Search

Abundances of light elements in dwarf stars of different ages are important constraints for stellar yields, Galactic chemical evolution and exoplanet chemical composition studies. We have measured C and N abundances and $^{12}$C/$^{13}$C ratios for a sample of 63 solar twins spanning a wide range in age, based on spectral synthesis of a comprehensive list of CH\,A-X and CN\,B-X features using HARPS spectra. The analysis of 55 thin disc solar twins confirms the dependences of [C/Fe] and [N/Fe] on [Fe/H]. [N/Fe] is investigated as a function of [Fe/H] and age for the first time for these stars. Our derived correlation [C/Fe]-age agrees with works for solar-type stars and solar twins, but the [N/Fe]-age correlation does not. The relations [C,N/Fe]-[Fe/H] and [C,N/Fe]-age for the solar twins lay under-solar. $^{12}$C/$^{13}$C is found correlated with [Fe/H] and seems to have decreased along the evolution of the local thin disc. Predictions from chemical evolution models for the solar vicinity corroborate the relations [C,N/Fe]-[Fe/H], $^{12}$C/$^{13}$C-age and [N/O]-[O/H], but do not for the $^{12}$C/$^{13}$C-[Fe/H] and [C/O]-[O/H] relations. The N/O ratio in the Sun is placed at the high end of the homogeneous distribution of solar twins, which suggests uniformity in the N-O budget for the formation of icy planetesimals, watery super-earths and giant planets. C and N had different nucleosynthetic origins along the thin disc evolution, as shown by the relations of [C/N], [C/O] and [N/O] against [O/H] and age. [C/N] and [C/O] are particularly observed increasing in time for solar twins younger than the Sun., Comment: Accepted for publication in MNRAS. The paper covers 20 pages, having 8 figures and 10 tables

We determine lithium (Li) abundances for solar twins in the M67 open cluster to add valuable information about the correlation between Li depletion and stellar age and, then, better understand stellar structure and evolution. We use high resolution and good signal-to-noise ratio spectra to characterize Li depletion in three solar twins from M67, using spectral synthesis in the region of the asymmetric 6707.75 \AA \, Li I feature. The mean Li abundance value of A(Li)$=1.6\pm0.2$ dex for our sample of M67 solar twins (our three stars plus a fourth solar twin from a previous analysis in the literature) presents Li abundance expected for its age. Also, the scatter estimated from the standard deviation of the Li abundances in this work is similar to the typical scatter found in a sample of field solar twins presented in the literature., Comment: 6 pages, 3 figures. Accepted for publication at MNRAS

Post-starburst galaxies, identified by their unusually strong Balmer absorption lines and weaker than average emission lines, have traditionally been selected based on their central stellar populations. Here we identify 360 galaxies with post-starburst regions from the MaNGA integral field survey and classify these galaxies into three types: 31 galaxies with central post-starburst regions (CPSB), 37 galaxies with off-center ring-like post-starburst regions (RPSB) and 292 galaxies with irregular post-starburst regions (IPSB). Focussing on the CPSB and RPSB samples, and comparing their radial gradients in D$_n$4000, H$\delta_{\rm A}$ and W(H$\alpha$) to control samples, we find that while the CPSBs have suppressed star formation throughout their bulge and disk, and clear evidence of rapid decline of star formation in the central regions, the RPSBs only show clear evidence of recently rapidly suppressed star formation in their outer regions and an ongoing central starburst. The radial profiles in mass-weighted age and stellar $v/\sigma$ indicate that CPSBs and RPSBs are not simply different evolutionary stages of the same event, rather that CPSB galaxies are caused by a significant disruptive event, while RPSB galaxies are caused by disruption of gas fuelling to the outer regions. Compared to the control samples, both CPSB and RPSB galaxies show a higher fraction of interactions/mergers, misaligned gas or bars that might be the cause of the gas inflows and subsequent quenching., Comment: 17 pages, 10 figures

Negative feedback from accretion onto super-massive black holes (SMBHs), that is to remove gas and suppress star formation in galaxies, has been widely suggested. However, for Seyfert galaxies which harbor less active, moderately accreting SMBHs in the local universe, the feedback capability of their black hole activity is elusive. We present spatially-resolved H$\alpha$ measurements to trace ongoing star formation in Seyfert galaxies and compare their specific star formation rate with a sample of star-forming galaxies whose global galaxy properties are controlled to be the same as the Seyferts. From the comparison we find that the star formation rates within central kpc of Seyfert galaxies are mildly suppressed as compared to the matched normal star forming galaxies. This suggests that the feedback of moderate SMBH accretion could, to some extent, regulate the ongoing star formation in these intermediate to late type galaxies under secular evolution., Comment: 12 pages, 7 figures, accepted by MNRAS

