arXiv subjects

Francois Boulanger

Publications and source records attributed to Francois Boulanger.

36 records · Page 2Linked to original sources

arXiv2015

BFORE: The B-mode Foreground Experiment

The B-mode Foreground Experiment (BFORE) is a proposed NASA balloon project designed to make optimal use of the sub-orbital platform by concentrating on three dust foreground bands (270, 350, and 600 GHz) that complement ground-based cosmic microwave background (CMB) programs. BFORE will survey ~1/4 of the sky with 1.7 - 3.7 arcminute resolution, enabling precise characterization of the Galactic dust that now limits constraints on inflation from CMB B-mode polarization measurements. In addition, BFORE's combination of frequency coverage, large survey area, and angular resolution enables science far beyond the critical goal of measuring foregrounds. BFORE will constrain the velocities of thousands of galaxy clusters, provide a new window on the cosmic infrared background, and probe magnetic fields in the interstellar medium. We review the BFORE science case, timeline, and instrument design, which is based on a compact off-axis telescope coupled to >10,000 superconducting detectors.

astro-ph.IM↗

arXiv2015

X-ray Emission from the Taffy (VV254) Galaxies and Bridge

We present the first X-ray observations of the Taffy galaxies (UGC 12914/5) with the Chandra observatory, and detect soft X-ray emission in the region of the gas-rich, radio-continuum-emitting Taffy bridge. The results are compared to Herschel observations of dust and diffuse [CII] line-emitting gas. The diffuse component of the Taffy bridge has an X-ray luminosity of L(X) (0.5-8keV) =5.4 x 10^39 erg s^-1, which accounts for 19% of the luminosity of the sum for the two galaxies. The total mass in hot gas is (0.8--1.3) x 10^8 M_sun, which is approximately 1% of the total (HI~+~H2) gas mass in the bridge, and ~11% of the mass of warm molecular hydrogen discovered by Spitzer. The soft X-ray and dense CO-emitting gas in the bridge have offset distributions, with the X-rays peaking along the north-western side of the bridge in the region where stronger far-IR dust and diffuse [CII] gas is observed by Herschel. We detect nine Ultra Luminous X-ray sources (ULXs) in the system, the brightest of which is found in the bridge, associated with an extragalactic HII region. We suggest that the X-ray--emitting gas has been shocked--heated to high temperatures and "splashed" into the bridge by the collision. The large amount of gas stripped from the galaxies into the bridge and its very long gas depletion timescale (>10 Gyr) may explain why this system, unlike most major mergers, is not a powerful IR emitter.

astro-ph.GA↗

arXiv2014

A dynamical transition from atomic to molecular intermediate-velocity clouds

Towards the high galactic latitude sky, the far-infrared (FIR) intensity is tightly correlated to the total hydrogen column density which is made up of atomic (HI) and molecular hydrogen (H$_{2})$. Above a certain column density threshold, atomic hydrogen turns molecular. We analyse gas and dust properties of intermediate-velocity clouds (IVCs) in the lower galactic halo to explore their transition from the atomic to the molecular phase. Driven by observations, we investigate the physical processes that transform a purely atomic IVC into a molecular one. Data from the Effelsberg-Bonn HI-Survey (EBHIS) are correlated to FIR wavebands of the Planck satellite and IRIS. Modified black-body emission spectra are fitted to deduce dust optical depths and grain temperatures. We remove the contribution of atomic hydrogen to the FIR intensity to estimate molecular hydrogen column densities. Two IVCs show different FIR properties, despite their similarity in HI, such as narrow spectral lines and large column densities. One FIR bright IVC is associated with H$_{2}$, confirmed by $^{12}$CO $(1\rightarrow0)$ emission; the other IVC is FIR dim and shows no FIR excess, which indicates the absence of molecular hydrogen. We propose that the FIR dim and bright IVCs probe the transition between the atomic and molecular gas phase. Triggered by dynamical processes, this transition happens during the descent of IVCs onto the galactic disk. The most natural driver is ram pressure exerted onto the cloud by the increasing halo density. Because of the enhanced pressure, the formation timescale of H$_{2}$ is reduced, allowing the formation of large amounts of H$_{2}$ within a few Myr.

astro-ph.GA↗

arXiv2013

PRISM (Polarized Radiation Imaging and Spectroscopy Mission): A White Paper on the Ultimate Polarimetric Spectro-Imaging of the Microwave and Far-Infrared Sky

