Search

We present photometric and morphological analyses of nuclear star clusters (NSCs) -- very dense, massive star clusters present in the central regions of most galaxies -- in a sample of 33 massive disk galaxies within 20 Mpc, part of the "Composite Bulges Survey." We use data from the Hubble Space Telescope including optical (F475W and F814W) and near-IR (F160W) images from the Wide Field Camera 3. We fit the images in 2D to take into account the full complexity of the inner regions of these galaxies (including the contributions of nuclear disks and bars), isolating the nuclear star cluster and bulge components. We derive NSC radii and magnitudes in all 3 bands, which we then use to estimate NSC masses. Our sample significantly expands the sample of massive late-type galaxies with measured NSC properties. We clearly identify nuclear star clusters in nearly 80% of our galaxies, putting a lower limit on the nucleation fraction in these galaxies that is higher than previous estimates. We find that the NSCs in our massive disk galaxies are consistent with previous NSC mass-NSC radius and Galaxy Mass-NSC Mass relations. However, we also find a large spread in NSC masses, with a handful of galaxies hosting very low-mass, compact clusters. Our NSCs are aligned in PA with their host galaxy disks but are less flattened. They show no correlations with bar or bulge properties. Finally, we find the ratio of NSC to BH mass in our massive disk galaxy sample spans a factor of $\sim$300., Comment: Accepted to The Astrophysical Journal

Using VLT/MUSE integral-field spectroscopic data for the barred spiral galaxy NGC 1097, we explore techniques that can be used to search for extended coherent shocks that can drive gas inflows in centres of galaxies. Such shocks should appear as coherent velocity jumps in gas kinematic maps, but this appearance can be distorted by inaccurate extraction of the velocity values and dominated by the global rotational flow and local perturbations like stellar outflows. We include multiple components in the emission-line fits, which corrects the extracted velocity values and reveals emission associated with AGN outflows. We show that removal of the global rotational flow by subtracting the circular velocity of a fitted flat disk can produce artefacts that obscure signatures of the shocks in the residual velocities if the inner part of the disk is warped or if gas is moving around the centre on elongated (non-circular) trajectories. As an alternative, we propose a model-independent method which examines differences in the LOSVD moments of H$\alpha$ and [N II]$\lambda$6583. This new method successfully reveals the presence of continuous shocks in the regions inward from the nuclear ring of NGC 1097, in agreement with nuclear spiral models., Comment: MNRAS accepted (19 pages, 18 figures, 1 table)

We present detailed morphological, photometric, and stellar-kinematic analyses of the central regions of two massive, early-type barred galaxies with nearly identical large-scale morphologies. Both have large, strong bars with prominent inner photometric excesses that we associate with boxy/peanut-shaped (B/P) bulges; the latter constitute ~ 30% of the galaxy light. Inside its B/P bulge, NGC 4608 has a compact, almost circular structure (half-light radius R_e approx. 310 pc, S\'ersic n = 2.2) we identify as a classical bulge, amounting to 12.1% of the total light, along with a nuclear star cluster (R_e ~ 4 pc). NGC 4643, in contrast, has a nuclear disc with an unusual broken-exponential surface-brightness profile (13.2% of the light), and a very small spheroidal component (R_e approx. 35 pc, n = 1.6; 0.5% of the light). IFU stellar kinematics support this picture, with NGC 4608's classical bulge slowly rotating and dominated by high velocity dispersion, while NGC 4643's nuclear disc shows a drop to lower dispersion, rapid rotation, V-h3 anticorrelation, and elevated h4. Both galaxies show at least some evidence for V-h3 correlation in the bar (outside the respective classical bulge and nuclear disc), in agreement with model predictions. Standard 2-component (bulge/disc) decompositions yield B/T ~ 0.5-0.7 (and bulge n > 2) for both galaxies. This overestimates the true "spheroid" components by factors of four (NGC 4608) and over 100 (NGC 4643), illustrating the perils of naive bulge-disc decompositions applied to massive barred galaxies., Comment: Revised with final proof corrections; pdflatex, 29 pages, 22 figures; accepted by MNRAS. Data, code, and Jupyter notebooks available at https://github.com/perwin/n4608-n4643