Spectra of galaxies contain a wealth of information about the stellar populations from which they are made. With integral-field unit (IFU) surveys, such data can be used to map out stellar population properties across the face of a galaxy, allowing one to go beyond simple radial profiles and study details of non-axisymmetric structure. To-date, however, such studies have been limited by the quality of available data and the power of spectral analysis tools. We now take the next step and study the barred spiral galaxy MCG+07-28-064 from observations obtained as part of the SDSS-IV MaNGA project. We find that we can decompose this galaxy into "time slices," which reveal the varying contributions that stars of differing ages make to its bar and spiral structure, offering new insight into the evolution of these features. We find evidence for the ongoing growth of the bar, including the most recent star formation on its leading edge, and for the underlying density wave responsible for spiral structure. This pilot study indicates that there is a wealth of untapped information on the spatial distribution of SFHs available in the current generation of IFU galaxy surveys., Comment: 6 pages, 3 figures. Accepted for publication in MNRAS. Figure 1 is an animation and requires a PDF viewer such as Adobe Acrobat Reader to properly render. An stand-alone version of the animation is available as supplementary material at https://doi.org/10.1093/mnras/stz2204

We analyze the intrinsic velocity dispersion properties of 648 star-forming galaxies observed by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, to explore the relation of intrinsic gas velocity dispersions with star formation rates (SFRs), SFR surface densities ($\rm{\Sigma_{SFR}}$), stellar masses and stellar mass surface densities ($\rm{\Sigma_{*}}$). By combining with high z galaxies, we found that there is a good correlation between the velocity dispersion and the SFR as well as $\rm{\Sigma_{SFR}}$. But the correlation between the velocity dispersion and the stellar mass as well as $\rm{\Sigma_{*}}$ is moderate. By comparing our results with predictions of theoretical models, we found that the energy feedback from star formation processes alone and the gravitational instability alone can not fully explain simultaneously the observed velocity-dispersion/SFR and velocity-dispersion/$\rm{\Sigma_{SFR}}$ relationships., Comment: 11 pages, 11 figures. Accepted for publication in MNRAS

We have obtained Gemini/GMOS spectra of 28 regions located across the interacting group NGC 6845, spanning from the inner regions of the four major galaxies (NGC 6845A, B, C, D) to the tidal tails of NGC 6845A. All regions in the tails are star-forming objects with ages younger than 10 Myr. We derived the gas-phase metallicity gradients across NGC 6845A and its two tails and we find that these are shallower than those for isolated galaxies. NGC 6845A has a gas-phase oxygen central metallicity of \mbox{12+log(O/H)$\sim$8.5} and a flat gas-phase metallicity gradient ($\beta$=0.002$\pm$0.004 dex kpc$^{-1}$) out to $\sim$4 $\times$ R$_{25}$ (to the end of the longest tidal tail). Considering the mass-metallicity relation, the central region of NGC 6845A displays a lower oxygen abundance than the expected for its mass. Taking into account this fact and considering the flat oxygen distribution measured along the eastern tidal tail, we suggest that an interaction event has produced a dilution in the central metallicity of this galaxy and the observed flattening in its metal distribution. We found that the star formation process along the eastern tidal structure has not been efficient enough to increase the oxygen abundances in this place, suggesting that this structure was formed from enriched material., Comment: Accepted for publication in MNRAS, 18 pages, 11 figures, 6 tables

We present here a systematic search for cyanopolyynes in the shock region L1157-B1 and its associated protostar L1157-mm in the framework of the Large Program "Astrochemical Surveys At IRAM" (ASAI), dedicated to chemical surveys of solar-type star forming regions with the IRAM 30m telescope. Observations of the millimeter windows between 72 and 272 GHz permitted the detection of HC$_3$N and its $^{13}$C isotopologues, and HC$_5$N (for the first time in a protostellar shock region). In the shock, analysis of the line profiles shows that the emission arises from the outflow cavities associated with L1157-B1 and L1157-B2. Molecular abundances and excitation conditions were obtained from analysis of the Spectral Line Energy Distributions under the assumption of Local Thermodynamical Equilibrium or using a radiative transfer code in the Large Velocity Gradient approximation. Towards L1157mm, the HC$_3$N emission arises from the cold envelope ($T_{rot}=10$ K) and a higher-excitation region ($T_{rot}$= $31$ K) of smaller extent around the protostar. We did not find any evidence of $^{13}$C or D fractionation enrichment towards L1157-B1. We obtain a relative abundance ratio HC$_3$N/HC$_5$N of 3.3 in the shocked gas. We find an increase by a factor of 30 of the HC$_3$N abundance between the envelope of L1157-mm and the shock region itself. Altogether, these results are consistent with a scenario in which the bulk of HC$_3$N was produced by means of gas phase reactions in the passage of the shock. This scenario is supported by the predictions of a parametric shock code coupled with the chemical model UCL_CHEM., Comment: 13 pages, 12 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

