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تسجيل مستخدم جديدThe Herschel Dwarf Galaxy Survey: I. Properties of the low-metallicity ISM from PACS spectroscopy
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The far-infrared (FIR) lines are key tracers of the physical conditions of the interstellar medium (ISM) and are becoming workhorse diagnostics for galaxies throughout the universe. Our goal is to explain the differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies. We present Herschel PACS spectroscopic observations of the CII157um, OI63 and 145um, OIII88um, NII122 and 205um, and NIII57um fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models. We find that the FIR lines together account for up to 3 percent of LTIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of OIII/NII122 and NIII/NII122 are high, indicative of hard radiation fields. In the photodissociation region (PDR), the CII/OI63 ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the OI145/OI63 ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The OIII/OI63 ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found ~4 times higher in the dwarfs than in metal-rich galaxies. The high CII/LTIR, OI/LTIR, and OIII/LTIR ratios, which decrease with increasing LTIR and LTIR/LB, are interpreted as a combination of moderate FUV fields and low PDR covering factor. Harboring compact phases of low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that in metal-rich galaxies.
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The sensitive infrared telescopes, Spitzer and Herschel, have been used to target low-metallicity star-forming galaxies, allowing us to investigate the properties of their interstellar medium (ISM) in unprecedented detail. Interpretation of the obser
vations in physical terms relies on careful modeling of those properties. We have employed a multiphase approach to model the ISM phases (HII region and photodissociation region) with the spectral synthesis code Cloudy. Our goal is to characterize the physical conditions (gas densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel Dwarf Galaxy Survey. We are particularly interested in correlations between those physical conditions and metallicity or star-formation rate. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [CII]157um line, and the characterization of the porosity of the ISM. We find that the lower-metallicity galaxies of our sample tend to have higher ionization parameters and galaxies with higher specific star-formation rates have higher gas densities. The [CII] emission arises mainly from PDRs and the contribution from the ionized gas phases is small, typically less than 30% of the observed emission. We also find correlation - though with scatter - between metallicity and both the PDR covering factor and the fraction of [CII] from the ionized gas. Overall, the low metal abundances appear to be driving most of the changes in the ISM structure and conditions of these galaxies, and not the high specific star-formation rates. These results demonstrate in a quantitative way the increase of ISM porosity at low metallicity. Such porosity may be typical of galaxies in the young Universe.
We present new photometric data from our Herschel Key Programme, the Dwarf Galaxy Survey (DGS), dedicated to the observation of the gas and dust in 48 low-metallicity environments. They were observed with PACS and SPIRE onboard Herschel at 70,100,160
,250,350, and 500 microns. We focus on a systematic comparison of the derived FIR properties (FIR luminosity, dust mass, dust temperature and emissivity index) with more metal-rich galaxies and investigate the detection of a potential submm excess. The data reduction method is adapted for each galaxy to derive the most reliable photometry from the final maps. PACS flux densities are compared with the MIPS 70 and 160 microns bands. We use colour-colour diagrams and modified blackbody fitting procedures to determine the dust properties of the DGS galaxies. We also include galaxies from the Herschel KINGFISH sample, containing more metal-rich environments, totalling 109 galaxies. The location of the DGS galaxies on Herschel colour-colour diagrams highlights the differences in global environments of low-metallicity galaxies. The dust in DGS galaxies is generally warmer than in KINGFISH galaxies (T_DGS~32 K, T_KINGFISH~23 K). The emissivity index, beta, is ~1.7 in the DGS, but metallicity does not make a strong effect on beta. The dust-to-stellar mass ratio is lower in low-metallicity galaxies: M_dust/M_star~0.02% for the DGS vs 0.1% for KINGFISH. Per unit dust mass, dwarf galaxies emit ~6 times more in the FIR than higher metallicity galaxies. Out of the 22 DGS galaxies detected at 500 micron, 41% present an excess in the submm not explained by our dust SED model. The excess mainly appears in lower metallicity galaxies (12+log(O/H) < 8.3), and the strongest excesses are detected in the most metal-poor galaxies. We stress the need for observations longwards of the Herschel wavelengths to detect any submm excess appearing beyond 500 micron.
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A detailed analysis of Herschel-PACS observations at the North Ecliptic Pole is presented. High quality maps, covering an area of 0.44 square degrees, are produced and then used to derive potential candidate source lists. A rigorous quality control p
ipeline has been used to create final legacy catalogues in the PACS Green 100 micron and Red 160 micron bands, containing 1384 and 630 sources respectively. These catalogues reach to more than twice the depth of the current archival Herschel/PACS Point Source Catalogue, detecting 400 and 270 more sources in the short and long wavelength bands respectively. Galaxy source counts are constructed that extend down to flux densities of 6mJy and 19mJy (50% completeness) in the Green 100 micron and Red 160 micron bands respectively. These source counts are consistent with previously published PACS number counts in other fields across the sky. The source counts are then compared with a galaxy evolution model identifying a population of luminous infrared galaxies as responsible for the bulk of the galaxy evolution over the flux range (5-100mJy) spanned by the observed counts, contributing approximate fractions of 50% and 60% to the cosmic infrared background (CIRB) at 100 microns and 160 microns respectively.
Context: The emission line of [CII] at 158 micron is one of the strongest cooling lines of the interstellar medium (ISM) in galaxies. Aims: Disentangling the relative contributions of the different ISM phases to [CII] emission, is a major topic of th
e HerM33es program, a Herschel key project to study the ISM in the nearby spiral galaxy M33. Methods: Using PACS, we have mapped the emission of [CII] 158 micron, [OI] 63 micron, and other FIR lines in a 2x2 region of the northern spiral arm of M33, centered on the HII region BCLMP302. At the peak of H-alpha emission, we have observed in addition a velocity resolved [CII] spectrum using HIFI. We use scatterplots to compare these data with PACS 160 micron continuum maps, and with maps of CO and HI data, at a common resolution of 12 arcsec or 50 pc. Maps of H-alpha and 24 micron emission observed with Spitzer are used to estimate the SFR. We have created maps of the [CII] and [OI] 63 micron emission and detected [NII] 122 micron and NIII 57 micron at individual positions. Results: The [CII] line observed with HIFI is significantly broader than that of CO, and slightly blue-shifted. In addition, there is little spatial correlation between [CII] observed with PACS and CO over the mapped region. There is even less spatial correlation between [CII] and the atomic gas traced by HI. Detailed comparison of the observed intensities towards the HII region with models of photo ionization and photon dominated regions, confirms that a significant fraction, 20--30%, of the observed [CII] emission stems from the ionized gas and not from the molecular cloud. The gas heating efficiency, using the ratio between [CII] and the TIR as a proxy, varies between 0.07 and 1.5%, with the largest variations found outside the HII region.
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