Interarm Regions Research Articles

Models for Interstellar Extinction in the Galaxy

We present two models for the interstellar extinction in the Galaxy that are based on the hypothesis that the interstellar dust is well mixed with the gas, with a constant ratio (except for a small dependence of metallicity on the Galactic radius), and therefore that the extinction is proportional to the column density of the gas. In the first model we assume that the Galaxy is axisymmetric; the gas density in the disk is a function of the Galactic radius and of the distance perpendicular to the Galactic plane, and the extinction is proportional to the column density of the gas. In the second model we take into account the spiral structure of the Galaxy. In this case, instead of increasing almost linearly with distance, the extinction increases by steps each time a spiral arm is crossed, but only increases slowly in the interarm regions. The gas density distribution is obtained from the Berkeley and Parkes H I surveys and from the Columbia University CO survey. The IRAS 100 μm brightness distribution is also used as a tracer of the interstellar dust column density. The predictions of the models are compared with data taken from a number of catalogs that present color excess and distances for large samples of stars. Our models are useful for estimating distances of objects and color corrections for objects for which the distance can be estimated by some other method, and also for star counts and brightness models of the Galaxy, among other applications.

HCN AND HCO+EMISSION IN M31 : TRACING THE DENSE MOLECULAR GAS IN A GALACTIC DISK

We present mm observations with the IRAM 30m radiotelescope of the HCN (J=1-0) and HCO+ (J=1-0) emission lines from Giant Moleculat Clouds (GMC) in the disk of the Andromeda Galaxy, The selected GMC targets have various morphology and environments, including locations within spiral arms or in interarm regions and with galactocentric radii ranging from 2.4 to 15.5 kpc over the disk. The radial distributions of the ratios HCN/CO and HCO+ /CO are discussed and their values are compared to other galaxies.

A Comparative Study of the Spatial Distribution of Ultraviolet and Far-Infrared Fluxes from M101

The total ultraviolet (UV) flux (from 1412 to 2718 A) of M101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 μm) using the maps of the galaxy taken by the Galaxy Evolution Explorer (GALEX) in the near-UV and far-UV and by the Infrared Space Observatory (ISO) at 60, 100, and 170 μm. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from ~4 in the nuclear region to nearly zero toward the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth that decreases with radius and that dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio of taking on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

Disruption time scales of star clusters in different galaxies

The observed average lifetime of the population of star clusters in the Solar Neighbourhood, the Small Magellanic Cloud and in selected regions of M 51 and M 33 is compared with simple theoretical predictions and with the results of N-body simulations. The empirically derived lifetimes (or disruption times) of star clusters depend on their initial mass as tdis in all four galaxies. N-body simulations have shown that the predicted disruption time of clusters in a tidal field scales as tdis, where trh is the initial half-mass relaxation time and tcr is the crossing time for a cluster in equilibrium. We show that this can be approximated accurately by tdis for clusters in the mass range of about 103 to 106 , in excellent agreement with the observations. Observations of clusters in different extragalactic environments show that tdis also depends on the ambient density in the galaxies where the clusters reside. Linear analysis predicts that the disruption time will depend on the ambient density of the cluster environment as tdis. This relation is consistent with N-body simulations. The empirically derived disruption times of clusters in the Solar Neighbourhood, in the SMC and in M 33 agree with these predictions. The best fitting expression for the disruption time is tdis where Mcl is the initial mass of the cluster and Myr. The disruption times of star clusters in M 51 within 1-5 kpc from the nucleus, is shorter than predicted by about an order of magnitude. This discrepancy might be due to the strong tidal field variations in M 51, caused by the strong density contrast between the spiral arms and interarm regions, or to the disruptive forces from giant molecular clouds.

