Signatures of recent star formation in ring S0 galaxies

We present a study of the stellar populations of ring and/or arm-like structures in a sample of S0 galaxies using GALEX far- and near-ultraviolet imaging and SDSS optical data. Such structures are prominent in the UV and reveal recent star formation. We quantitatively characterize these rejuvenation events, estimating the average age and stellar mass of the ring structures, as well as of the entire galaxy. The mass fraction of the UV-bright rings is a few percent of the total galaxy mass, although the UV ring luminosity reaches 70% of the galaxy luminosity. The integrated colors of these S0s locates them in the red sequence (NGC 2962) and in the so-called green valley. We suggest that the star formation episodes may be induced by different triggering mechanisms, such as the inner secular evolution driven by bars, and interaction episodes.     

Gas Flows and Star Formation in the Ringed Galaxy NGC 4736

We present high-resolution (∼5″) BIMA CO observations of the ringed galaxy NGC 4736, along with previously published VLA HI data (Braun, 1995). Strong CO emission is detected from the star-forming ring at r=45″ and in the central region, where a molecular bar is apparent. The azimuthally averaged gas surface density is still much less than the Toomre critical density within r=60″, despite the starburst conditions in the ring (gas depletion time ≲1Gyr). Both CO and HI velocity fields show strong departures from a circular rotating disc model. The velocity residuals are consistent with inflowing gas near the ends of the central bar, outflowing gas between the bar and the ring, and inflowing gas outside the ring. We propose that the high star formation efficiency in the ring results from gas being driven out towards the OLR of the bar and in towards the ILR of the larger oval distortion. However, the strong signature of inflow outside the ring is probably due in part to gas motion in elliptical orbits. This revised version was published online in July 2006 with corrections to the Cover Date.     

Warped polar ring in the Arp 212 galaxy

The Fabry-Perot scanning interferometer mounted on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences is used to study the distribution and kinematics of ionized gas in the peculiar galaxy Arp 212 (NGC 7625, IIIZw 102). Two kinematically distinct subsystems—the inner disk and outer emission filaments—are found within the optical radius of the galaxy. The first subsystem, at galactocentric distances r < 3.5 kpc, rotates in the plane of the stellar disk. The inner part of the ionized-gas disk (r<1.5–2 kpc) exactly coincides with the previously known disk consisting of molecular gas. The second subsystem of ionized gas is located at galactocentric distances 2–6 kpc. This subsystem rotates in a plane tilted by a significant angle to the stellar disk. The angle of orbital inclination in the outer disk increases with galactocentric distance and reaches 50° at r ≈ 6 kpc. The ionized fraction of the gaseous disk does not show up beyond this galactocentric distance, but we believe that the HI disk continues to warp and approaches the plane that is polar with respect to the inner disk of the galaxy. Hence Arp 212 can be classified as a galaxy with a polar ring (or a polar disk). The observed kinematics of the ionized and neutral gas can be explained assuming that the distribution of gravitational potential in the galaxy is not spherically symmetric. Most probably, the polar ring have formed via accretion of gas from the dwarf satellite galaxy UGC 12549.     

Dark matter haloes in interacting isolated galaxy pairs: The Importance of the Hα rotation curve

We present Hα scanning Fabry-Perot observations of the interacting galaxy pair NGC 3893/96 (Kar 302), an M51-type galaxy pair. The velocity field and rotation curve of the main galaxy (NGC 3893) were derived. These show the galaxy follows a rather axisymmetric behaviour. Together with HI observations, several mass models were adjusted in order to study the nature of the dark halo as well as the mass-to-light ratio of the galaxy. We find that in order to constrain these models, it is important to have a high resolution Hα rotation curve for the inner parts of the galaxy. This revised version was published online in September 2006 with corrections to the Cover Date.     

