We have observed the Arp 270 system (NGC 3395 & NGC 3396) in H{alpha} emission using the GH{alpha}FaS Fabry-Perot spectrometer on the 4.2m William Herschel Telescope (La Palma). In NGC 3396, which is edge-on to us, we detect gas inflow towards the ce
ntre, and also axially confined opposed outflows, characteristic of galactic superwinds, and we go on to examine the possibility that there is a shrouded AGN in the nucleus. The combination of surface brightness, velocity and velocity dispersion information enabled us to measure the radii, FWHM, and the masses of 108 HII regions in both galaxies. We find two distinct modes of physical behaviour, for high and lower luminosity regions. We note that the most luminous regions show especially high values for their velocity dispersions and hypothesize that these occur because the higher luminosity regions form from higher mass, gravitationally bound clouds while those at lower luminosity HII regions form within molecular clouds of lower mass, which are pressure confined.
We present the most complete atlas of nuclear rings to date. We include 113 rings found in 107 galaxies, six of which are elliptical galaxies, five are highly inclined disc galaxies, 18 are unbarred disc galaxies, and 78 are barred disc galaxies. Sta
r-forming nuclear rings occur in 20% of disc galaxies with types between T=-3 and T=7. We aim to explore possible relationships between the size and morphology of the rings and various galactic parameters. We produce colour index and structure maps, as well as Halpha and Paalpha continuum-subtracted images from HST archival data. We derive ellipticity profiles from H-band 2MASS images in order to detect bars and find their metric parameters. We measure the non-axisymmetric torque parameter, Qg, and search for correlations between bar, ring metric parameters, and Qg. Our atlas of nuclear rings includes star-forming and dust rings. Nuclear rings span a range from a few tens of parsecs to a few kiloparsecs in radius. Star-forming nuclear rings can be found in a wide range of morphological types, from S0 to Sd, with a peak in the distribution between Sab and Sb, and without strong preference for barred galaxies. Dust nuclear rings are found in elliptical and S0 galaxies. For barred galaxies, the maximum radius that a nuclear ring can reach is a quarter of the bar radius. We found a nearly random distribution of PA offsets between nuclear rings and bars. There is some evidence that nuclear ring ellipticity is limited by bar ellipticity. We confirm that the maximum relative size of a star-forming nuclear ring is inversely proportional to the non-axisymmetric torque parameter, Qg, and that the origin of nuclear rings, even the ones in non-barred hosts, are closely linked to the existence of dynamical resonances.
This article reviews observations and models of the diffuse ionized gas that permeates the disk and halo of our Galaxy and others. It was inspired by a series of invited talks presented during an afternoon scientific session of the 65th birthday cele
bration for Professor Carl Heiles held at Arecibo Observatory in August 2004. This review is in recognition of Carls long standing interest in and advocacy for studies of the ionized as well as the neutral components of the interstellar medium.
Context: Local reductions of the stellar velocity dispersion in the central regions of galaxies are known as sigma-drops. Knowing the origin of these features can lead to better understanding of inner galactic dynamics. Aims: We present a sample of
20 sigma-drop galaxies matched with a control sample of galaxies without sigma-drop in order to search for correlations between sigma-drops and the properties, primarily morphological, of the nuclear zones and discs of their host galaxies. Methods: We study the dust and Halpha distribution at 0.1 arcsec scale, using Hubble Space Telescope imaging, in the circumnuclear zones of the two samples of galaxies, searching for differences and trying to establish a link between the nuclear kinematics and the host morphology. We have also considered the CO and HI emission of the galaxies and their luminosity profiles. Results: We classify the two samples following both morphological parameters and the luminosity profiles. We find a larger fraction of nuclear dust spirals and Halpha rings in the sigma-drop sample. We also find that the fraction of Seyfert galaxies in the sigma-drop sample is bigger than that of LINERs and that the reverse is true for the control sample. Conclusions: Our findings are evidence that a $sigma$-drop is very probably due to inflow-induced star formation in a dynamically cool disc, or in a gas ring, shock focused by an inner Lindblad resonance above a certain critical density level. The same mechanism that feeds the nuclear ring or the nuclear disc is probably reponsible for the higher rate of Seyfert galaxies among the $sigma$-drop hosts.
Ring-shaped morphologies of nuclear star-forming regions within the central 40-200 pc of disk galaxies have been barely resolved so far in three composite Sy2 nuclei, the Sy2 Circinus galaxy and in three non-AGN galaxies. Such morphologies resemble t
hose of the standard 1 kpc-size nuclear rings that lie in the inner Lindblad resonance regions of disk galaxies and, if they have a similar origin, represent recent radial gas inflows tantalisingly close to the central supermassive black holes. We aim to identify the population of such ultra-compact nuclear rings (UCNRs) and study their properties in relation to those of the host galaxies. From archival Hubble Space Telescope UV and Halpha images and from dust structure maps of the circumnuclear regions in nearby galaxies, we analyse the morphology of the star formation and dust, specifically searching for ring structures on the smallest observable scales. In a sample of 38 galaxies studied, we have detected a total of four new UCNRs, 30-130 pc in radius, in three different galaxies. Including our confirmation of a previous UCNR detection, this yields a UCNR fraction of roughly 10%, although our sample is neither complete nor unbiased. For the first time we resolve UCNRs in two LINERs. Overall the UCNR phenomenon appears widespread and limited neither to late-type galaxies nor exclusively to AGN hosts.
The accretion of the intergalactic medium onto the gaseous disc is used to explain the generation of galactic warps. A cup-shaped distortion is expected, due to the transmission of the linear momentum; but, this effect is small for most incident infl
ow angles and the predominant effect turns out to be the transmission of angular momentum, i.e. a torque giving an integral-sign shaped warp. The torque produced by a flow of velocity ~100 km/s and baryon density ~10^{-25} kg/m^3, which is within the possible values for the intergalactic medium, is enough to generate the observed warps and this mechanism offers quite a plausible explanation. The inferred rate of infall of matter, ~1 M_sun/yr, to the Galactic disc that this theory predicts agrees with the quantitative predictions of chemical evolution resolving key issues, notably the G-dwarf problem. Sanchez-Salcedo (2006) suggests that this mechanism is not plausible because it would produce a dependence of the scaleheight of the disc with the Galactocentric azimuth in the outer disc, but rather than being an objection this is another argument in favour of the mechanism because this dependence is actually observed in our Galaxy.