ON THE GALACTIC SPIRAL PATTERNS: STELLAR AND GASEOUS

The gas response to a proposed spiral stellar pattern for our Galaxy is presented here as calculated via 2D hydrodynamic calculations utilizing the ZEUS code in the disk plane. The locus is that found by Drimmel (2000) from emission profiles in the K band and at 240 <TEX>${\mu}m$</TEX>. The self-consistency of the stellar spiral pattern was studied in previous work (see Martos et al. 2004). It is a sensitive function of the pattern rotation speed, <TEX>$\Omega$</TEX>p, among other parameters which include the mass in the spiral and its pitch angle. Here we further discuss the complex gaseous response found there for plausible values of <TEX>$\Omega$</TEX>p in our Galaxy, and argue that its value must be close to <TEX>$20 km s^{-l}\;kpc^{-1}$</TEX> from the strong self-consistency criterion and other recent, independent studies which depend on such parameter. However, other values of <TEX>$\Omega$</TEX>p that have been used in the literature are explored to study the gas response to the stellar (K band) 2-armed pattern. For our best fit values, the gaseous response to the 2-armed pattern displayed in the K band is a four-armed pattern with complex features in the interarm regions. This response resembles the optical arms observed in the Milky Way and other galaxies with the smooth underlying two-armed pattern of the old stellar disk populations in our interpretation. The complex gaseous response appears to be related to resonances in stellar orbits. Among them, the 4:1 resonance is paramount for the axisymmetric Galactic model employed, and the set of parameters explored. In the regime seemingly proper to our Galaxy, the spiral forcing appears to be marginally strong in the sense that the 4:1 resonance terminates the stellar pattern, despite its relatively low amplitude. In current work underway, the response for low values of <TEX>$\Omega$</TEX>p tends to remove most of the rich structure found for the optimal self-consistent model and the gaseous pattern is ring-like. For higher values than the optimal, more features and a multi-arm structure appears.

Three‐Dimensional Magnetohydrodynamic Modeling of the Gaseous Structure of the Galaxy: Synthetic Observations

We generate synthetic observations from the four-arm model we presented previously for the Galactic ISM in the presence of a spiral gravitational perturbation. We find that velocity crowding and diffusion have a strong effect in the longitude-velocity (l-v) diagram. The velocity-latitude (v-b) diagram presents structures at the expected spiral arm velocities, which can be explained by the off-the-plane structure of the arms presented in previous papers of this series. Such structures are observed in the Leiden/Dwingeloo H I survey. The rotation curve, as measured from the inside of the modeled galaxy, shows similarities with the observed one for the Milky Way, although it has large deviations from the smooth circular rotation corresponding to the background potential. The magnetic field inferred from a synthetic synchrotron map shows a largely circular structure but with interesting deviations in the midplane due to distortion of the field from circularity in the interarm regions.

Three‐Dimensional Magnetohydrodynamic Modeling of the Gaseous Structure of the Galaxy: Description of the Simulations

As we have discussed previously, the extra stiffness that a magnetic field adds to the interstellar medium (ISM) changes the way the ISM reacts to the presence of a spiral perturbation. At intermediate to high z, the gas shoots up before the arm, then flows over and falls behind the arm as it approaches the next arm. This generates a multicell circulation pattern, within each of which the net radial mass flux is positive near the midplane and negative at higher z. The flow distorts the magnetic field lines. In the arm region, the gas flows nearly parallel to the arm, and therefore the magnetic field adopts a similar pitch angle. Between the arms, the gas flows out in radius, generating a negative pitch angle in the magnetic field. The intensity and direction of the field yield synthetic synchrotron maps that reproduce some features of the synchrotron maps of external galaxies, such as the islands of emission and the displacement between the gaseous and synchrotron arms. When comparing the magnitude of the field with the local gas density, two distinctive relations appear, depending on whether the magnetic pressure is dominant. Above the plane, the density structure develops a shape resembling a breaking wave. This structure collapses and rises again with a period of about 60 Myr, similar to that of a vertical oscillation mode. The falling gas plays an important part in the overall hydrostatics, since its deceleration compresses the low-z gas, raising the average midplane pressure in the interarm region above that provided by the weight of the material above.

Molecular gas in the galaxy M 83

We present the kinematics of the molecular gas in the barred spiral galaxy M 83 (NGC 5236). The study is based on 12CO( 1–0 and 2–1) observations with the Swedish-ESO Submillimetre Telescope (SEST). Iso-velocity maps of the entire optical disk, 10′ 10′ or 13 13 kpc, are produced. They show the pattern of an inclined, rotating disk, but also the effects of streaming motions along the spiral arms. A dynamical mass of about 6 10 is estimated by fitting the rotation curve of an exponential disk model to these data. The gas constitutes about 13% of the disk mass. The pattern speed is determined from the residual velocity pattern. The locations of various resonances are discussed. The molecular gas velocity dispersion is determined, and a trend of decreasing dispersion with increasing galactocentric radius is found. A total gas (H2 + + He) mass surface density map is presented, and compared to the critical density for star formation of an isothermal gaseous disk. The critical density is exceeded in the spiral arms, but not in the interarm regions. The locations of Giant Molecular Associations (GMAs) and regions are consistent with this scenario of dynamically induced star formation.

Are galactic disks dynamically influenced by dust?

