Received 23 May 2016; accepted 26 July 2016; published 29 July 2016
The unified standard model for active galactic nuclei stands on a geometric paradigm, which implies that many of the observed properties among the different kinds of objects arise from the observer position and not from intrinsic properties of the host galaxy or their environment; in particular this statement is essential when explaining the differences among Seyfert types or among radio galaxy types.
This strong constraint makes important any observational suggestion about systematic differences in the host galaxy properties that could not depend on the observer line of sight. A variety of statistical studies were carried out without conclusive results (e.g.  -  ). The confirmation of systematic differences in the intrinsic properties would introduce a new ingredient in the unified standard model, an evolution process from less energetic nuclei (Type 2 AGN) toward more energetic ones (Type 1 AGN), making the study of feeding processes crucial for understanding the nature of AGN classes  -  .
Consequently, the connection between nuclear activity, star formation and infalling gas has received growing attention over the past ten years, with nuclear bars and circumnuclear disk instabilities being invoked as preferred mechanisms for removing angular momentum from the gaseous fuel (e.g.  -  ). In order to find any relation between AGN activity and environment, some structures present in the central regions of galaxies are particularly interesting; they are the star-forming circumnuclear rings and ring-like circumnuclear spirals (hereafter CNRs), which arise from the accumulation of infalling material at certain radius proving the active and effective removing of the gas angular momentum. Moreover, CNRs are morphological structures which are radially well differentiated from the active nucleus itself and other structures that can be associated to the active nuclei, like outflows or extended ionization regions. On the contrary, other circumnuclear star formation features as, for example, nuclear bars and hot spots can be confused at low resolution with AGN-related structures. Besides, CNRs represent a defined stage in the secular evolution of barred systems (e.g.  ) making more probable the detection of any correlation with a defined AGN feeding stage  .
The relationship between CNRs and the resonances (Inner Lindblad Resonance, ILR) that help to accumulate gas in these rings has been extensively discussed from both the theoretical (e.g.   ) and the observational (e.g.  -  ) standpoints. These resonances are a dynamical consequence of the bar perturbation but, although bars seem to contribute significantly to circumnuclear star formation, there are no robust evidences of a close relation between bars and nuclear “non-stellar” activity   . Furthermore, some non-barred galaxies exhibit CNRs in their nuclear region, suggesting differences in time scale evolution of bars and CNRs  .
In the present study we analyze a sample of nearby galaxies with CNRs and compare the properties of the host galaxy with that of a control sample, including the presence of AGNs and their type of activity. This work is organized as follow: In Section 2, we present the selected sample of galaxies with CNRs, we describe their properties that were considered to construct the control sample with similar properties than first one but without CNRs. In Section 3, we present the main results obtained from the comparison of both samples of galaxies. In Section 4, we discuss the results presented in the previous section and offer the final remarks. The catalogue presented here contains the most comprehensive sample of circumnuclear rings extracted from the literature until the year 2009. The preliminary version of this work is available at 2004astro.ph.9101.
2. CNR Galaxy Sample
2.1. Sample Selection and Description
We selected galaxies harboring CNRs in their central region. The CNR galaxy compilation was made based on images, color maps and radial profiles from literature up to 2009. Radial profiles are required in order to be able to measure the CNR dimensions. The starting point in the search was the Catalog of CNR galaxies from  , which was based on plate observations. The authors  present 64 CNR galaxies of which we managed to get visual confirmation of 51 of them.
In spite of the rings being well-defined structures, the classification of inner or circumnuclear rings is difficult when the global bar is not detected. For that reason we define some selection criteria in order to avoid misclassification of the observed ring. They are:
(1) The diameter ratio of CNR respect to the host galaxy should be less than 0.2.
(2) CNR radius between 100pc and 2 kpc. These restrictions were imposed based on the typical Inner Lindblad Resonances, limited for the Nuclear Lindblad Resonance and the inner ring at small and larger radii respectively.
A third criterion was added from the observational point of view. We are looking for well-resolved features. Additionally, we are considering a follow-up study of CNR galaxies, so we are interested in CNRs that can be observed from ground-based facilities and that could be compared with the ones detected in massive surveys. Therefore, we selected CNRs with:
(3) Apparent CNR radius larger than 1.5 arcsec (Distance up to 275 Mpc).
