and invisible at 21 cm as they leave the disc but become visible at some point on their orbits. We speculate that these failures indicate the need for accreted material from the intergalactic medium that could provide the low angular momentum and inflow required.

Molecular Gas in Spiral Galaxies: A New Warm Phase at Large Galactocentric Distances?

There is now strong evidence suggesting that the 12CO J = 1-0 transition, widely used to trace H2 gas, significantly underestimates its mass in metal-poor regions. In spiral disks such regions are found in large galactocentric distances where we show that any unaccounted H2 gas phase is likely to be diffuse (~5-20 cm^-3) and warmer (T(kin) ~ 50-100 K) than the cool (T(kin) ~ 15-20 K) CO-luminous one. Moreover we find that a high value of the H2 formation rate on grains, suggested by recent observational work, can compensate for the reduction of the available grain surface in the metal-poor part of typical galactic disks and thus enhance this CO-poor H2 component which may be contributing significantly to the mass and pressure of spiral disks beyond their optical radius.

A Search for Extraplanar Dust in Nearby Edge-on Spirals

We present high-resolution (06 to ∼ 10) BV images of 12 edge-on spiral galaxies observed with the WIYN 3.5 m telescope. These images were obtained to search for extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those previously found in NGC 891 (Howk & Savage). Many of these galaxies have been previously searched for diffuse ionized gas at high z. Our imaged galaxies include a sample of seven massive L*-like spiral galaxies within D ≲ 25 Mpc that have inclinations i ≳ 87° from the plane of the sky. We find that five of these seven systems show extraplanar dust, visible as highly structured absorbing clouds against the background stellar light of the galaxies. These dust structures lie at heights |z| ≳ 0.4 kpc, which should be above most of the thin disk molecular material in these galaxies. The more prominent structures are estimated to have associated gas masses ≳105 M⊙; the implied potential energies are ≳1052 ergs. All of the galaxies in our sample that show detectable Hα emission at large z also show extraplanar dust structures. None of those galaxies for which extraplanar Hα searches were negative show evidence for extensive high-z dust. The existence of extraplanar dust is a common property of massive spiral galaxies. We discuss several mechanisms for shaping the observed dust features. We emphasize in this discussion the possibility that these dusty clouds represent the dense phase of a multiphase medium at high z in spiral galaxies. In a few cases interactions with close galaxy companions could be responsible for the high-z dust, either through dynamical stripping or triggered star formation. We can rule out warps as the source of the observed high-z dust. Flaring gas layers seem an unlikely source of the observed material but cannot be ruled out at this time, except for those features that clearly connect to energetic processes in the disk. The correlation between high-z dust and extraplanar Hα emission may simply suggest that both trace the high-z interstellar medium in its various forms (or phases), the existence of which may ultimately be driven by vigorous star formation in the underlying disk. The absorption produced by high-z dust and associated gas in spiral galaxies must be accounted for when studying extraplanar emission from spiral galaxies over much of the electromagnetic spectrum.

Diffuse Ionized Gas in Spiral Galaxies: Probing Lyman Continuum Photon Leakage From H II Regions?

view Abstract Citations (176) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffuse Ionized Gas in Spiral Galaxies: Probing Lyman Continuum Photon Leakage From H II Regions? Ferguson, Annette M. N. ; Wyse, Rosemary F. G. ; Gallagher, J. S., III ; Hunter, Deidre A. Abstract As part of a large study to map the distribution of star formation across galactic disks, we have obtained deep Hα images of the nearby Sculptor Group spirals NGC 247 and NGC 7793. These images are of sufficiently high quality that they allow identification and analysis of diffuse Hα emission at surface brightness levels ranging from those of extremely low density H II regions down to those of the local Galactic disk diffuse emission. This paper presents a study of the large scale distribution and global energetics of diffuse ionized gas (DIG) in these galaxies and investigates the association between DIG and discrete H II regions. The radial and azimuthal intensity distributions of the DIG reveal that it is highly correlated with bright H II regions over both small and large scales. The observed Hα luminosities of the DIG are considerable and account for 30%-50% of the total Hα emission of each galaxy. The integrated minimum Lyman continuum (Lyc) power required to sustain the DIG in each galaxy is enormous and can only easily be met by the ionizing output from massive star formation; the mechanical energy from supernovae and stellar winds falls short of that required by at least a factor of two. Our results support the hypothesis that the DIG is photoionized by Lyc photons which leak out of traditional H II regions, and suggest that the local H I column density plays a role in regulating the amount of leakage which can occur. This interpretation has profound implications for the derivation of global and radial star-formation rates based on Hα emission-line fluxes, leading to qualitatively and quantitatively different behaviors depending on whether the Hα emission from the DIG is included or not. The contribution of the diffuse Hα component to the total Hα emission; i.e., the diffuse fraction, in these galaxies is similar to values found in other galaxies with differing Hubble types and star-formation rates. The constancy of the diffuse fraction is rather unexpected and implies that the overall fraction of photons which can leak out of H II regions and ionize the ISM over large scales is relatively invariant from one galaxy to another. Publication: The Astronomical Journal Pub Date: June 1996 DOI: 10.1086/117961 Bibcode: 1996AJ....111.2265F Keywords: GALAXIES: SPIRAL; GALAXIES: ISM; HII REGIONS full text sources ADS | data products SIMBAD (9) NED (2)

