HST observations have revealed that compact sources exist at the centers of many, maybe even most, galaxies across the Hubble sequence. These sources are called “nuclei” or also “nuclear star clusters” (NCs), given that their structural properties and position in the fundamental plane are similar to those of globular clusters. Interest in NCs increased recently due to the independent and contemporaneous finding of three groups (Rossa et al. for spiral galaxies; Wehner & Harris for dE galaxies; and Côté et al. for elliptical galaxies) that NC masses obey similar scaling relationships with host galaxy properties as do supermassive black holes. Here we summarize the results of our group on NCs in spiral galaxies. We discuss the implications for our understanding of the formation and evolution of NCs and their possible connection to supermassive black holes.

We report preliminary results of the characterization of bulge and inner disk stellar populations for 8 nearby spiral galaxies using Gemini/GMOS. The long-slit spectra extend out to 1–2 disk scale lengths with S/N/Å>50. Two different model fitting techniques, absorption-line indices and full spectral synthesis, are found to weigh age, metallicity, and abundance ratios differently, but with careful attention to the data/model matching (resolution & flux calibration), we are able constrain real signatures of age and metallicity gradients in star-forming galaxies.

We discuss SAURON absorption line strength maps of a sample of 24 early-type spirals, mostly Sa. From the Lick indices Hβ, Mgb and Fe 5015 we derive SSP-ages and metallicities. By comparing the scaling relations of Mg b and Hβ and central velocity dispersion with the same relation for the edge-on sample of Falcón-Barroso et al. (2002) we derive a picture in which the central regions of Sa galaxies contain at least 2 components: one (or more) thin, disc-like component, often containing recent star formation, and another, elliptical-like component, consisting of old stars and rotating more slowly, dominating the light above the plane. If one defines a bulge to be the component responsible for the light in excess of the outer exponential disc, then many Sa-bulges are dominated by a thin, disc-like component containing recent star formation.

We present a preliminary analysis of the stellar populations in the central parsecs of a sample of 22 Seyfert 2 galaxies, based on a careful separation of nebular emission and stellar light in high-spatial resolution HST-STIS spectra. 10% of the surveyed nuclei display stellar populations of intermediate age, ∼1-2 Gyr old, whereas the remaining targets appear to be evenly split between objects showing only very old stellar populations and nuclei requiring also an additional blue featureless component, which we characterise by means of very young, few-Myr-old stars. The small fraction of stellar population of intermediate age seems to argue against the presence of such a young component, however, since the short lifetime of O-stars would imply recurrent star-formation episodes and the build-up over the last 1-2 Gyr of a detectable intermediate-age population. Additionally, a correlation between the luminosity of such a blue component and the emission from highly-ionised species, together with the general absence of Wolf-Rayet features, further suggests that the featureless continuum arises from the central engine rather than from star-forming regions. We discuss our results in the framework of the unification paradigm and of models for star formation close to supermassive black holes.

We present a brief, comparative, study of the morphology of a sample of 9 late type, star forming, local galaxies, as seen in different bands. The objects are present in the “The Hα Galaxy Survey” by James et al., 2004 (J04 hereafter), in the Sloan Digital Sky Survey (“SDSSS”, York et al., 2000), and also in data from GALEX (Martin et al. 2005). We tested the pixel to pixel proportionality v. Hα of images in broad bands from FUV to NIR. We also analyzed the differences in the radial profiles of light in different bands. Finally, we also measured several parameters of the morphology of the distributions of light, as detected in bands of SDSS (u,g,r,i,z), GALEX (NUV, FUV) and in R and Hα from J04, aiming to test which bands better mimic the spatial distribution of the SF as traced by Hα. Our goal is to acquire as accurate a determination as possible of the spatial distribution of the massive star formation in local galaxies, to compare with the analogous distribution in galaxies at increasing redshift.

We have derived radial sky-subtracted surface brightness profiles of a sample of 42 face-on nearby early-type spiral galaxies, classified as unbarred, from SDSS data as well as our own observations (INT/WFC) at La Palma. A key objective was to derive the fraction of truncated profiles. We find that only a small minority 7% of the galaxies are obviously truncated within the limits of the measured surface brightness range, while some 20% show exponential disc profiles but with a shallower inner portion near the centre: classical “Freeman Type II” profiles (Freeman, 197). Half of the discs show a single unbroken exponential (“type I”) brightness profile with no evidence of truncation out to the edge of detectability, while the remainder show “antitruncations”: external profiles less steep than those of the internal disc (Type III). A general profile classification scheme based on these and other recent related measurements is presented, and will be of use in probing disc formation scenarios.

