103 research outputs found
Herschel SPIRE-FTS Observations of Excited CO and [CI] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas
We present Herschel SPIRE-FTS observations of the Antennae (NGC 4038/39), a
well studied, nearby ( Mpc) ongoing merger between two gas rich spiral
galaxies. We detect 5 CO transitions ( to ), both [CI]
transitions and the [NII] transition across the entire system, which
we supplement with ground based observations of the CO , and
transitions, and Herschel PACS observations of [CII] and [OI].
Using the CO and [CI] transitions, we perform both a LTE analysis of [CI], and
a non-LTE radiative transfer analysis of CO and [CI] using the radiative
transfer code RADEX along with a Bayesian likelihood analysis. We find that
there are two components to the molecular gas: a cold ( K)
and a warm ( K) component. By comparing the warm gas mass
to previously observed values, we determine a CO abundance in the warm gas of
. If the CO abundance is the same in the warm and
cold gas phases, this abundance corresponds to a CO luminosity-to-mass
conversion factor of $\alpha_{CO} \sim 7 \ M_{\odot}{pc^{-2} \ (K \ km \
s^{-1})^{-1}}_263\mu m\sim 0.01 L_{\odot}/M_{\odot}G_0\sim 1000$. Finally, we find
that a combination of turbulent heating, due to the ongoing merger, and
supernova and stellar winds are sufficient to heat the molecular gas.Comment: 50 pages, 15 figures, 8 tables, Accepted for publication in The
Astrophysical Journa
Gas-to-Dust mass ratios in local galaxies over a 2 dex metallicity range
This paper analyses the behaviour of the gas-to-dust mass ratio (G/D) of
local Universe galaxies over a large metallicity range. We combine three
samples: the Dwarf Galaxy Survey, the KINGFISH survey and a subsample from
Galametz et al. (2011) totalling 126 galaxies, covering a 2 dex metallicity
range, with 30% of the sample with 12+log(O/H) < 8.0. The dust masses are
homogeneously determined with a semi-empirical dust model, including submm
constraints. The atomic and molecular gas masses are compiled from the
literature. Two XCO are used to estimate molecular gas masses: the Galactic
XCO, and a XCO depending on the metallicity (as Z^{-2}). Correlations with
morphological types, stellar masses, star formation rates and specific star
formation rates are discussed. The trend between G/D and metallicity is
empirically modelled using power-laws (slope of -1 and free) and a broken
power-law. We compare the evolution of the G/D with predictions from chemical
evolution models. We find that out of the five tested galactic parameters,
metallicity is the galactic property driving the observed G/D. The G/D versus
metallicity relation cannot be represented by a power-law with a slope of -1
over the whole metallicity range. The observed trend is steeper for
metallicities lower than ~ 8.0. A large scatter is observed in the G/D for a
given metallicity, with a dispersion of 0.37 dex in metallicity bins of ~0.1
dex. The broken power-law reproduces best the observed G/D and provides
estimates of the G/D that are accurate to a factor of 1.6. The good agreement
of the G/D and its scatter with the three tested chemical evolution models
shows that the scatter is intrinsic to galactic properties, reflecting the
different star formation histories, dust destruction efficiencies, dust grain
size distributions and chemical compositions across the sample. (abriged)Comment: 23 pages, 12 figures, accepted in Astronomy & Astrophysic
Anarchism, Utopianism and Hospitality: The Work of René Schérer
RenĂ© SchĂ©rer (born 1922) is lamentably almost unknown to the Anglo-American world as his work has, as yet, not been translated . He is one of the main specialists of the French âutopian socialistâ, Charles Fourier (1772-1837), and a major thinker in his own right. He is the author of more than twenty books and co-editor of the journal ChimĂšres. Colleague and friend at Vincennes university (Paris 8) of Michel Foucault, Gilles Deleuze, FĂ©lix Guattari, Jacques Derrida, Jacques RanciĂšre, Jean-François Lyotard, François ChĂąletet, Alain Brossat, Georges Navet, Miguel Abensour, Pierre Macherey⊠he continues to host seminars at Paris 8 (now located at St. Denis). He is a living testimony to a radical past, and a continuing inspiration to a new generation of young thinkers. This article aims to convey the original specificity of his understanding of anarchism. By so doing, it will stress the importance of his work for any thinking concerned with a politicised resistance to social conformity and the supposed âstate of thingsâ today
Spatially resolved physical conditions of molecular gas and potential star formation tracers in Mâ83, revealed by the Herschel SPIRE FTS
