[en] Ultra-compact nuclear rings (UCNRs) can be defined as those galactic circumnuclear rings with a radius inferior to 200 parsecs. Because very high spatial resolution is needed, this population has not been studied systematically until now. One important issue that has to be solved is whether UCNRs have an origin similar to larger circumnuclear rings, which are related to inner Lindblad resonances, or whether they have a different origin. We present the first results of a survey performed on 487 galaxies imaged by HST, to find and characterize UCNRs. These results include the detection of new rings as well as the confirmation of rings that were already known. We present a list of 23 reliably detected UCNRs and discuss their origin.

[en] We test the theoretical prediction that the straightest dust lanes in bars are found in strongly barred galaxies, or more specifically, that the degree of curvature of the dust lanes is inversely proportional to the strength of the bar. The test uses archival images of barred galaxies for which a reliable nonaxisymmetric torque parameter (Q _b) and the radius at which Q _b has been measured (r(Q _b)) have been published in the literature. Our results confirm the theoretical prediction but show a large spread that cannot be accounted for by measurement errors. We simulate 238 galaxies with different bar and bulge parameters in order to investigate the origin of the spread in the dust lane curvature versus Q _b relation. From these simulations, we conclude that the spread is greatly reduced when describing the bar strength as a linear combination of the bar parameters Q _b and the quotient of the major and minor axes of the bar, a/b. Thus, we conclude that the dust lane curvature is predominantly determined by the parameters of the bar.

[en] We consider the massive star formation properties, radial profiles, and atomic gas masses of those galaxies in our Hα Galaxy Survey that have close companion galaxies, in comparison with a matched control sample of galaxies without companions. Our analysis is based on Hα and continuum images of 327 disk galaxies that form a representative sample of the local universe. We find that the presence of a close companion raises the star formation rate by a factor of just below two, while hardly at all increasing the equivalent width of the Hα emission. This means that although statistically galaxies with close companions form stars at a higher rate, they do this over extended periods of time and not as bursts. We find no significant increase in the central concentration of the star formation as a result of the presence of a close companion. The fraction of truly interacting or merging galaxies is very small in the local universe, at around 2%, and possibly 4% of bright galaxies. Most of these interacting galaxies currently have unremarkable star formation properties. We also study the properties of the galaxies in the survey with the most extreme values for star formation indicators such as rate, equivalent width, star formation rate per area, and gas-depletion timescale. We find that each of these indicators favors a different subset of galaxies, and use this information to discuss critically the possible definitions of the term starburst to describe galaxies with enhanced star formation activity. We conclude that no one starburst definition can be devised that is objective and generally discriminant. Unless one restricts the use of the term 'starburst' to a very small number of galaxies indeed, the term will continue to be used for a heterogeneous and wide-ranging collection of objects with no physical basis for their classification as starburst. Our overall conclusions are that (1) whereas the rare interacting and merging galaxies may have enhanced star formation, and vice versa, those galaxies with the highest star formation are often interacting or merging, the influence of the presence of a close companion on the star formation in galaxies is in general very small, and long lasting; and (2) the term 'starburst' cannot be easily defined objectively and generally in physical terms.

[en] We combine DensePak integral field unit and TAURUS Fabry-Perot observations of 13 nuclear rings to show an interconnection between the kinematic properties of the rings and their resonant origin. The nuclear rings have regular and symmetric kinematics, and lack strong non-circular motions. This symmetry, coupled with a direct relationship between the position angles and ellipticities of the rings and those of their host galaxies, indicates that the rings are in the same plane as the disk and are circular. From the rotation curves derived, we have estimated the compactness (v²/r) up to the turnover radius, which is where the nuclear rings reside. We find that there is evidence of a correlation between compactness and ring width and size. Radially wide rings are less compact, and thus have lower mass concentration. The compactness increases as the ring width decreases. We also find that the nuclear ring size is dependent on the bar strength, with weaker bars allowing rings of any size to form.