HI Observations of NGC 1068

Neutral hydrogen is an important tracer of galactic dynamics. Hence, observations of the detailed structure and kinematics of HI are vital in order to determine the relationship between AGN and their host galaxy. We describe high resolution VLA emission line observations of NGC 1068 at about 600 pc linear resolution and 5.2 km s-1 velocity resolution. We present the HI morphology of this Seyfert galaxy and discuss its peculiar rotation curve and speculate how its shape might be related to the Seyfert activity. Related to this, we will highlight the pronounced HI ring within which the tightly wound CO spiral arms are found, and discuss the high, up to 55 km s-1, velocity dispersions which are found there. This revised version was published online in July 2006 with corrections to the Cover Date.     

Revisiting the formation history of the minor-axis dust lane galaxy NGC 1947

In this paper, we present a detailed study of the peculiar early-type galaxy NGC 1947. The main goal of this work is to constrain the dynamical status and the formation history of NGC 1947 by comparing the observed properties with the predictions derived from different galaxy formation scenarios. To this aim, we derived the photometric and kinematical properties of NGC 1947. Due to the presence of an extended dust lane, which crosses the galaxy centre along the photometric minor axis, we used near-infrared (NIR) images ( J and K bands) to derive an accurate analysis of the stellar light distribution. Optical images (in the V and R bands) are used to derive the colour profiles and colour maps to study the structure of the dust lane. The observed kinematics confirm the presence of two components with decoupled angular momentum: gas and dust rotate along the minor axis, while the rotation velocities of the stars are observed along the major axis. The complex structure observed in NGC 1947 supports the hypothesis that some kind of interactions happened in the evolution of this object. We analysed two alternatives: a merging process and an accretion event. We discussed how the observed properties strongly suggest that the decoupled ring of gas and dust has been accreted from outside.     

Near-Infrared Spectral Mapping of NGC 1614

We present the spatial distribution of the bright near-infrared emission lines, Br, H₂, He I, [Fe II], and the CO band longwards of 2.3 m, for the luminous infrared galaxy NGC 1614. The morphology of the ionised gas is different from that of the stellar light, and possibly forms a circumnuclear ring. Our data imply that the stellar population is older and the extinction is lower in the nucleus relative to the surrounding circumnuclear ring. We suggest that NGC 1614 is a galaxy whose recent interaction triggered massive star formation in the nucleus, which in turn caused a radially outward progression of star formation thereby producing the circumnuclear ring we observe today. There is no evidence for a buried AGN, and it is difficult to reconcile our data with the simple evolutionary model of ultraluminous galaxies proposed by Sanders et al (1988).     

Vertical Velocity Dispersion of the Ionized Gas In NGC 3938

We have studied the kinematics of the ionized gas in the nearly face-on galaxy NGC 3938 by means of observations made with theFabry–Perot interferometer TAURUS II at the William Herschel Telescope, using the H_α line. We have been able to produce high-resolution velocity and velocity-dispersion maps which allow us to make a detailed study of the kinematics of the ionized gas. In particular we have found that the vertical velocity dispersion is constant with galactocentric radius, as has already been found for the atomic and molecular gas in this galaxy. This suggests the existence of several heating mechanisms in the disc acting simultaneously to produce the observed behaviour. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ionized gas characteristics in the cavities of the gas and dust disc of the spiral galaxy NGC 6946

The parameters of the ionized gas in NGC 6946 (in the [NII] λλ6548, 6583, H _α and [SII] λλ6717, 6731 lines) are investigated with the SAO RAS BTA telescope along three positions of the long slit of the SCORPIO focal reducer, passing through a number of large and small cavities of the gaseous disc of the galaxy. These cavities correspond exactly to the cavities in warm dust, visible at 5 − 8μm. We found that everywhere in the direction of NGC 6946 the lines of ionized gas are decomposed into two Gaussians, one of which shows almost constant [SII]/H _α and [NII]/H _α ratios, as well as an almost constant radial velocity within the measurement errors (about −35… − 50 km/s). This component is in fact the foreground radiation from the diffuse ionized gas of our Galaxy, which is not surprising, given the low (12°) latitude of NGC 6946; a similar component is also present in the emission of neutral hydrogen. The analysis of the component of ionized gas, occurring inNGC 6946, has revealed that it shows signs of shock excitation in the cavities of the gaseous disc of the galaxy. This shock excitation is as well typical for the extraplanar diffuse ionized gas (EDIG), observed in a number of spiral galaxies at their high Z-coordinates. This can most likely be explained by low density of the gas in the NGC 6946 disc (with the usual photoionization) inside the cavities, due to what we see the spectral features of the EDIG gas of NGC 6946, projected onto them, and located outside the plane of the galaxy. In the absence of separation of ionized gas into two components by radial velocities, there is an increasing contribution to the integral line parameters by the EDIG of our Galaxy when the gas density in NGC 6946 decreases, which explains some strange results, obtained in the previous studies. Themorphology of warmdust, visible in the infrared range and HI is almost the same (except for the peripheral parts of the galaxy, where there are no sources of dust heating).Moreover, the shock excitation of the ionized gas is detected in the smallest holes, distinguishable only in the IR images.     

