arXiv subjects

A. C. Quillen

Publications and source records attributed to A. C. Quillen.

At least 37 records · Page 2Linked to original sources

arXiv2012

Radial Migration Does Little for Galactic Disc Thickening

Non-axisymmetric components, such as spirals and central bars, play a major role in shaping galactic discs. An important aspect of the disc secular evolution driven by these perturbers is the radial migration of stars. It has been suggested recently that migration can populate a thick-disc component from inner-disc stars with high vertical energies. Since this has never been demonstrated in simulations, we study in detail the effect of radial migration on the disc velocity dispersion and disc thickness, by separating simulated stars into migrators and non-migrators. We apply this method to three isolated barred Tree-SPH N-body galaxies with strong radial migration. Contrary to expectations, we find that as stellar samples migrate, on the average, their velocity dispersion change (by as much as 50%) in such a way as to approximately match the non-migrating population at the radius at which they arrive. We show that, in fact, migrators suppress heating in parts of the disc. To confirm the validity of our findings, we also apply our technique to three cosmological re-simulations, which use a completely different simulation scheme and, remarkably, find very similar results. We believe the inability of migration to thicken discs is a fundamental property of internal disc evolution, irrespective of the migration mechanism at work. We explain this with the approximate conservation of the (average) vertical and radial actions rather than the energy. This "action mixing" can be used to constrain the migration rate in the Milky Way: estimates of the average vertical action in observations for different populations of stars should reveal flattening with radius for older groups of stars.

astro-ph.GA↗

arXiv2012

Evolution of Galactic Discs: Multiple Patterns, Radial Migration and Disc Outskirts

We investigate the evolution of galactic disks in N-body Tree-SPH simulations. We find that disks, initially truncated at three scale-lengths, can triple their radial extent, solely driven by secular evolution. Both Type I (single exponential) and Type II (down-turning) observed disk surface-brightness profiles can be explained by our findings. We relate these results to the strong angular momentum outward transfer, resulting from torques and radial migration associated with multiple patterns, such as central bars and spiral waves of different multiplicity. We show that even for stars ending up on cold orbits, the changes in angular momentum exhibit complex structure as a function of radius, unlike the expected effect of transient spirals alone. Focussing on one of our models, we find evidence for non-linear coupling among m=1, 2, 3 and 4 density waves, where m is the pattern multiplicity. We suggest that the naturally occurring larger resonance widths at galactic radii beyond four scale-lengths may have profound consequences on the formation and location of breaks in disk density profiles, provided spirals are present at such large distances. We also consider the effect of gas inflow and show that when in-plane smooth gas accretion of ~5 M_sun/yr is included, the outer disks become more unstable, leading to a strong increase in the stellar velocity dispersion. This, in turn, causes the formation of a Type III (up-turning) profile in the old stellar population. We propose that observations of Type III surface brightness profiles, combined with an up-turn in the stellar velocity dispersions beyond the disk break, could be a signature of ongoing gas-accretion. The results of this study suggest that disk outskirts comprised of stars migrated from the inner disk would have relatively large radial velocity dispersions, and significant thickness when seen edge-on. [Abridged]

astro-ph.GA↗

arXiv2012

Residual Cooling and Persistent Star Formation amid AGN Feedback in Abell 2597

New Chandra X-ray and Herschel FIR observations enable a multiwavelength study of active galactic nucleus (AGN) heating and intracluster medium (ICM) cooling in the brightest cluster galaxy of Abell 2597. The new Chandra observations reveal the central < 30 kiloparsec X-ray cavity network to be more extensive than previously thought, and associated with enough enthalpy to theoretically inhibit the inferred classical cooling flow. Nevertheless, we present new evidence, consistent with previous results, that a moderately strong residual cooling flow is persisting at 4%-8% of the classically predicted rates in a spatially structured manner amid the feedback-driven excavation of the X-ray cavity network. New Herschel observations are used to estimate warm and cold dust masses, a lower-limit gas-to-dust ratio, and a star formation rate consistent with previous measurements. The cooling time profile of the ambient X-ray atmosphere is used to map the locations of the observational star formation entropy threshold as well as the theoretical thermal instability threshold. Both lie just outside the < 30 kpc central region permeated by X-ray cavities, and star formation as well as ionized and molecular gas lie interior to both. The young stars are distributed in an elongated region that is aligned with the radio lobes, and their estimated ages are both younger and older than the X-ray cavity network, suggesting both jet-triggered as well as persistent star formation over the current AGN feedback episode. Bright X-ray knots that are coincident with extended Ly-alpha and FUV continuum filaments motivate a discussion of structured cooling from the ambient hot atmosphere along a projected axis that is perpendicular to X-ray cavity and radio axis. We conclude that the cooling ICM is the dominant contributor of the cold gas reservoir fueling star formation and AGN activity in the Abell 2597 BCG.

