The Extreme Hosts of Extreme Supernovae

We use GALEX ultraviolet (UV) and optical integrated photometry of the hosts of seventeen luminous supernovae (LSNe, having peak M_V < -21) and compare them to a sample of 26,000 galaxies from a cross-match between the SDSS DR4 spectral catalog and GALEX interim release 1.1. We place the LSNe hosts on the galaxy NUV-r versus M_r color magnitude diagram (CMD) with the larger sample to illustrate how extreme they are. The LSN hosts appear to favor low-density regions of the galaxy CMD falling on the blue edge of the blue cloud toward the low luminosity end. From the UV-optical photometry, we estimate the star formation history of the LSN hosts. The hosts have moderately low star formation rates (SFRs) and low stellar masses (M_*) resulting in high specific star formation rates (sSFR). Compared with the larger sample, the LSN hosts occupy low-density regions of a diagram plotting sSFR versus M_* in the area having higher sSFR and lower M_*. This preference for low M_*, high sSFR hosts implies the LSNe are produced by an effect having to do with their local environment. The correlation of mass with metallicity suggests that perhaps wind-driven mass loss is the factor that prevents LSNe from arising in higher-mass, higher-metallicity hosts. The massive progenitors of the LSNe (>100 M_sun), by appearing in low-SFR hosts, are potential tests for theories of the initial mass function that limit the maximum mass of a star based on the SFR.Comment: 8 pages, 3 figures, 2 tables, accepted to ApJ, amended references and updated SN designation

Effects of the integrated galactic IMF on the chemical evolution of the solar neighbourhood

The initial mass function determines the fraction of stars of different intial mass born per stellar generation. In this paper, we test the effects of the integrated galactic initial mass function (IGIMF) on the chemical evolution of the solar neighbourhood. The IGIMF (Weidner & Kroupa 2005) is computed from the combination of the stellar intial mass function (IMF), i.e. the mass function of single star clusters, and the embedded cluster mass function, i.e. a power law with index beta. By taking into account also the fact that the maximum achievable stellar mass is a function of the total mass of the cluster, the IGIMF becomes a time-varying IMF which depends on the star formation rate. We applied this formalism to a chemical evolution model for the solar neighbourhood and compared the results obtained by assuming three possible values for beta with the results obtained by means of a standard, well-tested, constant IMF. In general, a lower absolute value of beta implies a flatter IGIMF, hence a larger number of massive stars and larger metal ejection rates. This translates into higher type Ia and II supernova rates, higher mass ejection rates from massive stars and a larger amount of gas available for star formation, coupled with lower present-day stellar mass densities. (abridged) We also discuss the importance of the present day stellar mass function (PDMF) in providing a way to disentangle among various assumptions for beta. Our results indicate that the model adopting the IGIMF computed with beta ~2 should be considered the best since it allows us to reproduce the observed PDMF and to account for most of the chemical evolution constraints considered in this work.Comment: 22 pages, 19 figure

Investigating the Arctic phytoplankton variability and diversity based on modeling and satellite retrievals

In our study we focus on improving our understanding of possible interactions between the open water and sea ice and the surface ocean biogeochemistry under the recently observed sea ice decline in the Arctic. In particular, the analysis of changes in phytoplankton functional types (PFTs) over 2002 to 2012 based on long-term time series of satellite retrievals and supported by a modeling study is presented. The phytoplankton dynamics as well as phytoplankton diversity in response to Arctic Amplification is simulated with the DARWIN biogeochemical model (Follows et al., 2007, Dutkiewicz et al., 2015) coupled to the Massachusetts Institute of Technology general circulation model (MITgcm) with a configuration based on a cubed‐sphere grid (Menemenlis et al. 2008). The model results are complemented with information on phytoplankton compositions retrieved with PhytoDOAS (Bracher et al. 2009, Sadeghi et al. 2012) from available hyper-spectral optical satellite measurements (SCIAMACHY and OMI), which are synergistically combined via an optimal interpolation technique with multi-spectral optical satellite data (OC-CCI)

