arXiv subjects

Richard J. Rand

Publications and source records attributed to Richard J. Rand.

At least 37 records · Page 2Linked to original sources

arXiv2014

Investigating Disk-Halo Flows and Accretion: A Kinematic and Morphological Analysis of Extraplanar HI in NGC 3044 and NGC 4302

To further understand the origins of and physical processes operating in extra-planar gas, we present observations and kinematic models of HI in the two nearby, edge-on spiral galaxies NGC 3044 and NGC 4302. We model NGC 3044 as a single, thick disk. Substantial amounts of extra-planar HI are also detected. We detect a decrease in rotation speed with height (a lag) that shallows radially, reaching zero at approximately R25. The large-scale kinematic asymmetry of the approaching and receding halves suggests a recent disturbance. The kinematics and morphology of NGC 4302, a Virgo Cluster member, are greatly disturbed. We model NGC 4302 as a combination of a thin disk and a second, thicker disk, the latter having a hole near the center. We detect lagging extra-planar gas, with indications of shallowing in the receding half, although its characteristics are difficult to constrain. A bridge is detected between NGC 4302 and its companion, NGC 4298. We explore trends involving the extra-planar HI kinematics of these galaxies, as well as galaxies throughout the literature, as well as possible connections between lag properties with star formation and environment. Measured lags are found to be significantly steeper than those modeled by purely ballistic effects, indicating additional factors. Radial shallowing of extra-planar lags is typical and occurs between 0.5R25 and R25, suggesting internal processes are important in dictating extra-planar kinematics.

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arXiv2014

The Interstellar Medium and Star Formation in Edge-On Galaxies. II. NGC 4157, 4565, and 5907

We present a study of the vertical structure of the gaseous and stellar disks in a sample of edge-on galaxies (NGC 4157, 4565, and 5907) using BIMA/CARMA 12CO (J = 1 --> 0), VLA H I, and Spitzer 3.6 micron data. In order to take into account projection effects when we measure the disk thickness as a function of radius, we first obtain the inclination by modeling the radio data. Using the measurement of the disk thicknesses and the derived radial profiles of gas and stars, we estimate the corresponding volume densities and vertical velocity dispersions. Both stellar and gas disks have smoothly varying scale heights and velocity dispersions, contrary to assumptions of previous studies. Using the velocity dispersions, we find that the gravitational instability parameter Q follows a fairly uniform profile with radius and is greater than or equal to 1 across the star forming disk. The star formation law has a slope that is significantly different from those found in more face-on galaxy studies, both in deprojected and pixel-by-pixel plots. Midplane gas pressure based on the varying scale heights and velocity dispersions appears to roughly hold a power-law correlation with the midplane volume density ratio.

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arXiv2013

CHANG-ES III: UGC10288 -- An Edge-on Galaxy with a Background Double-lobed Radio Source

This 3rd paper in the CHANG-ES series shows the first results from our regular data taken with the Karl G. Jansky Very Large Array (JVLA). The edge-on galaxy, UGC 10288, has been observed in the B, C, and D configurations at L-band (1.5 GHz) and in the C and D configurations at C-band (6 GHz) in all polarization products. We show the first spatially resolved images in these bands,the first polarization images, and the first composed image at an intermediate frequency (4.1 GHz) which has been formed from a combination of all data sets. A surprising new result is the presence of a strong, polarized, double-lobed extragalactic radio source ({\it CHANG-ES A}) almost immediately behind the galaxy and perpendicular to its disk. The core of {\it CHANG-ES A} has an optical counterpart at a photometric redshift of $z_{phot}\,=\,0.39$; the southern radio lobe is behind the disk of UGC 10288 and the northern lobe is behind the halo region. This background `probe' has allowed us to do a preliminary Faraday Rotation analysis of the foreground galaxy, putting limits on the regular magnetic field and electron density in the halo of UGC 10288 in regions in which there is no direct detection of a radio continuum halo. We have revised the flux densities of the two sources individually as well as the star formation rate (SFR) for UGC 10288. UGC 10288 would have fallen well below the CHANG-ES flux density cutoff, had it been considered without the brighter contribution of the background source. UGC 10288 shows discrete high-latitude radio continuum features, but it does not have a {\it global} radio continuum halo. The total minimum magnetic field strength at a sample position in the arc is $\sim$ 10 $μ$G. Thus, this galaxy still appears to be able to form substantial high latitude, localized features in spite of its relatively low SFR.