ESO 565-G11 was exhaustively analyzed by  who determine that this galaxy harbors an exceptionally large CNR with a radius of 2.6 kpc. In spite of the CNR radius is out of the selection criterion, the circumnuclear classification of this ring is certain, and then this galaxy was included in the sample. ESO 138-G1 is a E/S0 galaxy with no apparent bar, which present a relative radius of 0.23, just above the imposed limit of 0.20, but the absolute radius is 1.25 kpc, well below the upper radius limit, whereby ESO 565-11 was incorporated anyway.
Galaxies without a CNR published image, but with CNR parameters from two or more published sources are included, adopting the mean CNR radius value.
In Table 1, we list the 94 selected CNR galaxies. Over 30 additional galaxies seemed good candidates to have CNRs, however some of them do not fully satisfy our selection criteria and some others require detailed observations to confirm the presence of the ring.
In Figure 1, we present the absolute and relative radius distribution of the sample. The median radius of a circumnuclear ring is 0.7 kpc (mean 0.8 kpc, rms 0.4 kpc). When considering only non-barred hosts the average is 0.87 kpc, there is a slight trend of larger CNR’s residing in galaxies without bar. 91% of the catalogued CNR’s (up to 2010) are between 200 pc and 1.85 kpc, reinforcing the election of radius limits adopted in the sample selection criteria. Additionally, the upper limit of 20% in relative size is in the tail of the distribution, warranting the loss of a few objects because of it.
The morphological type, bar classification, and nuclear activity for the selected galaxies are listed in Table 2. The first two characteristics were obtained from  and, for galaxies that were not catalogued by  , from NED. The nuclear activity classification was based on the catalog of  who classify galaxies with strong nuclear activity, and we use  for classification of nuclei with lower activity. For the objects common to both catalogs, the activity type was almost coincident. This suggests that the relative distribution of Seyfert types is not strongly dependent on the two classification sources used here. The used nuclear activity classification is in agreement with that assigned by NED in all cases except for NGC 5850, which is classified as LINER in  but has no nuclear activity classification in NED.
CNRs are thought to be related with the ILRs which are dynamically generated by the bar perturbation. However, some CNR galaxies are un-barred galaxies. In Figure 3, we display the distribution of bar intensity classification for CNR galaxies in comparison with the expected bar distribution from  . While the CNR galaxy sample presents, as expected, an excess of barred galaxies, the number of strong bars is the expected one. Indeed, the weak barred galaxies are out of the expectation respect of the  sample (16 objects above the expectation value, N1/2 = 6.4).
2.2. Nuclear Activity
The nuclear activity distribution of the sample is shown in Figure 4. The intermediate class of Seyfert galaxies,
Table 1. CNR galaxy sample.
Note: Col. 1: Galaxy Name. Col. 2: Radius of galaxy to the 25th mag isophote in B-band in Kpc (NED). Col. 3: CNR Radius in Kpc. Col. 4: CNR classification: “H” star-forming CNR; “D” Dust CNR, “S” Stellar CNR. Col. 5: Reference for images, color maps or radial profiles where the CNR was identified.
Figure 1. Distribution of circumnuclear ring radius for the sample of 94 CNR.
Sy1.2 and Sy1.5, were included in Sy1 class, while the intermediate class, Sy1.8 and Sy1.9 were added to the Sy2 class, as well as the Seyfert galaxies classified as Sy1 only in polarized light because this specific technique studies are available only for few galaxies, being classified as Sy2 in classic spectroscopic studies. The Sy2 galaxies are predominant in the CNR sample, even more frequent than nuclear star-forming galaxies.
Reference  established the frequency of Seyfert galaxies per morphological type for a sample of 1246 galaxies, whose nuclear activity classification was based on  catalog (Frequency of Sy1-Sy2: 2% - 0% in E/S0; 6.5% - 6.5% in Sa and Sab; 3.2% - 3.9% in Sb and Sbc; 0.5% - 0.8% in Sc). Considering the morphological distribution of the CNR galaxy sample shown in Figure 2, we determine the expected number of Sy1 and Sy2 galaxies in the sample, resulting in 3 Sy1 and 3 Sy2 galaxies. These expected values are very low respect to the observed ones of 6 Sy1 and 16 Sy2 galaxies. Thanks to the observational improvements, the number of discoveries of weak active nuclei was increasing respect to the active nuclei of higher activity. For that reason, we make a new comparison using as reference a study searching for signs of low activity in 486 nearby galactic nuclei  . For this study the frequency is Sy1-Sy2: 0% - 7% in E; 2% - 7% in S0; 7% - 12% in S0/Sab; 2% - 12.6% in Sb and Sbc; 1% - 4.5% in Sc and Scd; 0% - 5% in Sd/Sm.