Dynamically cold components are well known to destabilize hotter, even much more massive components. E.g. stellar disks can become unstable by a small admixture of cold gas or proto-planetary disks might be destabilized by a small fraction of dust. In this paper we studied the dynamical influence of a cold dust component on the gaseous phase in the central regions of galactic disks. We performed two-dimensional hydrodynamical simulations for flat multi-component disks embedded in a combined static stellar and dark matter potential. The pressure-free dust component is coupled to the gas by a drag force depending on their velocity difference. It turned out that the most unstable regions are those with either a low or near to minimum Toomre parameter or with rigid rotation, i.e. the central area. In that regions the dust-free disks become most unstable for high azimuthal modes (m ∼ 8), whereas in dusty disks all modes have a similar amplitude resulting in a patchy appearance. The structures in the dust have a larger contrast between arm and inter-arm regions than those of the gas. The dust peaks are frequently correlated with peaks of the gas distribution, but they do not necessarily coincide with them. Therefore, a large scatter in the dust-to-gas ratios is expected. The appearance of the dust is more cellular (i.e. sometimes connecting different spiral features), whereas the gas is organized in a multi-armed spiral structure. We found that an admixture of 2% dust (relative to the mass of the gas) destabilizes gaseous disks substantially, whereas dust-to-gas ratios below 1% have no influence on the evolution of the gaseous disk. For a high dust-to-gas ratio of 10% the instabilities reach a saturation level already after 30 Myr. The stability of the gaseous disks also strongly depends on their Toomre parameter. But even in hot gaseous disks a destabilizing influence of the dust component has been found.

Can Dust Destabilise Galactic Disks?

AbstractWe studied the dynamic influence of a dust component on the gaseous phase in central regions of galactic disks. We performed two-dimensional hydrodynamic simulations for flat, multicomponent disks embedded in a stellar and dark matter potential. The pressure-free dust component is coupled to the gas by a drag force depending on their velocity difference. The most unstable regions are those with either a low or near-to-minimum Toomre parameter or with rigid rotation, i.e. the central area. In those regions the dust-free disks become most unstable for a small range of high azimuthal modes (m ∼ 8), whereas in dusty disks all modes have similar amplitudes resulting in a patchy appearance. The structures in the dust have a larger contrast between arm and interarm regions than those of the gas. The dust peaks are frequently correlated with peaks of the gas distribution, but they do not necessarily coincide with them. This leads to a large scatter in the dust to gas ratios. The appearance of the dust is more cellular (i.e. sometimes connecting different spiral features), whereas the gas is organised in a multi-armed spiral structure. We found that an admixture of 2% dust (relative to the mass of the gas) destabilises gaseous disks substantially, whereas dust to gas ratios below 1% have no influence on the evolution of the gaseous disk. For a high dust to gas ratio of 10% the instabilities reach the saturation level after 30 Myr.

The Recent Star Formation History of the M31 Disk

The star formation history of the northern and southern M31 disk is measured using samples of BV photometry for 4' × 4' regions taken from the KPNO/CTIO Local Group Survey. The distances, mean reddening values, and age distributions of the stars in these regions were measured using the routines of Dolphin. Independent measurements of overlapping fields show that the results are stable for most samples. A slight distance gradient is seen across the major axis of the southern disk, and a mean distance of 24.47 ± 0.03 is found by combining the results. Higher mean reddening values are seen to follow the spiral structure. The stellar age distributions are consistent with episodic star formation confined mainly to the gas-rich arm regions. If these episodes were caused by propagating density waves, the waves did not cause significant star formation episodes in the gas-poor interarm regions. A combination of all of the results provides the total star formation rate for 1.4 deg2 of the M31 disk for six epochs. These results suggest that star formation in the disk declined by ~50% from ~250 to ~50 Myr ago. The lowest star formation rate occurred ~25 Myr ago, followed by a ~20% increase to the present. The mean star formation rate for this large portion of M31 over the past 60 Myr is 0.63 ± 0.07 M⊙ yr-1, suggesting a total mean rate for the disk of ~1 M⊙ yr-1.

Resistive MHD simulations of the Parker instability in galactic disks

Parker instability leads to the formation of tangential discontinuities in a magnetic field and subsequent magnetic reconnection due to a numerical and/or an explicit resistivity. In this paper we investigate the role of the uniform, localized and numerical resistivity on the diusion of magnetic field lines during the growth phase of Parker instability modes. We propose a new method to quantify the diusion of magnetic field lines which is attributed to the presence of resistivity in ideal and non- ideal MHD codes. The method relies (1) on integration of magnetic lines in between periodic boundaries, (2) on measurements of the dispersion of magnetic field lines with the left and the right periodic boundaries and (3) on a statistical analysis of shifts of a large set of magnetic lines. The proposed method makes it possible to detect topological evolution of magnetic field. We perform a series of resistive MHD simulations of the Parker instability in uniformly rotating galactic disks. We follow the topological evolution of the magnetic field evolving due to the Parker instability and relate it to the ratio of total to uniform magnetic field in galactic disks. We find that after the onset of the Parker instability, the magnetic field becomes first tangled and later on it evolves toward a uniform state due to the presence of resistivity. A similar eect of a varying contribution of a turbulent magnetic field is observed in arms and inter-arm regions of galaxies.

