Richard J. Rand

The local galactic magnetic field

The Galactic magnetic field is analyzed based on the large amount of pulsar rotation measures (RMs) now available, and regions of the sky suspected of producing anomalous RMs are discussed. The North Polar Spur appears to cause large positive RMs in the region l between 0 and 60 deg and b between 0 and 60 deg, and thus causes a systematic bias in modeling if the pulsars in the region are not flagged. Using only unflagged pulsars within D = 3 kpc and a longitudinal model of the local field, the strength is found to be B(0) = 1.6 + or - 0.3 microgauss in the direction l(B) = 96 deg + or - 4 deg, with a reversal at D(r) = 600 + or - 80 pc toward the inner Galaxy. A concentric-ring model is found to be superior to a bisymmetric spiral model as a fit to the data without the 3 kpc restriction, with a strength of B0 = 1.3 + or - 0.2 microgauss at the position of the sun and a reversal at D(r) = 650 + or - 90 pc toward the Galactic center. 33 refs.

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.

Diffuse Ionized Gas in Edge-on Spiral Galaxies: Extraplanar and Outer Disk Hα Emission

We present Hα images of five edge-on galaxies: NGC 891, NGC 4631, NGC 4244, NGC 3003, and UGC 9242. We also analyze [S II] λλ6717, 6731 and [O III] λ5007 images of NGC 4631. For several of these galaxies, these images are the most sensitive to date. We analyze the ionized gas content, with particular attention to the diffuse ionized gas (DIG). The DIG layer in NGC 891 is traced out to at least 5 kpc from the midplane, confirming an earlier spectroscopic detection. The DIG in four of these galaxies contributes 40%-50% of the total Hα luminosity, similar to the contribution in face-on galaxies, but in NGC 891 the DIG contributes 80%-90%. This is likely due to the higher dust content in the disk of NGC 891, which obscures the H II regions but may also reflect the extraordinary prominence of the DIG layer in that galaxy. Our very deep image of UGC 9242 shows very low surface brightness emission, as low as 0.3 pc cm-6, reaching 4-5 kpc above the midplane. This galaxy also exhibits filaments near the bright Hα nucleus, an indication of a starburst superwind. In NGC 4631 we see a very large shell of emission extending 3.5 kpc into the halo. The [S II]/Hα and [O III]/Hα ratios in NGC 4631 are consistent with the ratios seen in other galaxies, and with photoionization models. There is a region on the southeast side of disk where the [O III]/(Hα + [N II]) ratio reaches over 1.0 in the DIG, which coincides with an H I supershell. We use our very deep images of NGC 3003 and UGC 9242 to search for ionized gas in the outer disks as a test of the strength of the metagalactic ionizing radiation field. We find no outer disk emission down to our 1 σ limit of 0.13 pc cm-6 on scales of 1.5 kpc in NGC 3003. Based on this limit we rule out a metagalactic ionizing radiation field stronger than 11 × 10-23 ergs cm-2 s-1 Hz-1 sr-1. There is an indication of extended disk emission in UGC 9242, which would imply a stronger radiation field, but various concerns, most importantly flat-fielding uncertainties due to foreground stars in the image, lead us to question whether this feature is real.

DEEP SUBMILLIMETER IMAGES OF NGC 891—COLD DUST AT LARGER GALACTIC RADII

We present images in the 450 and 850 μm continuum of the edge-on galaxy NGC 891. These measurements, carried out with the recently commissioned Submillimeter Common-User Bolometer Array, provide the deepest images yet of a nearby galaxy in the submillimeter wave band. We detect dust emission from 2/3 of the optical disk and confirm the presence of dust chimneys escaping from the main absorption layer up to z-heights of nearly 2 kpc. A comparison between the submillimeter surface brightness along the major axis with that corresponding to the IRAS 60 and 100 μm filters implies that large amounts of cold dust (~15 K) are present in the disk (in fact, an order of magnitude more grain material than the warm dust detected by IRAS). These cold grains predominate at larger galactic radii.

The Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey - I. Survey description and pilot observations

We introduce a new, very deep neutral hydrogen (H i) survey being performed with the Westerbork Synthesis Radio Telescope (WSRT). The Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey is producing an archive of some of the most sensitive H i observations available, on the angular scales which are most useful for studying faint, diffuse gas in and around nearby galaxies. The survey data are being used to perform careful modeling of the galaxies, characterizing their gas content, morphology, and kinematics, with the primary goal of revealing the global characteristics of cold gas accretion onto spiral galaxies in the local Universe. In this paper, we describe the survey sample selection, the data acquisition, reduction, and analysis, and present the data products obtained during our pilot program, which consists of UGC 2082, NGC 672, NGC 925, and NGC 4565. The observations reveal a first glimpse of the picture that the full HALOGAS project aims to illuminate: the properties of accreting H i in different types of spirals, and across a range of galactic environments. None of the pilot survey galaxies hosts an H i halo of the scale of NGC 891, but all show varying indications of halo gas features. We compare the properties of detected features in the pilot survey galaxies with their global characteristics, and discuss similarities and differences with NGC 891 and NGC 2403.

Further Spectroscopy of the Diffuse Ionized Gas in NGC 891 and Evidence for a Secondary Source of Ionization

Two long-slit spectra of the diffuse ionized gas (DIG) in NGC 891 are presented. The first reveals variations parallel to the major axis in emission-line ratios in the halo gas at z = 700 pc. It is found that filaments of H? emission show lower values of [N II]/H?, [S II]/H?, and [O I]/H?. Although this result is expected if the filaments represent the walls of evacuated chimneys, it merely reflects a more general correlation of these ratios with H? surface brightness along the slit and may simply arise from radiation dilution effects. Halo regions showing low line ratios are probably relatively close to ionizing sources in the disk below. The results highlight difficulties inherent in observations of edge-on galaxies caused by lack of knowledge of structure in the in-plane directions. The [S II]/[N II] ratio shows almost no dependence on distance along the major axis or H? surface brightness. Values of [O I]/H? indicate that H is 80%-95% ionized (assuming T = 104 K), with the higher ionization fractions correlating with higher surface brightness. Much more interesting information on the nature of this gaseous halo comes from the second observation, which shows the vertical dependence of [N II]/H?, [S II]/H?, [O I]/H?, and [O III]/H? through the brightest region of the DIG halo. The most surprising result, in complete contradiction to models in which the DIG is ionized by massive stars in the disk, is that [O III]/H? rises with height above the plane for z > 1 kpc (even as [N II]/H?, [S II]/H?, and [O I]/H? are rising, in line with expectations from such models). The run of [S II]/[N II] is also problematic, showing essentially no contrast with z. The [O III] emission probably arises from shocks, turbulent mixing layers, or some other secondary source of ionization. Composite models in which the line emission comes from a mix of photoionized gas and shocks or turbulent mixing layers are considered in diagnostic diagrams, with the result that many aspects of the data can be explained. Problems with the run of [S II]/[N II] still remain, however. There is a reasonably large parameter space allowed for the second component. For the photoionized component, only matter-bounded models succeed, putting a fairly strong restriction on the clumpiness of the halo gas. Given the many uncertainties, the composite models can do little more than demonstrate the feasibility of these processes as secondary sources of energy input. A fairly robust result, however, is that the fraction of H? emission arising from the second component probably increases with z. From values of [O I]/H?, H is essentially 100% ionized at z = 0 kpc and 90% ionized at z = 1 kpc (again assuming T = 104 K).

Ionization Sources and Physical Conditions in the Diffuse Ionized Gas Halos of Four Edge-On Galaxies

Deep long-slit spectra of the diffuse ionized gas halos of the edge-on spiral galaxies NGC 4302 and UGC 10288 are presented. Emission lines are detected up to about z = 2 kpc in NGC 4302 and to nearly z = 3 kpc on the north side of UGC 10288. For both galaxies, the line ratios [N II]/Hα and [S II]/Hα increase with z in accordance with dilute photoionization models. Runs of [S II]/[N II] and for UGC 10288 the run of [O III]/Hα, however, are not explained by the models. Scale height determinations of their diffuse ionized gas (DIG) halos are generally lower than those of galaxies with more prominent extraplanar DIG features. These data, along with previously presented data for NGC 5775 and NGC 891, are used to address the issue of how DIG halos are energized. Composite photoionization/shock models are generally better at explaining runs of line ratios in these galaxies than photoionization models alone. Models of line ratios in NGC 5775 require a greater contribution from shocks for filamentary regions than for nonfilamentary regions to explain the run of [O III]/Hα. In either case, the [S II]/[N II] ratio is not well fit by the models. Composite models for UGC 10288 are successful at reproducing the run of [S II]/[N II] for all but the the highest values of [N II]/Hα; however, the run of [O III]/Hα versus [N II]/Hα does not show any discernible trend, making it difficult to determine whether or not shocks contribute to the layers maintenance. We also examine whether the data can be explained simply by an increase in temperature with z in a pure photoionization model without a secondary source of ionization. Runs of [S II]/Hα, [N II]/Hα, and [S II]/[N II] in each of the four galaxies are consistent with such an increase. However, the runs of [O III]/Hα versus z in NGC 5775 and UGC 10288 require unusually high-ionization fractions of O++ that cannot be explained without invoking a secondary ionization source or at the very least a much higher temperature for the [O III]-emitting component than for the [S II]- and [N II]-emitting component. An increase in temperature with z is generally more successful at explaining the [O III]/Hα run in NGC 891, with the ionization fraction of O++ remaining relatively low and nearly constant with z.