The $M_{BH}$ - $\sigma_{\star}$ relation is considered a result of co-evolution between the host galaxies and their super-massive black holes. For elliptical-bulge hosting inactive galaxies, this relation is well established, but there is still a debate whether active galaxies follow the same relation. In this paper, we estimate black hole masses for a sample of 19 local luminous AGNs (LLAMA) in order to test their location on the $M_{BH}$ - $\sigma_{\star}$ relation. Super-massive black hole masses ($M_{BH}$) were derived from the broad-line based relations for H$\alpha$, H$\beta$ and Pa$\beta$ emission line profiles for the Type 1 AGNs. We compare the bulge stellar velocity dispersion ($\sigma_{\star}$) as determined from the Ca II triplet (CaT) with the dispersion measured from the near-infrared CO (2-0) absorption features for each AGN and find them to be consistent with each other. We apply an extinction correction to the observed broad line fluxes and we correct the stellar velocity dispersion by an average rotation contribution as determined from spatially resolved stellar kinematic maps. The H$\alpha$-based black hole masses of our sample of AGNs were estimated in the range 6.34 $\leq$ $\log{M_{BH}}$ $\leq$ 7.75 M$_\odot$ and the $\sigma_{\star CaT}$ estimates range between 73 $\leq$ $\sigma_{\star CaT}$ $\leq$ 227 km s$^{-1}$. From the so-constructed $M_{BH}$ - $\sigma_{\star}$ relation for our Type 1 AGNs, we estimate the black hole masses for the Type 2 AGNs and the inactive galaxies in our sample. In conclusion, we find that our sample of local luminous AGNs is consistent with the $M_{BH}$ - $\sigma_{\star}$ relation of lower luminosity AGNs and inactive galaxies, after correcting for dust extinction and the rotational contribution to the stellar velocity dispersion., Comment: Accepted by A&A, 30 pages, 19 figures

Using a sample of 472 local Universe (z<0.06) galaxies in the stellar mass range 10.25 < log M*/M_sun < 10.75, we explore the variation in galaxy structure as a function of morphology and galaxy colour. Our sample of galaxies is sub-divided into red, green and blue colour groups and into elliptical and non-elliptical (disk-type) morphologies. Using KiDS and VIKING derived postage stamp images, a group of eight volunteers visually classified bars, rings, morphological lenses, tidal streams, shells and signs of merger activity for all systems. We find a significant surplus of rings ($2.3\sigma$) and lenses ($2.9\sigma$) in disk-type galaxies as they transition across the green valley. Combined, this implies a joint ring/lens green valley surplus significance of $3.3\sigma$ relative to equivalent disk-types within either the blue cloud or the red sequence. We recover a bar fraction of ~44% which remains flat with colour, however, we find that the presence of a bar acts to modulate the incidence of rings and (to a lesser extent) lenses, with rings in barred disk-type galaxies more common by ~20-30 percentage points relative to their unbarred counterparts, regardless of colour. Additionally, green valley disk-type galaxies with a bar exhibit a significant $3.0\sigma$ surplus of lenses relative to their blue/red analogues. The existence of such structures rules out violent transformative events as the primary end-of-life evolutionary mechanism, with a more passive scenario the favoured candidate for the majority of galaxies rapidly transitioning across the green valley., Comment: 17 pages, 9 figures, accepted for publication in MNRAS

Although nearly one-third of barred galaxies host an inner, secondary bar, the formation and evolution of double barred galaxies remain unclear. We show here an example model of a galaxy, dominated by a live dark matter halo, in which double bars form naturally, without requiring gas, and we follow its evolution for a Hubble time. The inner bar in our model galaxy rotates almost as slowly as the outer bar, and it can reach up to half of its length. The route to the formation of a double bar may be different from that of a single strong bar. Massive dark matter halo or dynamically hot stellar disc may play an important role in the formation of double bars and their subsequent evolution., Comment: 5 pages, 7 figures, Accepted for publication in MNRAS Letters

In a matched sample of Seyfert and quiescent galaxies we simultaneously probe the stellar and molecular gas kinematics from 1 kpc down to 50 pc with the aim of identifying the dynamical processes dictating black hole accretion rates. This first paper compares the global characteristics of a sample of ten galaxies. We find several differences within a radius of 500 pc that are correlated with Active Galactic Nucleus (AGN) activity. The Seyferts have: (1) a more centrally concentrated nuclear stellar surface brightness with lower stellar luminosities beyond a radius of 100 pc, (2) a lower stellar velocity dispersion within a radius of 200 pc, (3) elevated H2 luminosity out to a radius of at least 250 pc, and (4) more centrally concentrated H2 surface brightness profiles. These observed differences can be interpreted as evidence for Seyfert galaxies having a dynamically cold (in comparison to the bulge) nuclear structure composed of a significant gas reservoir and a relatively young stellar population. This structure is undetected (and possibly does not exist) in quiescent galaxies. The presence of such a nuclear structure in Seyfert galaxies provides evidence for inflow of the surrounding interstellar medium since the nuclear stellar population requires a supply of gas from which to form. The fueling of a Seyfert AGN is thus associated with the formation of a dynamically cold component of gas and stars on scales of hundreds of parsecs., Comment: 18 pages, 22 figures. Accepted for publication in the Astrophysical Journal. Minor revisions to match published version