It is worth to mention that this study was carried out with thorough spectroscopic analysis looking for weak nuclear activity. However the CNR sample was not analyzed as thoroughly as that of  . Nonetheless, the expectation from the thorough  sample is 3 Sy1 and 9 Sy2 galaxies, still below the observed values. One could expect that thorough spectroscopy of the CNR sample would add even more weak AGN’s, enlarging the frequency of CNR’s with AGN’s respect to the expected from the galaxy sample of  .
Considering the very low number of active nuclei in the CNR galaxy sample, we designed another test trying to determine if the excess of active nuclei is significant. We built a comparison galaxy sample as a control sample.
3. Galaxies with and without a CNR
As we want to assess the incidence of nuclear activity in the CNR galaxy sample, we excluded seven objects of the CNR galaxy sample because their CNRs were detected in studies of active galactic nuclei. Their active galaxy condition was decisive in the discovery of the CNR, conditioning the CNR and nuclear activity relationship. In order to avoid biases in the nuclear activity classification or global properties of CNR and comparison galaxies, we restricted the sample to those galaxies catalogued in  and we use exclusively the  catalog for nuclear activity classification.
The resulting non-biased subsample of CNR galaxies has 67 objects. For the assignment of the comparison galaxy we consider the following aspects:
Observationally, the probability to detect a CNR depends on the inclination of the galaxy and the apparent diameter. Therein, we want the same CNR detection probability that the hitherto known CNR galaxies.
Table 2. CNR galaxy properties.
Note: Col. 1: Galaxy Name. Col. 2: Morphological Type from Tully (1988: E = −5; S0 = −2; Sa = 1; Sb = 3; Sc = 7). Col. 3: Bar Intensity Classification (“B”: Strong; “X” Weak; “A” Absent). Col. 4: Nuclear Activity Classification: (left) Veron-Cetty & Veron (2006) nuclear classification (“H2” nuclear star-formation; “S1”, “S1.5”, “S1.8”, “S1.9”, and “S2” Seyfert nuclei; “S1h” Sy2 nuclei with broad emission lines only in polarized light; “S3” LINER/Transition); (right) Ho et al. (1997) nuclear classification (“H” nuclear star-formation; “L2” LINER; “T2” Transition).
Figure 2. (a) Distribution of Morphological Type from  for the host galaxies of the CNR sample; (b) comparison between the observed morphological distribution with that expected for a random sample of 94 galaxies from  Catalog distribution.
Consequently, we impose the following conditions for comparison galaxies:
(1) The departure in inclination from the CNR galaxy must be ∆i < 10º;
(2) The difference in corrected apparent sizes must be ∆D25 < 0.2 D25CNR
We aim to have comparison galaxies with similar global properties than the CNR galaxies in order to guarantee the same probability to have an active nucleus. Therefore, we designated the following criteria:
(3) The departure in morphological type numerical code from the CNR galaxy must be ∆T ≤ 1.
(4) The departure in B absolute magnitude from the CNR galaxy must be ∆MB < 0.3;
(5) The difference in projected real sizes must be ∆R25 (kpc) < 0.4 R25CNR (kpc);
We define a selection criteria index that includes the five selection criteria with the same weight each one. The galaxy in the  general galaxy catalog that has the lowest selection index was assigned as the comparison galaxy for each CNR host galaxy. The selected comparison galaxy was examined in the literature in order to guarantee that there is no indication of the presence of an unreported CNR. In Table 3, we list the selection properties for CNR and comparison galaxies. The mean differences between both samples are (−4˚ ± 10˚) in inclination; (0.8’ ± 1.7’) in apparent diameter; (0.0 ± 0.2) mag in B-band absolute magnitude; (0 ± 1) in morpho-
Figure 3. Distribution of bar classification for the host galaxies of the CNR sample. The horizontal marks indicate the expected distribution for a random sample of 94 galaxies from  Catalog distribution.