The Canis Major OB1 Association has an intriguing scenario of star formation, especially in the Canis Major R1 (CMa R1) region traditionally assigned to a reflection nebula, but in reality an ionized region. This work is focused on the young stellar population associated to CMa R1, for which our previous results from ROSAT, optical and near-infrared data had revealed two stellar groups with different ages, suggesting a possible mixing of populations originated from distinct star-formation episodes. The X-ray data allow the detected sources to be characterized according to hardness ratios, light curves and spectra. Estimates of mass and age were obtained from the 2MASS catalogue, and used to define a complete subsample of stellar counterparts, for statistical purposes. A catalogue of 387 XMM-Newton sources is provided, 78% being confirmed as members or probable members of the CMa R1 association. Flares were observed for 13 sources, and the spectra of 21 bright sources could be fitted by a thermal plasma model. Mean values of fits parameters were used to estimate X-ray luminosities. We found a minimum value of log(L$_X$[erg/s]) = 29.43, indicating that our sample of low-mass stars (M$_\star$ $\leq$ 0.5 M$_\odot$), being faint X-ray emitters, is incomplete. Among the 250 objects selected as our complete subsample (defining our best sample), 171 are found to the East of the cloud, near Z CMa and dense molecular gas, 50% of them being young (< 5 Myr) and 30% being older (> 10 Myr). The opposite happens to the West, near GU CMa, in areas lacking molecular gas: among 79 objects, 30% are young and 50% are older. These findings confirm that a first episode of distributed star formation occurred in the whole studied region ~10 Myr ago and dispersed the molecular gas, while a second, localized episode (< 5 Myr) took place in the regions where molecular gas is still present., 38 pages, 21 figures, accepted for A&A

We present new maps of emission-line flux distributions and kinematics in both ionized (traced by HI and [Fe II] lines) and molecular (H2) gas of the inner 0.7 × 0.7 kpc2 of the galaxy NGC 4303, with a spatial resolution 40-80 pc and velocity resolution 90-150 km s-1 obtained from near-IR integral field spectroscopy using the Very Large Telescope instrument SINFONI. The most prominent feature is a 200-250 pc ring of circumnuclear star-forming regions. The emission from ionized and molecular gas shows distinct flux distributions: while the strongest HI and [Fe II] emission comes from regions in the west side of the ring (ages ~ 4 Myr), the H2 emission is strongest at the nucleus and in the east side of the ring (ages > 10 Myr). We find that regions of enhanced hot H2 emission are anti-correlated with those of enhanced [Fe II] and HI emission, which can be attributed to post-starburst regions that do not have ionizing photons anymore but still are hot enough (~2000 K) to excite the H2 molecule. The line ratios are consistent with the presence of an active galactic nucleus at the nucleus. The youngest regions have stellar masses in the range 0.3-1.5 × 105 M and ionized and hot molecular gas masses of ~0.25-1.2 × 104 M and ~2.5-5M, respectively. The stellar and gas velocity fields show a rotation pattern, with the gas presenting larger velocity amplitudes than the stars, with a deviation observed for the H2 along the nuclear bar, where increased velocity dispersion is also observed, possibly associated with non-circular motions along the bar. The stars in the ring show smaller velocity dispersion than the surroundings, which can be attributed to a cooler dynamics due to their recent formation from cool gas., RAR acknowledges support from FAPERGS (project no. 2366-2551/14-0) and CNPq (project no. 470090/2013-8 and 302683/2013-5). LC acknowledges support from CNPq special visitor fellowship PVE 313945/2013-6 under the Brazilian programme Science without Borders. LC, JP and SA are supported by grants AYA2012-32295, AYA2012-39408 and ESP2015-68964 from the Ministerio de Economía y Competitividad of Spain. DAS, RR and TSB thank CNPq for financial support.

We present deep SINFONI K-band integral field spectra of two submillimeter galaxy systems (SMG): BR 1202-0725 and J1000+0234, at z = 4.69 and 4.55, respectively. Spectra extracted for each object in the two systems do not show any signature of the [O ii]λλ3726, 29 Å emission-lines, placing upper flux limits of 3.9 and 2.5 × 10erg s cmfor BR 1202-0725 and J1000+0234, respectively. Using the relation between the star formation rate (SFR) and the luminosity of the [O ii] doublet, we estimate unobscured SFR upper limits of ∼ 10-15 M yrand ∼30-40 M yrfor the objects of the two systems, respectively. For the SMGs, these values are at least two orders of magnitude lower than those derived from SED and IR luminosities. The differences on the SFR values would correspond to internal extinction of, at least, 3.4-4.9 and 2.1-3.6 mag in the visual for BR 1202-0725 and J1000+0234 SMGs, respectively. The upper limit for the [O ii]-derived SFR in one of the LAEs (Lyα2) in the BR1202-0725 system is at least one order of magnitude lower than the previous SFR derived from infrared tracers, while both estimates are in good agreement for Lyα1. The lower limits to the internal extinction in these two Lyman-alpha emitters are 0.6 mag and 1.3 mag, respectively. No evidence for [O ii] emission associated with Lyα1 is identified in our data, implying that residuals of the K-band sky emission lines after subtraction in medium-band imaging data could provide the adequate flux., G.S.C. and L.C. acknowledge support from CNPq special visitor fellowship PVE 313945/2013-6 under the Brazilian program Science without Borders. L.C. acknowledges support by grant AYA2012-32295. J.P.L. acknowledges support by grant AYA-2012-39408-C02-1. L.C. acknowledges support by grant AYA2012-39408-C02-01 and AYA2015-68964

