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.
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.
We present an analysis of the star formation properties of field galaxies within the local volume out to a recession velocity limit of 3000 km/s. A parent sample of 863 star-forming galaxies is used to calculate a B-band luminosity function. This is
then populated with star formation information from a subsample of 327 galaxies, for which we have H alpha imaging, firstly by calibrating a relationship between galaxy B-band luminosity and star formation rate, and secondly by a Monte Carlo simulation of a representative sample of galaxies, in which star formation information is randomly sampled from the observed subset. The total star formation rate density of the local Universe is found to be between 0.016 and 0.023 MSun/yr/cubic Mpc, with the uncertainties being dominated by the internal extinction correction used in converting measured H alpha fluxes to star formation rates. If our internally derived B-band luminosity function is replaced by one from the Sloan Digital Sky Survey blue sequence, the star formation rate densities are approx. 60% of the above values. We also calculate the contribution to the total star formation rate density from galaxies of different luminosities and Hubble T-types. The largest contribution comes from bright galaxies with B absolute mag of approx. -20 mag, and the total contribution from galaxies fainter than -15.5 mag is less than 10%. Almost 60% of the star formation rate density comes from galaxies of types Sb, Sbc or Sc; 9% from galaxies earlier than Sb and 33% from galaxies later than Sc. Finally, 75 - 80% of the total star formation in the local Universe is shown to be occurring in disk regions, defined as being >1 kpc from the centres of galaxies.
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.
Many barred galaxies show a set of symmetric enhancements at the ends of the stellar bar, called {it ansae}, or the ``handles of the bar. The ansa bars have been in the literature for some decades, but their origin has still not been specifically add
ressed, although, they could be related to the growth process of bars. But even though ansae have been known for a long time, no statistical analysis of their relative frequency of occurrence has been performed yet. Similarly, there has been no study of the varieties in morphology of ansae even though significant morphological variations are known to characterise the features. In this paper, we make a quantitative analysis of the occurrence of ansae in barred galaxies, making use of {it The de Vaucouleurs Atlas of Galaxies} by Buta and coworkers. We find that $sim 40%$ of SB0s show ansae in their bars, thus confirming that ansae are common features in barred lenticulars. The ansa frequency decreases dramatically with later types, and hardly any ansae are found in galaxies of type Sb or later. The bars in galaxies with ansae are stronger in the median than those in galaxies without ansae, but the presence of inner and outer rings is not related to the presence of ansae. Implications of these results and theories for the possible origin of ansae are discussed briefly.
(Abridged) We present a study of the optical spectra of a sample of eight star-forming nuclear rings and the nuclei of their host galaxies. The spectra were obtained with the ISIS spectrograph on the William Herschel Telescope and cover a wide range
in wavelength, enabling the measurement of several stellar absorption features and gas emission lines. We compared the strength of the absorption lines to a variety of population synthesis models for the star-formation history in the nuclear rings, including also the contribution of the older bulge and disc stellar components. We find that the stars in our sample of nuclear rings have most likely formed over a prolonged period of time characterised by episodic bursts of star-formation activity. Constant star formation is firmly ruled out by the data, whereas a one-off formation event is an unlikely explanation for a common galactic component such as nuclear rings. We have used emission-line measurements to constrain the physical conditions of the ionised gas within the rings. Emission in all nuclear rings originates from HII-regions with electron densities typical for these kinds of objects, and that the rings are characterised by values for the gas metallicity ranging from slightly below to just above solar. As 20% of nearby spiral galaxies hosts nuclear rings that are currently forming massive stars, our finding of an episodic star formation history in nuclear rings implies that a significant population remains to be identified of young nuclear rings that are not currently in a massive star formation phase.