We present a new method that derives both velocity components in the equatorial plane of a barred stellar disc from the observed line-of-sight velocity, assuming geometry of a thin disc. The method can be applied to large departures from circular motion, and does not require multipole decomposition. It is based on assumptions that the bar is close to steady-state (i.e. does not evolve fast), and that both morphology and kinematics are symmetrical with respect to the major axis of the bar. We derive the equations used in the method, and analyze the effect of observational errors on the inferred velocity fields. We show that this method produces meaningful results via a simple toy model. We also apply the method on integral-field data of NGC 936, for which we recover both velocity components in the disc. Knowing both velocity components in the disc, i.e. the non-observable transverse velocity in addition to the line-of-sight velocity, puts additional constraints on dynamical models and allows for new ways of determining parameters that are crucial in characterizing galaxies., Comment: 8 pages, 6 figures, accepted by MNRAS on 3 Sept 2012

Using the Inamori Magellan Areal Camera and Spectrograph (IMACS) integral-field unit (IFU) on the 6.5m Magellan telescope, we have designed the first statistically significant investigation of the two-dimensional distribution and kinematics of ionized gas and stars in the central kiloparsec regions of a well-matched sample of Seyfert and inactive control galaxies selected from the Sloan Digital Sky Survey. The goals of the project are to use the fine spatial sampling (0.2 arcsec/pixel) and large wavelength coverage (4000-7000A) of the IMACS-IFU to search for dynamical triggers of nuclear activity in the central region where active galactic nucleus (AGN) activity and dynamical timescales become comparable, to identify and assess the impact of AGN-driven outflows on the host galaxy and to provide a definitive sample of local galaxy kinematics for comparison with future three-dimensional kinematic studies of high-redshift systems. In this paper, we provide the first detailed description of the procedure to reduce and calibrate data from the IMACS-IFU in `long mode' to obtain two-dimensional maps of the distribution and kinematics of ionized gas and stars. The sample selection criteria are presented, observing strategy described and resulting maps of the sample galaxies presented along with a description of the observed properties of each galaxy and the overall observed properties of the sample., Comment: 62 pages. 41 figures. 5 tables. Accepted for publication in ApJS. High-resolution version available at: http://www.astro.ljmu.ac.uk/~pbw/IMACS-IFU/IMACS-1-highRes.pdf

We report the discovery of a two-armed mini-spiral structure within the inner kiloparsec of the E0 LINER/Seyfert 1 galaxy Arp102B. The arms are observed in H-alpha emission and located East and West of the nucleus, extending up to about 1 kpc from it. We use narrow-band imaging from the Hubble Space Telescope Advanced Camera for Surveys, in combination with archival VLA radio images at 3.6 and 6 cm to investigate the origin of the nuclear spiral. From the H-alpha luminosity of the spiral, we obtain an ionized gas mass of the order of one million solar masses. One possibility is that the nuclear spiral represents a gas inflow triggered by a recent accretion event which has replenished the accretion disk, giving rise to the double-peaked emission-line profiles characteristic of Arp102B. However, the radio images show a one-sided curved jet which correlates with the eastern spiral arm observed in the H-alpha image. A published milliarcsecond radio image also shows one-sided structure at position angle about 40 degrees, approximately aligned with the inner part of the eastern spiral arm. The absence of a radio counter-part to the western spiral arm is tentatively interpreted as indicating that the jet is relativistic, with an estimated speed of 0.45c. Estimates of the jet kinetic energy and the ionizing luminosity of the active nucleus indicate that both are capable of ionizing the gas along the spiral arms. We conclude that, although the gas in the nuclear region may have originated in an accretion event, the mini-spiral is most likely the result of a jet-cloud interaction rather than an inflowing stream., Comment: 9 pages, 5 figures, accepted for publication in ApJ

We analyze how the orbital support of the inner bar in a double-barred galaxy (nested bars) depends on the angular velocity (i.e. pattern speed) of this bar. We study orbits in seven models of double bars using the method of invariant loops. The range of pattern speed is covered exhaustively. We find that not all pattern speeds are allowed when the inner bar rotates in the same direction as the outer bar. Below a certain minimum pattern speed orbital support for the inner bar abruptly disappears, while at high values of this speed the orbits indicate an increasingly round bar that looks more like a twist in the nuclear isophotes than a dynamically independent component. For values between these two extremes, orbits supporting the inner bar extend further out as the bar's pattern speed decreases, their corresponding loops become more eccentric, pulsate more, and their rotation becomes increasingly non-uniform, as they speed up and slow down in their motion. Lower pattern speeds also lead to a less coherent bar, as the pulsation and acceleration increasingly varies among the loops supporting the inner bar. The morphologies of fast and slow inner bars expected from the orbital structure studied here are recently recovered observationally by decomposition of double barred galaxies. Our findings allow us to link the observed morphology to the dynamics of the inner bar., Comment: ApJ accepted