Large-Scale Structure and Dynamics of Cassiopeia OB7

A large region (9° × 9°) surrounding the OB association Cassiopeia OB7 has been studied for signs of interaction between the stars and the ambient gas. A large half-shell has been detected in H I around Cas OB7, as well as in the infrared and partially in CO. This half-shell covers a range of 20 km s-1 but could reach 40 km s-1 when extrapolating to the whole shell. The structure is 120 pc across if located at 2 kpc, the distance to Cas OB7. At this distance, we estimate the mass of the shell to be approximately 30,000 M⊙. Interestingly, all Cas OB7 members are located well within the edge of this shell. We believe that there is a strong hint of interaction between the gas and the stars based on morphological evidence. The velocity of both objects (association and shell) and their respective distance are also in agreement. We explain the missing half by an expansion into the interarm region.

The Virgo High-Resolution CO Survey: III. NGC 4254

Abstract We present high-angular-resolution (${1\rlap {.}{}^{\mathrm {\prime \prime }}5}$-${5{}^{\mathrm {\prime \prime }}}$) interferometer observations of the ${{12\atop} \mathrm{CO}}$ ($J=1 \hbox{--} 0$) line emission in the central region of the SA(s)c galaxy NGC 4254 (M 99). The observations were obtained using the Nobeyama Millimeter-wave Array (NMA) during the course of a long-term CO-line survey of Virgo spirals. We present the spectra, channel maps, integrated intensity distributions, velocity fields, position–velocity diagrams, and compare the data with various optical images. The rotation velocity is already finite at the nucleus, or at least it rises steeply to $80 \,\mathrm{km} \,\mathrm{s}^{-1}$ within the central $1^{\prime\prime}$, indicating the existence of a massive core of $10^8 \,{{{M}_{\odot}}}$ within $1^{\prime\prime}$ (80 pc) radius. The CO intensity maps show that the inner disk has well-developed multiple spiral arms, winding out from a bar-shaped elongated molecular complex. In addition to the bisymmetric spiral arms, an asymmetric tightly wound arm with high molecular gas density is found to wind out from the molecular bar. The molecular spiral arms, particularly the tightly wound arm, well traces optical dark lanes, and are associated with $\mathrm{H}\alpha$ arms having many H ii regions. The inner asymmetric spiral structures can be explained by ram-pressure distortion of inter-arm low density regions of the inner disk by the intra-cluster gas wind, and is indeed reproduced by a hydrodynamical simulation.

The effects of spiral arms on the multi-phase ISM

Statistical parameters of the ISM driven by thermal energy injections from supernova explosions have been obtained from 3D, nonlinear, magnetohydrodynamic, shearing-box simulations for spiral arm and interarm regions. The density scale height obtained for the interarm regions is 50% larger than for the spiral arms because of the higher gas temperature. The filling factor of the hot gas is also significantly larger between the arms and depends sensitively on magnetic field strength.

Three‐dimensional Magnetohydrodynamic Modeling of the Gaseous Structure of the Galaxy: Setup and Initial Results

We show the initial results of our three-dimensional MHD simulations of the flow of the Galactic atmosphere as it responds to a spiral perturbation in the potential. In our standard case, as the gas approaches the arm, there is a downward converging flow that terminates in a complex of shocks just ahead of the midplane density peak. The density maximum slants forward at high z, preceded by a similarly leaning shock. The latter diverts the flow upward and over the arm, as in a hydraulic jump. Behind the gaseous arm, the flow falls again, generating further secondary shocks as it approaches the lower z material. In cases with two arms in the perturbing potential, the gaseous arms tends to lie somewhat downstream of the potential minimum. In the four-arm case, this is true at large r or early evolution times. At smaller r, the gaseous arms follow a tighter spiral, crossing the potential maximum and fragmenting into sections arranged on average to follow the potential spiral. Structures similar to the high-z part of the gaseous arms are found in the interarm region of our two-arm case, while broken arms and low column density bridges are present in the four-arm case. Greater structure is expected when we include cooling of denser regions. We present three examples of what can be learned from these models. We compare the velocity field with that of purely circular rotation and find that an observer inside the Galaxy should see radial velocity deviations typically greater than 20 km s-1. Synthetic spectra, vertical from the midplane, show features at velocities ?-20 km s-1, which do not correspond to actual density concentrations. Placing the simulated observer outside the Galaxy, we find velocity structure and arm corrugation similar to those observed in H? in NGC 5427.

