Spatially Resolved Spectroscopic Star Formation Histories of Nearby Disks: Hints of Stellar Migration

We use the Mitchell Spectrograph (formerly VIRUS-P) to observe 12 nearby disk galaxies. We successfully measure ages in the outer disk in six systems. In three cases (NGC 2684, NGC 6155, and NGC 7437), we find that a downward break in the disk surface brightness profile corresponds with a change in the dominant stellar population with the interior being dominated by active star formation and the exterior having older stellar populations that are best-fit with star formation histories that decline with time. The observed increase in average stellar ages beyond a profile break is similar to theoretical models that predict surface brightness breaks are caused by stellar migration, with the outer disk being populated from scattered old interior stars. In three more cases (IC 1132, NGC 4904, and NGC 6691), we find no significant change in the stellar population as one crosses the break radius. In these galaxies, both the inner and outer disks are dominated by active star formation and younger stellar populations. While radial migration can contribute to the stellar populations beyond the break, it appears more than one mechanism is required to explain all of our observed stellar profile breaks.Comment: 18 pages, 21 figures. Accepted for publication in Ap

Radial Distribution of Near-UV Flux in Disc Galaxies in the range 0<z<1

(Abridged) The goal of this paper is to quantify the changes on the SF distribution within the disc galaxies in the last ~8 Gyr. We use as a proxy for the SF radial profile the Near-UV surface brightness distributions, allowing suitably for extinction. We compare the effective radii (R_eff) and concentration of the flux distribution in the rest-frame Near-UV for a sample of 270 galaxies in the range 0<z<1. This radial distribution is compared to that measured in the rest-frame B-band, which traces older stellar populations. The analysis is performed using deep, high resolution, multi-band images from GALEX, SDSS, and HST/ACS - GOODS-South. The relation R_eff(NUV)- M* suffers a moderate change between z~1 and z~0: at a fixed stellar mass of 1E10 M_sun, galaxies increase their effective radii by a factor 1.18+/-0.06. Median profiles in NUV show signs of truncation at R~R_eff, and median colour profiles (NUV-B) show a minimum (a "bluest" point) also around R~1-1.5 R_eff. The distributions of NUV flux are more compact at z~1 than nowadays, in terms of the fraction of flux enclosed in a specific radius (in kpc). Our results indicate that the SF surface density has decreased dramatically in discs since z~1, and this decline has been more intense in the central parts (<~R_eff) of the galaxies. In addition, our data suggest that the bulges/pseudo-bulges have grown in surface brightness with regard to the discs since z~1.Comment: 26 pages, 21 figures, accepted for publication in Astronomy & Astrophysics on March 21s

The joint evolution of baryons and dark matter haloes

We have studied the dark matter (DM) distribution in a approx 10^12 h^-1 M_sun mass halo extracted from a simulation consistent with the concordance cosmology, where the physics regulating the transformation of gas into stars was allowed to change producing galaxies with different morphologies. The presence of baryons produces the concentration of the DM halo with respect to its corresponding dissipationless run, but we found that this response does not only depend on the amount of baryons gathered in the central region but also on the way they have been assembled. DM and baryons affect each other in a complex way so the formation history of a galaxy plays an important role on its final total mass distribution. Supernova (SN) feedback regulates the star formation and triggers galactic outflows not only in the central galaxy but also in its satellites. Our results suggest that, as the effects of SN feedback get stronger, satellites get less massive and can even be more easily disrupted by dynamical friction, transferring less angular momentum. We found indications that this angular momentum could be acquired not only by the outer part of the DM halo but also by the inner ones and by the stellar component in the central galaxy. The latter effect produces stellar migration which contributes to change the inner potential well, probably working against further DM contraction. As a consequence of the action of these processes, when the halo hosts a galaxy with an important disc structure formed by smooth gas accretion, it is more concentrated than when it hosts a spheroidal system which experienced more massive mergers and interactions. (abridged)Comment: 16 pages, 16 figures, accepted to MNRA

Towards a fully consistent Milky Way disc model: Part 1 The local model based on kinematic and photometric data

We present a fully consistent evolutionary disc model of the solar cylinder. The model is based on a sequence of stellar sub-populations described by the star formation history (SFR) and the dynamical heating law (given by the age-velocity dispersion relation AVR). The combination of kinematic data from Hipparcos and the finite lifetimes of main sequence (MS) stars enables us to determine the detailed vertical disc structure independent of individual stellar ages and only weakly dependent on the IMF. The disc parameters are determined by applying a sophisticated best fit algorithm to the MS star velocity distribution functions in magnitude bins. We find that the AVR is well constrained by the local kinematics, whereas for the SFR the allowed range is larger. A simple chemical enrichment model is included in order to fit the local metallicity distribution of G dwarfs. In our favoured model A the power law index of the AVR is 0.375 with a minimum and maximum velocity dispersion of 5.1 km/s and 25.0 km/s, respectively. The SFR shows a maximum 10 Gyr ago and declines by a factor of four to the present day value of 1.5 M_sun/pc^2/Gyr. A best fit of the IMF leads to power-law indices of -1.46 below and -4.16 above 1.72 M_sun avoiding a kink at 1 M_sun. An isothermal thick disc component with local density of ~6% of the stellar density is included. A thick disc containing more than 10% of local stellar mass is inconsistent with the local kinematics of K and M dwarfs.Comment: 20 pages, 18 figs., accepted by MNRA