Molecular Bars in Spiral Galaxies

Images of molecular gas in spiral galaxies show two types of bar structure: large-scale bars, which are correlated with the presence of stellar bars, and generally follow the spiral structure, and nuclear “bars” of a few hundred parsecs in size, which show no direct correlation with the presence of a stellar bar. The dynamical origin of nuclear molecular bars is unclear, although they are consistent with radial inflow of gas into the nucleus, and subsequent evolution of the gas structure.

Dark matter, not magnetism

Battaner et al. have recently argued that interstellar magnetic fields can significantly affect the circular motion of gas in spiral galaxies and explain their observed flat rotation curves. This interpretation of galaxy kinematics challenges the conventional dark matter (DM) picture. However, the evidence for DM in galaxies is not restricted to gas motions. Measurements of velocity differences within galaxy pairs (not based on gas rotation and hence not affected by magnetic fields) imply the existence of massive haloes, whose properties are in total agreement with those derived from the DM interpretation of the observed rotation curves. On the other hand, the predictions of the magnetic-field, no-DM scenario cannot be reconciled with observations of pairwise galaxy dynamics

The ratio of molecular to atomic gas in spiral galaxies as a function of morphological type

view Abstract Citations (223) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Ratio of Molecular to Atomic Gas in Spiral Galaxies as a Function of Morphological Type Young, Judith S. ; Knezek, Patricia M. Abstract As part of the FCRAO Extragalactic CO Survey, we have observed λ2.6 mm CO emission along the major axis in 142 galaxies. We find that the ratio of molecular to atomic gas among spiral galaxies decreases by a factor of ~ 20 as a function of morphological type; the mean M(H_2_)/M(H I) ratio is 4.0 +/- 1.9 for S0-Sa galaxies and 0.2 +/- 0.1 for Sd-Sm galaxies. The decrease in the M(H_2_)/M(H I) ratio with type is not due to a Malmqulst bias, since a volume-limited subset of the galaxies also displays a decrease in the M(H_2_)/M(H I) ratio. Furthermore, we argue that the observed M(H_2_)/M(H I) decrease is not due to variations in molecular cloud properties as a function of type. For all of the spiral galaxies in our sample, we find the mean value of the ratio of total ISM gas mass to optical area to be 17 M_sun_ pc^-2^, within a factor of 2 of the mean value for each type. We find only factor of 2 increase in the mean gas surface density of spiral galaxies from early to late morphological type. The principal effect we find is that the dominant phase of the gas changes as a function of type. This may indicate that atomic gas can become molecular at a lower critical column density in early- than late-type spiral galaxies. Publication: The Astrophysical Journal Pub Date: December 1989 DOI: 10.1086/185606 Bibcode: 1989ApJ...347L..55Y Keywords: Carbon Monoxide; Galactic Structure; Interstellar Gas; Molecular Gases; Monatomic Gases; Spiral Galaxies; Abundance; Infrared Astronomy; Molecular Clouds; Astrophysics; GALAXIES: GENERAL; GALAXIES: INTERSTELLAR MATTER full text sources ADS | data products SIMBAD (1)

The peculiar motions of the gas in spiral galaxies

The peculiar motions of the gas in spiral galaxies

A possible relationship between ring magnetism and H I gas in spiral galaxies

view Abstract Citations (18) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A possible relationship between ring magnetism and HI gas in spiral galaxies. Vallee, J. P. Abstract Polarization observations of spiral galaxies have revealed so far only two types of large-scale distribution for their magnetic fields, either mostly ring shaped or mostly spiral shaped. A newly found relationship is reported here, namely: the observation of ring magnetism in the presence of a 'large' quantity of H I gas mass, and the observation of spiral magnetism in the presence of a 'small' quantity of H I gas mass. A previously known relationship is strengthened here, namely: the observation of spiral magnetism in the presence of a large tidal interaction with a companion galaxy, and the observation of ring magnetism in the presence of a small tidal interaction. Both relationships appear to be physically reasonable, and complementary. Publication: The Astronomical Journal Pub Date: March 1986 DOI: 10.1086/114034 Bibcode: 1986AJ.....91..541V Keywords: Hydrogen Clouds; Interstellar Gas; Interstellar Magnetic Fields; Spiral Galaxies; Density Wave Model; Polarimetry; Tides; Astrophysics full text sources ADS | data products SIMBAD (8) NED (8)