The method to study oscillating potentials of double bars, based on invariant loops, is introduced here in a new way, intended to be more intelligible. Using this method, I show how the orbital structure of a double-barred galaxy (nested bars) changes with the variation of nuclear bar's pattern speed. Not all pattern speeds are allowed when the inner bar rotates in the same direction as the outer bar. Below certain minimum pattern speed orbital support for the inner bar abruptly disappears, while high values of this speed lead to loops that are increasingly round. For values between these two extremes, loops supporting the inner bar extend further out as its pattern speed decreases, and they become more eccentric and pulsate more. These findings do not apply to counter-rotating inner bars., Comment: 6 pages; to appear in "Pattern Speeds along the Hubble Sequence", E. M. Corsini and V. P. Debattista (eds.), Memorie della Societa` Astronomica Italiana

We show that stable double-frequency orbits form the backbone of double bars, because they trap around themselves regular orbits, as stable closed periodic orbits do in single bars, and in both cases the trapped orbits occupy similar volume of phase-space. We perform a global search for such stable double-frequency orbits in a model of double bars by constructing maps of trajectories with initial conditions well sampled over the available phase-space. We use the width of a ring sufficient to enclose a given map as the indicator of how tightly the trajectory is trapped around a double-frequency orbit. We construct histograms of these ring widths in order to determine the fraction of phase-space occupied by ordered motions. We build 22 further models of double bars, and we construct histograms showing the fraction of the phase-space occupied by regular orbits in each model. Our models indicate that resonant coupling between the bars may not be the dominant factor reducing chaos in the system., Comment: 15 pages, 9 figures, MNRAS accepted

We report on our attempts to achieve a nearly steady-state gas flow in hydrodynamical simulations of doubly barred galaxies. After exploring the parameter space, we construct two models, for which we evaluate the photometric and the kinematic integrals, present in the Tremaine-Weinberg method, in search of observational signatures of two rotating patterns. We show that such signatures are often present, but a direct fit to data points is likely to return incorrect pattern speeds. However, for a particular distribution of the tracer, presented here, the values of the pattern speeds can be retrieved reliably even with the direct fit., Comment: 4 pages, 2 figures; to appear in "Formation and Evolution of Galaxy Disks", ASP Conf. Ser., eds. J.G. Funes & E.M. Corsini

The backbone of double bars is made out of double-frequency orbits, and loops, their maps, indicate the bars' extent, morphology and dynamics., Comment: 2 pages, 1 figure; to appear in "Formation and Evolution of Galaxy Disks", ASP Conf. Ser., eds. J.G. Funes & E.M. Corsini

In central regions of non-axisymmetric galaxies high-resolution hydrodynamical simulations indicate spiral shocks, which are capable of transporting gas inwards. The efficiency of transport is lower at smaller radii, therefore instead of all gas dropping onto the galactic centre, a roughly uniform distribution of high-density gas develops in the gaseous nuclear spiral downstream from the shock, and the shear in gas is very low there. These are excellent conditions for star formation. This mechanism is likely to contribute to the process of (pseudo-) bulge formation., Comment: 4 pages, 2 figures, to appear in the proceedings of the IAU Symposium 245, "Formation and Evolution of Galaxy Bulges"

Bars in galaxies are mainly supported by particles trapped around stable periodic orbits. These orbits represent oscillatory motion with only one frequency, which is the bar driving frequency, and miss free oscillations. We show that a similar situation takes place in double bars: particles get trapped around parent orbits, which in this case represent oscillatory motion with two frequencies of driving by the two bars, and which also lack free oscillations. Thus the parent orbits, which constitute the backbone of an oscillating potential of two independently rotating bars, are the double-frequency orbits. These orbits do not close in any reference frame, but they map onto loops, first introduced by Maciejewski & Sparke (1997). Trajectories trapped around the parent double-frequency orbit map onto a set of points confined within a ring surrounding the loop., Comment: 10 pages, 6 figures, submitted to MNRAS