The structure of spiral galaxies: radial profiles in stellar Mass-to-Light ratio and the Dark Matter distribution

The colour and metallicity gradients observed in spiral galaxies suggest that the mass-to-light ratio (M*/L) of the stellar disc is a function of radius. This is indeed predicted by chemo-photometric models of galactic discs. We investigate the distribution of luminous and dark matter in spiral galaxies, taking into account the radial dependence of the stellar M*/L - which is usually assumed to be constant in studies of the mass structure. From the chemo-photometric models of Portinari et al. (2004) and in agreement with the observed radial profiles of galaxy colours, we derive the typical average M*/L profile of the stellar discs of spiral galaxies. We compute the corresponding Variable Mass-to-Light (VML) stellar surface density profile and then the VML disc contribution to the circular velocity. We use the latter, combined with a well studied dark matter velocity profile, to mass model the co-added rotation curves of Persic et al. (1996). (See http://www.facebook.com/home.php#!/group.php?gid=310260450630 to retrieve extra information on the kinematics of galaxies) By investigating rotation curves in the framework of VML stellar discs, we confirm, to a first approximation, the scenario obtained with the constant M*/L assumption: a dark matter halo with a shallow core, an inner baryon-dominated region and a larger proportion of Dark Matter in smaller objects. However, the resulting size of the the dark halo core and of the inner baryon dominance region are somewhat smaller. The stronger role that VML discs have in the innermost regions is important to constrain the galaxy mass structure in both Lambda Cold Dark Matter and MOND scenarios.Comment: 12 pages, 10 figures, in press on Astronomy and Astrophysics. Minor text revisions to match published version. Reference added to web-link to http://www.facebook.com/home.php#!/group.php?gid=31026045063

Tropical cyclone impacts on seagrass-associated fishes in a temperate-subtropical estuary

Major storms can alter coastal ecosystems in several direct and indirect ways including habitat destruction, stormwater-related water quality degradation, and organism mortality. From 2010–2020, ten tropical cyclones impacted coastal North Carolina, providing an opportunity to explore ecosystem responses across multiple storms. Using monthly trawl and contemporaneous seagrass surveys conducted in Back Sound, NC, we evaluated how cyclones may affect the nursery role of shallow-water biogenic habitats by examining seagrass-associated fish responses within a temperate-subtropical estuary. We employed a general before-after-control-impact approach using trawls conducted prior (before) and subsequent (after) to storm arrival and years either without (control) or with (impact) storms. We examined whether effects were apparent over short (within ~three weeks of impact) and seasonal (May-October) timescales, as well as if the magnitude of storm-related shifts varied as a function of storm intensity. Our findings suggest that the ability of these shallow-water habitats to support juvenile fishes was not dramatically altered by hurricanes. The resilience exhibited by fishes was likely underpinned by the relative persistence of the seagrass habitat, which appeared principally undamaged by storms based upon review of available–albeit limited seagrass surveys. Increasing cyclone intensity, however, was correlated with greater declines in catch and may potentially underlie the emigration and return rate of fish after cyclones. Whether estuarine fishes will continue to be resilient to acute storm impacts despite chronic environmental degradation and predicted increases major tropical cyclone frequency and intensity remains a pressing question

The interaction of dark matter cusp with the baryon component in disk galaxies

In this paper we examine the effect of the formation and evolution of the disk galaxy on the distribution of dark halo matter. We have made simulations of isolated dark matter (DM) halo and two component (DM + baryons). N-body technique was used for stellar and DM particles and TVD MUSCL scheme for gas-dynamic simulations. The simulations include the processes of star formation, stellar feedback, heating and cooling of the interstellar medium. The results of numerical experiments with high spatial resolution let us to conclude in two main findings. First, accounting of star formation and supernova feedback resolves the so-called problem of cusp in distribution of dark matter predicted by cosmological simulations. Second, the interaction of dark matter with dynamic substructures of stellar and gaseous galactic disk (e.g., spiral waves, bar) has an impact on the shape of the dark halo. In particular, the in-plane distribution of dark matter is more symmetric in runs, where the baryonic component was taken into account.Comment: 7 pages, 6 figure