Figure 4. Distribution of nuclear emission spectroscopic classification for the CNR sample hosts. “S1” class includes Sy1 galaxies, Sy1.2, and Sy1.5; “S2” class includes Sy2 galaxies, Sy1.8, Sy1.9 and galaxies classified as Sy1 only in polarized light (Sy1i, Sy1h); “L” class includes LINER galaxies; “HII” class includes galaxies with star formation in the nucleus; “no” includes galaxies with stellar nuclei. Marks indicate the expected Seyfert galaxy numbers from the  active galaxy frequency distribution considering the morphological distribution of the CNR galaxy sample.
logical type; and (−0.9 ± 4.1) Kpc in absolute diameter.
The bar intensity distribution for both samples is shown in Figure 5. While the tendency is the same than the one observed in the full sample (an excess of weak bars in CNR galaxies respect to the comparison galaxies), this result is not conclusive due to the low number of objects.
In Figure 6, the incidence of AGN’s is plotted for both samples. Unexpectedly, there are more galaxies with active and star-forming nuclei in the CNR subsample than in the comparison one. The expectation of Sy1 and Sy2 galaxies obtained from  is of 2 Sy1 and 2 Sy2 for both samples (they have the same morphological distribution). Therefore, the number of Sy1 galaxies observed in both samples (3 in CNR galaxy subsample and
Table 3. Properties of CNR galaxy subsample.
Note: Col. 1: CNR Galaxy Name. Col. 2: Comparison Galaxy Name. Col. 3: Morphological Type (E = −5; S0 = −2; Sa = 1; Sb = 3; Sc = 7). Col. 4: Bar Classification. Col 5: galaxy inclination [deg]. Col. 6: Diameter of galaxy to the 25th mag isophote in B-band [arcmin].Col. 7: Diameter of galaxy to the 25th mag isophote in B-band corrected by projection and extinction [Kpc]. Col. 8: Absolute Magnitude in B-band. Col. 9: Absolute Magnitude in K-band. Col. 10: Galaxy Distance (h = 0.75) [Mpc]. Col. 11: Local Density of Galaxies brighter than MB = −16 [Galaxy/Mpc3]. Col. 12: Kinematic WidthWiR = 2 Vmax [Km/s]. Col. 13: Mass of neutral gas [M]. Col. 14: Total Mass [M]. Col. 15: Fraction of nuclear gas. Col. 16: Mass-Luminosity ratio for neutral gas in B-band [M/L]. Col. 17: Total Mass-Luminosity ratio in B-band [M/L].
Figure 5. Distribution of bar classification for CNR and comparison galaxy samples.
Figure 6. Distribution of nuclear classification for CNR and Comparison galaxy samples. “S1” class includes Sy1 galaxies, Sy1.2, and Sy1.5; “S2” class includes Sy2 galaxies, Sy1.8, Sy1.9 and galaxies classified as Sy1 only in polarized light (Sy1i, Sy1h); “L” class includes LINER galaxies; “HII” class includes galaxies with star formation in the nucleus; “no” includes galaxies with stellar nuclei.
1 in the comparison sample) is in agreement with the expected one, although the low number of objects make difficult to detect any different trend. However, the incidence of Sy2 galaxies in the CNR galaxy subsample is significantly higher than that of the comparison sample and the expected value, being 14 against 4 Sy2 galaxies.
Some properties that could be linked with the CNR formation and evolution are available in the  catalog. We compare these properties for both samples. For instance, the tidal interaction with nearby bright galaxies could favor the in falling gas and therefore the formation of the CNRs. However, very close encounters could perturb the dynamics of the disk deleting the ILR and the consequent accumulation of material in the CNR. The galaxy local density distribution for both samples is shown in Figure 7. There are no apparent differences in the environment of CNR and comparison galaxies.
The content of gas may promote the loss of angular momentum and the consequent feeding of the CNR. Therefore, we compare the amount of neutral gas (M(HI)/Mtot and M(HI)/LB) without finding significant differences between both samples. Actually, the M(HI)/LB could present a tendency to be lower for CNR galaxies (Figure 8) although the mean values are indistinguishable.
The presence of massive dark matter halos is usually invoked for disk stability disfavoring the generation of bars and resonances. The kinematic width (W20) distribution has a peak in 300 km/s for both samples. Nevertheless, the comparison distribution has a high velocity tail, which impacts the median W20 value (307 km/s for the CNR sample and 410 km/s for the comparison one). Therefore CNR host galaxies might have smaller dark matter halos, fairly consistent with the theoretical predictions.