[en] We investigate the nature of the relations between black hole (BH) mass (M _BH) and the central velocity dispersion (σ) and, for core-Sérsic galaxies, the size of the depleted core (R _b). Our sample of 144 galaxies with dynamically determined M _BH encompasses 24 core-Sérsic galaxies, thought to be products of gas-poor mergers, and reliably identified based on high-resolution Hubble Space Telescope imaging. For core-Sérsic galaxies, i.e., combining normal-core (R _b < 0.5 kpc) and large-core galaxies (R _b ≳ 0.5 kpc), we find that M _BH correlates remarkably well with R _b such that $M_{\mathrm{B}\mathrm{H}}\propto <!--\; ???\; -->R_{\mathrm{b}}^{1.20\pm <!--\; ??\; -->0.14}$ (rms scatter in log M _BH of Δ_rms ∼ 0.29 dex), confirming previous works on the same galaxies apart from three new ones. Separating the sample into Sérsic, normal-core and large-core galaxies, we find that Sérsic and normal-core galaxies jointly define a single log-linear M _BH–σ relation M _BH ∝ σ ^4.88±0.29 with Δ_rms ∼ 0.47 dex; however, at the high-mass end, large-core galaxies (four with measured M _BH) are offset upward from this relation by (2.5–4) × σ _s, explaining the previously reported steepening of the M _BH–σ relation for massive galaxies. Large-core spheroids have magnitudes M _V ≲ −23.50 mag, half-light radii R _e ≳ 10 kpc, and are extremely massive, M _* ≳ 10¹² M _☉. Furthermore, these spheroids tend to host ultramassive BHs (M _BH ≳ 10¹⁰ M _☉) tightly connected with their R _b rather than σ. The less popular M _BH–R _b relation exhibits ∼62% less scatter in log M _BH than the M _BH–σ relations. Our findings suggest that large-core spheroids form via multiple major “dry” merger events involving super/ultramassive BHs, consistent with the flattening of the σ–L _V relation observed at M _V ≲ −23.5 mag.

[en] The tight correlations between supermassive black hole (SMBH) mass (M _BH) and the properties of the host galaxy have useful implications for our understanding of the growth of SMBHs and of the evolution of galaxies. Here, we present newly observed correlations between M _BH and the host galaxy total UV−[3.6] color (${\mathcal{C}}_{\mathrm{U}\mathrm{V},\mathrm{t}\mathrm{o}\mathrm{t}}$, Pearson's r = 0.6–0.7) for a sample of 67 galaxies (20 early-type galaxies and 47 late-type galaxies) with directly measured M _BH in the Galaxy Evolution Explorer/S⁴G survey. The colors are carefully measured in a homogeneous manner using the far-UV, near-UV, and 3.6 μm magnitudes of the galaxies and their multicomponent structural decompositions in the literature. We find that more massive SMBHs are hosted by (early- and late-type) galaxies with redder colors, but the $M_{\mathrm{B}\mathrm{H}}-<!--\; ???\; -->{\mathcal{C}}_{\mathrm{U}\mathrm{V},\mathrm{t}\mathrm{o}\mathrm{t}}$ relations for the two morphological types have slopes that differ at ∼2σ level. Early-type galaxies define a red sequence in the $M_{\mathrm{B}\mathrm{H}}-<!--\; ???\; -->{\mathcal{C}}_{\mathrm{U}\mathrm{V},\mathrm{t}\mathrm{o}\mathrm{t}}$ diagrams, while late-type galaxies trace a blue sequence. Within the assumption that the specific star formation rate of a galaxy (sSFR) is well traced by L _UV/L _3.6, it follows that the SMBH masses for late-type galaxies exhibit a steeper dependence on sSFR than those for early-type galaxies. The $M_{\mathrm{B}\mathrm{H}}-<!--\; ???\; -->{\mathcal{C}}_{\mathrm{U}\mathrm{V},\mathrm{t}\mathrm{o}\mathrm{t}}$ and M _BH − L _3.6,tot relations for the sample galaxies reveal a comparable level of vertical scatter in the log M _BH direction, approximately 5%–27% more than the vertical scatter of the M _BH − σ relation. Our $M_{\mathrm{B}\mathrm{H}}-<!--\; ???\; -->{\mathcal{C}}_{\mathrm{U}\mathrm{V},\mathrm{t}\mathrm{o}\mathrm{t}}$ relations suggest different channels of SMBH growth for early- and late-type galaxies, consistent with their distinct formation and evolution scenarios. These new relations offer the prospect of estimating SMBH masses reliably using the galaxy color alone. Furthermore, we show that they are capable of estimating intermediate black hole masses in low-mass early- and late-type galaxies.