The Galactic Distribution of Large HiShells

We report the discovery of 19 new H I shells in the Southern Galactic Plane Survey (SGPS). These shells, which range in radius from 40 pc to 1 kpc, were found in the low-resolution Parkes portion of the SGPS data set covering Galactic longitudes l = 253° to l = 358°. Here we give the properties of individual shells, including positions, physical dimensions, energetics, masses, and possible associations. We also examine the distribution of these shells in the Milky Way and find that several of the shells are located between the spiral arms of the Galaxy. We offer possible explanations for this effect, in particular, that the density gradient away from spiral arms, combined with the many generations of sequential star formation required to create large shells, could lead to a preferential placement of shells on the trailing edges of spiral arms. Spiral density wave theory is used in order to derive the magnitude of the density gradient behind spiral arms. We find that the density gradient away from spiral arms is comparable to that out of the Galactic plane and therefore suggest that this may lead to exaggerated shell expansion away from spiral arms and into interarm regions.

The Large‐Scale Distribution and Properties of Carbon Monoxide in a Sample of Nearby Spiral Galaxies

Galaxies are defined by a cumulative history of converting of gas into stars, and molecular gas (H2) is the fuel for this star formation. Because H2 cannot be observed directly, we observe the “tracer” CO and derive the H2 mass indirectly via a “conversion” factor. To move beyond global correlations and the limitations of undersampled, limited-coverage maps, this study has used the NRAO 12 m telescope and its “on-the-fly” observing mode to efficiently and reliably map cold CO emission over very large areas in five spiral galaxies. The galaxies were selected based on previous CO detections, large angular size, low inclination, and morphology to form an illustrative sample ranging from big to small, starburst to nonstarburst, and actively interacting to passively quiescent. CO was mapped in roughly regions covering the optical disks of the selected ′ ′ 15 # 15 galaxies. This experiment has the following major results: (1) We now have large-scale, fully sampled maps of CO in IC 342, M83, NGC 1097, NGC 4736, and NGC 6946 (Fig. 1). The first largescale, total neutral gas (H2 H i) surface density maps Sgas of these galaxies are now available. (2) The CO maps confirm the correlation with the optical disks in overall extent and gross morphology, although significant morphological differences exist. Interarm molecular gas is common in the sample. In the case of NGC 1097, interarm regions in the grand-design pattern,

The Parker Instability in Three Dimensions: Corrugations and Superclouds along the Carina‐Sagittarius Arm

Here we present three-dimensional MHD models for the Parker instability in a thick magnetized disk, including the presence of a spiral arm. The B field is assumed parallel to the arm, and the model results are applied to the optical segment of the Carina-Sagittarius arm. The characteristic features of the undular and interchange modes are clearly apparent in the simulations. The interchange mode appears first and generates small interstellar structures in the interarm regions, but its development inside the arm is hampered by the acceleration of the spiral wave. In contrast, the undular mode follows its normal evolution inside the spiral wave, creating large gas concentrations distributed along the arm. This results in a clear arm/interarm difference; the instability triggers the formation of large interstellar clouds (with masses in the range of 106-107 M☉) inside the arms but generates only small structures with slight density enhancements in the interarm regions. The resulting clouds are distributed in an antisymmetric way with respect to the midplane, and their masses are similar to those inferred for H I superclouds in our Galaxy. Such a cloud distribution results in an azimuthal corrugation along the arm, and for conditions similar to those of the optical segment of the Carina-Sagittarius arm, it has a wavelength of about 2.4 kpc. This structuring, then, can explain the origin of both H I superclouds and the azimuthal corrugations in spiral arms. In particular, the wavelength of the fastest growing undular mode matches the corrugation length derived with the young stellar groups located in the optical segment of the Carina-Sagittarius arm.

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

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 × 109 M⊙, 4% of the total dynamical mass. Molecular gas constitutes 80% of total gas mass within the inner 10' and 30% overall. CO (1–0) emission and 21 cm continuum emission are extremely well correlated spatially in regions where H2 dominates the gas surface density. The spatial correlation between CO (1–0) and 100 μm 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'. 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' 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.