Finally, we compare the Mass-Luminosity Ratio (Figure 9) observing no apparent differences between samples. The Tully-Fisher relation in B and K bands is consistent with the standard relation of  for both samples.
4. Discussion and Final Remarks
We carried out an exhaustive search of galaxies harboring CNRs in their central region from images, color maps and radial profiles available in the literature, which allowed determining the dimensions of the ringed structures. Additionally, we impose some conditions in order to guarantee the circumnuclear nature of the observed ring. Thus, we have set up the most complete sample of 94 CNR galaxies known up to 2009. The sample is useful for studying the CNR and host galaxy properties in order to understand the origin and evolution of such structures and the potential connection with the feeding of the central objects.
The CNRs in this sample exhibit a mean diameter of 1.6 Kpc, being the most common value (mode) of 1 Kpc. These results are in agreement with that of  who determine a CNR mean diameter of 1.1 Kpc for 20 bared galaxies. The linear diameter of the characteristic CNR is 6% of the host galaxy diameter.
Figure 7. Distribution of local density of galaxies brighter than-16 B-mag for CNR and Comparison galaxy samples.
Figure 8. Distribution of Mas-Luminosity ratio in B-band for the neutral gas mass component.
The presence of CNR is more frequent in Sb-Sbc galaxies, and, comparing with the  catalog, the CNR galaxy sample present an excess of Sa and Sb galaxies respect to a random sample of 94 spiral galaxies. This morphological type excess could be explained if evolved disks favor these ringed structures. The CNR galaxies also prefer weak bars, opposite than expected taking into account that the bar perturbation is invoked as the mechanism of generation of the resonances associated to the CNRs. This result could indicate different evolution time scale for bar and CNR structures.
Dynamical studies propose that the galactic disks can be stabilized readjusting the mass distribution in the central region, increasing the central rotation velocity, which would lead in the ILR development. As a consequence, the density wave propagation is decelerated and the loss of angular momentum is favored  . Therefore,
Figure 9. Distribution of mas-luminosity ratio in B-band for the galaxy total mass.
the incidence of CNRs, as evidence of the ILR presence, would be greater in galaxies with massive central regions. Considering that the mass of bulge component is related with the mass of the nuclear supermassive black hole  , a potential connection between the CNRs and the nuclear activity is suspected. Indeed, a larger number of active nuclei are observed in the CNR galaxy sample respect to that expected for a random sample with the same morphological type distribution. Particularly, the main excess is presented in the number of Sy2 active galaxies.
Taking into account the possible selection bias of the sample, we design a comparison galaxy sample with galaxies having similar observational and physical characteristics. CNRs discovered in AGN searches were excluded in order to avoid the bias toward active objects. Moreover, single source catalogs of galaxy properties and nuclear activity classification were used, yielding to a 67 CNR galaxy subsample with their respectively comparison galaxy.
Although the same tendency of having more weak bars, observed in the full sample, is also insinuated in the subsample, this excess is not conclusive due to the low number of objects. However, the excess of active nuclei respect to the control sample is more robust (17 vs 5). Worth noting that the active galaxies observed in the comparison galaxy sample is in accordance with the expectation obtained from  (5 vs 4). Again, the excess of active nuclei is notorious in the Sy2 type, being 14 in the subsample against 4 in the control sample.
Within the framework of the classical unified model for Seyfert galaxies, the Sy1 and Sy2 galaxies are the same phenomenon, differing only in the orientation of the supermassive black hole accretion disk respect to the observer line of sight. Taking into account that the CNRs are more probably observed in galaxies with low inclination, in this sample, we would expect positive selection effect for Sy1 type, unless the accretion disks were not coplanar with the galactic disk in a statistical sense. Some authors propose an evolutionary scenario for Seyfert galaxies in which the active nuclei is powered as the material inflow is increasing (e.g.     ). In this context, as the gas is led to the central region and the star formation is triggered; the supermassive black hole is fed and the Sy2 stage is activated. As already mentioned, this increasing central mass concentration promotes the CNR formation. And therefore, the coexistence of the CNR and Sy2 phase is expected. However, the increasing inflows toward the nucleus should attenuate the CNR structures in order to when the Sy1 phase is triggered, the CNR is barely observed, as a possible explanation for the lack of excess of Sy1 in the CNR galaxy sample. However, no evolved CNRs are observed in Sy1 galaxies, being 4 CNRs of star formation and 2 dusty CNRs. The  catalog allowed us to study some other properties in the CNR galaxy subsample and the comparison one. The CNR existence seems to be unrelated with the presence of a bright companion galaxy, whereby the CNR formation would be related to intrinsic galaxy properties or to the merging with satellite dwarf galaxies.