Modelling gaseous and stellar kinematics in the disc galaxies NGC 772, 3898 and 7782

We present V -band surface photometry and major-axis kinematics of stars and ionized gas of three early-type spiral galaxies, namely NGC 772, 3898 and 7782. For each galaxy we present a self-consistent Jeans model for the stellar kinematics, adopting the light distribution of bulge and disc derived by means of a two-dimensional parametric photometric decomposition. This allows us to investigate the presence of non-circular gas motions, and derive the mass distribution of luminous and dark matter in these objects.
NGC 772 and 7782 have apparently normal kinematics with the ionized gas tracing the gravitational equilibrium circular speed. This is not true in the innermost region (| r |≲8 arcsec) of NGC 3898, where the ionized gas is rotating more slowly than the circular velocity predicted by dynamical modelling. This phenomenon is common in the bulge-dominated galaxies for which dynamical modelling enables us to make the direct comparison between the gas velocity and the circular speed, and it poses questions about the reliability of galaxy mass distributions derived by the direct decomposition of the observed ionized-gas rotation curve into the contributions of luminous and dark matter.     

Star formation history in the central region of the barred galaxy NGC 7177

Using the method of two-dimensional spectroscopy, we have investigated the kinematics and distribution of the gas and stars at the center of the early-type spiral galaxy NGC 7177 with a mediumscale bar as well as the change in the mean age of the stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius R = 1″.5−2″, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate—basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar age inside the star formation ring, in the galactic nucleus, is old, ∼10 Gyr.Outside, at a distance R = 6″−8″ from the nucleus, the mean age of the stellar population is ∼2 Gyr. If we agree that the bar in NGC 7177 is old, then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions of some dynamical models.     

Kinematics of an Anaemic Cluster Galaxy

We present new HI and CO data including velocity information of NGC 4548, an anaemic galaxy in the Virgo Cluster. The distribution of the atomic gas shows a ring-like structure. The HI rotation curve is deduced and can be extrapolated inwards with the help of the CO data. The derived molecular fraction decreases continuously up to a radius of about 40″ showing a rather sharp transition between the molecular and the atomic disc. A three-dimensional visualization of the data cube permits us to detect a perturbation of the velocity field in the northern part of the galaxy. In a first approach we reconstruct the possible sites of HI emission with the help of a simple kinematical model in three dimensions. The reconstructed model shows a ridge which is believed to be due to ram-pressure stripping. In addition, we show first results of a dynamical model to simulate the interaction between the intra-cluster medium and the galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

The phenomenon of the galaxy NGC 6286: A forming polar ring or a superwind?