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arXiv2012

HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4565

We present 21-cm observations and models of the neutral hydrogen in NGC 4565, a nearby, edge-on spiral galaxy, as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. These models provide insight concerning both the morphology and kinematics of HI above, as well as within, the disk. NGC 4565 exhibits a distinctly warped and asymmetric disk with a flaring layer. Our modeling provides no evidence for a massive, extended HI halo. We see evidence for a bar and associated radial motions. Additionally, there are indications of radial motions within the disk, possibly associated with a ring of higher density. We see a substantial decrease in rotational velocity with height above the plane of the disk (a lag) of -40 +5/-20 km/s/kpc and -30 +5/-30 km s/kpc in the approaching and receding halves, respectively. This lag is only seen within the inner ~4.75' (14.9 kpc) on the approaching half and ~4.25' (13.4 kpc) on the receding, making this a radially shallowing lag, which is now seen in the HI layers of several galaxies. When comparing results for NGC 4565 and those for other galaxies, there are tentative indications of high star formation rate per unit area being associated with the presence of a halo. Finally, HI is found in two companion galaxies, one of which is clearly interacting with NGC 4565.

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arXiv2012

Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- I: Introduction to the Survey

We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky Very Large Array (i.e. the Expanded Very Large Array, or EVLA) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared - radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of AGNs. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.

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arXiv2012

Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- II: First Results on NGC 4631

We present the first results from the CHANG-ES survey, a new survey of 35 edge-on galaxies to search for both in-disk as well as extra-planar radio continuum emission. The motivation and science case for the survey are presented in a companion paper (Paper I). In this paper (Paper II), we outline the observations and data reduction steps required for wide-band calibration and mapping of EVLA data, including polarization, based on C-array test observations of NGC 4631. With modest on-source observing times (30 minutes at 1.5 GHz and 75 minutes at 6 GHz for the test data) we have achieved best rms noise levels of 22 and 3.5 $μ$Jy beam$^{-1}$ at 1.5 GHz and 6 GHz, respectively. New disk-halo features have been detected, among them two at 1.5 GHz that appear as loops in projection. We present the first 1.5 GHz spectral index map of NGC 4631 to be formed from a single wide-band observation in a single array configuration. This map represents tangent slopes to the intensities within the band centered at 1.5 GHz, rather than fits across widely separated frequencies as has been done in the past and is also the highest spatial resolution spectral index map yet presented for this galaxy. The average spectral index in the disk is $\barα_{1.5 GHz}\,=\,-0.84\,\pm\,0.05$ indicating that the emission is largely non-thermal, but a small global thermal contribution is sufficient to explain a positive curvature term in the spectral index over the band. Two specific star forming regions have spectral indices that are consistent with thermal emission. Polarization results (uncorrected for internal Faraday rotation) are consistent with previous observations and also reveal some new features. On broad scales, we find strong support for the notion that magnetic fields constrain the X-ray emitting hot gas.

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arXiv2012

Wheels of Fire IV. Star Formation and the Neutral Interstellar Medium in the Ring Galaxy AM0644-741

We combine data from the ATNF and the SEST to investigate the neutral ISM in AM0644-741, a large and robustly star-forming ring galaxy. The galaxy's ISM is concentrated in the 42-kpc diameter starburst ring, but appears dominated by atomic gas, with a global molecular fraction (f_mol) of only 7.9%. Apart from the starburst peak, the gas ring is stable against the growth of gravitational instabilities (Q_gas=2-7). Including stars lowers Q overall, but not enough to make Q<1 everywhere. The ring's global star formation efficiency (SFE) appears somewhat elevated, but varies around the ring by more than an order of magnitude, peaking where star formation is most intense. AM0644-741's star formation law is peculiar: HI follows a Schmidt law while H2 is uncorrelated with SFR/area. Photodissociation models yield low volume densities in the ring, particularly in the starburst quadrant (n~2 cm^-3), implying a warm neutral medium dominated ISM. At the same time, the ring's pressure and ambient far-ultraviolet radiation field lead to the expectation of a predominantly molecular ISM. We argue that the ring's peculiar star formation law, n, SFE, and f_mol result from the ISM's >100 Myr confinement time in the starburst ring, which enhances the destructive effects of embedded massive stars and supernovae. As a result, the ring's molecular ISM becomes dominated by small clouds where star formation is most intense, causing H2 to be underestimated by 12CO line fluxes: in effect X(CO) >> X(Gal) despite the ring's solar metallicity. The observed large HI component is primarily a low density photodissociation product, i.e., a tracer rather than a precursor of massive star formation. Such an "over-cooked" ISM may be a general characteristic of evolved starburst ring galaxies.