We present new Space Telescope Imaging Spectrograph (STIS) observations of three spiral galaxies, NGC 4303, NGC 3310 and NGC 4258. The bright optical emission lines H$\alpha$ $\lambda$ $6564 \AA$, [NII] $\lambda$$\lambda$ $6549,6585 \AA$ and [SII] $\lambda$$\lambda$ $ 6718,6732 \AA$ were used to study the kinematics of the ionized gas in the nuclear region of each galaxy with a $\sim 0.07\arcsec$ spatial resolution. In NGC 3310, the observed gas kinematics is well matched by a circularly rotating disk model but we are only able to set an upper limit to the BH mass which, taking into account the allowed disk inclinations, varies in the range $5.0 \times 10^{6} - 4.2 \times 10^{7} M_{\odot}$ at the 95% confidence level. In NGC 4303 the kinematical data require the presence of a BH with mass $M_{BH}=(5.0)^{+0.87}_{-2.26}\times 10^{6}M_{\odot}$ (for a disk inclination $i=70$ deg).In NGC 4258, the observed kinematics require the presence of a black hole with $M_{BH}= (7.9)^{+6.2}_{-3.5} \times 10^{7}M_{\odot}$ ($i=60$ deg). This result is in good agreement with the published value $(3.9 \pm 0.1) \times 10^{7} M_{\odot}$, derived from $H_{2}O$-maser observations. Our attempt at measuring BH masses in these 3 late type Sbc spiral galaxies has shown that these measurements are very challenging and at the limit of the highest spatial resolution currently available. Nonetheless our estimates are in good agreement with the scaling relations between black holes and their host spheroids suggesting that (i) they are reliable and (ii) black holes in spiral galaxies follows the same scaling relations as those in more massive early-type galaxies. A crucial test for the gas kinematical method, the correct recovery of the known BH mass in NGC 4258, has been successful. [abridged], Comment: 24 pages, 21 figures, A&A in press

Recent high-resolution observations indicate that nuclear spirals are often present in the innermost few hundred parsecs of disc galaxies. My models show that nuclear spirals form naturally as a gas response to non-axisymmetry in the gravitational potential. Some nuclear spirals take the form of spiral shocks, resulting in streaming motions in the gas, and in inflow comparable to the accretion rates needed to power local Active Galactic Nuclei. Recently streaming motions of amplitude expected from the models have been observed in nuclear spirals, confirming the role of nuclear spirals in feeding of the central massive black holes., Comment: 6 pages, 4 figures, to appear in ""Mapping the Galaxy and Nearby Galaxies", Eds. K. Wada and F. Combes, Springer, in press

When integrals in the standard Tremaine-Weinberg method are evaluated for the case of a realistic model of a doubly barred galaxy, their modifications introduced by the second rotating pattern are in accord with what can be derived from a simple extension of that method, based on separation of tracer's density. This extension yields a qualitative argument that discriminates between prograde and retrograde inner bars. However, the estimate of the value of inner bar's pattern speed requires further assumptions. When this extension of the Tremaine-Weinberg method is applied to the recent observation of the doubly barred galaxy NGC 2950, it indicates that the inner bar there is counter-rotating, possibly with the pattern speed of -140 +/- 50 km/s/arcsec. The occurrence of counter-rotating inner bars can constrain theories of galaxy formation., Comment: accepted by MNRAS; Appendix A added, Discussion expanded

If supermassive black holes in centres of galaxies form by merging of black-hole remnants of massive Population III stars, then there should be a few black holes of mass one or two orders of magnitude smaller than that of the central ones, orbiting around the centre of a typical galaxy. These black holes constitute a weak perturbation in the gravitational potential, which can generate wave phenomena in gas within a disc close to the centre of the galaxy. Here we show that a single orbiting black hole generates a three-arm spiral pattern in the central gaseous disc. The density excess in the spiral arms in the disc reaches values of 3-12% when the orbiting black hole is about ten times less massive than the central black hole. Therefore the observed density pattern in gas can be used as a signature in detecting the most massive orbiting black holes., Comment: accepted by MNRAS, 8 pages, 4 figures

Nuclear spirals naturally form as a gas response to non-axisymmetry in the galactic potential, even if the degree of this asymmetry is very small. Linear wave theory well describes weak nuclear spirals, but spirals induced by stronger asymmetries in the potential are clearly beyond the linear regime. Hydrodynamical models indicate spiral shocks in this latter case that, depending on how the spiral intersects the x2 orbits, either get damped, leading to the formation of the nuclear ring, or get strengthened, and propagate towards the galaxy centre. Central massive black hole of sufficient mass can allow the spiral shocks to extend all the way to its immediate vicinity, and to generate gas inflow up to 0.03 M_sun/yr, which coincides with the accretion rates needed to power luminous local Active Galactic Nuclei., Comment: 14 pages, 10 figures, MNRAS, accepted

