Context. Observations of nearby starburst and spiral galaxies have revealed that molecular gas is the driver of star formation. However, some nearby low-metallicity dwarf galaxies are actively ...forming stars, but CO, the most common tracer of this reservoir, is faint, leaving us with a puzzle about how star formation proceeds in these environments. Aims. We aim to quantify the molecular gas reservoir in a subset of 6 galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data and to link this reservoir to the observed star formation activity. Methods. We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at the ATNF Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as CII 157 mu m and OI 63 mu m observations obtained with the Herschel/PACS spectrometer in the 6 low-metallicity dwarf galaxies: Haro 11, Mrk 1089, Mrk 930, NGC 4861, NGC 625, and UM 311. We derived their molecular gas masses from several methods, including using the CO-to-H sub(2) conversion factor X sub(CO) (both Galactic and metallicity-scaled values) and dust measurements. The molecular and atomic gas reservoirs were compared to the star formation activity. We also constrained the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. Results. We detect CO in 5 of the 6 galaxies, including first detections in Haro 11 (Z ~ 0.4 Zmiddot in circle), Mrk 930 (0.2 Zmiddot in circle), and UM 311 (0.5 Zmiddot in circle), but CO remains undetected in NGC 4861 (0.2Zmiddot in circle). The CO luminosities are low, while CII is bright in these galaxies, resulting in CII/CO(1-0) > or = 10 000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion timescales, even when considering metallicity-scaled X sub(CO) factors. Those galaxies are dominated by their HIgas, except Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro 11 using Cloudy and estimate an equivalent X sub(CO) factor that is 10 times higher than the Galactic value. Overall, our results confirm the emerging picture that CO suffers from significant selective photodissociation in low-metallicity dwarf galaxies.
Context. Low-metallicity dwarf galaxies often show no or little CO emission, despite the intense star formation observed in local samples. Both simulations and resolved observations indicate that ...molecular gas in low-metallicity galaxies may reside in small dense clumps, surrounded by a substantial amount of more diffuse gas that is not traced by CO. Constraining the relative importance of CO-bright versus CO-dark H2 star-forming reservoirs is crucial to understanding how star formation proceeds at low metallicity. Aims. We test classically used single component radiative transfer models and compare their results to those obtained on the assumption of an increasingly complex structure of the interstellar gas, mimicking an inhomogeneous distribution of clouds with various physical properties. Methods. Using the Bayesian code MULTIGRIS, we computed representative models of the interstellar medium as combinations of several gas components, each with a specific set of physical parameters. We introduced physically motivated models assuming power-law distributions for the density, ionization parameter, and the depth of molecular clouds. Results. This new modeling framework allows for the simultaneous reproduction of the spectral constraints from the ionized gas, neutral atomic gas, and molecular gas in 18 galaxies from the Dwarf Galaxy Survey. We confirm the presence of a predominantly CO-dark molecular reservoir in low-metallicity galaxies. The predicted total H2 mass is best traced by C II158 μm and, to a lesser extent, by C I 609 μm, rather than by CO(1–0). We examine the CO-to-H2 conversion factor (αCO) versus metallicity relation and find that its dispersion increases significantly when different geometries of the gas are considered. We define a “clumpiness” parameter that is anti-correlated with C II/CO and explains the dispersion of the αCO versus metallicity relation. We find that low-metallicity galaxies with high clumpiness parameters may have αCO values as low as the Galactic value, even at low metallicity. Conclusions. We identify the clumpiness of molecular gas as a key parameter for understanding variations of geometry-sensitive quantities, such as αCO. This new modeling framework enables the derivation of constraints on the internal cloud distribution of unresolved galaxies, based solely on their integrated spectra.
We combine new Herschel/SPIRE sub-millimeter observations with existing multiwavelength data to investigate the dust scaling relations of the Herschel Reference Survey, a magnitude-, volume-limited ...sample of ~300 nearby galaxies in different environments. We show that the dust-to-stellar mass ratio anti-correlates with stellar mass, stellar mass surface density and NUV − r colour across the whole range of parameters covered by our sample. Moreover, the dust-to-stellar mass ratio decreases significantly when moving from late- to early-type galaxies. These scaling relations are similar to those observed for the Hi gas-fraction, supporting the idea that the cold dust is tightly coupled to the cold atomic gas component in the interstellar medium. We also find a weak increase of the dust-to-Hi mass ratio with stellar mass and colour but no trend is seen with stellar mass surface density. By comparing galaxies in different environments we show that, although these scaling relations are followed by both cluster and field galaxies, Hi-deficient systems have, at fixed stellar mass, stellar mass surface density and morphological type systematically lower dust-to-stellar mass and higher dust-to-Hi mass ratios than Hi-normal/field galaxies. This provides clear evidence that dust is removed from the star-forming disk of cluster galaxies but the effect of the environment is less strong than what is observed in the case of the Hi disk. Such effects naturally arise if the dust disk is less extended than the Hi and follows more closely the distribution of the molecular gas phase, i.e., if the dust-to-atomic gas ratio monotonically decreases with distance from the galactic center.
