RINGS: the RSS Imaging Spectroscopy Nearby Galaxies Survey |
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Simulations of structure formation within the canonical cosmological framework now explain an impressive array of global galaxy properties. In concert with prescriptions for baryonic feedback processes, for instance, recent models reproduce the redshift dependence of the colours, luminosities, and morphologies of the bright galaxy population (e.g. Dekel & Birnboim 2006). By contrast, there has been little growth of knowledge regarding the internal structure and dynamics of galaxies in the ``era of precision cosmology". This is particularly the case for disk galaxies, where contributions to the mass budget from the dark matter and baryons therein can be constrained from mass models of measured rotation curves. While the average halo concentrations inferred from the kinematics of dark matter-dominated galaxies agree with those implied by recent cosmological measurements (Spergel et al. 2006), the dynamics of some well-studied systems are still strongly inconsistent with the latter (e.g. Simon et al. 2003). The interplay between dark matter and baryons during galaxy formation is also poorly understood. For a range of stellar mass-to-light ratios, standard corrections for halo contraction due to infalling baryons produce underdense inner halos profiles in {\it all} normal spirals (McGaugh et al. 2007). The structure and dynamics of disk galaxies thus remain among the most persistent and serious observational challenges for the LCDM framework. The problems described above may indicate fundamental flaws in our understanding of disk galaxy formation, or the need for exotic new physics on galaxy scales. There are also avenues for understanding them within the current paradigm, however: non-circular motions, gas pressure, projection and resolution effects, halo triaxiality, and dynamical friction have all been invoked to reconcile observed kinematics with predicted galaxy structure (e.g. Valenzuela et al. 2007). While a consensus on the importance of these processes has yet to be reached, it is now clear that progress on this front requires higher quality data than have been obtained in the past. In particular, line-of-sight velocity fields that combine kinematics beyond the optical disk with high-resolution tracers across it are needed, in order to constrain the shape of the dark matter halo and asymmetries in the baryons, respectively. This is best achieved with a multi-wavelength approach: HI observations are essential for tracing the potential in the outer regions of spirals, and Halpha imaging spectroscopy maps their inner disk dynamics on arcsecond scales. With the advent of high-throughput optical detectors and the sensitivity of existing radio ones, hybrid HI + Halpha kinematics for large numbers of systems are now within reach. The field of disk galaxy dynamics is therefore poised for a renaissance, in which long-standing problems can be effectively tackled with new observational tools. To take advantage of these opportunities, I have initiated a program in collaboration with T. B. Williams and J. A. Sellwood to produce homogeneous, multi-wavelength velocity fields for a large sample of normal spirals. I am spearheading RINGS: the Rutgers Imaging spectroscopy Nearby Galaxies Survey, that will deliver aperture synthesis HI observations from the Very Large Array and Fabry-Perot Halpha kinematics from the Robert Stobie Spectrograph (RSS) on the new 10-m South African Large Telescope (SALT) for 24 nearby disk galaxies. Observing time has also been allocated to carry out broad-band photometry for each system, in order to constrain the morphologies and mass-to-light ratios of their stellar populations. In addition, we plan to search for low column-density gas in a subset of the sample via deep single-dish HI observations (see below). RINGS will thus double the number of local spirals with kinematics and photometry suitable for detailed dynamical studies. We are also developing new techniques to model RINGS data products. The high throughput and large field-of-view of RSS yield inner disk velocity fields with 1"-2" resolution rather than the standard 4"-5", facilitating studies of non-circular motions within the optical disk. We have devised a new model for these asymmetries, under the assumption that they stem from a bar-like or oval distortion to the potential (see below). Mass models of the resulting kinematics together with photometric constraints will allow for quantitative estimates of the impact of these asymmetries on the inferred halo profiles. The analysis will culminate with numerical simulations of the optimal mass model outputs to examine their stability, which might help mitigate the degeneracy between solutions with different dark and luminous contributions to the potential (e.g. Widrow & Dubinsky 2005). RINGS will thus place the tightest possible constraints on the halo profiles of a large number of normal spirals, addressing the key questions raised above: how do the baryons in spiral galaxies influence their dark matter halos? Are their halo density distributions compatible with LCDM predictions when these processes are taken into account? |