PRISM (Polarized Radiation Imaging and Spectroscopy Mission) was proposed to ESA in response to the Call for White Papers for the definition of the L2 and L3 Missions in the ESA Science Programme. PRISM would have two instruments: (1) an imager with a 3.5m mirror (cooled to 4K for high performance in the far-infrared---that is, in the Wien part of the CMB blackbody spectrum), and (2) an Fourier Transform Spectrometer (FTS) somewhat like the COBE FIRAS instrument but over three orders of magnitude more sensitive. Highlights of the new science (beyond the obvious target of B-modes from gravity waves generated during inflation) made possible by these two instruments working in tandem include: (1) the ultimate galaxy cluster survey gathering 10e6 clusters extending to large redshift and measuring their peculiar velocities and temperatures (through the kSZ effect and relativistic corrections to the classic y-distortion spectrum, respectively) (2) a detailed investigation into the nature of the cosmic infrared background (CIB) consisting of at present unresolved dusty high-z galaxies, where most of the star formation in the universe took place, (3) searching for distortions from the perfect CMB blackbody spectrum, which will probe a large number of otherwise inaccessible effects (e.g., energy release through decaying dark matter, the primordial power spectrum on very small scales where measurements today are impossible due to erasure from Silk damping and contamination from non-linear cascading of power from larger length scales). These are but a few of the highlights of the new science that will be made possible with PRISM.

astro-ph.CO↗

arXiv2013

Millimeter dust emission compared with other mass estimates in N11 molecular clouds in the LMC

CO and dust emission at millimeter wavelengths are independent tracers of cold interstellar matter, which have seldom been compared on the scale of GMCs in other galaxies. In this study, and for the first time in the Large Magellanic Cloud, we compute the molecular cloud masses from the mm emission of the dust and compare them with the masses derived from their CO luminosity and virial theorem. We present CO (J=1-0,2-1) and 1.2 mm continuum observations of the N11 star forming region in the LMC obtained with the SEST telescope and the SIMBA bolometer, respectively. We use the CO data to identify individual molecular clouds and measure their physical properties. The correlations between the properties of the N11 clouds are in agreement with those found in earlier studies in the LMC that sample a larger set of clouds and a larger range of cloud masses. For the N11 molecular clouds, we compare the masses estimated from the CO luminosity (Xco\Lco), the virial theorem (Mvir) and the millimeter dust luminosity (L_d). The measured ratios Lco/Mvir and L_d/Mvir constrain the Xco and K_d (dust emissivity at 1.2 mm per unit gas mass) parameters as a function of the virial parameter a_vir. The comparison between the different mass estimates yields a Xco-factor of 8.8x10^20 cm-2/(K km s-1) x a_vir and a K_d parameter of 1.5x10^-3 cm2/g x a_vir. We compare our N11 results with a similar analysis for molecular clouds in the Gould's Belt. We do not find in N11 a large discrepancy between the dust mm and virial masses as reported in earlier studies of molecular clouds in the SMC. The ratio between L_d and Mvir in N11 is half of that measured for Gould's Belt clouds, which can be accounted for by a factor of two lower gas-to-dust mass ratio, as the difference in gas metallicities. If the two samples have similar a_vir values, this result implies that their dust far-IR properties are also similar.

astro-ph.CO↗

arXiv2011

Dense gas without star formation: The kpc-sized molecular disk in 3C326 N

We report the discovery of a 3 kpc disk of few 10^9 Ms of dense, warm H_2 in the nearby radio galaxy 3C326 N, which shows no signs of on-going or recent star formation and falls a factor 60 below the Schmidt-Kennicutt law. VLT/SINFONI imaging spectroscopy shows broad (FWHM \sim 500 km/s) ro-vibrational H_2 lines across all of the disk, with irregular profiles and line ratios consistent with shocks. The ratio of turbulent and gravitational energy suggests that the gas is highly turbulent and not gravitationally bound. In absence of the driving by the jet, short turbulent dissipation times suggest the gas should collapse rapidly and form stars, at odds with the recent star-formation history. Motivated by hydrodynamic models of rapid H_2 formation boosted by turbulent compression, we propose that the molecules formed from diffuse atomic gas in the turbulent jet cocoon. Since the gas is not self-gravitating, it cannot form molecular clouds or stars while the jet is active, and is likely to disperse and become atomic again after the nuclear activity ceases. We speculate that very low star-formation rates are to be expected under such conditions, provided that the large-scale turbulence sets the gas dynamics in molecular clouds. Our results illustrate that jets may create large molecular reservoirs as expected in 'positive feedback' scenarios of AGN-triggered star formation, but that this alone is not sufficient to trigger star formation.

