On the nature of the near-UV extended light in Seyfert galaxies

We study the nature of the extended near-UV emission in the inner kiloparsec of a sample of 15 Seyfert galaxies which have both near-UV (F330W) and narrow band [OIII] high resolution Hubble images. For the majority of the objects we find a very similar morphology in both bands. From the [OIII] images we construct synthetic images of the nebular continuum plus the emission line contribution expected through the F330W filter, which can be subtracted from the F330W images. We find that the emission of the ionised gas dominates the near-UV extended emission in half of the objects. A further broad band photometric study, in the bands F330W (U), F547M (V) and F160W (H), shows that the remaining emission is dominated by the underlying galactic bulge contribution. We also find a blue component whose nature is not clear in 4 out of 15 objects. This component may be attributed to scattered light from the AGN, to a young stellar population in unresolved star clusters, or to early-disrupted clusters. Star forming regions and/or bright off-nuclear star clusters are observed in 4/15 galaxies of the sample.Comment: 23 pages, 6 figures, 3 tables; accepted for publication in MNRA

VLT observations of metal-rich extra galactic HII regions. I. Massive star populations and the upper end of the IMF

We have obtained high quality FORS1/VLT optical spectra of 85 disk HII regions several nearby spiral galaxies. Our sample of metal-rich HII regions with metallicities close to solar and higher reveal the presence of Wolf-Rayet (WR) stars in 27 objects from the blue WR bump and 15 additional candidate WR regions. This provides for the first time a large set of metal-rich WR regions. Approximately half of the WR regions also show broad CIV emission attributed to WR stars of the WC subtype (...). The WR regions show quite clear trends between their observed WR features and the Hbeta emission line. Detailed synthesis models are presented to understand/interpret these observations. (...) The availability of a fairly large sample of metal-rich WR regions allows us to improve existing estimates of the upper mass cut-off of the IMF in a robust way and independently of detailed modeling: from the observed maximum Hbeta equivalent width of the WR regions we derive a LOWER LIMIT for M_up of 60-90 Msun in the case of a Salpeter slope and larger values for steeper IMF slopes. From our direct probe of the massive star content we conclude that there is at present no evidence for systematic variations of the upper mass cut-off of the IMF in metal-rich environments, in contrast to some claims based on indirect nebular diagnostics. (abridged abstract

Synthesis models in a probabilistic framework: metrics of fitting

In general, synthesis models provide the mean value of the distribution of possible integrated luminosities, this distribution (and not only its mean value) being the actual description of the integrated luminosity. Therefore, to obtain the closest model to an observation only provides confi- dence about the precision of such a fit, but not information about the accuracy of the result. In this contribution we show how to overcome this drawback and we propose the use of the theoretical mean-averaged dispersion that can be produced by synthesis models as a metric of fitting to infer accurate physical parameters of observed systems.Comment: Invited talk in "New Quests in stellar astrophysics II. Ultraviolet properties of evolved stellar populations" in press. 8 pages, 5 figures. (Part II on metric of fitting for population synthesis

The Starburst-AGN connection: The role of stellar clusters in AGNs

Nuclear stellar clusters are a common phenomenon in spirals and in starbursts galaxies, and they may be a natural consequence of the star formation processes in the central regions of galaxies. HST UV imaging of a few Seyfert 2 galaxies have resolved nuclear starbursts in Seyfert 2 revealing stellar clusters as the main building blocks of the extended emission. However, we do not know whether stellar clusters are always associated with all types of nuclear activity. We present NUV and optical images provided by HST to find out the role that stellar clusters play in different types of AGNs (Seyferts and LLAGNs). Also with these images, we study the circumnuclear dust morphology as a probe of the circumnuclear environment of AGNs. Here we present a summary of the the first results obtained for the sample of Seyferts and LLAGN galaxies.Comment: Contribution to the conference proceedings "Space Astronomy: The UV window to the Universe", El Escorial (Spain), May 28-June 1 2007, submitted to Ap&SS, invited ed. Gomes de Castro, A.I. Further explanations are in Mu\~noz Marin, et al (2007) and Gonzalez Delgado et al (2007); and the full collection of figures are at the ULR: http://www.iaa.es/~rosa/research/LLAGNs2007/LLAGNs-HSTIma1.html http://www.iaa.es/~manuel/publications/paper01.htm

The cosmic evolution of the spatially-resolved star formation rate and stellar mass of the CALIFA survey