astro-ph.CO↗

arXiv2012

Multiphase Signatures of AGN Feedback in Abell 2597

We present new Chandra X-ray observations of the brightest cluster galaxy (BCG) in the cool core cluster Abell 2597. The data reveal an extensive kpc-scale X-ray cavity network as well as a 15 kpc filament of soft-excess gas exhibiting strong spatial correlation with archival VLA radio data. In addition to several possible scenarios, multiwavelength evidence may suggest that the filament is associated with multiphase (10^3 - 10^7 K) gas that has been entrained and dredged-up by the propagating radio source. Stemming from a full spectral analysis, we also present profiles and 2D spectral maps of modeled X-ray temperature, entropy, pressure, and metal abundance. The maps reveal an arc of hot gas which in projection borders the inner edge of a large X-ray cavity. Although limited by strong caveats, we suggest that the hot arc may be (a) due to a compressed rim of cold gas pushed outward by the radio bubble or (b) morphologically and energetically consistent with cavity-driven active galactic nucleus (AGN) heating models invoked to quench cooling flows, in which the enthalpy of a buoyant X-ray cavity is locally thermalized as ambient gas rushes to refill its wake. If confirmed, this would be the first observational evidence for this model.

astro-ph.CO↗

arXiv2010

The 1.6 micron near infrared nuclei of 3C radio galaxies: Jets, thermal emission or scattered light?

Using HST NICMOS 2 observations we have measured 1.6-micron near infrared nuclear luminosities of 100 3CR radio galaxies with z<0.3, by modeling and subtracting the extended emission from the host galaxy. We performed a multi-wavelength statistical analysis (including optical and radio data) of the properties of the nuclei following classification of the objects into FRI and FRII, and LIG (low-ionization galaxies), HIG (high-ionization galaxies) and BLO (broad-lined objects) using the radio morphology and optical spectra, respectively. The correlations among near infrared, optical, and radio nuclear luminosity support the idea that the near infrared nuclear emission of FRIs has a non-thermal origin. Despite the difference in radio morphology, the multi-wavelength properties of FRII LIG nuclei are statistically indistinguishable from those of FRIs, an indication of a common structure of the central engine. All BLOs show an unresolved near infrared nucleus and a large near infrared excess with respect to FRII LIGs and FRIs of equal radio core luminosity. This requires the presence of an additional (and dominant) component other than the non-thermal light. Considering the shape of their spectral energy distribution, we ascribe the origin of their near infrared light to hot circumnuclear dust. A near infrared excess is also found in HIGs, but their nuclei are substantially fainter than those of BLO. This result indicates that substantial obscuration along the line-of-sight to the nuclei is still present at 1.6 micron. Nonetheless, HIGs nuclei cannot simply be explained in terms of dust obscuration: a significant contribution from light reflected in a circumnuclear scattering region is needed to account for their multiwavelength properties.

astro-ph.CO↗

arXiv2010

Herschel observations of FIR emission lines in brightest cluster galaxies

The question of how much gas cools in the cores of clusters of galaxies has been the focus of many, multiwavelength studies in the past 30 years. In this letter we present the first detections of the strongest atomic cooling lines, [C II], [O I] and [N I] in two strong cooling flow clusters, A1068 and A2597, using Herschel PACS. These spectra indicate that the substantial mass of cold molecular gas (>10^9 Mo) known to be present in these systems is being irradiated by intense UV radiation, most probably from young stars. The line widths of these FIR lines indicate that they share dynamics similar but not identical to other ionised and molecular gas traced by optical, near-infrared and CO lines. The relative brightness of the FIR lines compared to CO and FIR luminosity is consistent with other star-forming galaxies indicating that the properties of the molecular gas clouds in cluster cores and the stars they form are not unusual. These results provide additional evidence for a reservoir of cold gas that is fed by the cooling of gas in the cores of the most compact clusters and provide important diagnostics of the temperature and density of the dense clouds this gas resides in.