Nuclear spirals can provide a wealth of information about the nuclear potential in disc galaxies. They are unlikely to form in nuclei with solid-body rotation, yet they are present in a majority of galactic centres. Their morphology varies depending on whether a central massive black hole (MBH) is present or absent in the galaxy. In this paper I consider predictions of the linear theory for waves induced in gas by asymmetric gravitational potential, which are applicable to the nuclear spirals observed in galaxies. Generation and propagation of waves is governed by dynamical resonances, and inclusion of a MBH can move or even create resonances, greatly altering the extent and shape of the nuclear spiral. I will use predictions of the linear theory presented here as a guideline when interpreting hydrodynamical models in the second paper of this series. I also comment on modifications that self-gravity in gas imposes on the induced waves., Comment: 10 pages, 3 figures, MNRAS, accepted

Nuclear spirals can provide a wealth of information about the nuclear potential in disc galaxies. They form naturally as a gas response to non-axisymmetry in the gravitational potential, even if the degree of this asymmetry is very small. Linear wave theory well describes weak nuclear spirals, but stronger asymmetries in the potential induce waves beyond the linear regime, which appear as spiral shocks. If a central massive black hole (MBH) is present, spiral shocks can extend all the way to its immediate vicinity, and generate gas inflow up to 0.03 Msun/yr. This coincides with the accretion rates needed to power local Active Galactic Nuclei., Comment: 4 pages, 3 figures, to appear in the proceedings of the IAU Symp.222 "The Interplay among Black Holes, Stars and ISM in Galactic Nuclei"

Bars in galaxies are mainly supported by particles trapped around closed periodic orbits. These orbits respond to the bar's forcing frequency only and lack free oscillations. We show that a similar situation takes place in double bars: particles get trapped around orbits which only respond to the forcing from the two bars and lack free oscillations. We find that writing the successive positions of a particle on such an orbit every time the bars align generates a closed curve, which we call a loop. Loops allow us to verify consistency of the potential. As maps of doubly periodic orbits, loops can be used to search the phase-space in double bars in order to determine the fraction occupied by ordered motions., Comment: 10 pages, 5 figures, to appear in Lecture Notes in Physics, vol. "Galaxies and Chaos", eds. G. Contopoulos & N. Voglis

Asymmetries in galactic potentials, either self-induced, or caused by a passing companion, play an important role in global gas dynamics in galaxies. In particular, they are able to trigger gas inflow, which in turn feeds nuclear activity. In the inner kiloparsec, the inflowing gas becomes subject to various resonances induced by asymmetry in the potential. This produces complicated gas morphology and dynamics seen in the nuclear rings and spirals. Formation of nuclear rings is related to barred galaxies, and explained by the orbital structure in bars. One can get basic understanding of weak nuclear spirals from the linear wave theory, but stronger spirals are likely to be out of the linear regime, and they appear as spiral shocks in hydrodynamical models. In addition, nuclear stellar bars are observed in a considerable fraction of disc galaxies, often nested inside the large bar; their orbital structure further modifies the nuclear gas dynamics. If a massive black hole (MBH) is present in the galactic centre, it governs the resonances even beyond its classically defined sphere of influence. Since resonances shape gas dynamics in the nuclear region, the central black hole should be able to regulate gas flow around itself., Comment: 13 pages, 4 figures; invited review to appear in the proceedings of the JENAM 2002 Workshop "Galactic Dynamics", eds. C.Boily, P. Patsis, C. Theis, S. Portegies Zwart, R. Spurzem; EDP Sciences, in press

A wide-spread belief that nested bars enhance gas inflow to the galactic centre has recently been contradicted by dynamical models in which inner bars seem to prohibit such inflow. Can the existing models of dynamically possible double bars be modified to enable strong inflow in the secondary bar? I present here simple dynamical arguments which imply that in general, double bars in resonant coupling do not enhance gas inflow. However, stronger inflow with straight shocks in the inner bar can occur if there is no resonant coupling of the commonly assumed form between the bars., Comment: 4 pages, 1 figure, to appear in "Disks of Galaxies : Kinematics, Dynamics and Perturbations" (ASP Conference Series), eds E.Athanassoula and A.Bosma

We report here results of high-resolution hydrodynamical simulations of gas flows in barred galaxies, with a focus on gas dynamics in the central kiloparsec. In a single bar with an Inner Lindblad Resonance, we find either near-circular motion of gas in the nuclear ring, or a spiral shock extending towards the galaxy center, depending on the sound speed in the gas. From a simple model of a dynamically-possible doubly barred galaxy with resonant coupling, we infer that the secondary bar is likely to end well inside its corotation. Such a bar cannot create shocks in the gas flow, and therefore will not reveal itself in color maps through straight dust lanes: the gas flows induced by it are different from those caused by the rapidly rotating main bars. In particular, we find that secondary stellar bars are unlikely to increase the mass inflow rate into the galactic nucleus., Comment: Accepted for publication in MNRAS, 12 pages, 5 figures. Because of space limitations, Fig.5 has low resolution in the paper. For better quality, see http://www-thphys.physics.ox.ac.uk/users/WitoldMaciejewski/pub.html