Post-Starburst Quasars (PSQs) are hypothesized to represent a stage in the evolution of massive galaxies in which the star formation has been recently quenched due to the feedback of the nuclear activity. In this paper our goal is to test this scenario with a resolved stellar population study of the PSQ J0210-0903, as well as of its emitting gas kinematics and excitation. We have used optical Integral Field Spectroscopy obtained with the Gemini GMOS instrument at a velocity resolution of ~120 km/s and spatial resolution of ~0.5 kpc. We find that old stars dominate the luminosity (at 4700 \AA) in the inner 0.3 kpc (radius), while beyond this region (at ~0.8 kpc) the stellar population is dominated by both intermediate age and young ionizing stars. The gas emission-line ratios are typical of Seyfert nuclei in the inner 0.3 kpc, where an outflow is observed. Beyond this region the line ratios are typical of LINERs and may result from the combination of diluted radiation from the nucleus and ionization from young stars. The gas kinematics show a combination of rotation in the plane of the galaxy and outflows, observed with a maximum blueshift of -670 km/s. We have estimated a mass outflow rate in ionized gas in the range 0.3--1.1 M_sun/yr and a kinetic power for the outflow of dE/dt ~ 1.4--5.0 x 10^40 erg/s ~0.03% - 0.1% x L_bol. This outflow rate is two orders of magnitude higher than the nuclear accretion rate of ~8.7 x 10^-3 M_sun/yr, thus being the result of mass loading of the nuclear outflow by circumnuclear galactic gas. Our observations support an evolutionary scenario in which the feeding of gas to the nuclear region has triggered a circumnuclear starburst 100's Myr ago, followed by the triggering of the nuclear activity, producing the observed gas outflow which may have quenched further star formation in the inner 0.3 kpc., Comment: 17 pages, 9 Figures, 2 Tables

We study the central (inner few hundred parsecs) stellar populations of four starburst galaxies (NGC34, NGC1614, NGC3310 and NGC7714) in the near-infrared (NIR), from 0.8 to 2.4microns, by fitting combinations of stellar population models of various ages and metallicities. The NIR spectra of these galaxies feature many absorption lines. For the first time, we fit simultaneously as much as 15 absorption features in the NIR. The observed spectra are best explained by stellar populations containing a sizable amount (20 to 56 % by mass) of ~1Gyr old stellar population with Thermally Pulsing-Asymptotic Giant Branch stars. We found that the metallicity of the stars which dominates the light is solar. Metallicities substantially different from solar give a worse fit. Though the ages and metallicities we estimate using the NIR spectroscopy are in agreement with values from the literature based on the UV/optical, we find older ages and a larger age spread. This may be due to the fact that the optical is mostly sensitive to the last episode of star formation, while the NIR better maintains the record of previous stellar generations. Another interesting result is that the reddening estimated from the whole NIR spectrum is considerably lower than that based on emission lines. Finally, we find a good agreement of the free emission line spectrum with photoionization models, using as input spectral energy distribution the synthetic composite template we derived as best-fit., 11 pages, 7 figures, accepted for publication on MNRAS

We show that the microquasar LSI+61303 is running away from its birth place in a young complex of massive stars. The supernova explosion that formed the compact object shot out the x-ray binary with a linear momentum of 430 +/- 140 Msun km/s, which is comparable to the linear momenta found in solitary runaway neutron stars and millisecond pulsars. The properties of the binary system and its runaway motion of 27 +/- 6 km/s imply that the natal supernova was asymmetric and that the upper limit for the mass that could have been suddenly ejected in the explosion is ~2 Msun. The initial mass of the progenitor star of the compact object that is inferred depends on whether the formation of massive stars in the parent stellar cluster was coeval or a sequential process., 4 pages, 1 figure. Published in A&A, see http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004A%26A...422L..29M and the NRAO press release at http://www.nrao.edu/pr/2004/shotout/