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Bisymmetric Distortions in Disk Galaxy Kinematics |
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Velocity field studies have demonstrated that kinematic asymmetries are more important in low-mass galaxies than previously thought. They are typically characterized as radial motions, although coherent flows of the magnitudes reported require prohibitively large gas transfers in a dynamical time (Simon et al. 2005). In collaboration with J. A. Sellwood, I am testing the hypothesis that these asymmetries stem from bar-like or oval distortions rather than from pure radial flows. We have adapted the minimization technique of Barnes & Sellwood (2003) to model disk galaxy velocity fields as a superposition of an axisymmetric flow and elliptical stream lines, and have studied the kinematics of the nearby dwarf spiral NGC~2976 in detail. We found that while the data accommodate non-circular flow models with either radial motions or a bisymmetric distortion, only the latter model reflects the true physical structure of the system: the baryons in NGC 2976 therefore dominate its kinematics out to much larger radii than previously thought (Spekkens & Sellwood 2007). We are in the process of carrying out self-consistent hydrodynamic simulations of this system, which will enable a detailed study of the dark and luminous contributions to its kinematics. |
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Breaking the Disk-Halo Degeneracy in Huchra's Lens |
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I am interested in exploiting the properties of extreme extragalactic systems to gain insight into galaxy structure. I am coordinating the Rutgers early science project for the Robert Stobie Spectrograph on the Southern African Large Telescope to analyse long-slit spectra of CGCG 378-015, a spiral galaxy whose bulge lenses a background quasar into 4 images. "Huchra's Lens'' is one of the few objects for which a joint analysis of the disk dynamics and gravitational lensing is possible: this may allow for the well-known degeneracy between the baryonic and dark matter contributions to the kinematics of disk galaxies may be broken. The expertise at Rutgers in disk dynamics and gravitational lensing will be tapped to carry out a definitive mass decomposition of the system with these two complementary techniques. |
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Kinematics of Rapidly Rotating Late-Type Spirals |
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The kinematics of spirals at the high-mass end of the galaxy scaling relations are also powerful probes of disk galaxy structure. In collaboration with R. Giovanelli, I obtained HI aperture synthesis observations of a sample of rapidly rotating spiral galaxies with extant I-band photometry and long-slit Halpha spectra (Spekkens & Giovanelli 2006). Contrary to previous claims, we found that the rotation curves of these ``fast rotators'' remain flat well beyond their optical radii, implying that the global properties of high surface brightness disks do not correlate with rotation curve shape. I am currently working with the dark matter group at the International School for Advanced Studies (SISSA) to corroborate this result using high-quality rotation curves from the literature. The angular momentum distributions of the fast rotators also test an explanation for the observed upper bound to spiral galaxy rotation velocities. If disks are stabilized by their parent halos and galaxy baryonic masses scale with halo masses, then a stability criterion of the form lambda > md arises, where lambda is the halo spin parameter and md is the baryon fraction of the system (e.g. Mo et al. 1998). Since collisionless halos with lambda > 0.1 are rare, massive systems that retain their baryons during galaxy formation seldom have large enough lambda to form stable disks. In this scenario, massive spirals have large lambda. However, mass models of the fast rotator kinematics produce values of lambda that are comparable to those of low-mass galaxies for a wide range of stellar mass-to-light ratios. Our findings thus imply an alternate mechanism for disk stability in spirals and/or a disconnect between the angular momenta of disks and that of their parent halos (Spekkens & Giovanelli, in preparation). |
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The Cusp/Core Problem in Low-Mass Galaxies |