Integral Field Unit Observations of NGC 891: Kinematics of the Diffuse Ionized Gas Halo

We present high and moderate spectral resolution spectroscopy of diffuse ionized gas (DIG) emission in the halo of NGC 891. The data were obtained with the SparsePak integral field unit at the WIYN Observatory. The wavelength coverage includes the [N II] λλ6548, 6583, Hα, and [S II] λλ6716, 6731 emission lines. Position-velocity (PV) diagrams, constructed using spectra extracted from four SparsePak pointings in the halo, are used to examine the kinematics of the DIG. Using two independent methods, a vertical gradient in azimuthal velocity is found to be present in the northeast quadrant of the halo, with magnitude approximately 15-18 km s-1 kpc-1, in agreement with results from H I observations. The kinematics of the DIG suggests that this gradient begins at approximately 1 kpc above the midplane. In another part of the halo, the southeast quadrant, the kinematics is markedly different and suggest rotation at about 175 km s-1, much slower than the disk but with no vertical gradient. We use an entirely ballistic model of disk-halo flow in an attempt to reproduce the kinematics observed in the northeast quadrant. Analysis shows that the velocity gradient predicted by the ballistic model is far too shallow. Based on intensity cuts made parallel to the major axis in the ballistic model and an Hα image of NGC 891 from the literature, we conclude that the DIG halo is much more centrally concentrated than the model, suggesting that hydrodynamics dominate over ballistic motion in shaping the density structure of the halo. Velocity dispersion measurements along the minor axis of NGC 891 seem to indicate a lack of radial motions in the halo, but the uncertainties do not allow us to set firm limits.

Kinematics of Diffuse Ionized Gas Halos: A Ballistic Model of Halo Rotation

To better understand diffuse ionized gas (DIG) kinematics and halo rotation in spiral galaxies, we have developed a model in which clouds are ejected from the disk and follow ballistic trajectories through the halo. The behavior of clouds in this model has been investigated thoroughly through a parameter space search and a study of individual cloud orbits. Synthetic velocity profiles have been generated in z (height above the plane) from the models for the purpose of comparing with velocity centroid data from previously obtained long-slit spectra of the edge-on spirals NGC 891 (one slit) and NGC 5775 (two slits). In each case, a purely ballistic model is insufficient to explain observed DIG kinematics. In the case of NGC 891, the observed vertical velocity gradient is not as steep as predicted by the model, possibly suggesting a source of coupling between disk and halo rotation or an outwardly directed pressure gradient. The ballistic model more successfully explains DIG kinematics observed in NGC 5775; however, it cannot explain the observed trend of high-z gas velocities nearly reaching the systemic velocity. Such behavior can be attributed to either an inwardly directed pressure gradient or a possible tidal interaction with its companion, NGC 5774. In addition, the ballistic model predicts that clouds move radially outward as they cycle through the halo. The mass and energy fluxes estimated from the model suggest that this radially outward gas migration leads to a redistribution of material that may significantly affect the evolution of the interstellar medium.

The pattern speeds of m51, m83 and ngc 6946 using co and the tremaine-Weinberg method

In spiral galaxies in which the molecular phase dominates the ISM, the molecular gas as traced by CO emission will approximately obey the continuity equation on orbital timescales. The Tremaine-Weinberg method can then be used to determine the pattern speed of such galaxies. We have applied the method to single-dish CO maps of three nearby spirals, M51, M83, and NGC 6946, to obtain estimates of their pattern speeds: 38 ± 7, 45 ± 8, and 39 ± 8 km s-1 kpc-1, respectively, and we compare these results to previous measurements. We also analyze the major sources of systematic errors in applying the Tremaine-Weinberg method to maps of CO emission.