We present a multiwavelength study of the OH Megamaser galaxy (OHMG) IRAS16399-0937, based on new HST/ACS F814W and H$\alpha$+[NII] images and archive data from HST, 2MASS, Spitzer, Herschel and the VLA. This system has a double nucleus, whose northern (IRAS16399N) and southern (IRAS16399S) components have a projected separation of $\sim$ 6'' (3.4 kpc) and have previously been identified based on optical spectra as a Low Ionization Nuclear Emission Line Region (LINER) and starburst nucleus, respectively. The nuclei are embedded in a tidally distorted common envelope, in which star formation is mostly heavily obscured. The infrared spectrum is dominated by strong polycyclic aromatic hydrocarbon (PAH), but deep silicate and molecular absorption features are also present, and are strongest in the IRAS16399N nucleus. The 0.435 - 500$\mu$m SED was fitted with a model including stellar, ISM and AGN torus components using our new MCMC code, clumpyDREAM. The results indicate that the IRAS16399N contains an AGN (L$_{bol} \sim 10^{44}$ ergs/s) deeply embedded in a quasi-spherical distribution of optically-thick clumps with a covering fraction $\approx1$. We suggest that these clumps are the source of the OHM emission in IRAS16399-0937. The high torus covering fraction precludes AGN-photoionization as the origin of the LINER spectrum, however, the spectrum is consistent with shocks (v $\sim100-200$ km s$^{-1}$). We infer that the $\sim10^8$ M$_{\odot}$ black-hole in IRAS16399N is accreting at a small fraction ($\sim1$%) of its Eddington rate. The low accretion-rate and modest nuclear SFRs suggest that while the gas-rich major merger forming the IRAS16399-0937 system has triggered widespread star formation, the massive gas inflows expected from merger simulations have not yet fully developed., Comment: Paper was accepted for publication in The Astrophysical Journal. 72 pages, 18 figures

We report our study of features at the observed red end of the white dwarf cooling sequences for three Galactic globular clusters: NGC\,6397, 47\,Tucanae and M\,4. We use deep colour-magnitude diagrams constructed from archival Hubble Space Telescope (ACS) to systematically investigate the blue turn at faint magnitudes and the age determinations for each cluster. We find that the age difference between NGC\,6397 and 47\,Tuc is 1.98$^{+0.44}_{-0.26}$\,Gyr, consistent with the picture that metal-rich halo clusters were formed later than metal-poor halo clusters. We self-consistently include the effect of metallicity on the progenitor age and the initial-to-final mass relation. In contrast with previous investigations that invoked a single white dwarf mass for each cluster, the data shows a spread of white dwarf masses that better reproduce the shape and location of the blue turn. This effect alone, however, does not completely reproduce the observational data - the blue turn retains some mystery. In this context, we discuss several other potential problems in the models. These include possible partial mixing of H and He in the atmosphere of white dwarf stars, the lack of a good physical description of the collision-induced absorption process and uncertainties in the opacities at low temperatures. The latter are already known to be significant in the description of the cool main sequence. Additionally, we find that the present day local mass function of NGC\,6397 is consistent with a top-heavy type, while 47\,Tuc presents a bottom-heavy profile., Comment: Accepted for publication in MNRAS (16 pages, 19 figures)

We present our study on the spatially resolved H_alpha and M_star relation for 536 star-forming and 424 quiescent galaxies taken from the MaNGA survey. We show that the star formation rate surface density (Sigma_SFR), derived based on the H_alpha emissions, is strongly correlated with the M_star surface density (Sigma_star) on kpc scales for star- forming galaxies and can be directly connected to the global star-forming sequence. This suggests that the global main sequence may be a consequence of a more fundamental relation on small scales. On the other hand, our result suggests that about 20% of quiescent galaxies in our sample still have star formation activities in the outer region with lower SSFR than typical star-forming galaxies. Meanwhile, we also find a tight correlation between Sigma_H_alpha and Sigma_star for LI(N)ER regions, named the resolved "LI(N)ER" sequence, in quiescent galaxies, which is consistent with the scenario that LI(N)ER emissions are primarily powered by the hot, evolved stars as suggested in the literature., Comment: 6 pages, 4 figures. ApJ Letter accepted