Families of dynamically hot stellar systems over ten orders of magnitude in mass

Dynamically hot stellar systems, whether star clusters or early-type galaxies, follow well-defined scaling relations over many orders of magnitudes in mass. These fundamental plane relations have been subject of several studies, which have been mostly confined to certain types of galaxies and/or star clusters so far. Here, we present a complete picture of hot stellar systems ranging from faint galaxies and star clusters of only a few hundred solar masses up to giant ellipticals (gEs) with 10^12 M_sun, in particular including large samples of compact ellipticals (cEs), ultra-compact dwarf galaxies (UCDs), dwarf ellipticals (dEs) of nearby galaxy clusters and Local Group ultra-faint dwarf spheroidals (dSphs). For all those stellar systems we show the effective radius-luminosity, effective radius-stellar mass, and effective mass surface density-stellar mass plane. Two families of hot stellar systems can be differentiated: the 'galaxian' family, ranging from gEs over Es and dEs to dSphs, and the 'star cluster' family, comprising globular clusters (GCs), UCDs and nuclear star clusters (NCs). Interestingly, massive ellipticals have a similar size-mass relation as cEs, UCDs and NCs, with a clear common boundary towards minimum sizes. No object of either family is located in the 'zone of avoidance' beyond this limit. Even the majority of early-type galaxies at high redshift obeys this relation. The sizes of dEs and dSphs as well as GCs barely vary with mass over several orders of magnitude. We use the constant galaxy sizes to derive the distances of several local galaxy clusters. Both, galaxies and star clusters, do not exceed a surface density of \Sigma_eff = 3.17*10^{10}*M^{-3/5} M_sun pc^{-2}, causing an orthogonal kink in the galaxy sequence for ellipticals more massive than 10^{11} M_sun. The densest stellar systems (within their effective radius) are nuclear star clusters. (abridged)Comment: 13 pages, 7 figures, accepted for publication in MNRAS. Errors for mean effective radii and related quantities in Table 3 corrected, references added and affiliation changed in the replaced versio

A GALEX Ultraviolet Imaging Survey of Galaxies in the Local Volume

We present results from a GALEX ultraviolet (UV) survey of a complete sample of 390 galaxies within ~11 Mpc of the Milky Way. The UV data are a key component of the composite Local Volume Legacy (LVL), an ultraviolet-to-infrared imaging program designed to provide an inventory of dust and star formation in nearby spiral and irregular galaxies. The ensemble dataset is an especially valuable resource for studying star formation in dwarf galaxies, which comprise over 80% of the sample. We describe the GALEX survey programs which obtained the data and provide a catalog of far-UV (~1500 Angstroms) and near-UV (~2200 Angstroms) integrated photometry. General UV properties of the sample are briefly discussed. We compute two measures of the global star formation efficiency, the SFR per unit HI gas mass and the SFR per unit stellar mass, to illustrate the significant differences that can arise in our understanding of dwarf galaxies when the FUV is used to measure the SFR instead of H-alpha. We find that dwarf galaxies may not be as drastically inefficient in coverting gas into stars as suggested by prior H-alpha studies. In this context, we also examine the UV properties of late-type dwarf galaxies that appear to be devoid of star formation because they were not detected in previous H-alpha narrowband observations. Nearly all such galaxies in our sample are detected in the FUV, and have FUV SFRs that fall below the limit where the H-alpha flux is robust to Poisson fluctuations in the formation of massive stars. The UV colors and star formation efficiencies of H-alpha-undetected, UV-bright dwarf irregulars appear to be relatively unremarkable with respect to those exhibited by the general population of star-forming galaxies.Comment: submitted to ApJS, revised per referee's comments; accepted Oct. 30 w/o further revision; 37 pages; figure 6 omitted due to size; figure available from http://users.obs.carnegiescience.edu/jlee/paper

High mass star formation in normal late-type galaxies: observational constraints to the IMF

We use Halpha and FUV GALEX data for a large sample of nearby objects to study the high mass star formation activity of normal late-type galaxies. The data are corrected for dust attenuation using the most accurate techniques at present available, namely the Balmer decrement and the total far-infrared to FUV flux ratio. The sample shows a highly dispersed distribution in the Halpha to FUV flux ratio indicating that two of the most commonly used star formation tracers give star formation rates with uncertainties up to a factor of 2-3. The high dispersion is due to the presence of AGN, where the UV and the Halpha emission can be contaminated by nuclear activity, highly inclined galaxies, for which the applied extinction corrections are probably inaccurate, or starburst galaxies, where the stationarity in the star formation history required for transforming Halpha and UV luminosities into star formation rates is not satisfied. Excluding these objects we reach an uncertainty of ~50% on the SFR. The Halpha to FUV flux ratio increases with their total stellar mass. If limited to normal star forming galaxies, however, this relationship reduces to a weak trend that might be totally removed using different extinction correction recipes. In these objects the Halpha to FUV flux ratio seems also barely related with the FUV-H colour, the H band effective surface brightness, the total star formation activity and the gas fraction. The data are consistent with a Kroupa and Salpeter initial mass function in the high mass stellar range and imply, for a Salpeter IMF, that the variations of the slope cannot exceed 0.25, from g=2.35 for massive galaxies to g=2.60 in low luminosity systems. We show however that these observed trends, if real, can be due to the different micro history of star formation in massive galaxies with respect to dwarf.Comment: Accepted for publication on Ap