astro-ph.CO↗

arXiv2010

Herschel photometry of brightest cluster galaxies in cooling flow clusters

The dust destruction timescales in the cores of clusters of galaxies are relatively short given their high central gas densities. However, substantial mid-infrared and sub-mm emission has been detected in many brightest cluster galaxies. In this letter we present Herschel PACS and SPIRE photometry of the brightest cluster galaxy in three strong cooling flow clusters, A1068, A2597 and Zw3146. This photometry indicates that a substantial mass of cold dust is present (>3 x 10^7 Mo) at temperatures significantly lower (20-28K) than previously thought based on limited MIR and/or sub-mm results. The mass and temperature of the dust appear to match those of the cold gas traced by CO with a gas-to-dust ratio of 80-120.

astro-ph.CO↗

arXiv2009

Is the Milky Way ringing? The hunt for high velocity streams

We perform numerical simulations of a stellar galactic disk with initial conditions chosen to represent an unrelaxed population which might have been left following a merger. Stars are unevenly distributed in radial action angle, though the disk is axisymmetric. The velocity distribution in the simulated Solar neighborhood exhibits waves traveling in the direction of positive v, where u,v are the radial and tangential velocity components. As the system relaxes and structure wraps in phase space, the features seen in the uv-plane move closer together. We show that these results can be obtained also by a semi-analytical method. We propose that this model could provide an explanation for the high velocity streams seen in the Solar neighborhood at approximate v in km/s, of -60 (HR 1614), -80 (Arifyanto and Fuchs 2006), -100 (Arcturus), and -160 (Klement et al. 2008). In addition, we predict four new features at v ~ -140, -120, 40 and 60 km/s. By matching the number and positions of the observed streams, we estimate that the Milky Way disk was strongly perturbed ~1.9 Gyr ago. This event could have been associated with Galactic bar formation.

astro-ph.GA↗

arXiv2007

The Vertical Structure of Planet-induced Gaps in Proto-Planetary Discs

Giant planets embedded in circumstellar discs are expected to open gaps in these discs. We examine the vertical structure of the gap edges. We find that the planet excites spiral arms with significant (Mach number of a half) vertical motion of the gas, and discuss the implications of these motions. In particular, the spiral arms will induce strong vertical stirring of the dust, making the edge appeared `puffed up' relative to the bulk of the disc. Infra-red observations (sensitive to dust) would be dominated by the light from the thick inner edge of the disc. Sub-millimetre observations (sensitive to gas velocities) would appear to be hot in `turbulent' motions (actually the ordered motion caused by the passage of the spiral arms), but cold in chemistry. Resolved sub-millimetre maps of circumstellar discs might even be able to detect the spiral arms directly.

astro-ph↗

arXiv2007

New Constraints on the Galactic Bar

Previous work has related the Galactic Bar to structure in the local stellar velocity distribution. Here we show that the Bar also influences the spatial gradients of the velocity vector via the Oort constants. By numerical integration of test-particles we simulate measurements of the Oort C value in a gravitational potential including the Galactic Bar. We account for the observed trend that C is increasingly negative for stars with higher velocity dispersion. By comparing measurements of C with our simulations we improve on previous models of the Bar, estimating that the Bar pattern speed is Omega_b/Omega_0=1.87\pm0.04, where Omega_0 is the local circular frequency, and the Bar angle lies within 20<phi_0<45 deg. We find that the Galactic Bar affects measurements of the Oort constants A and B less than ~2 km/s/kpc for the hot stars.

astro-ph↗

arXiv2006

The Radial Velocity Experiment (RAVE): first data release

We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, surface gravity) of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). The RAVE program started in 2003, obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region ($λλ$ 8,410--8,795 Å) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9<I<12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including re-observations). Those data were obtained on 67 nights between 11 April 2003 to 03 April 2004. The total sky coverage within this data release is $\sim$4,760 square degrees. The average signal to noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km/s. Combining internal errors and zero-point errors, the mode is found to be 2 km/s. Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km/s. We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal to noise. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2 and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS and 2MASS). The data release can be accessed via the RAVE webpage: http://www.rave-survey.org.