With a help of hydrodynamical models, we explore fundamental modes of gas flow in central parts of a galaxy potential with non-axisymmetric perturbations of various strengths. Our grid-based algorithm allows us to get a detailed picture of gas dynamics down to about 10 pc from the galaxy center, where the central black hole becomes dominant. Gas inflow in a bar with an Inner Lindblad Resonance often stops at the nuclear ring. We find that embedded bars in resonant coupling are unlikely to increase the inflow, but for certain gas parameters in a single bar, inflow to the center develops in a spiral shock, similar to the patterns observed in nuclear discs of real galaxies. The spiral pattern persists for very weak non-axisymmetries of the potential., Comment: 6 pages, 4 figures, to appear in ``The central kpc of starbursts and AGN: the La Palma connection'', Eds. J.H. Knapen, et al., ASP conf. series, in press (2001)

Motivated by STIS observations of more than 50 nearby galactic nuclei, we consider long-slit emission-line spectra when the slit is wider than the instrumental PSF, and the target has arbitrarily large velocity gradients. The finite width of the slit generates complex patterns in the spectra that can be misinterpreted as coming from various physically distinct nuclear components, but when interpreted correctly, they can have considerable diagnostic power. For a thin disk in circular motion around a central galactic black hole (BH), a characteristic artifact occurs in the spectrum at the outer edge of the BH's sphere of influence. It betrays the presence of a BH, and allows us to develop a new method for estimating its mass, which gives higher sensitivity to BH detection than traditional methods., Comment: 4 pages, 2 figures, to appear in the proceedings of IAU Symposium 205 'Galaxies and their Constituents at the Highest Angular Resolution'

We consider long-slit emission-line spectra of galactic nuclei when the slit is wider than the instrumental PSF, and the target has large velocity gradients. The finite width of the slit generates complex distributions of brightness at a given spatial point in the measured spectrum, which can be misinterpreted as coming from additional physically distinct nuclear components. We illustrate this phenomenon for the case of a thin disc in circular motion around a nuclear black hole (BH). We develop a new method for estimating the mass of the BH that exploits a feature in the spectrum at the outer edge of the BH's sphere of influence, and therefore gives higher sensitivity to BH detection than traditional methods. Moreover, with this method we can determine the black hole mass and the inclination of the surrounding disc separately, whereas the traditional approach to black-hole estimation requires two long-slit spectra to be taken. We show that with a given spectrograph, the detectability of a BH depends on the sense of rotation of the nuclear disc. We apply our method to estimate the BH mass in M84 from a publicly available spectrum, and recover a value 4 times lower than that published previously from the same data., Comment: submitted to MNRAS; 9 pages, 8 figures; Fig.8 on the last page in colour; for full resolution version, see http://www-thphys.physics.ox.ac.uk/users/WitoldMaciejewski/pub.html

Motivated by STIS observations of more than 50 nearby galactic nuclei, we consider long-slit emission-line spectra when the slit is wider than the instrumental PSF. The finite width of the slit generates complex patterns in the spectra that can have considerable diagnostic power if they are modeled with adequate sophistication. They allowed us to develop a new method for estimating the BH mass that gives fuller information and higher sensitivity to BH detection than traditional methods., Comment: 2 pages, 1 figure. To appear in ASP Conference Series "Galaxy Disks and Disk Galaxies", J.G. Funes S.J. and E.M. Corsini, eds

High-resolution observations of the inner regions of barred disk galaxies have revealed many asymmetrical, small-scale central features, some of which are best described as secondary bars. Because orbital time-scales in the galaxy center are short, secondary bars are likely to be dynamically decoupled from the main kiloparsec-scale bars. Here, we show that regular orbits exist in such doubly-barred potentials and that they can support the bars in their motion. We find orbits in which particles remain on loops: closed curves which return to their original positions after two bars have come back to the same relative orientation. Stars trapped around stable loops could form the building blocks for a long-lived, doubly-barred galaxy. Using the loop representation, we can find which orbits support the bars in their motion, and what are the constraints on the sizes and shapes of self-consistent double bars. In particular, it appears that a long-lived secondary bar may exist only when an Inner Lindblad Resonance is present in the primary bar, and that it would not extend beyond this resonance., Comment: 18 pages, 11 figures, to appear in MNRAS