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arXiv2011

HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4244

We present 21-cm observations and models of the HI kinematics and distribution of NGC 4244, a nearby edge-on Scd galaxy observed as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. Our models give insight into the HI kinematics and distribution with an emphasis on the potential existence of extra-planar gas as well as a negative gradient in rotational velocity with height above the plane of the disk (a lag). Our models yield strong evidence against a significantly extended halo and instead favor a warp component along the line of sight as an explanation for some of the observed thickening of the disk. Based on these models, we detect a lag of -9 +3/-2 km s-1 kpc-1 in the approaching half and -9 +/-2 km s-1 kpc-1 in the receding half. This lag decreases in magnitude to -5+/-2 km s-1 kpc-1 and -4+/-2 km s-1 kpc-1 near a radius of 10 kpc in the approaching and receding halves respectively. Additionally, we detect several distinct morphological and kinematic features including a shell that is probably driven by star formation within the disk.

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arXiv2011

Infrared Spectroscopy of Halos of Edge-on Galaxies

We present a study of ionized gas, PAHs, and molecular hydrogen emission in the halos of three edge-on galaxies, NGC 891, NGC 5775 and NGC 3044, based on 10-20 micron Spitzer Space Telescope spectra. The [Ne III]/[Ne II] ratio, an excellent measure of radiation hardness, rises with z in the halo of NGC 891. It is also higher in the halo of NGC 5775 than in the disk. NGC 3044 presents a more confusing situation. To explain the [Ne III]/[Ne II] as well as optical line ratio behavior in NGC 891, we carry out a simple exploration of parameter space with CLOUDY, which indicates a large increase in radiation temperature with height. Illustrative examples of physical models using a Monte Carlo radiative transfer code show that the rising neon ratio may be explained by adding a vertically extended, hot stellar source to a thin disk of massive stars. However, several other sources of hard spectra may be relevant. PAH features have scale heights of 430--530 pc in NGC 891 and 720--1080 pc in NGC 5775, suggesting they can be transported by disk-halo flows. Within NGC 891 and NGC 5775, scale heights are similar for all PAHs. For NGC 891, the scale heights exceed that of 8 micron emission, indicating a transition from more ionized to more neutral PAHs with height. Most PAH equivalent widths are higher in the halos. Molecular hydrogen 17.03 micron emission with scale heights of 550-580 pc in NGC 891 and 850 pc in NGC 5775 suggests a molecular component in a surprisingly thick layer.

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arXiv2009

Uncovering the Origins of Spiral Structure by Measuring Radial Variation in Pattern Speeds

Current theories of spiral and bar structure predict a variety of pattern speed behaviors, calling for detailed, direct measurement of the radial variation of pattern speeds. Our recently developed Radial Tremaine-Weinberg (TWR) method allows this goal to be achieved for the first time. Here we present TWR spiral pattern speed estimates for M101, IC 342, NGC 3938 and NGC 3344 in order to investigate whether spiral structure is steady or winding, whether spirals are described by multiple pattern speeds, and the relation between bar and spiral speeds. Where possible, we interpret our pattern speeds estimates according to the resonance radii associated with each (established with the disk angular rotation), and compare these to previous determinations. By analyzing the high-quality HI and CO data cubes available for these galaxies, we show that it is possible to determine directly multiple pattern speeds within these systems, and hence identify the characteristic signatures of the processes that drive the spiral structure. Even this small sample of galaxies reveals a surprisingly complex taxonomy, with the first direct evidence for the presence of resonant coupling of multiple patterns found in some systems, and the measurement of a simple single pattern speed in others. Overall, this study demonstrates that we are now in a position to uncover more of the apparently complex physics that lies behind spiral structure.