International audienceWe investigate the physical properties of the molecular and ionized gas, and their relationship to the star formation and dust properties in Mâ83, based on submillimeter imaging spectroscopy from within the central 3.5âČ (~4 kpc in diameter) around the starburst nucleus. The observations use the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) onboard the Herschel Space Observatory. The newly observed spectral lines include [CI] 370 ÎŒm, [CI] 609 ÎŒm, [NII] 205 ÎŒm, and CO transitions from J = 4â3 to J = 13â12. Combined with previously observed J = 1â0 to J = 3â2 transitions, the CO spectral line energy distributions are translated to spatially resolved physical parameters, column density of CO, N(CO), and molecular gas thermal pressure, Pth, with a non-local thermal equilibrium (non-LTE) radiative transfer model, RADEX. Our results show that there is a relationship between the spatially resolved intensities of [NII] 205 ÎŒm and the surface density of the star formation rate (SFR), ÎŁSFR. This relation, when compared to integrated properties of ultra-luminous infrared galaxies (ULIRGs), exhibits a different slope, because the [NII] 205 ÎŒm distribution is more extended than the SFR. The spatially resolved [CI] 370 ÎŒm, on the other hand, shows a generally linear relationship with ÎŁSFR and can potentially be a good SFR tracer. Compared with the dust properties derived from broad-band images, we find a positive trend between the emissivity of CO in the J = 1â0 transition with the average intensity of interstellar radiation field (ISRF), âš U â©. This trend implies a decrease in the CO-to-H2 conversion factor, XCO, when âš U â© increases. We estimate the gas-to-dust mass ratios to be 77 ± 33 within the central 2 kpc and 93 ± 19 within the central 4 kpc of Mâ83, which implies a Galactic dust-to-metal mass ratio within the observed region of Mâ83. The estimated gas-depletion time for the Mâ83 nucleus is 1.13 ± 0.6 Gyr, which is shorter than the values for nearby spiral galaxies found in the literature (~2.35 Gyr), most likely due to the young nuclear starbursts. A linear relationship between Pth and the radiation pressure generated by âš U â©, Prad, is found to be Pth â 30 Prad, which signals that the ISRF alone is insufficient to sustain the observed CO transitions. The spatial distribution of Pth reveals a pressure gradient, which coincides with the observed propagationof starburst activities and the alignment of (possibly background) radio sources. We discover that the off-centered (from the optical nucleus) peak of the molecular gas volume density coincides well with a minimum in the relative aromatic feature strength, indicating a possible destruction of their carriers. We conclude that the observed CO transitions are most likely associated with mechanical heating processes that are directly or indirectly related to very recent nuclear starbursts
The evolution of the dust and gas content in galaxies
We use deep Herschel observations taken with both PACS and SPIRE imaging cameras to estimate the dust mass of a sample of galaxies extracted from the GOODS-S, GOODS-N and the COSMOS fields. We divide the redshiftâstellar mass (M star )âstar formation rate (SFR) parameter space into small bins and investigate average properties over this grid. In the first part of the work we investigate the scaling relations between dust mass, stellar mass and SFR out to z = 2.5. No clear evolution of the dust mass with redshift is observed at a given SFR and stellar mass. We find a tight correlation between the SFR and the dust mass, which, under reasonable assumptions, is likely a consequence of the Schmidt-Kennicutt (S-K) relation. The previously observed correlation between the stellar content and the dust content flattens or sometimes disappears when considering galaxies with the same SFR. Our finding suggests that most of the correlation between dust mass and stellar mass obtained by previous studies is likely a consequence of the correlation between the dust mass and the SFR combined with the main sequence, i.e., the tight relation observed between the stellar mass and the SFR and followed by the majority of star-forming galaxies. We then investigate the gas content as inferred from dust mass measurements. We convert the dust mass into gas mass by assuming that the dust-to-gas ratio scales linearly with the gas metallicity (as supported by many observations). For normal star-forming galaxies (on the main sequence) the inferred relation between the SFR and the gas mass (integrated S-K relation) broadly