We present GALEX far-ultraviolet (FUV) and near-ultraviolet (NUV) as well as SDSS g, r, i photometry and structural parameters for the Herschel Reference Survey, a magnitude-, volume-limited sample ...of nearby galaxies in different environments. We use this unique dataset to investigate the ultraviolet (UV) structural scaling relations of nearby galaxies and to determine how the properties of the UV disk vary with atomic hydrogen content and environment. We find a clear change of slope in the stellar mass vs. effective surface brightness relation when moving from the optical to the UV, with more massive galaxies having brighter optical but fainter UV surface brightnesses than smaller systems. A similar change of slope is also seen in the radius vs. surface brightness relation. By comparing our observations with the predictions of a simple multi-zone chemical model of galaxy evolution, we show that these findings are a natural consequence of a much more efficient inside-out growth of the stellar disk in massive galaxies. We confirm that isophotal radii are always a better proxy for the size of the stellar/star-forming disk than effective quantities and we show that the extent of the UV disk (normalized to the optical size) is strongly correlated to the integrated Hi gas fraction. This relation still holds even when cluster spirals are considered, with Hi-deficient systems having less extended star-forming disks than Hi-normal galaxies. Interestingly, the star formation in the inner part of Hi-deficient galaxies is significantly less affected by the removal of the atomic hydrogen, as expected in a simple ram-pressure stripping scenario. These results suggest that it is the amount of Hi that regulates the growth of the star-forming disk in the outskirts of galaxies.
EGFRvIII is a key oncogene in glioblastoma (GBM). EGFRvIII results from an in-frame deletion in the extracellular domain of EGFR, does not bind ligand and is thought to be constitutively active. ...Although EGFRvIII dimerization is known to activate EGFRvIII, the factors that drive EGFRvIII dimerization and activation are not well understood. Here we present a new model of EGFRvIII activation and propose that oncogenic activation of EGFRvIII in glioma cells is driven by co-expressed activated EGFR wild type (EGFRwt). Increasing EGFRwt leads to a striking increase in EGFRvIII tyrosine phosphorylation and activation while silencing EGFRwt inhibits EGFRvIII activation. Both the dimerization arm and the kinase activity of EGFRwt are required for EGFRvIII activation. EGFRwt activates EGFRvIII by facilitating EGFRvIII dimerization. We have previously identified HB-EGF, a ligand for EGFRwt, as a gene induced specifically by EGFRvIII. In this study, we show that HB-EGF is induced by EGFRvIII only when EGFRwt is present. Remarkably, altering HB-EGF recapitulates the effect of EGFRwt on EGFRvIII activation. Thus, increasing HB-EGF leads to a striking increase in EGFRvIII tyrosine phosphorylation while silencing HB-EGF attenuates EGFRvIII phosphorylation, suggesting that an EGFRvIII-HB-EGF-EGFRwt feed-forward loop regulates EGFRvIII activation. Silencing EGFRwt or HB-EGF leads to a striking inhibition of EGFRvIII-induced tumorigenicity, while increasing EGFRwt or HB-EGF levels resulted in accelerated EGFRvIII-mediated oncogenicity in an orthotopic mouse model. Furthermore, we demonstrate the existence of this loop in human GBM. Thus, our data demonstrate that oncogenic activation of EGFRvIII in GBM is likely maintained by a continuous EGFRwt-EGFRvIII-HB-EGF loop, potentially an attractive target for therapeutic intervention.
We combine Spitzer and Herschel data of the star-forming region N11 in the Large Magellanic Cloud (LMC) to produce detailed maps of the dust properties in the complex and study their variations with ...the interstellar-medium conditions. We also compare Atacama Pathfinder EXperiment/Large APEX Bolometer Camera (APEX/LABOCA) 870 μm observations with our model predictions in order to decompose the 870 μm emission into dust and non-dust free–free emission and CO(3–2) line contributions. We find that in N11, the 870 μm can be fully accounted for by these three components. The dust surface density map of N11 is combined with H i and CO observations to study local variations in the gas-to-dust mass ratios. Our analysis leads to values lower than those expected from the LMC low-metallicity as well as to a decrease of the gas-to-dust mass ratio with the dust surface density. We explore potential hypotheses that could explain the low ‘observed’ gas-to-dust mass ratios (variations in the X
CO factor, presence of CO-dark gas or of optically thick H i or variations in the dust abundance in the dense regions). We finally decompose the local spectral energy distributions (SEDs) using a principal component analysis (i.e. with no a priori assumption on the dust composition in the complex). Our results lead to a promising decomposition of the local SEDs in various dust components (hot, warm, cold) coherent with that expected for the region. Further analysis on a larger sample of galaxies will follow in order to understand how unique this decomposition is or how it evolves from one environment to another.