Active Galactic Nuclei (AGN) require mass accretion onto the central engine. On large galactic scales, torques from a stellar bar can efficiently remove angular momentum from gas, and cause it to move inwards along two hydrodynamical shocks on the leading edges of the bar. Inflowing gas settles on near-circular orbits around the Inner Lindblad Resonance (ILR), and forms a nuclear ring, about 1 kpc in size. A secondary bar inside the main one, with its own pair of shocks, has been proposed to drive further inflow, and thus feed the AGN in a manner similar to the inflow on large scales. Here, we report results of high resolution hydrodynamical simulations, where we examine the nature of the nuclear ring, and check how efficient double bars can be in fueling AGNs., Comment: 2 pages, 1 figure, to appear in ASP Conference Series "Galaxy dynamics: from the early universe to the present", better resolution figures can be found at http://www-thphys.physics.ox.ac.uk/users/WitoldMaciejewski/pub.html

We propose simple, explicit, analytical approximations for the kinematics of an adiabatic blast wave propagating in an exponentially stratified ambient medium, and for the onset of radiative cooling, which ends the adiabatic era. Our method, based on the Kompaneets implicit solution and the Kahn approximation for the radiative cooling coefficient, gives straightforward estimates for the size, expansion velocity, and progression of cooling times over the surface, when applied to supernova remnants (SNRs). The remnant shape is remarkably close to spherical for moderate density gradients, but even a small gradient in ambient density causes the cooling time to vary substantially over the remnant's surface, so that for a considerable period there will be a cold dense expanding shell covering only a part of the remnant. Our approximation provides an effective tool for identifying the approximate parameters when planning 2-dimensional numerical models of SNRs, the example of W44 being given in a subsequent paper., Comment: ApJ accepted, 11 pages, 2 figures embedded, aas style with ecmatex.sty and lscape.sty packages

Inner regions of barred disk galaxies often include asymmetrical, small-scale central features, some of which are best described as secondary bars. Because orbital timescales in the galaxy center are short, secondary bars are likely to be dynamically decoupled from the main kiloparsec-scale bars. We found that non-chaotic multiply-periodic particle orbits can exist in potentials with two dynamically decoupled bars.Stars trapped around these orbits could form the building blocks for a long-lived, doubly-barred galaxy. A self-consistent secondary bar appears to induce formation of inner gaseous rings rather than shocks in gas flow., Comment: 4 pages, including 1 figure, pasp style. To appear in the proceedings of "Galaxy Dynamics" conference held in Rutgers, August 1998

(shortened version) We show that many observations of W44, a supernova remnant in the galactic plane at a distance of about 2500 pc, are remarkably consistent with the simplest realistic model. The model remnant is evolving in a smooth ambient medium of fairly high density, about 6 cm^-3 on average, with a substantial density gradient. At the observed time it has an age of about 20,000 years, consistent with the age of the associated pulsar, and a radius of 11 to 13 pc. Over most of the outer surface, radiative cooling has become important in the post shock gas; on the denser end there has been sufficient compression of the cooled gas to develop a very thin dense half shell of about 450 M_sun, supported against further compression by nonthermal pressure. The half shell has an expansion velocity of about 150 km s^-1, and is bounded on the outer surface by a radiative shock with that speed. We provide several analytic tools for the assembly of models of this type. We review the early evolution and shell formation analyses and their generalizations to evolution in a density gradient. We also calculate the density and temperature that should be present in the hot interior of a remnant with thermal conduction. We supply the van der Laan mechanism in a particularly useful form for the calculation of radio continuum from radiative remnants. Finally, we demonstrate a simple technique for estimating the optical emission expected. These tools are employed to choose parameters of models which we then explore with our 1d and 2d hydrocodes, providing, respectively, the detailed x-ray spectra and dynamical characteristics., Comment: 42 pages, latex, uses a4wide.sty, 18 figures; original figures available at ftp://ftp.astro.wisc.edu/outgoing/witold/w44/ ; submitted to ApJ

We show that non-chaotic multiply-periodic particle orbits can exist in a galaxy-like potential where a small fast-tumbling nuclear bar is nested inside a main bar which has a slower pattern speed. We introduce the concept of a loop: a one-dimensional curve such that particles distributed along it at some initial instant return to the same curve (as viewed in the rotating frame of one of the bars) after the bars return to the same relative position. Just as particles following regular orbits in a simply-barred potential are trapped around closed periodic orbits, so regular orbits in a doubly-barred potential oscillate about stable loops. We find both loops which remain aligned with the inner bar and loops aligned with the outer bar: particles trapped around these loops could form the building blocks for a long-lived, self-consistent, doubly-barred galaxy. In a realistic doubly-barred galaxy potential, we demonstrate the existence of stable loops which support the structure of the inner bar. We use the epicyclic approximation to preview the pattern of gas flow in a doubly-barred potential., Comment: 6 pages, 1 figure, submitted to ApJ Letters