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Register a new userLarge Metallicity Variations in the Galactic Interstellar Medium
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The Interstellar Medium (ISM) comprises gases at different temperatures and densities, including ionized, atomic, molecular species, and dust particles. The neutral ISM is dominated by neutral hydrogen and has ionization fractions up to 8%. The concentration of chemical elements heavier than helium (metallicity) spans orders of magnitudes in Galactic stars, because they formed at different times. Instead, the gas in the Solar vicinity is assumed to be well mixed and have Solar metallicity in traditional chemical evolution models. The ISM chemical abundances can be accurately measured with UV absorption-line spectroscopy. However, the effects of dust depletion, which removes part of the metals from the observable gaseous phase and incorporates it into solid grains, have prevented, until recently, a deeper investigation of the ISM metallicity. Here we report the dust-corrected metallicity of the neutral ISM measured towards 25 stars in our Galaxy. We find large variations in metallicity over a factor of 10 (with an average 55 +/- 7% Solar and standard deviation 0.28 dex) and including many regions of low metallicity, down to ~17% Solar and possibly below. Pristine gas falling onto the disk in the form of high-velocity clouds can cause the observed chemical inhomogeneities on scales of tens of pc. Our results suggest that this low-metallicity accreting gas does not efficiently mix into the ISM, which may help us understand metallicity deviations in nearby coeval stars.
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Some very large (>0.1 um) presolar grains are sampled in meteorites. We reconsider the lifetime of very large grains (VLGs) in the interstellar medium focusing on interstellar shattering caused by turbulence-induced large velocity dispersions. This path has never been noted as a dominant mechanism of destruction. We show that, if interstellar shattering is the main mechanism of destruction of VLGs, their lifetime is estimated to be $gtrsim 10^8$ yr; in particular, very large SiC grains can survive cosmic-ray exposure time. However, most presolar SiC grains show residence times significantly shorter than 1 Gyr, which may indicate that there is a more efficient mechanism than shattering in destroying VLGs, or that VLGs have larger velocity dispersions than 10 km s$^{-1}$. We also argue that the enhanced lifetime of SiC relative to graphite can be the reason why we find SiC among $mu$m-sized presolar grains, while the abundance of SiC in the normal interstellar grains is much lower than graphite.
In the framework of a systematic ALMA study of IR-selected main-sequence and starburst galaxies at z~1-1.7 at typical ~1 resolution, we report on the effects of mid-IR- and X-ray-detected active galactic nuclei (AGN) on the reservoirs and excitation of molecular gas in a sample of 55 objects. We find detectable nuclear activity in ~30% of the sample. The presence of dusty tori influences the IR SED of galaxies, as highlighted by the strong correlation among the AGN contribution to the total IR luminosity budget (fAGN = LIR,AGN/LIR), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR (S1.2mm/S24um) observed color, with consequences on the empirical SFR estimates. Nevertheless, we find only marginal effects of AGN on the CO (J=2,4,5,7) or neutral carbon ([CI](1-0), [CI](2-1)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full SLEDs. The [CI] and CO emission up to J=5,7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, we highlight the existence of a large variety of line luminosities and ratios despite the homogeneous selection. In particular, we find a sparse group of AGN-dominated sources with the highest LIR,AGN/LIR,SFR ratios, >3, that are more luminous in CO(5-4) than what is predicted by the LCO(5-4)-LIR,SFR relation, which might be the result of the nuclear activity. For the general population, our findings translate into AGN having minimal effects on quantities such as gas and dust fractions and SFEs. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8x larger dust optical depths. In general, this is consistent with a marginal impact of the nuclear activity on the gas reservoirs and star formation in average star-forming AGN hosts with LIR>5e11 Lsun, typically underrepresented in surveys of quasars and SMGs.
Signals from radio pulsars show a wavelength-dependent delay due to dispersion in the interstellar plasma. At a typical observing wavelength, this delay can vary by tens of microseconds on five-year time scales, far in excess of signals of interest to pulsar timing arrays, such as that induced by a gravitational-wave background. Measurement of these delay variations is not only crucial for the detection of such signals, but also provides an unparallelled measurement of the turbulent interstellar plasma at au scales. In this paper we demonstrate that without consideration of wavelength- independent red-noise, simple algorithms to correct for interstellar dispersion can attenuate signals of interest to pulsar timing arrays. We present a robust method for this correction, which we validate through simulations, and apply it to observations from the Parkes Pulsar Timing Array. Correction for dispersion variations comes at a cost of increased band-limited white noise. We discuss scheduling to minimise this additional noise, and factors, such as scintillation, that can exacerbate the problem. Comparison with scintillation measurements confirms previous results that the spectral exponent of electron density variations in the interstellar medium often appears steeper than expected. We also find a discrete change in dispersion measure of PSR J1603-7202 of ~2x10^{-3} cm^{-3}pc for about 250 days. We speculate that this has a similar origin to the extreme scattering events seen in other sources. In addition, we find that four pulsars show a wavelength-dependent annual variation, indicating a persistent gradient of electron density on an au spatial scale, which has not been reported previously.
This review describes where we are today in light of the dust and gas properties and their relation to star formation, in low metallicity galaxies of the local universe following recent surveys from sensitive infrared space telescopes, mainly Spitzer and Herschel space observatories as well as ground-based observations of the molecular gas reservoir. Models to interpret the ISM properties are gaining sophistication in order to account for the wide range of valuable observational diagnostics that we have today to trace the different gas phases, the broad range of photometry we have, from mid-infrared to submillimetre dust emission and the various galactic size scales that we can sample today. This review summarizes the rich multi-phase observations we can exploit today, and the multi-phase modeling approach to interpret the observations.
We fit the near-infrared to radio spectral energy distributions of a sample of 30 luminous and ultra-luminous infrared galaxies with models that include both starburst and AGN components. The aim of the work was to determine important physical parameters for this kind of objects such as the optical depth towards the luminosity source, the star formation rate, the star formation efficiency and the AGN fraction. We found that although about half of our sample have best-fit models that include an AGN component, only 30 % have an AGN which accounts for more than 10 % of the infrared luminosity whereas all have an energetically dominant starburst. Our models also determine the mass of dense molecular gas. Assuming that this mass is that traced by the HCN molecule, we reproduce the observed linear relation between HCN luminosity and infrared luminosity found by Gao and Solomon (2004). However, our derived conversion factor between HCN luminosity and the mass of dense molecular gas is a factor of 2 smaller than that assumed by these authors. Finally, we find that the star formation efficiency falls as the starburst ages.
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