astro-ph.CO↗

arXiv2010

Excitation of H$_2$ in photodissociation regions as seen by Spitzer

We present spectroscopic observations obtained with the infrared Spitzer Space Telescope, which provide insight into the H$_2$ physics and gas energetics in photodissociation Regions (PDRs) of low to moderate far-ultraviolet (FUV) fields and densities. We analyze data on six well known Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling a poorly explored range of excitation conditions ($χ\sim 5-10^3$), relevant to the bulk of molecular clouds in galaxies. Spitzer observations of H$_2$ rotational lines are complemented with H$_2$ data, including ro-vibrational line measurements, obtained with ground-based telescopes and ISO, to constrain the relative contributions of ultraviolet pumping and collisions to the H$_2$ excitation. The data analysis is supported by model calculations with the Meudon PDR code. The observed column densities of rotationally excited H$_2$ are observed to be much higher than PDR model predictions. In the lowest excitation PDRs, the discrepancy between the model and the data is about one order of magnitude for rotational levels $J \ge $3. We discuss whether an enhancement in the H$_2$ formation rate or a local increase in photoelectric heating, as proposed for brighter PDRs in former ISO studies, may improve the data-model comparison. We find that an enhancement in the H$_2$ formation rates reduces the discrepancy, but the models still fall short of the data. This large disagreement suggests that our understanding of the formation and excitation of H$_2$ and/or of PDRs energetics is still incomplete. We discuss several explanations, which could be further tested using the Herschel Space Telescope

astro-ph.GA↗

arXiv2010

Jet-Powered Molecular Hydrogen Emission from Radio Galaxies

H2 pure-rotational emission lines are detected from warm (100-1500 K) molecular gas in 17/55 (31% of) radio galaxies at redshift z<0.22 observed with the Spitzer IR Spectrograph. The summed H2 0-0 S(0)-S(3) line luminosities are L(H2)=7E38-2E42 erg/s, yielding warm H2 masses up to 2E10 Msun. These radio galaxies, of both FR radio morphological types, help to firmly establish the new class of radio-selected molecular hydrogen emission galaxies (radio MOHEGs). MOHEGs have extremely large H2 to 7.7 micron PAH emission ratios: L(H2)/L(PAH7.7) = 0.04-4, up to a factor 300 greater than the median value for normal star-forming galaxies. In spite of large H2 masses, MOHEGs appear to be inefficient at forming stars, perhaps because the molecular gas is kinematically unsettled and turbulent. Low-luminosity mid-IR continuum emission together with low-ionization emission line spectra indicate low-luminosity AGNs in all but 3 radio MOHEGs. The AGN X-ray emission measured with Chandra is not luminous enough to power the H2 emission from MOHEGs. Nearly all radio MOHEGs belong to clusters or close pairs, including 4 cool core clusters (Perseus, Hydra, A 2052, and A 2199). We suggest that the H2 in radio MOHEGs is delivered in galaxy collisions or cooling flows, then heated by radio jet feedback in the form of kinetic energy dissipation by shocks or cosmic rays.

astro-ph.CO↗

arXiv2010

Mid-infrared spectroscopy of Spitzer-selected ultra-luminous starbursts at z~2

We want to study the mid-infrared properties and the starburst and AGN contributions, of 24um sources at z~2, through analysis of mid-infrared spectra combined with millimeter, radio, and infrared photometry. Mid-infrared spectroscopy allows us to recover accurate redshifts. A complete sample of 16 Spitzer-selected sources (ULIRGs) believed to be starbursts at z~2 ("5.8um-peakers") was selected in the (0.5 sq.deg.) J1064+56 SWIRE Lockman Hole field. These sources have S(24um)>0.5mJy, a stellar emission peak redshifted to 5.8um, and r'(Vega)>23. The entire sample was observed with the low resolution units of the Spitzer/IRS infrared spectrograph. These sources have 1.2mm observations with IRAM 30m/MAMBO and very deep 20cm observations from the VLA. Nine of our sources also benefit from 350um observation and detection from CSO/SHARC-II. The entire sample shows good quality IRS spectra dominated by strong PAH features. The main PAH features at 6.2, 7.7, 8.6, and 11.3um have high S/N average luminosities of 2.90, 10.38, 3.62, and 2.29x10^{10}Lsun, respectively. We derived accurate redshifts spanning from 1.75 to 2.28. The average of these redshifts is 2.017. This result confirms that the selection criteria of "5.8um-peakers" associated with a strong detection at 24um are reliable to select sources at z~2. We have analyzed the different correlations between PAH emission and infrared, millimeter, and radio emission. Practically all our sources are strongly dominated by starburst emission. We have also defined two subsamples based on the equivalent width at 7.7um to investigate AGN contributions. Our sample contains strong starbursts and represents a particularly 24um-bright class of SMGs. The very good correlation between PAH and far-IR luminosities is now confirmed in high-z starburst ULIRGs. These sources show a small AGN contribution to the mid-IR, around ~20% in most cases.