We present our observations of the pair of interacting galaxies NGC 6285/86 carried out with the 6-m Special Astrophysical Observatory (SAO) telescope using 1D and 2D spectroscopy. The observations of NGC 6286 with a long-slit spectrograph (UAGS) near the Hα line revealed the rotation of the gaseous disk around an axis offset by 5″–7″ from the photometric center and a luminous gas at a distance up to 9 kpc in a direction perpendicular to the galactic plane. Using a multipupil fiber spectrograph (MPFS), we constructed the velocity fields of the stellar and gaseous components in the central region of this galaxy, which proved to be similar. The close radial velocities of the pair and the wide (5′×5′) field of view of the scanning Fabry-Perot interferometer (IFP) allowed us to simultaneously obtain images in the Hα and [N II]λ6583 lines and in the continuum, as well as to construct the radial velocity fields and to map the distribution of the [N II]λ6583/Hα ratio for both galaxies. Based on all these data, we studied the gas kinematics in the galaxies, constructed their rotation curves, and estimated their masses (2 × 10¹¹M_⊙ for NGC 6286 and 1.2 × 10¹⁰M_⊙ for NGC 6285). We found no evidence of gas rotation around the major axis of NGC 6286, which argues against the assumption that this galaxy has a forming polar ring. The IFP observations revealed an emission nebula around this galaxy with a structure characteristic of superwind galaxies. The large [N II]λ6583/Hα ratio, which suggests the collisional excitation of its emission, and the high infrared luminosity are additional arguments for the hypothesis of a superwind in the galaxy NGC 6286. A close encounter between the two galaxies was probably responsible for the starburst and the bipolar outflow of hot gas from the central region of the disk.     

Orientation dependency of broad-line widths in quasars and consequences for black hole mass estimation

We have identified two new galaxies with gas counter-rotation (NGC 1596 and 3203) and have confirmed similar behaviour in another one (NGC 128), this using results from separate studies of the ionized-gas and stellar kinematics of a well-defined sample of 30 edge-on disc galaxies. Gas counter-rotators thus represent 10 ± 5 per cent of our sample, but the fraction climbs to 21 ± 11 per cent when only lenticular (S0) galaxies are considered and to 27 ± 13 per cent for S0 galaxies with detected ionized gas only. Those fractions are consistent with but slightly higher than previous studies. A compilation from well-defined studies of S0 galaxies in the literature yields fractions of 15 ± 4 and 23 ± 5 per cent, respectively. Although mainly based on circumstantial evidence, we argue that the counter-rotating gas originates primarily from minor mergers and tidally induced transfer of material from nearby objects. Assuming isotropic accretion, twice those fractions of objects must have undergone similar processes, underlining the importance of (minor) accretion for galaxy evolution. Applications of gas counter-rotators to barred galaxy dynamics are also discussed.     

RAM-Pressure Stripping of a Galaxy's ISM

NGC 4485 is a late-type galaxy moving through the very extended HI envelope of its companion, NGC 4490. NGC 4485 is undetected in HI,CO and IRAS wavebands, but a peak at all of these frequencies is seen displaced from the optical disk. HI data show a prominent bow-shock ahead of this displaced gas. It is argued that almost the entire ISM, including molecular clouds, has been stripped from this galaxy byram-pressure, and that it will emerge from the encounter with very little star forming potential. This revised version was published online in July 2006 with corrections to the Cover Date.     

Probing the physics of interacting galaxies

The morphological and velocity structures in the gaseous (HI and CO) and stellar components of two interacting systems are examined. Both Arp 140 and Arp 104 reveal extended tidal tails in the HI. The Hα and FIR fluxes of Arp 140 yield similar SFR of ∼ 0.8 M_⊙ yr^-1. In contrast the Hα flux of Arp 104 yields a SFR of ∼ 0.05 M_⊙ yr^-1, ∼ 20 times smaller than that obtained from the FIR flux. Spectra were used to examine the changing velocity of atomic and molecular gas in NGC 5218 (Arp 104). The atomic and molecular gas were found to be dynamically similar with comparable velocities and velocity widths across the galaxy; consistent with the two phases responding similarly to the interaction, or enhanced HI to CO conversion in the centre of the galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Star formation history in the central region of the barred galaxy NGC 4245