[en] We present empirical constraints on the influence of stellar bars on the fueling of active galactic nuclei (AGNs) out to z = 0.84 using a sample of X-ray-selected AGNs hosted in luminous non-interacting face-on and moderately inclined disk galaxies from the Chandra COSMOS survey. Using high-resolution Hubble Space Telescope imaging to identify bars, we find that the fraction of barred active galaxies displays a similar behavior as that of inactive spirals, declining with redshift from 71% at $z\sim <!--\; ???\; -->0.3$, to 35% at $z\sim <!--\; ???\; -->0.8$. With active galaxies being typically massive, we compare them against a mass-matched sample of inactive spirals and show that, while at face value the AGN bar fraction is slightly higher at all redshifts, we cannot rule out that the bar fractions of active and inactive galaxies are the same. The presence of a bar has no influence on the AGN strength, with barred and unbarred active galaxies showing equivalent X-ray luminosity distributions. From our results, we conclude that the occurrence and the efficiency of the fueling process is independent of the large scale structure of a galaxy. The role of bars, if any, may be restricted to providing the suitable conditions for black hole fueling to occur, i.e., bring a fresh supply of gas to the central 100 pc. At the high-redshift end, we find that roughly 60% of active disk galaxies are unbarred. We speculate this to be related with the known dynamical state of disks at higher redshifts—more gas-rich and prone to instabilities than local spirals—which could also lead to gas inflows without the need of bars.

[en] We present deep near-infrared K_s -band Anglo-Australian Telescope Infrared Imager and Spectrograph observations of a selected sample of nearby barred spiral galaxies, including some with the strongest known bars. The sample covers a range of Hubble types from SB0^- to SBc. The goal is to determine if the torque strengths of the spirals correlate with those of the bars, which might be expected if the bars actually drive the spirals as has been predicted by theoretical studies. This issue has implications for interpreting bar and spiral fractions at high redshift. Analysis of previous samples suggested that such a correlation exists in the near-infrared, where effects of extinction and star formation are less important. However, the earlier samples had only a few excessively strong bars. Our new sample largely confirms our previous studies, but still any correlation is relatively weak. We find two galaxies, NGC 7513 and UGC 10862, where there is only a weak spiral in the presence of a very strong bar. We suggest that some spirals probably are driven by their bars at the same pattern speed, but that this may be only when the bar is growing or if there is abundant gas and dissipation.

[en] We obtained GALEX FUV, NUV, and Spitzer/IRAC 3.6 μm photometry for >2000 galaxies, available for 90% of the S⁴G sample. We find a very tight GALEX blue sequence (GBS) in the (FUV–NUV) versus (NUV–[3.6]) color–color diagram, which is populated by irregular and spiral galaxies, and is mainly driven by changes in the formation timescale (τ) and a degeneracy between τ and dust reddening. The tightness of the GBS provides an unprecedented way of identifying star-forming galaxies and objects that are just evolving to (or from) what we call the GALEX green valley (GGV). At the red end of the GBS, at (NUV–[3.6]) > 5, we find a wider GALEX red sequence (GRS) mostly populated by E/S0 galaxies that has a perpendicular slope to that of the GBS and of the optical red sequence. We find no such dichotomy in terms of stellar mass (measured by M_[3.6]) since both massive (M_⋆>10¹¹M_⊙) blue- and red-sequence galaxies are identified. The type that is proportionally more often found in the GGV is the S0-Sa’s, and most of these are located in high-density environments. We discuss evolutionary models of galaxies that show a rapid transition from the blue to the red sequence on a timescale of 10⁸ yr

[en] We examine the effect of circumstellar dust extinction on the near-IR (NIR) contribution of asymptotic giant branch (AGB) stars in intermediate-age clusters throughout the disk of M100. For our sample of 17 AGB-dominated clusters we extract optical-to-mid-IR spectral energy distributions (SEDs) and find that NIR brightness is coupled to the mid-IR dust emission in such a way that a significant reduction of AGB light, of up to 1 mag in the K band, follows from extinction by the dust shell formed during this stage. Since the dust optical depth varies with AGB chemistry (C-rich or O-rich), our results suggest that the contribution of AGB stars to the flux from their host clusters will be closely linked to the metallicity and the progenitor mass of the AGB star, to which dust chemistry and mass-loss rate are sensitive. Our sample of clusters—each the analogue of a ∼1 Gyr old post-starburst galaxy—has implications within the context of mass and age estimation via SED modeling at high-z: we find that the average ∼0.5 mag extinction estimated here may be sufficient to reduce the AGB contribution in the (rest-frame) K band from ∼70%, as predicted in the latest generation of synthesis models, to ∼35%. Our technique for selecting AGB-dominated clusters in nearby galaxies promises to be effective for discriminating the uncertainties associated with AGB stars in intermediate-age populations that plague age and mass estimation in high-z galaxies.