We find in a detailed morphological study (at 15 pc resolution) that the extended massive star formation process, shows: (i) extended triple asymmetrical spiral arms structure (r = 5 kpc); and (ii) the spiral arms emanate from three different nuclei. The main optical nucleus shows a small spiral-disk (r = 500 pc) which is a continuation of the external one and reach the very nucleus. And this very nucleus shows blue elongate structure (63 pc x 30 pc), and luminous blue star cluster properties. We study the kinematics of this system and present a detailed Halpha velocity field for the central region (r = 5 kpc). In the main optical nucleus we found a clear "outflow component" associated to galactic-winds and a "inflow radial motion" (in the spiral-disk nuclear structure, r = 700 pc). In addition we detected the outflow component in the central and external regions (r < 5-6 kpc), with a very wide opening angle of 140. We found that the mean value of the inflow region (at PA = 80) is practically perpendicular to the axis of the bipolar outflow (at PA = 160). We analyze in detail the physical conditions in the giant H II regions located in the asymmetric spiral arms, the two main optical nuclei, and the outflow component. In the central region (r = 5-6 kpc) we detected that the nuclear starburst and the extended giant H II regions have very similar properties. We suggest that the interaction between dynamical effects, the galactic--wind (outflow), low-energy cosmic rays, and the molecular+ionized gas (probably in the inflow phase) could be the possible mechanism that generate the "similar extended properties in the massive star formation, at scale of 5-6 kpc!". We analyze for NGC3256 the possible evolution from luminous IR galaxy to QSOs., 86 pages, 36 figures

We study the stellar population of galaxies with active star formation, determining ages of the stellar components by means of spectral population synthesis of their absorption spectra. The data consist of optical spectra of 185 nearby ($z \leq 0.075$) emission line galaxies. They are mostly HII galaxies, but we also include some Starbursts and Seyfert 2s, for comparison purposes. They were grouped into 19 high signal-to-noise ratio template spectra, according to their continuum distribution, absorption and emission line characteristics. The templates were then synthesized with a star cluster spectral base. The synthesis results indicate that HII galaxies are typically age-composite stellar systems, presenting important contribution from generations up to as old as 500 Myr. We detect a significant contribution of populations with ages older than 1 Gyr in two groups of HII galaxies. The age distributions of stellar populations among Starbursts can vary considerably despite similarities in the emission line spectra. In the case of Seyfert 2 groups we obtain important contributions of old population, consistent with a bulge. From the diversity of star formation histories, we conclude that typical HII galaxies in the local universe are not systems presently forming their first stellar generation., Comment: 12 pages, 4 figures, MNRAS in press

We use long-slit spectroscopic data to study in detail the extended narrow-line regions (ENLRs) of the Seyfert 1 galaxy ESO 362-G18 and Seyfert 2 galaxy ESO 362-G8. These two galaxies have similar emission-line luminosities and extents of the ENLR (D4 kpc), whose shapes in previous narrowband (O III) images suggest anisotropic escape of the nuclear ionizing radiation as expected for shadowing by a nuclear torus in the framework of the uni—ed model. In the Seyfert 1 galaxy the high-excitation gas shows an approximately cone-shaped morphology. From the observed kinematics, we conclude that the gas within the cone most probably belongs to the galaxy disk, which implies that the collimation axis is closer to the disk than half the opening angle of the cone of ionizing radiation. In the Seyfert 2 galaxy, the main structure in the high-excitation gas is an emission blob which apparently consists of a high- latitude cloud being blown away from the nuclear region and ionized by the nuclear source. We use the radial distribution of stellar population features in order to extrapolate this population to the nucleus and isolate the optical continuum of the nuclear source. We obtain a featureless power-law continuum for the Seyfert 1 galaxy, while for the Seyfert 2 galaxy we conclude that the F l P l~0.76 nuclear bluer color and smaller equivalent widths of the absorption lines are due to an aging burst of star formation (age B 300 Myr) and that the nuclear source is hidden from direct view. Using the photoionization code MAPPINGS Ic and a mixture of matter-bounded (MB) and ionization-bounded (IB) clouds, we model the ENLRs of the two galaxies. We use all the observables, mostly the emission-line —uxes as a function of distance from the nucleus and the optical nuclear contin- uum observed in the Seyfert 1 galaxy as well as its X-ray —ux, to constrain the parameters of a self- consistent model for the ENLR. For both galaxies, we conclude that a power-law ionizing continuum better reproduces the high-excitation lines near the nucleus than a multisegmented power law F l P l~1.2 used in previous works. For the Seyfert 1 galaxy ESO 362-G18, the inferred luminosity of the ionizing continuum can be reconciled with the —ux observed in the optical, while in the X-rays the observed —ux is D100 times weaker than that necessary to reproduce the line —uxes, suggesting that the X-ray contin- uum is absorbed toward Earth. For the Seyfert 2 galaxy ESO 362-G8, the inferred ionizing continuum when extrapolated to the optical implies a minimum obscuration toward the nuclear source of A V B 4.0 mag. In the hypothesis of an isotropic nuclear source, in order to better constrain the model parameters, we have adopted symmetrical physical conditions as a function of distance on both sides of the nucleus: namely, the ionizing —ux, the temperature, density, and ionization parameter of the MB gas, and the metallicity. The radial density behavior of the IB gas was observationally inferred from the (S II) doublet ratio. The only free parameter, which was allowed to vary independently, was the relative proportion of the MB and IB emission-line components along the ENLR. The high-excitation gas within the cone of ESO 362-G18 and within the blob of ESO 362-G8 have been modeled as regions of larger mass contri- bution from the MB component relative to other locations of the ENLR. We derive the —lling factors, covering factors, and gas masses along the ENLR as a function of dis- tance from the nucleus. A comparison between the model results for the two galaxies shows that, around the nucleus, the Seyfert 1 galaxy has a larger excitation due to a larger contribution of the MB com- ponent. However, in the cone, the excitation is lower than in the blob of the Seyfert 2 galaxy due to a combination of a lower ionizing —ux and larger gas density in the disk of the Seyfert 1 galaxy. The total ionized gas mass derived for the blob in the Seyfert 2 galaxy is 105.8 consistent with its proposed M _ , origin in a nuclear superwind which probably occurred D300 Myr ago, while the ionized gas mass in the disk of the Seyfert 1 galaxy is 1 order of magnitude smaller. Subject headings: galaxies: activegalaxies: ISMgalaxies: nucleigalaxies: Seyfert