We have investigated the gas and stellar kinematics and the stellar population properties at the center of the early-type galaxy NGC 4245 with a large-scale bar by the method of two-dimensional spectroscopy. The galaxy has been found to possess a pronounced chemically decoupled compact stellar nucleus, which is at least a factor of 2.5 richer in metals than the stellar population of the bulge, and a ring of young stars with a radius of 300 pc. Star formation goes on in the ring even now; its location corresponds to the inner Lindblad resonance of the large-scale bar. According to Hubble Space Telescope data, the mean stellar age in the chemically decoupled nucleus is significantly younger than that within 0″.25 of the center. It may be concluded that we take the former ultracompact star formation ring with a radius of no more than 100 pc located at the inner Lindblad resonance of the now disappeared nuclear bar as the chemically decoupled nucleus. On the whole, the picture of star formation at the center of this gas-poor galaxy is consistent with theoretical predictions of the consequences of the secular evolution of a stellar-gaseous disk under the action of a bar or bars.     

The shape of the dark matter halo in the early-type galaxy NGC 2974

We present H  i observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected H  i disc in this galaxy is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape.
We construct mass models of NGC 2974 by combining the H  i rotation curve with the central kinematics of the ionized gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionized gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the H  i gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and modified Newtonian dynamics, which fit the data marginally worse.
The mass-to-light ratio M / L _I increases in NGC 2974 from 4.3 M_⊙/L_⊙, _I at one effective radius to 8.5 M_⊙/L_{⊙, I} at 5  R _e. This increase of M / L already suggests the presence of dark matter: we find that within 5  R _e at least 55 per cent of the total mass is dark.     

Observational evidence for AGN fueling: I. The case of NGC 6104—Merging with a companion

We investigate in detail the kinematics and morphology of the Seyfert galaxy NGC 6104 in order to identify the mechanism of gas transportation to the active galactic nucleus (AGN). Our observational data were obtained at the 6-m Special Astrophysical Observatory telescope with the MPFS integral-field spectrograph and the SCORPIO universal device in three modes: direct imaging, a scanning Fabry—Perot interferometer, and long-slit spectroscopy. Images from the HST archive were invoked to study the structure of the circumnuclear region. An analysis of deep images has shown for the first time that NGC 6104 is in the phase of active merging with a companion galaxy. We have been able to study the detailed picture of ionized gas motions up to galactocentric distances of 14 kpc and to construct the stellar velocity field for the inner region. The radial gas motions toward the AGN along the central bar play a significant role at galactocentric distances of 1–5 kpc. In addition, we have detected an outflow of ionized gas from the nucleus that presumably resulted from the intrusion of a radio jet into the ambient interstellar medium. Using diagnostic diagrams, we estimate the contributions from the AGN and star formation to the galactic gas ionization. We estimate the bar pattern speed by the Tremaine-Weinberg method and show that the inner ring observed in the galaxy’s images has a resonant nature. Two possible ring formation scenarios, before and during the interaction with a companion, are discussed.     

12CO, 13CO and HCN in the Barred Galaxy NGC 1530

Bars probably have a great importance in galactic evolution. The barred potential is able to concentrate large quantities of interstellar gas in the vicinity of the nucleus, feeding any nuclear activity, be it central starbursts or black hole accretion discs. The IRAM millimeter interferometer and 30 m telescope have allowed a precise analysis of the molecular gas in the bar and the nucleus of a typical barred spiral galaxy, NGC 1530. In this galaxy, I have detected CO(1→0) along two lanes that trace shocks in the molecular gas. In these lanes, the gas moves toward the centre of the galaxy, with typical in fall velocities of 100 km s^-1. I have shown in these shocks an anticorrelation between shear in the gas and star formation efficiency by comparing H^α and CO maps. I have also studied the centre of this galaxy at higher resolution in¹²CO(1→0), ¹²CO(2→1),¹³CO(1→0) and HCN(1→0). In the central region, the gas distribution is a ring or an unresolved spiral, surrounded by two curved arcs. The nuclear ring contains large amounts of dense gas traced by HCN and ¹³CO, and shows intense star formation, as indicated by the non-thermal centimetre continuum. The arcs, in contrast, are poor in dense gas and form few stars. This revised version was published online in July 2006 with corrections to the Cover Date.