agrees with the results of previous studies based on CO measurements, despite the completely different approaches. We observe that all galaxies in the sample follow, within uncertainties, the same S-K relation. However, when investigated in redshift intervals, the S-K relation shows a moderate, but significant redshift evolution. The bulk of the galaxy population at z ⌠2 converts gas into stars with an efficiency (star formation efficiency, SFE = SFR/M gas , equal to the inverse of the depletion time) about 5 times higher than at z ⌠0. However, it is not clear what fraction of such variation of the SFE is due to an intrinsic redshift evolution and what fraction is simply a consequence of high-z galaxies having, on average, higher SFR, combined with thesuper-linear slope of the S-K relation (whileother studies finda linear slope). We confirm that the gas fraction (f gas = M gas /(M gas + M star )) decreases with stellar mass and increases with the SFR. We observe no evolution with redshift once M star and SFR are fixed. We explain these trends by introducing a universal relation between gas fraction, stellar mass and SFR that does not evolve with redshift, at least out to z ⌠2.5. Galaxies move across this relation as their gas content evolves across the cosmic epochs. We use the 3D fundamental f gas âM star âSFR relation, along with the evolution of the main sequence with redshift, to estimate the evolution of the gas fraction in the average population of galaxies as a function of redshift and as a function of stellar mass: we find that M star > ⌠10 11 M ? galaxies show the strongest evolution at z > ⌠1.3 and a flatter trend at lower redshift, while f gas decreases more regularly over the entire redshift range probed in M star < ⌠10 11 Mo galaxies, in agreement with a downsizing scenario
Theory of the formation of a collisionless Weibel shock: pair vs. electron/proton plasmas
Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies
Aims. The purpose of this work is the characterization of the radial distribution of dust, stars, gas, and star-formation rate (SFR) in a sub-sample of 18 face-on spiral galaxies extracted from the DustPedia sample.
Methods. This study is performed by exploiting the multi-wavelength DustPedia database, from ultraviolet (UV) to sub-millimeter bands, in addition to molecular (12CO) and atomic (Hi) gas maps and metallicity abundance information available in the literature. We fitted the surface-brightness profiles of the tracers of dust and stars, the mass surface-density profiles of dust, stars, molecular gas, and total gas, and the SFR surface-density profiles with an exponential curve and derived their scale-lengths. We also developed a method to solve for the CO-to-H2 conversion factor (αCO) per galaxy by using dust- and gas-mass profiles.
Results. Although each galaxy has its own peculiar behavior, we identified a common trend of the exponential scale-lengths versus wavelength. On average, the scale-lengths normalized to the B-band 25 mag/arcsec2 radius decrease from UV to 70 ÎŒm, from 0.4 to 0.2, and then increase back up to ~0.3 at 500 microns. The main result is that, on average, the dust-mass surface-density scale-length is about 1.8 times the stellar one derived from IRAC data and the 3.6 ÎŒm surface brightness, and close to that in the UV. We found a mild dependence of the scale-lengths on the Hubble stage T: the scale-lengths of the Herschel bands and the 3.6 ÎŒm scale-length tend to increase from earlier to later types, the scale-length at 70 ÎŒm tends to be smaller than that at longer sub-mm wavelength with ratios between longer sub-mm wavelengths and 70 ÎŒm that decrease with increasing T. The scale-length ratio of SFR and stars shows a weak increasing trend towards later types. Our αCO determinations are in the range (0.3â9) Mâ pc-2 (K km s-1)-1, almost invariant by using a fixed dust-to-gas ratio mass (DGR) or a DGR depending on metallicity gradient
Electron-ion temperature relaxation in warm dense hydrogen observed with picosecond resolved X-Ray scattering
Angularly resolved X-ray scattering measurements from fs-laser heated hydrogen have been used to determine the equilibration of electron and ion temperatures in the warm dense matter regime. The relaxation of rapidly heated cryogenic hydrogen is visualized using 5.5 keV X-ray pulses from the Linac Coherent Light (LCLS) source in a 1 Hz repetition rate pump-probe setting. We demonstrate that the electron-ion energy transfer is faster than quasi-classical Landau-Spitzer models that use ad hoc cutoffs in the Coulomb logarithm
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