The detected trends and their consequences seem significant enough to encourage researchers in the field to perform high quality observations of a large and complete set of galaxies with circumnuclear rings and its matched control sample.
 Dultzin-Hacyan, D., Krongold, Y., Fuentes-Guridi, I. and Marziani, P. (1999) The Close Environment of Seyfert Galaxies and Its Implication for Unification Models. The Astrophysical Journal, 513, L111.
 Storchi-Bergmann, T., Gonzalez-Delgado, R.M., Schmitt, H.R., Fernades, R.C. and Heckman, T. (2001) Circumnuclear Stellar Population, Morphology, and Environment of Seyfert 2 Galaxies: An Evolutionary Scenario. The Astrophysical Journal, 559, 147.
 Schmitt, H.R. (2001) The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus. The Astronomical Journal, 122, 2243.
 Virani, S.N., De Robertis, M.M. and van Dalfsen, M.L. (2000) A CCD Study of the Environment of Seyfert Galaxies. III. Host Galaxies and the Nearby Environments. The Astronomical Journal, 120, 1739.
 Martini, P., Regan, M.W., Mulchaey, J.S. and Pogge, R.W. (2003) Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei. The Astrophysical Journal, 589,774.
 Maiolino, R., Alonso-Herrero, A., Anders, S., Quillen, A., Rieke, M.J., Rieke, G.H. and Tacconi-Garman, L.E. (2000) Discovery of a Nuclear Gas Bar Feeding the Active Nucleus in Circinus. The Astrophysical Journal, 531,219-231.
 Combes, F (2000) Bar Driven Galaxy Evolution and Time-Scales to Feed AGN. In Combes, F., Mamon, G.A. and Charmandaris, V., Eds., Dynamics of Galaxies: From the Early Universe to the Present, Astronomical Society of the Pacific Conference Series 197, 15.
 Piner, B.G., Stone, J.M. and Teuben, P.J. (1995) Nuclear Rings and Mass Inflow in Hydrodynamic Simulations of Barred Galaxies. The Astrophysical Journal, 449, 508.
 Laine, S., Shlosman, I., Knapen, J.H. and Peletier, R.F. (2002) Nested and Single Bars in Seyfert and Non-Seyfert Galaxies. The Astrophysical Journal, 567, 97-117.
 Moles, M., Marquez, I. and Perez, E. (1995) The Relation between Dynamical Perturbations, Morphology, and Nuclear Activity in Spiral Galaxies. The Astrophysical Journal, 438, 604-609.
 Buta, R., Crocker, D.A. and Byrd, G.G. (1999) A Hubble Space Telescope Optical and Ground-Based Near-Infrared Study of the Giant Nuclear Ring in ESO 565-11. The Astronomical Journal, 118, 2071-2100.
 Ho, L.C., Filippenko, A.V. and Sargent, W.L.W. (1997) A Search for “Dwarf” Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies. The Astrophysical Journal Supplement Series, 112, 315-390.
 Tully, R.B. and Pierce, M.J. (2000) Distances to Galaxies from the Correlation between Luminosities and Line Widths. III. Cluster Template and Global Measurement of H0. The Astrophysical Journal, 533, 744-780.
 Magorrian, J., Tremaine, S., Richstone, D., Bender, R., Bower, G., Dressler, A., Faber, S.M., Gebhardt, K., Green, R., et al. (1998) The Demography of Massive Dark Objects in Galaxy Centers. The Astronomical Journal, 115, 2285-2305.
 Levenson, N.A., Weaver, K.A. and Heckman, T.M. (2001) The Seyfert-Starburst Connection in X-Rays. II. Results and Implications. The Astrophysical Journal, 550, 230-242.
 Ferruit, P., Wilson, A. and Mulchaey, J. (2000) Hubble Space Telescope WFPC2 Imaging of a Sample of Early-Type Seyfert Galaxies. Astrophysical Journal Supplement, 128, 139-169.