A recent JHKs study of several grand-design spiral galaxies shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J-H) vs (H-Ks) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H-Ks) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram MK vs (J-Ks), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Qd = (H-Ks) - 0.884 (J-H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. We investigate the effectiveness of the Qd index in sorting out clusters of different ages at their early evolutionary stages. Surface photometry was carried out for 2MASS images of populous clusters younger than ~100Myr whose ages were available. Some clusters, particularly the embedded ones, were studied for the first time using this method. The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group shows the expected bimodal distribution in the colour-colour and colour-magnitude diagrams. In particular, we confirm the index Qd as a powerful tool for distinguishing clusters younger than about 7Myr from older clusters. (abridged), Comment: 10 pages, 11 figures; accepted for publication in A&A

It is often argued that young stellar clusters suffer a significant infant mortality that is partly related to the expulsion of dust and gas in their early phases caused by radiation pressure from hot stars and supernovae. Near-infrared (J-K)-Mk diagrams of young stellar clusters in nearby spiral galaxies show a bi-modal distribution that is consistent with a fast decline of their intrinsic extinction at an early epoch. The distinct features in the color-magnitude diagrams (CMD) and the fast change of colors for the youngest clusters allow us to place constraints on their early evolutionary phases, including the time scale for the decreasing extinction caused for instance by gas and dust expulsion. Monte Carlo simulations of cluster populations were performed using the power-law distribution function g(M, t) ~ M^a t^gam. Integrated colors were computed from Starburst99 models. The simulated near-infrared CMD were compared with those observed for six grand-design, spiral galaxies using statistical goodness-of-fit tests. The CMDs indicate a significant mortality of young, massive clusters with gam = -1.4 +-0.5. High initial extinction Av = 8-11m and strong nebular emission are required to reproduce the bi-modal color distributions of the clusters. An extended star formation phase of longer than 5 Myr is suggested. The reduction of the internal extinction of the clusters starts during their active star formation and lasts for a period of 5-10 Myr., Comment: 5 pages, 3 figures; accepted for publication in A&A Letters

Tidal debris which are rich in HI gas, formed in interacting and merging systems, are suitable laboratories to study star formation outside galaxies. Recently, several such systems were observed, which contained many young star forming regions outside the galaxies. In previous works, we have studied young star forming regions outside galaxies in different systems with optical and/or gaseous tidal debris, all of them with available archive GALEX/UV images, in order to understand how often they occur and in which type of environments. In this paper we searched for star forming regions around the galaxy NGC2865, a shell galaxy which is circled by a ring of HI, with a total mass of 1.2 x 10$^9$ M$_\odot$. Using the Multi-Slit Imaging Spectroscopy Technique with the Gemini telescope, we detected all H$\alpha$ emitting sources in the surroundings of the galaxy NGC2865, down to a flux limit of 10$^{-18}$ erg cm$^{-2}$ s$^{-1}$ \AA$^{-1}$. Together with Near and Far-Ultraviolet flux information we characterize the star formation rates, masses, ages, and metallicities for these HII regions. In total, we found 26 emission-line sources in a 60 $\times$ 60 Kpc field centered over the southeastern tail of the HI gas present around the galaxy NGC2865. Out of the 26 H$\alpha$ emitters, 19 are in the satellite galaxy FGCE 0745 and seven are intergalactic HII regions scattered over the south tail of the HI gas around NGC2865. We found that the intergalactic HII regions are young ($, Comment: 13 pages, 13 figures, Accepted in A&A