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arXiv2008

Radial Dependence of the Pattern Speed of M51

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

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arXiv2008

Infrared Spectroscopy of the Diffuse Ionized Halo of NGC 891

We present infrared spectroscopy from the Spitzer Space Telescope at one disk position and two positions at a height of 1 kpc from the disk in the edge-on spiral NGC 891, with the primary goal of studying halo ionization. Our main result is that the [Ne III]/[Ne II] ratio, which provides a measure of the hardness of the ionizing spectrum free from the major problems plaguing optical line ratios, is enhanced in the extraplanar pointings relative to the disk pointing. Using a 2D Monte Carlo-based photo-ionization code which accounts for the effects of radiation field hardening, we find that this trend cannot be reproduced by any plausible photo-ionization model, and that a secondary source of ionization must therefore operate in gaseous halos. We also present the first spectroscopic detections of extraplanar PAH features in an external normal galaxy. If they are in an exponential layer, very rough emission scale-heights of 330-530 pc are implied for the various features. Extinction may be non-negligible in the midplane and reduce these scale-heights significantly. There is little significant variation in the relative emission from the various features between disk and extraplanar environment. Only the 17.4 micron feature is significantly enhanced in the extraplanar gas compared to the other features, possibly indicating a preference for larger PAHs in the halo.

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arXiv2008

Vertically Extended Neutral Gas in the Massive Edge-on Spiral NGC 5746

We present Very Large Array 21-cm observations of the massive edge-on spiral galaxy NGC 5746. This galaxy has recently been reported to have a luminous X-ray halo, which has been taken as evidence of residual hot gas as predicted in galaxy formation scenarios. Such models also predict that some of this gas should undergo thermal instabilities, leading to a population of warm clouds falling onto the disk. If so, then one might expect to find a vertically extended neutral layer. We detect a substantial high-latitude component, but conclude that almost all of its mass of 1.2-1.6 billion solar masses most likely resides in a warp. Four features far from the plane containing about 100 million solar masses are found at velocities distinct from this warp. These clouds may be associated with the expected infall, although an origin in a disk-halo flow cannot be ruled out, except for one feature which is counter-rotating. The warp itself may be a result of infall according to recent models. But clearly this galaxy lacks a massive, lagging neutral halo as found in NGC 891. The disk HI is concentrated into two rings of radii 1.5 and 3 arcminutes. Radial inflow is found in the disk, probably due to the bar in this galaxy. A nearby member of this galaxy group, NGC 5740, is also detected. It shows a prominent one-sided extension which may be the result of ram pressure stripping.

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arXiv2007

Tests of the Radial Tremaine-Weinberg Method

At the intersection of galactic dynamics, evolution and global structure, issues such as the relation between bars and spirals and the persistence of spiral patterns can be addressed through the characterization of the angular speeds of the patterns and their possible radial variation. The Radial Tremaine-Weinberg (TWR) Method, a generalized version of the Tremaine-Weinberg method for observationally determining a single, constant pattern speed, allows the pattern speed to vary arbitrarily with radius. Here, we perform tests of the TWR method with regularization on several simulated galaxy data sets. The regularization is employed as a means of smoothing intrinsically noisy solutions, as well as for testing model solutions of different radial dependence (e.g. constant, linear or quadratic). We test these facilities in studies of individual simulations, and demonstrate successful measurement of both bar and spiral pattern speeds in a single disk, secondary bar pattern speeds, and spiral winding (in the first application of a TW calculation to a spiral simulation). We also explore the major sources of error in the calculation and find uncertainty in the major axis position angle most dominant. In all cases, the method is able to extract pattern speed solutions where discernible patterns exist to within 20% of the known values, suggesting that the TWR method should be a valuable tool in the area of galactic dynamics. For utility, we also discuss the caveats in, and compile a prescription for, applications to real galaxies.