 Buta, R., Alpert, A.J., Cobb, M.L., Crocker, D.A. and Purcell, G.B. (1998) An Optical, Near-Infrared, and Kinematic Study of Four Early-Type Resonance Ring Galaxies. The Astronomical Journal, 116, 1142-1162.
 Crocker, D.A., Baugus, P.D. and Buta, R. (1996) The Distribution and Properties of H II Regions in Early-to-Interme-diate Hubble Type Ringed Galaxies. The Astrophysical Journal Supplement, 105, 353.
 Regan, M. and Mulchaey, J. (1999) Using Hubble Space Telescope Imaging of Nuclear Dust Morphology to Rule out Bars Fueling Seyfert Nuclei. The Astronomical Journal, 117, 2676-2694.
 Knapen, J., Whyte, L., De Blok, W. and van der Hulst, J. (2004) The Nuclear Ring in the Unbarred Galaxy NGC 278: Result of a Minor Merger? Astronomy and Astrophysics, 423, 481-493.
 Knapen, J.H., Mazzuca, L.M., BÖker, T., Shlosman, I., Colina, L., Combes, F. and Axon, D.J. (2006) Massive Star Formation in the Central Regions of Spiral Galaxies. Astronomy and Astrophysics, 448, 489-498.
 Wozniak, H., Friedli, D., Martinet, L., Martin, P. and Bratschi, P. (1995) Disc Galaxies with Multiple Triaxial Structures. I. BVRI and Hα Surface Photometry. Astronomy and Astrophysics Supplement, 111, 115-152.
 Knapen, J.H. (2005) Structure and Star Formation in Disk Galaxies. III. Nuclear and Circumnuclear Hα Emission. Astronomy and Astrophysics, 429, 141-151.
 Erwin, P. (2004) Double-Barred Galaxies. I. A Catalog of Barred Galaxies with Stellar Secondary Bars and Inner Disks. Astronomy and Astrophysics, 415, 941-957.
 Maoz, D., Barth, A., Filippenko, A.V., Ho, L., Machetto, D., Rix, H.-W. and Schneider, D.P. (1996) Hubble Space Telescope Ultraviolet Images of Five Circumnuclear Star-Forming Rings. The Astronomical Journal, 111, 2248-2264.
 BÖker, T., Calzetti, D., Sparks, W., Axon, D., Bergeron, L.E., Bushouse, H., Colina, L., Daou, D., Gilmore, D., et al. (1999) The NICMOS Snapshot Survey of Nearby Galaxies. The Astrophysical Journal Supplement Series, 124, 95-126.
 Díaz, R.J., Dottori, H., Vera-Villamizar, N. and Carranza, G. (2003) Circumnuclear Structures in the Interacting Seyfert Galaxy NGC 1241: Kinematics and Optical/Infrared Morphology. The Astrophysical Journal, 597, 860-869.
 Pérez-Ramírez, D., Knapen, J.H., Peletier, R.F., Laine, S., Doyon, R. and Nadeau, D. (2000) Circumnuclear Regions in Barred Spiral Galaxies—I. Near-Infrared Imaging. Monthly Notices of the Royal Astronomical Society, 317, 234-248.
 Buta, R. (1986) The Structure and Dynamics of Ringed Galaxies. II. UBVRI Surface Photometry and H Alpha Kinematics of the Ringed Barred Spiral NGC 1433. The Astrophysical Journal Supplement, 61, 631-665.
 Knapen, J.H., Pérez-Ramírez, D. and Laine, S. (2002) Circumnuclear Regions in Barred Spiral Galaxies—II. Relations to Host Galaxies. Monthly Notice of the Royal Astronomical Society, 337, 808-828.
 Agüero, E.L., Díaz, R.J. and Bajaja, E. (2004) NGC 1566: Spectroscopy of a Symmetric System with Seyfert Nucleus. Astronomy and Astrophysics, 414, 453-461.
 Tacconi-Garman, L.E., Sternberg, A. and Eckart, A. (1996) Sub-Arcsecond 2.2 Micron Imaging of the Starburst Galaxy NGC 1808: Infrared Observations of Super Star Clusters. The Astronomical Journal, 112, 918.
 González Delgado, R., Pérez, E., Tandhunter, C., Vilchez, J. and Rodriguez Espinosa, J. (1997) H II Region Population in a Sample of Nearby Galaxies with Nuclear Activity. I. Data and General Results. The Astrophysical Journal Supplement, 108, 155.