We present preliminary results from a near-infrared, integral-field study of the central 500 pc in the nearby Sbc spiral NGC 613. More specifically, we present maps of various gaseous emission lines in which the ring structure can be identified. The ring consists of at least seven “hot spots” that are actively forming stars and thus can be identified in recombination lines of hydrogen (H) and Helium (He). The molecular gas (traced by emission from the H2 molecule) is more evenly distributed over the entire ring. We also present H- and K-band spectra of one of the ring hot spots and the nucleus of NGC 613 (i.e. the central 1′′). The nucleus emits little or no flux in H or He recombination lines. Instead, its spectrum is dominated by H2 and [FeII] emission, suggesting that NGC 613 is currently in a phase in which gas is accumulating within the central 50 pc. Moreover, we find clear evidence that the ring structure is disturbed by an energetic outflow from the nucleus, supporting evidence that NGC 613 harbors an AGN.

We characterize the stellar populations in the nuclear region of the barred spiral galaxy NGC 4900 using the integral field spectrometer OASIS and the synthesis code LavalSB and the code from Mollá & García-Vargas (2000) for the young (<10 Myr) and the old stellar populations, respectively. The high spatial resolution of the instrument allows us to find an old population uniformely distributed and younger regions located at the end of the galaxy bar and on each side of a nuclear bar.

The nearby Virgo Cluster provides an ideal laboratory to study galaxy-galaxy and galaxy-cluster interactions at a level of detail impossible at higher redshift. In Virgo, there exists a large population of spiral galaxies with mostly undisturbed stellar disks, but truncated gas disks. We present results of an observational study of several of these galaxies, utilizing optical and UV imaging, and optical spectroscopy. By combining optical spectroscopy and UV imaging, we are able to constrain the time since star formation ended in the outer disk and, therefore, constrain the time since the galaxies were stripped. Our results show that while most of the galaxies in our sample are consistent with being stripped near the cluster center, several show evidence for being stripped well outside the core, suggesting that the “reach” of the intracluster medium is greater than is suggested by simple ICM models.

We show initial results from our ongoing HST/ACS GHOSTS survey of the resolved stellar envelopes of 14 nearby, massive disk galaxies. In hierarchical galaxy formation the stellar halos and thick disks of galaxies are formed by accretion of minor satellites and therefore contain valuable information about the (early) assembly process of galaxies. We detect for the first time the very small halo of NGC 4244, a low mass edge-on galaxy. We find that massive galaxies have very extended halos, with equivalent surface brightnesses of 28-29 V-mag arcsec−2 at 20-30 kpc from the disk. The old RGB stars of the thick disk in the NGC 891 and NGC 4244 edge-on galaxies truncate at the same radius as the young thin disk stars, providing insights into the formation of both disk truncations and thick disks. We furthermore present the stellar populations of a very low surface brightness stream around M83, the first such a stream resolved into stars beyond those of the Milky Way and M31.

We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628. We investigate the role of the individual Hii regions together with the large-scale gravitational mechanisms which govern star formation and overall evolution in spiral galaxies. Our kinematical analysis (reinforced by literature maps in HI and CO at lower angular resolution) enables us to verify the presence of an inner rapidly rotating inner disk-like component which we attribute to long term secular evolution of the large-scale spiral arms and oval structure. We find that gas is falling in from the outer parts towards the bluer central regions. This could be an early phase in the formation of a pseudo-bulge. We find signatures of radial motions caused by an m = 2 perturbation, which are likely to be responsible for the inflow of material forming the circumnuclear ring and the rapidly rotating inner structure.

The structural parameters of bulges, disks and bars of a sample of nearly 1000 nearby galaxies are being determined through sophisticated image decomposition in the g, r and i bands. The sample is carefully drawn from the Sloan Digital Sky Survey Data Release 2 (SDSS DR2), contains 963 galaxies, of which 407 host AGN, and we show that it is representative of the galaxy and AGN populations in the local universe. A large number of other physical properties have also been determined for these galaxies within the SDSS collaboration. With these data, we reinforce several recent studies and find a number of results leading to new insights into how the different galaxy components relate to each other and the extent to which the hosts galaxies of AGN differ from quiescent galaxies.

We present new results on stellar populations of bulges in spiral galaxies, from optical and near-IR data (images and spectra). In particular, we analyze a sample of bulges hosted by low surface brightness galaxies (LSBs). Correlations between metallicity, scale lengths of disk and bulges, and HI content are reported, supporting the bulge secular evolution hypothesis, as well as systematics between the derived evolutionary parameters of the embedded stellar populations. Using spectra of moderate resolution, we also investigate the stellar histories of bulges using Lick/IDS indices and recent α-enhancement models. Results indicate that small bulges tend to be smaller and metal poor, compared to larger ones. However, all bulges appear to have been formed in the same time-scale. We present some ongoing studies and some prospects of our research.