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arXiv2007

Integral Field Unit Observations of NGC 4302: Kinematics of the Diffuse Ionized Gas Halo

We present moderate resolution spectroscopy of extraplanar diffuse ionized gas (EDIG) emission in the edge-on spiral galaxy NGC 4302. The spectra were obtained with the SparsePak integral field unit (IFU) at the WIYN Observatory. The spectra are used to construct position-velocity (PV) diagrams at several ranges of heights above the midplane. Azimuthal velocities are directly extracted from the PV diagrams using the envelope tracing method, and indicate an extremely steep dropoff in rotational velocity with increasing height, with magnitude ~30 km/s/kpc. We find evidence for a radial variation in the velocity gradient on the receding side. We have also performed artificial observations of galaxy models in an attempt to match the PV diagrams. The results of a statistical analysis also favor a gradient of ~30 km/s/kpc. We compare these results with an entirely ballistic model of disk-halo flow, and find a strong dichotomy between the observed kinematics and those predicted by the model. The disagreement is worse than we have found for other galaxies in previous studies. The conclusions of this paper are compared to results from two other galaxies, NGC 5775 and NGC 891. We find that the vertical gradient in rotation speed, per unit EDIG scale height, for all three galaxies is consistent with a constant magnitude (within the errors) of approximately 15-25 km/s/scaleheight, independent of radius. This relationship is also true within the galaxy NGC 4302. We also discuss how the gradient depends on the distribution and morphology of the EDIG and the star formation rates of the galaxies, and consequences for the origin of the gas.

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arXiv2006

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 [NII]6548,6583, Halpha, and [SII]6716,6731 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/kpc, in agreement with results from HI observations. The kinematics of the DIG suggest that this gradient begins at approximately 1 kpc above the midplane. In another part of the halo, the southeast quadrant, the kinematics are markedly different, and suggest rotation at about 175 km/s, much slower than the disk but with no vertical gradient. We utilize 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 Halpha 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.

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arXiv2004

Pattern Speeds of BIMA-SONG Galaxies with Molecule-Dominated ISMs Using the Tremaine-Weinberg Method

We apply the Tremaine-Weinberg method of pattern speed determination to data cubes of CO emission in six spiral galaxies from the BIMA SONG survey each with an ISM dominated by molecular gas. We compare derived pattern speeds with estimates based on other methods, usually involving the identification of a predicted behavior at one or more resonances of the pattern(s). In two cases (NGC 1068 and NGC 4736) we find evidence for a central bar pattern speed that is greater than that of the surrounding spiral and roughly consistent with previous estimates. However, the spiral pattern speed in both cases is much larger than previous determinations. For the barred spirals NGC 3627 and NGC 4321, the method is insensitive to the bar pattern speed (the bar in each is nearly parallel to the major axis; in this case the method will not work), but for the former galaxy the spiral pattern speed found agrees with previous estimates of the bar pattern speed, suggesting that these two structures are part of a single pattern. For the latter, the spiral pattern speed found is in agreement with several previous determinations. For the flocculent spiral NGC 4414 and the ``Evil Eye'' galaxy NGC 4826, the method does not support the presence of a large-scale coherent pattern. We also apply the method to a simulated barred galaxy in order to demonstrate its validity and to understand its sensitivity to various observational parameters. In addition, we study the results of applying the method to a simulated, clumpy axisymmetric disk with no wave present. The TW method in this case may falsely indicate a well-defined pattern.

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arXiv2000

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. 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 photo-ionization/shock models are generally better at explaining runs of line ratios in these galaxies than photo-ionization models alone. Models of line ratios in NGC 5775 require a greater contribution from shocks for filamentary regions than for non-filamentary regions to explain the run of [OIII]/Halpha. In either case, the [SII]/[NII] ratio is not well fit by the models. Composite models for UGC 10288 are successful at reproducing the run of [SII]/[NII] for all but the the highest values of [NII]/Halpha; however, the run of [OIII]/Halpha vs. [NII]/Halpha does not show any discernible trend, making it difficult to determine whether or not shocks make a contribution. We also examine whether the data can be explained simply by an increase in temperature with z in a pure photo-ionization model. Runs of [SII]/Halpha, [NII]/Halpha, and [SII]/[NII] in each of the four galaxies are consistent with such an increase. However, the runs of [OIII]/Halpha vs. z in NGC 5775 and UGC 10288 require unusually high ionization fractions of O^{++} that can not be explained without invoking a secondary ionization source or at the very least a much higher temperature for the [OIII]-emitting component than for the [SII]- and [NII]-emitting component. An increase in temperature with z is generally more successful at explaining the [OIII]/Halpha run in NGC 891.

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