Runaway stars as progenitors of supernovae and gamma-ray bursts

When a core collapse supernova occurs in a binary system, the surviving star as well as the compact remnant emerging from the SN, may reach a substantial space velocity. With binary population synthesis modelling at solar and one fifth of solar metallicity, we predict the velocities of such runaway stars or binaries. We compile predictions for runaway OB stars, red supergiants and Wolf-Rayet stars. For those stars or binaries which undergo a second stellar explosion we compute their further evolution and the distance travelled until a Type II or Type Ibc SN or a long or short gamma-ray burst occurs. We find our predicted population of OB runaway stars broadly matches the observed population of stars but, to match the fastest observed WR runaway stars, we require that black holes receive an asymmetric kick upon formation. We find that at solar metallicity Type Ic SN progenitors travel shorter distances than the progenitors of other SN types because they are typically more massive and thus have shorter lifetimes. Those of Type IIP SN can fly farthest about 48 pc on average at solar metallicity. In considering the consequences of assuming that the progenitors of long GRBs are spun-up secondary stars that experience quasi-homogeneous evolution, we find that such evolution has a dramatic effect on the population of runaway WR stars and that some 30 per cent of GRBs could occur a hundred parsecs or more from their initial positions. We also consider mergers of double compact object binaries consisting of neutron stars and/or black holes. We find the most common type of visible mergers are neutron star--black hole mergers that are roughly ten times more common than neutron star--neutron star mergers. We also find that there may be a population of low-velocity neutron stars that are ejected from a binary rather than by their own natal kick.Comment: Accepted for publication in MNRAS, 23 pages, 17 figures and 11 tables. Abstract was editted to fit within arXiv.org submission requirement

I Zw 18 as morphological paradigm for rapidly assembling high-z galaxies

IZw18, ever since regarded as the prototypical blue compact dwarf (BCD) galaxy, is, quite ironically, the most atypical BCD known. This is because its large exponential low-surface brightness envelope is not due to an old stellar host but entirely due to extended nebular emission (ne) (Papaderos et al. 2002; P02). We study IZw18 and IZw18C down to an unprecedently faint surface brightness level using HST ACS data. We argue that the properties of IZw18C can be consistently accounted for by propagating star formation over the past ~100 Myr, in combination with stellar diffusion and the associated radial stellar mass filtering effect (P02). As for IZw18, we find that ne extends out to ~16 stellar scale lengths and provides at least 1/3 of the total optical emission. The case of IZw18 suggests caution in studies of distant galaxies in dominant stages of their evolution, rapidly assembling their stellar mass at high specific star formation rates (SSFRs). It calls attention to the fact that ne is not necessarily cospatial with the underlying ionizing and non-ionizing stellar background, neither has to scale with its surface density. The prodigious energetic output during dominant phases of galaxy evolution may result in large exponential ne envelopes, extending much beyond the still compact stellar component, just like in IZw18. Therefore, the morphological paradigm of IZw18, while probably unique in the nearby Universe, may be ubiquitous among high-SSFR galaxies at high redshift. Using IZw18 as reference, we show that extended ne may introduce substantial observational biases and significantly affect fundamental galaxy relations. Among others, we show that the surface brightness profiles of distant morphological analogs to IZw18 may be barely distinguishable from Sersic profiles with an exponent 2<n<5, thus mimicking the profiles of massive galaxy spheroids. (abridged)Comment: 22 pages, 15 figures, Accepted for publication in Astronomy and Astrophysic

Young stellar clusters and associations in M33

We analyse multi-wavelength observations of 32 young star clusters and associations in M33 with known oxygen abundance (8 < 12 + log(O/H) < 8.7), using ultraviolet (UV), optical, mid-infrared (MIR), CO (1-0) and 21-cm line (HI) observations. We derive their spectral energy distribution, and we determine age, bolometric luminosities, masses and the extinction, by comparing the multi-band integrated photometry to single-age stellar population models. The stellar system ages range between 2 and 15 Myr, masses are between 3 x 10^2 and 4 x 10^4 M_sun, and the intrinsic extinction, A_V, varies from 0.3 to 1 mag. We find a correlation between age and extinction, and between the cluster mass and size. The MIR emission shows the presence of a dust component around the clusters whose fractional luminosity at 24 um, L_{24}/L_{Bol}, decreases with the galactocentric distance. However, the total IR luminosity inferred from L_{24} is smaller than what we derive from the extinction corrections. The Halpha luminosity predicted by population synthesis models is larger than the observed one, especially for low-mass systems (M < 10^4 M_sun). Such a difference is reduced, but not erased, when the incomplete sampling of the initial mass function (IMF) at the high-mass end is taken into account. Our results suggest that a non-negligible fraction of UV ionising and non-ionising radiation is leaking into the ISM outside the HII regions. This would be in agreement with the large UV and Halpha diffuse fractions observed in M33, but it implies that stellar systems younger than 3 Myr retain, on average, only 30% of their Lyman continuum photons. However, the uncertainties on cluster ages and the stochastic fluctuations of the IMF do not allow to accurately quantify this issue. We also consider the possibility that this discrepancy is the consequence of a suppressed or delayed formation of the most massive stars.Comment: 17 pages, 13 figures. Accepted for publications in A&A; v2 --> Table 2 corrected because of a misprint in the FUV magnitude