We present line-strength maps of a sample of 18 late-type spirals. The data have been taken using SAURON at the WHT. Apart from the maps, we present the central line indices, and estimate the star formation time-scale. In an exponentially declining SFR scenario, we find a trend between the time-scale τ and the velocity dispersion: more massive galaxies show shorter burst durations. This result is in agreement with the analysis of Hα emission in galaxies. A detailed study on these data will be published by Ganda et al. (2007).

We present our UBVR photometry observations of giant HII regions in NGC 628 in comparison with a detailed grid of evolutionary models of stellar clusters to interpret the observed colours of young stellar populations in terms of IMFs and SFRs parameters and dust content. Observed colours are complicate functions of searched IMF, age, regime of star formation as well as chemical abudances and interstellar reddening. A content of database of observed properties is crucial to get significant results. To exclude age-reddening and age-metallicity degenerations we carried out UBVR photometry of 127 giant HII regions are star formation complexes (SFCs) with independently measured chemical abundancies Z and interstellar reddening in NGC 628 by Belly & Roy (1992).

Massive star formation in the central regions of spiral galaxies plays an important role in the dynamical and secular evolution of their hosts. Here, we summarise a number of recent investigations of the star formation history and the physical conditions of the gas in circumnuclear regions, to illustrate not only the detailed results one can achieve, but also the potential of using state-of-the-art spectroscopic and analysis techniques in researching the central regions of galaxies in general. We review how the star formation history of nuclear rings confirms that they are long-lived and stable configurations. Gas flows in from the disk, through the bar, and into the ring, where successive episodes of massive star formation occur. A detailed analysis of the ring in NGC 7742, where we use similar data to determine the physical conditions of the line emitting gas using a combination of ionisation and stellar population modelling, is described in a second poster paper (these proceedings, p. 000; Mazzuca et al. 2006).

We present an emission-line diagnostic analysis of integral-field spectroscopic observations that cover the circumnuclear ring-like regions in a small number of spiral galaxies. We concentrate on the specific case of the Sa galaxy NGC 7742, which hosts a spectacular circumnuclear starburst ring and nuclear regions characterised by low-ionisation emission. The gas in the ring rotates in the opposite sense to the stars in the galaxy, suggesting a recent merging or acquisition event. The combination of integral-field measurements for the Hα+N[ii] emission lines from DensePak and the Hβ and [Oiii] emission from SAURON allows the construction of diagnostic diagrams that highlight the transition from star formation in the ring to excitation by high-velocity shocks or by an AGN near the centre. DensePak measurements for the [Sii] line ratio reveal very low gas densities in the ring, Ne<100 cm−3, characteristic of massive Hii regions. Comparison with MAPPINGS III models for starbursts with low gas densities shows that the ring is of roughly solar metallicity. We suggest that NGC 7742 cannibalised a smaller galaxy rich in metal-poor gas, and that star formation episodes in the ring have since increased the metallicity to its present value.

We present extensive photometric and spectroscopic study to give a new insight in the bulge stellar population. Super-solar α/Fe and its constant value along the radial profile, in most of the galaxies, suggest that the star formation in these objects has been fast and occurred at the same time in the whole bulge.

We present a grid of 440 spectro-photometric models for simulating spiral and irregular galaxies. They have been consistently calculated with evolutionary synthesis models which use as input the information proceeding from chemical evolution models. The model predictions are spectral energy distributions, brightness and color profiles and radial distributions of spectral absorption stellar indices which are in agreement with observations

We present the preliminary detection of a stellar halo in the nearby, edge-on galaxy, NGC 4244. Using data from the Hubble Space Telescope's Advanced Camera for Surveys (HST/ACS), we have examined number counts of red giant branch (RGB) stars along the minor axis of the galaxy out to 10 kpc, with a limiting surface brightness of μR ∼ 31 mag/arcsec2. At distances more than 2.5 kpc above the disk, a second stellar component is clearly visible. This component is well fit by either an exponential or a power law. The best-fit exponential scale height is similar to the scale length of the galaxy disk, suggesting that we are indeed detecting a halo component. Furthermore, the color of the RGB stars above 2.5 kpc are significantly bluer than those in the disk, suggesting a more metal-poor population. Outside the local group, this finding represents one of the first detections of a resolved stellar halo in a spiral galaxy.