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Collisionless simulations of the growth of structure in the LCDM framework produce ``cuspy'' halos with rho(r) ~ r^{-alpha} and alpha ~ 1 at small halo radii. Dwarf galaxies are well-suited to the search for cusps since their baryons are believed to have little impact on the structure of their dark matter halos; the latter should thus be directly comparable to theoretical expectations. However, kinematics derived from long-slit optical spectroscopy of low-mass systems generally produce ``cores" that depend little on radius. Contradictory conclusions regarding the validity of the standard cosmology on galaxy scales thus arose (e.g. Swaters et. al. 2003): while some advocated a theoretical crisis, other argued that long-slit observing biases turn intrinsic cusps into measured cores. To address this issue, we measured the inner halo profiles of 165 low-mass systems, and simulated observations of model galaxies to quantify the impact of the observing conditions on the resulting rotation curves (Spekkens et al. 2005). We concluded that the sample kinematics are consistent with any alpha <= 1 when observing biases are taken into account, and thus that the measured cores are not in conflict with LCDM predictions. Many of the limitations that hinder long-slit studies are avoided when inner halo profiles are measured from velocity fields, and assessing the extent to which cusps can be distinguished from cores with high-quality kinematics is a major science goal for RINGS. I am also collaborating with J. Simon and A. Bolatto to expand their sample of high-quality Halpha kinematics for local dwarf galaxies using the DesnePak and SparsePak arrays on WIYN. |
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Single-Dish HI Legacy Surveys with ALFA |
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The Arecibo L-band feed array (ALFA) allows for efficient HI mapping with the most sensitive radio telescope in the world. A series of legacy-style surveys are underway to carry out the key science enabled by ALFA, and their extensive data products will be accessible to the astronomical community at large. The ALFA surveys afford new opportunities to constrain galaxy formation and evolution from the morphology and dynamics of low column-density HI. The Arecibo Legacy Fast ALFA survey (ALFALFA; Giovanelli et al. 2005) is a blind HI survey that will map 7000 square degrees at high galactic latitudes with unprecedented sensitivity and resolution. Designed to constrain the HI mass function below 10^8 M_sun, it is particularly sensitive to low-mass, gas-rich systems. I have been a member of the ALFALFA survey team since its inception in 2003, and carried out some of the precursor observations that led to the development of the double-pass, limited-azimuth drift survey strategy ultimately adopted. I am currently spearheading follow-up aperture synthesis observations of an HI cloud complex in the Virgo cluster discovered with ALFALFA that has no obvious optical counterparts: are the clouds members of a long-sought, low-mass ``dark galaxy'' population (e.g. Minchin et al. 2005), or relics of galaxy harassment or ram pressure mechanisms in the cluster environment? I am also a member of the Arecibo Galaxy Environment Survey team (AGES; Auld et al. 2006). My particular interest lies in deep HI mapping of nearby spirals to probe the extent of their HI disks: the existence of reservoirs of low column-density atomic gas would impact the star formation and feedback processes in spirals, and their dynamics would place strong constraints on the underlying halo mass profiles. |
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Interacting, Isolated Spirals and the Hunt for Dark Galaxies |
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I am working with D. Maletic at Rutgers University to obtain aperture synthesis HI observations of the few isolated spiral galaxies in the local Universe that have disturbed optical disks. Their HI morphologies and dynamics are sensitive probes of tidal interactions, and any optically faint, gas-rich companions in their vicinity should be readily detected. The observations should thus help test the hypothesis that these systems are interacting with bona-fide dark galaxies (Karachentsev et al. 2006). |
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High-Latitude HI in NGC 5433 and its Environment |
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There is now considerable evidence for a disk-halo connection in spiral galaxies. The role of galaxy environment in fueling the outflows observed, however, is poorly understood. In collaboration with J. A. Irwin, I examined the HI morphology and kinematics of the edge-on starburst NGC 5433 (known to have a thick radio continuum disk) and its environment in an attempt to address this issue (Spekkens et al. 2004). In collaboration with J. A. Irwin, I derived HI synthesis maps of NGC 5433 from observations with the VLA in its D-configuration. We later obtained C-configuration follow-up observations, for which I planned and executed each run, reduced the resulting datasets and carried out the majority of the analysis. The resolution of the C-configuration data and the sensitivity of the combined C+D-configuration data allowed us to study the HI content of NGC 5433 in detail and to probe its environment. Spectral models of the main disk emission in NGC 5433 reveal 3 extra-planar features, including a complete loop in position-velocity space. We also find that NGC 5433 is the dominant member of a group of at least 5 galaxies; a variety of evidence suggests that interactions are occurring in this group. We therefore postulate that NGC 5433's environment may indeed play a role in the generation of the high-latitude features observed. |
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