We investigate the cosmic evolution of the absolute and specific star formation rate (SFR, sSFR) of galaxies as derived from a spatially-resolved study of the stellar populations in a set of 366 nearby galaxies from the CALIFA survey. The analysis combines GALEX and SDSS images with the 4000 break, H_beta, and [MgFe] indices measured from the datacubes, to constrain parametric models for the SFH, which are then used to study the cosmic evolution of the star formation rate density (SFRD), the sSFR, the main sequence of star formation (MSSF), and the stellar mass density (SMD). A delayed-tau model, provides the best results, in good agreement with those obtained from cosmological surveys. Our main results from this model are: a) The time since the onset of the star formation is larger in the inner regions than in the outer ones, while tau is similar or smaller in the inner than in the outer regions. b) The sSFR declines rapidly as the Universe evolves, and faster for early than for late type galaxies, and for the inner than for the outer regions of galaxies. c) SFRD and SMD agree well with results from cosmological surveys. At z< 0.5, most star formation takes place in the outer regions of late spiral galaxies, while at z>2 the inner regions of the progenitors of the current E and S0 are the major contributors to SFRD. d) The inner regions of galaxies are the major contributor to SMD at z> 0.5, growing their mass faster than the outer regions, with a lookback time at 50% SMD of 9 and 6 Gyr for the inner and outer regions. e) The MSSF follows a power-law at high redshift, with the slope evolving with time, but always being sub-linear. f) In agreement with galaxy surveys at different redshifts, the average SFH of CALIFA galaxies indicates that galaxies grow their mass mainly in a mode that is well represented by a delayed-tau model, with the peak at z~2 and an e-folding time of 3.9 Gyr.Comment: 23 pages, 16 figures, 6 tables, accepted for publication in Astronomy & Astrophysics. *Abridged abstract

The CaT strength in Seyfert nuclei revisited: analyzing young stars and non-stellar light contributions to the spectra

In a former paper (Garcia-Rissmann et al. 2005; hereafter Paper I), we have presented spectra of 64 active, 9 normal and 5 Starburst galaxies in the region around the near-IR Calcium triplet absorption lines and the [SIII]9069 line. In the present paper we analyze the CaT strength (WCaT), and kinematical products derived in that study, namely stellar and ionized gas velocity dispersions. Our main results may be summarized as follows: (1) Seyfert 2s show no sign of dilution in WCaT with respect to the values spanned by normal galaxies, even when optical absorption lines such as the CaII K band at 3933 A are much weaker than in old, bulge-like stellar populations. (2) The location of Seyfert 2s in the WCaT-WCaK plane is consistent with evolutionary synthesis models. The implication is that the source responsible for the dilution of optical lines in these AGN is a young stellar population, rather than an AGN featureless continuum, confirming the conclusion of the pioneer study of Terlevich, Diaz & Terlevich. (3) In Seyfert 1s, both W[SIII] and WCaT tend to be diluted due to the presence of a non-stellar component, in agreement with the unification paradigm. (4) A comparison of stellar and gas velocity dispersions confirms the existence of a correlation between the typical velocities of stars and clouds of the Narrow Line Region. The strength and scatter around this correlation are similar to those previously obtained from the [OIII]5007 line width.Comment: 14 pages, 15 figures. Paper accepted for publication in MNRA

Fluorescent Excitation of Balmer Lines in Gaseous Nebulae: Case D

Nonionizing stellar continua are a potential source of photons for continuum pumping in the hydrogen Lyman transitions. In the environments where these transitions are optically thick, de-excitation occurs through higher series lines. As a result, the emitted flux in the affected lines has a fluorescent contribution in addition to the usual recombination one; in particular, Balmer emissivities are systematically enhanced above case B predictions. The effectiveness of such a mechanism in H II regions and the adequacy of photoionization models as a tool to study it are the two main focuses of this work. We find that photoionization models of H II regions illuminated by low-resolution (λ/δλ ≲ 1000) population synthesis models significantly overpredict the fluorescent contribution to the Balmer lines; the bias has typical values of the order of a few hundredths of a dex, with the exact figure depending on the parameters of the specific model and the simulated aperture. Conversely, photoionization models in which the nonionizing part of the continuum is omitted or is not transferred significantly underpredict the fluorescent contribution to the Balmer lines, producing a bias of similar amplitude in the opposite direction. Realistic estimations of the actual fluorescent fraction of the Balmer intensity require photoionization models in which the relevant portion of the stellar continuum is adequately represented, that is, its resolution is high in the region of the Lyman lines. In this paper, we carry out such an estimation and discuss the variations to be expected as the simulated observational setup and the stellar population\u27s parameters are varied. In all the cases explored, we find that fluorescent excitation provides a significant contribution to the total Balmer emissivity. We also show that differential fluorescent enhancement may produce line-of-sight differences in the Balmer decrement, mimicking interstellar extinction. Fluorescent excitation emerges from our study as a small but important mechanism for the enhancement of Balmer lines. As such, we recommend to take it into account in the abundance analysis of photoionized regions, particularly in the case of high-precision applications such as the determination of primordial helium