astro-ph↗

arXiv2005

Disk heating by more than one spiral density wave

We consider a differentially rotating, 2D stellar disk perturbed by two steady state spiral density waves moving at different patterns speeds. Our investigation is based on direct numerical integration of initially circular test-particle orbits. We examine a range of spiral strengths and spiral speeds and show that stars in this time dependent gravitational field can be heated (their random motions increased).This is particularly noticeable in the simultaneous propagation of a 2-armed spiral density wave near the corotation resonance (CR), and a weak 4-armed one near the inner and outer 4:1 Lindblad resonances. In simulations with 2 spiral waves moving at different pattern speeds we find: (1) the variance of the radial velocity, sigma_R^2, exceeds the sum of the variances measured from simulations with each individual pattern; (2) sigma_R^2 can grow with time throughout the entire simulation; (3) sigma_R^2 is increased over a wider range of radii compared to that seen with one spiral pattern; (4) particles diffuse radially in real space whereas they don't when only one spiral density wave is present. Near the CR with the stronger, 2-armed pattern, test particles are observed to migrate radially. These effects take place at or near resonances of both spirals so we interpret them as the result of stochastic motions. This provides a possible new mechanism for increasing the stellar velocity dispersion in galactic disks. If multiple spiral patterns are present in the Galaxy we predict that there should be large variations in the stellar velocity dispersion as a function of radius.

astro-ph↗

arXiv2005

Radial disk heating by more than one spiral density wave

We consider a differentially rotating, 2D stellar disk perturbed by two steady state spiral density waves moving at different patterns speeds. Our investigation is based on direct numerical integration of initially circular test-particle orbits. We examine a range of spiral strengths and spiral speeds and show that stars in this time dependent gravitational field can be heated (their random motions increased). This is particularly noticeable in the simultaneous propagation of a 2-armed spiral density wave near the corotation resonance (CR), and a weak 4-armed one near the inner and outer 4:1 Lindblad resonances. In simulations with 2 spiral waves moving at different pattern speeds we find: (1) the variance of the radial velocity, sigma_R^2, exceeds the sum of the variances measured from simulations with each individual pattern; (2) sigma_R^2 can grow with time throughout the entire simulation; (3) sigma_R^2 is increased over a wider range of radii compared to that seen with one spiral pattern; (4) particles diffuse radially in real space whereas they don't when only one spiral density wave is present. Near the CR with the stronger, 2-armed pattern, test particles are observed to migrate radially. These effects take place at or near resonances of both spirals so we interpret them as the result of stochastic motions. This provides a possible new mechanism for increasing the stellar velocity dispersion in galactic disks. If multiple spiral patterns are present in the Galaxy we predict that there should be large variations in the stellar velocity dispersion as a function of radius.

nlin.CD↗

arXiv2005

870 micron observations of nearby 3CRR radio galaxies

We present submillimeter continuum observations at 870 microns of the cores of low redshift 3CRR radio galaxies, observed at the Heinrich Hertz Submillimeter Telescope. The cores are nearly flat spectrum between the radio and submillimeter which implies that the submillimeter continuum is likely to be synchrotron emission and not thermal emission from dust. The emitted power from nuclei detected at optical wavelengths and in the X-rays is similar in the submillimeter, optical and X-rays. The submillimeter to optical and X-ray power ratios suggest that most of these sources resemble misdirected BL Lac type objects with synchrotron emission peaking at low energies. However we find three exceptions, the FR I galaxy 3C264 and the FR II galaxies 3C390.3 and 3C338 with high X-ray to submillimeter luminosity ratios. These three objects are candidate high or intermediate energy peaked BL Lac type objects. With additional infrared observations and from archival data, we compile spectral energy distributions (SEDs) for a subset of these objects. The steep dips observed near the optical wavelengths in many of these objects suggest that extinction inhibits the detection and reduces the flux of optical continuum core counterparts. High resolution near or mid-infrared imaging may provide better measurements of the underlying synchrotron emission peak.

astro-ph↗

arXiv2005

The Evolution of Protoplanetary Disk Edges

We investigate gap formation in gaseous protostellar disks by a planet in a circular orbit in the limit of low disk viscosity. This regime may be appropriate to an aging disk after the epoch of planet formation. We find that the distance of planet to the gap outer boundary can be between the location of the $m=2$ and $m=1$ outer Lindblad resonances. This distance is weakly dependent upon both the planet's mass and disk viscosity. We find that the evolution of the disk edge takes place on two timescales. The first timescale is set by the spiral density waves driven by the planet. The second timescale depends on the viscosity of the disk. The disk approaches a state where the outward angular momentum flux caused by the disk viscosity is balanced by the dissipation of spiral density waves which are driven at the Lindblad resonances. This occurs inefficiently however because of the extremely low gas density near the planet. We find that the distance between the planet and the peak density at the disk outer edge is only weakly dependent on the viscosity and planet mass, however the ratio of the gas density near the planet to that in the disk (or the slope of density along the disk edge) is strongly dependent upon both quantities. We find that the disk density profile along the edge scales approximately with disk viscosity divided by the square of the planet mass. We account for this behavior with a simple scenario in which the dissipation of angular momentum from the spiral density waves is balanced against diffusion in the steep edge of the disk.