We investigate the relation between the characteristics of the circumnuclear stellar population and both the galaxy morphology and the presence of close companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present unambiguous signatures of recent episodes of star formation within $\approx$300 pc from the nucleus. When we relate this property with the Hubble type of the host galaxy, we find that the incidence of recent circumnuclear star formation increases along the Hubble sequence, and seems to be larger than in non-Seyfert galaxies for the early Hubble types S0 and Sa, but similar to that in non-Seyfert galaxies for later Hubble types. Both in early-type and late-type Seyferts, the presence of recent star-formation is related to the galaxy morphology in the inner few kiloparsecs, as observed in HST images through the filter F606W by Malkan et al., who has assigned a late ``inner Hubble type'' to most Seyfert 2s with recent nuclear star-formation. This new classification is due to the presence of dust lanes and spiral structures in the inner region. The presence of recent star formation in Seyfert 2 nuclei is also related to interactions: among the 13 galaxies of the sample with close companions or in mergers, 9 have recent star formation in the nucleus. These correlations between the presence of companions, inner morphology and the incidence of recent star formation suggest an evolutionary scenario in which the interaction is responsible for sending gas inwards which both feeds the AGN and triggers star-formation. The starburst then fades with time and the composite Seyfert 2 + Starburst nucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellar population., Comment: 12 pages, 6 figures, 2 tables

Spectral distributions from the UV to the near-IR of a sample of 44 star-forming galaxies are used to calculate the metallicity (O/H), star-formation rate (SFR) and age of the starbursts. The oxygen abundance covers the range 8.3 less than O/H less than 9.4 and nitrogen (N) is found to be mostly a product of secondary nucleosynthesis for O/H greater than 8.4. Due to its secondary origin, N/O ratios up to approximately equals 4 times the solar value can be obtained for metal-rich starbursts. The SFR ranges 0.01 to 100 solar mass/year. The lower metallicity galaxies seem to be experiencing an instantaneous burst of star formation, with ages ranging from under 5 x 10(exp 6) to 10(exp 7) yr. The highest metallicity galaxies are most probably experiencing a continuous burst. Correlations between the calculated quantities and several spectral features are investigated. We found a highly significant correlation between the equivalent width W(C IV lambda 1550)-a stellar (absorption) feature- and the oxygen abundance of the emitting gas (O/H). Thus we show for the first time that the stellar metallicity is well correlated with the gas metallicity in star-bursting galaxies. The equivalent width W(Si IV lambda 1400) and the emission line ratio (N II) lambda lambda 6548.84/H(sub alpha) also correlate well with O/H, and all three features can be used as metallicity indicators for star-forming galaxies. The continuum color between lambda 1400 and lambda 3500 (C(14 - 35)) is shown to correlate with O/H, although it is better correlated with E(B - V). It was not possible to disentangle the metallicity from the reddening effect in C(14- 35). We estimate that the reddening affecting the UV continuum is about half the one derived from the Balmer decrement of the emitting gas. The SFR correlates well with the galaxy luminosity and there is no dependence of the continuum color on the SFR. The higher metallicities are only found in the more luminous galaxies, while low metallicities are found over the whole luminosity interval (-16 less than M(sub B) less than -23, H(sub 0) = 50 km/sec/Mpc) covered by the sample.

We examine a representative sample of 35 Seyfert 2 nuclei. Previous work has shown that nearly 1/2 of these nuclei show the direct (but difficult-to-detect) spectroscopic signatures at optical-UV wavelengths of the hot massive stars that power circum-nuclear starbursts. In this paper we examine a variety of more-easily-measured quantities for this sample, such as the equivalent widths of strong absorption features, continuum colors, emission line equivalent widths, ratios and profiles, far-IR luminosities and near-UV surface brightness. We compare the composite starburst+Seyfert 2 nuclei to ``pure'' Seyfert 2's, Starburst galaxies and normal galactic nuclei. Our goals are to verify whether these properties in composite nuclei are consistent with the expected impact of a starburst, and to investigate alternative less-demanding methods to infer the presence of starbursts in Seyfert 2's, applicable to larger or more distant samples. We show that starbursts do indeed leave clear and easily quantifiable imprints on the near-UV to optical continuum range. Composite starburst+Seyfert 2 systems can be recognized by: (1) a strong ``Featureless Continuum'' which dilutes the CaII K line from the host's bulge to W_K < 10 A; (2) emission lines whose equivalent widths are intermediate between Starburst galaxies and ``pure'' Seyfert 2's; (3) relatively low excitation line-ratios, which indicate that part of the gas ionization in these Seyfert 2's (typically \sim 50% of Hbeta) is due to photoionization by OB stars; (4) large far IR luminosities (> 10^10 Lsun); (5) High near-UV surface brightness (~10^3 Lsun/pc^2). These characteristics are all consistent with the expected impact of circum-nuclear starbursts on the observed properties of Seyfert 2's. (abridged), ApJ in press - 67(!) pages (including 15 figures)