The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) is a complete blind survey of the Virgo cluster covering similar to 40 sq. deg in the far UV (FUV, lambda(eff) = 1539 angstrom, Delta lambda = 442 ...angstrom) and similar to 120 sq. deg in the near UV (NUV, lambda(eff) = 2316 angstrom, Delta lambda = 1060 angstrom). The goal of the survey is to study the ultraviolet (UV) properties of galaxies in a rich cluster environment, spanning a wide luminosity range from giants to dwarfs, and regardless of prior knowledge of their star formation activity. The UV data will be combined with those in other bands (optical: NGVS; far-infrared - submm: HeViCS; HI: ALFALFA) and with our multizone chemo-spectrophotometric models of galaxy evolution to make a complete and exhaustive study of the effects of the environment on the evolution of galaxies in high density regions. We present here the scientific objectives of the survey, describing the observing strategy and briefly discussing different data reduction techniques. Using UV data already in-hand for the central 12 sq. deg we determine the FUV and NUV luminosity functions of the Virgo cluster core for all cluster members and separately for early-and late-type galaxies and compare it to the one obtained in the field and other nearby clusters (Coma, A1367). This analysis shows that the FUV and NUV luminosity functions of the core of the Virgo clusters are flatter (alpha similar to -1.1) than those determined in Coma and A1367. We discuss the possible origin of this difference.
We present a line survey of the ultraluminous infrared galaxy Arp 220, taken with the newly installed SEPIA (Swedish-European Southern Observatory PI receiver for APEX) Band 5 instrument on APEX ...(Atacama Pathfinder Experiment). We illustrate the capacity of SEPIA to detect the 183.3 GHz H2O 31,3–22,0 line against the atmospheric H2O absorption feature. We confirm the previous detection of the HCN(2–1) line, and detect new transitions of standard dense gas tracers such as HNC(2–1), HCO+(2–1), CS(4–3), C34S(4–3) and HC3N(20–19). We also detect HCN(2–1) v
2 = 1 and the 193.5 GHz methanol (4–3) group for the first time. The absence of time variations in the megamaser water line compared to previous observations seems to rule out an AGN nuclear origin for the line. It could, on the contrary, favour a thermal origin instead, but also possibly be a sign that the megamaser emission is associated with star-forming cores washed out in the beam. We finally discuss how the new transitions of HCN, HNC and HCO+ refine our knowledge of the interstellar medium physical conditions in Arp 220.
We present the molecular cloud properties of N55 in the Large Magellanic Cloud using 12CO(1-0) and 13CO(1-0) observations obtained with Atacama Large Millimeter Array. We have done a detailed study ...of molecular gas properties, to understand how the cloud properties of N55 differ from Galactic clouds. Most CO emission appears clumpy in N55, and molecular cores that have young stellar objects (YSOs) show larger linewidths and masses. The massive clumps are associated with high and intermediate mass YSOs. The clump masses are determined by local thermodynamic equilibrium and virial analysis of the 12CO and 13CO emissions. These mass estimates lead to the conclusion that (a) the clumps are in self-gravitational virial equilibrium, and (b) the 12CO(1-0)-to-H2 conversion factor, X CO , is 6.5 × 1020 cm−2 (K km s−1)−1. This CO-to-H2 conversion factor for N55 clumps is measured at a spatial scale of ∼0.67 pc, which is about two times higher than the X CO value of the Orion cloud at a similar spatial scale. The core mass function of N55 clearly show a turnover below 200 M , separating the low-mass end from the high-mass end. The low-mass end of the 12CO mass spectrum is fitted with a power law of index 0.5 0.1, while for 13CO it is fitted with a power law index 0.6 0.2. In the high-mass end, the core mass spectrum is fitted with a power index of 2.0 0.3 for 12CO, and with 2.5 0.4 for 13CO. This power law behavior of the core mass function in N55 is consistent with many Galactic clouds.
We use Spitzer Space Telescope and Herschel Space Observatory far-infrared data along with ground-based optical and near-infrared data to understand how dust heating in the nearby face-on spiral ...galaxies M81, M83 and NGC 2403 is affected by the starlight from all stars and by the radiation from star-forming regions. We find that 70/160
m surface brightness ratios tend to be more strongly influenced by star-forming regions. However, the 250/350
m and 350/500
m surface brightness ratios are more strongly affected by the light from the total stellar populations, suggesting that the dust emission at >250
m originates predominantly from a component that is colder than the dust seen at <160
m and that is relatively unaffected by star formation activity. We conclude by discussing the implications of this for modelling the spectral energy distributions of both nearby and more distant galaxies and for using far-infrared dust emission to trace star formation.
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