astro-ph↗

arXiv2005

The Effect of Spiral Structure on the Stellar Velocity Distribution in the Solar Neighborhood

Clumps in the solar neighborhood's stellar velocity distribution could be caused by spiral density waves. In the solar neighborhood, stellar velocities corresponding to orbits that are nearly closed in the frame rotating with a spiral pattern represent likely regions for stellar concentrations. Via particle integration, we show that orbits can intersect the solar neighborhood when they are excited by Lindblad resonances with a spiral pattern. We find that a two-armed spiral density wave with pattern speed placing the Sun near the 4:1 Inner Lindblad Resonance (ILR) can cause two families of nearly closed orbits in the solar neighborhood. One family corresponds to square shaped orbits aligned so their peaks lie on top of, and support, the two dominant stellar arms. The second family correspond to orbits 45 degrees out of phase with the other family. Such a spiral density pattern could account for two major clumps in the solar neighborhood's velocity distribution. The Pleiades/Hyades moving group corresponds to the first family of orbits and the Coma Berenices moving group corresponds to the second family. This model requires a spiral pattern speed of approximately 0.66 +- 0.03 times the angular rotation rate of the Sun or 18.1 +- 0.8 km/s/kpc.

astro-ph↗

arXiv2005

Turbulence driven by outflow-blown cavities in the molecular cloud of NGC 1333

Outflows from young stellar objects have been identified as a possible source of turbulence in molecular clouds. To investigate the relationship between outflows, cloud dynamics and turbulence, we compare the kinematics of the molecular gas associated with NGC 1333, traced in 13CO(1-0), with the distribution of young stellar objects (YSOs) within. We find a velocity dispersion of ~ 1-1.6 km/s in 13CO that does not significantly vary across the cloud, and is uncorrelated with the number of nearby young stellar outflows identified from optical and submillimeter observations. However, from velocity channel maps we identify about 20 cavities or depressions in the 13CO intensity of scales > 0.1-0.2 pc and velocity widths 1-3 km/s. The cavities exhibit limb brightened rims in both individual velocity channel maps and position velocity diagrams, suggesting that they are slowly expanding. We interpret these cavities to be remnants of past YSO outflow activity: If these cavities are presently empty, they would fill in on time scales of a million years. This can exceed the lifetime of a YSO outflow phase, or the transit time of the central star through the cavity, explaining the the absence of any clear correlation between the cavities and YSO outflows. We find that the momentum and energy deposition associated with the expansion of the cavities is sufficient to power the turbulence in the cloud. In this way we conclude that the cavities are an important intermediary step between the conversion of YSO outflow energy and momentum into cloud turbulent motions.

astro-ph↗

arXiv2003

Diffuse X-ray emission in spiral galaxies

We compare the soft diffuse X-ray emission from Chandra images of 12 nearby intermediate inclination spiral galaxies to the morphology seen in Halpha, molecular gas, and mid-infrared emission. We find that diffuse X-ray emission is often located along spiral arms in the outer parts of spiral galaxies but tends to be distributed in a rounder morphology in the center. The X-ray morphology in the spiral arms matches that seen in the mid-infrared or Halpha and so implies that the X-ray emission is associated with recent active star formation. We see no strong evidence for X-ray emission trailing the location of high mass star formation in spiral arms. However, population synthesis models predict a high mechanical energy output rate from supernovae for a time period that is about 10 times longer than the lifetime of massive ionizing stars, conflicting with the narrow appearance of the arms in X-rays. The fraction of supernova energy that goes into heating the ISM must depend on environment and is probably higher near sites of active star formation. The X-ray estimated emission measures suggest that the volume filling factors and scale heights are high in the galaxy centers but low in the outer parts of these galaxies. The differences between the X-ray properties and morphology in the centers and outer parts of these galaxies suggest that galactic fountains operate in outer galaxy disks but that winds are primarily driven from galaxy centers.

astro-ph↗