 BÖker, T., Laine, S., van der Marel, R., Sarzi, M., Rix, H.-W., Ho, L. and Shields, J.C. (2002) A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. I. Observations and Image Analysis. The Astronomical Journal, 123, 1389-1410.
 Alonso-Herrero, A., Ryder, S.D. and Knapen, J.H. (2001) Nuclear Star Formation in the Hotspot Galaxy NGC 2903. Monthly Notice of the Royal Astronomical Society, 322, 757-769.
 Storchi-Bergmann, T., Rodriguez-Ardila, A., Schmitt, H.R., Wilson, A.S. and Baldwin, J.A. (1996) Circumnuclear Star Formation in Active Galaxies. The Astrophysical Journal, 472, 83.
 Carollo, C.M., Stiavelli, M., Seigar, M., de Zeeuw, P.T. and Dejonghe, H. (2002) Spiral Galaxies with HST/NICMOS. I. Nuclear Morphologies, Color Maps, and Distinct Nuclei. The Astronomical Journal, 123, 159-183.
 Meurer, G., Heckman, T., Leitherer, C., Kinney, A., Robert, C. and Garnett, D. (1995) Starbursts and Star Clusters in the Ultraviolet. The Astronomical Journal, 110, 2665.
 Windhorst, R., Taylor, V.A., Jansen, R.A., Odewahn, S.C., Chiarenza, C.A.T., et al. (2002) A Hubble Space Telescope Survey of the Mid-Ultraviolet Morphology of Nearby Galaxies. The Astrophysical Journal Supplement, 143, 113-158.
 Schinnerer, E., Maciejewski, W., Scoville, N. and Moustakas, L. (2002) Toward the Secondary Bar: Gas Morphology and Dynamics in NGC 4303. The Astrophysical Journal, 575, 826-844.
 Benedict, G.F., Howell, D.A., Jørgensen, I., Kenney, J.D.P. and Smith, B.J. (2002) NGC 4314. IV. Photometry of Star Clusters with the Hubble Space Telescope: History of Star Formation in the Vicinity of a Nuclear Ring. The Astronomical Journal, 123, 1411-1432.
 Tomita, A., Aoki, K., Watanabe, M., Takata, T. and Ichikawa, S. (2000) The Central Gas Systems of Early-Type Galaxies Traced by Dust Features, Based on the Hubble Space Telescope WFPC2 Archival Images. The Astronomical Journal, 120, 123-130.
 King, D.L., Vladilo, G., Lipman, K., de Boer, K.S., Centurion, M., Moritz, P. and Walton, N.A. (1995) NGC 4526 Gas, High Velocity Clouds, and Galactic Halo Gas: The Interstellar Medium towards SN 1994D. Astronomy and Astrophysics, 300, 881-889.
 Sakamoto, K., Okumura, S.K., Ishizuki, S. and Scoville, N.Z. (1999) CO Images of the Central Regions of 20 Nearby Spiral Galaxies. The Astrophysical Journal Supplement, 124, 403-437.
 Elmegreen, D., Chromey, F. and Warren, A. (1998) Discovery of a Double Circumnuclear Ring and Minibar in the Starburst Galaxy M83. The Astronomical Journal, 116, 2834-2840.
 Jogee, S., Shlosman, I., Laine, S., Englmaier, P., Knapen, J.H., Scoville, N. and Wilson, C.D. (2002) Gasdynamics in NGC 5248: Fueling a Circumnuclear Starburst Ring of Super-Star Clusters. The Astrophysical Journal, 575, 156-177.
 Quillen, A.C., Alonso-Herrero, A., Rieke, M.J., McDonald, C., Falcke, H. and Rieke, G.H. (1999) Dust Lanes Causing Structure in the Extended Narrow-Line Region of Early-Type Seyfert Galaxies. The Astrophysical Journal Supplement, 527, 685-690.
 Vega Beltran, J.C., Zeilinger, W.W., Amico, P., Schultheis, M., Corsini, E.M., Funes, J.G., Beckman, J. and Bertola, F. (1998) Mixed Early and Late-Type Properties in the Bar of NGC 6221: Evidence for Evolution along the Hubble Sequence? Astronomy and Astrophysics, 131, 105-114.