astro-ph.CO↗

arXiv2010

LABOCA observations of giant molecular clouds in the south west region of the Small Magellanic Cloud

The amount of molecular gas is a key for understanding the future star formation in a galaxy. Because H2 is difficult to observe directly in dense and cold clouds, tracers like CO are used. However, at low metallicities especially, CO only traces the shielded interiors of the clouds. mm dust emission can be used as a tracer to unveil the total dense gas masses. The comparison of masses deduced from the continuum SIMBA 1.2 mm emission and virial masses in a sample of giant molecular clouds (GMCs), in the SW region of the Small Magellanic Cloud (SMC), showed a discrepancy that is in need of an explanation. This study aims at better assessing possible uncertainties on the dust emission observed in the sample of GMCs from the SMC and focuses on the densest parts of the GMCs where CO is detected. New observations were obtained with the LABOCA camera on the APEX telescope. All GMCs previously observed in CO are detected and their emission at 870microns is compared to ancillary data. The different contributions to the sub-mmm emission are estimated, as well as dust properties, in order to deduce molecular cloud masses precisely. The (sub-)mm emission observed in the GMCs in the SW region of the SMC is dominated by dust emission and masses are deduced for the part of each cloud where CO is detected and compared to the virial masses. The mass discrepancy between both methods is confirmed at 870microns with the LABOCA observations: the virial masses are on average 4 times smaller than the masses of dense gas deduced from dust emission, contrary to what is observed for equivalent clouds in our Galaxy. The origin of this mass discrepancy in the SMC remains unkown. The direct interpretation of this effect is that the CO linewidth used to compute virial masses do not measure the full velocity distribution of the gas. Geometrical effects and uncertainties on the dust properties are also discussed.

astro-ph.CO↗

arXiv2010

Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4

We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6$-$0.4 as well as in the molecular gas directly associated with that source. The OH+ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O+ lines remains lower than 1 in the same velocity range, and the H3O+ line shows only weak absorption. For LSR velocities between 7 and 50 kms$^{-1}$ we estimate total column densities of $N$(OH+) $> 2.5 \times 10^{14}$ cm$^{-2}$, $N$(H2O+) $\sim 6 \times 10^{13}$ cm$^{-2}$ and $N$(H3O+) $\sim 4.0 \times 10^{13}$ cm$^{-2}$. These detections confirm the role of O$^+$ and OH$^+$ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH+ by the H2O+ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.

astro-ph.GA↗

arXiv2009

Serendipity observations of far infrared cirrus emission in the Spitzer Infrared Nearby Galaxies Survey: Analysis of far-infrared correlations

We present an analysis of far-infrared dust emission from diffuse cirrus clouds. This study is based on serendipitous observations at 160 microns at high galactic latitude with the Multiband Imaging Photometer (MIPS) onboard the Spitzer Space Telescope by the Spitzer Infrared Nearby Galaxies Survey (SINGS). These observations are complemented with IRIS data at 100 and 60 microns and constitute one of the most sensitive and unbiased samples of far infrared observations at small scale of diffuse interstellar clouds. Outside regions dominated by the cosmic infrared background fluctuations, we observe a substantial scatter in the 160/100 colors from cirrus emission. We compared the 160/100 color variations to 60/100 colors in the same fields and find a trend of decreasing 60/100 with increasing 160/100. This trend can not be accounted for by current dust models by changing solely the interstellar radiation field. It requires a significant change of dust properties such as grain size distribution or emissivity or a mixing of clouds in different physical conditions along the line of sight. These variations are important as a potential confusing foreground for extragalactic studies.

astro-ph.GA↗

arXiv2008

Modeling of diffuse molecular gas applied to HD 102065 observations

Aims. We model a diffuse molecular cloud present along the line of sight to the star HD 102065. We compare our modeling with observations to test our understanding of physical conditions and chemistry in diffuse molecular clouds. Methods. We analyze an extensive set of spectroscopic observations which characterize the diffuse molecular cloud observed toward HD 102065. Absorption observations provide the extinction curve, H2, C I, CO, CH, and CH+ column densities and excitation. These data are complemented by observations of CII, CO and dust emission. Physical conditions are determined using the Meudon PDR model of UV illuminated gas. Results. We find that all observational results, except column densities of CH, CH+ and H2 in its excited (J > 2) levels, are consistent with a cloud model implying a Galactic radiation field (G~0.4 in Draine's unit), a density of 80 cm-3 and a temperature (60-80 K) set by the equilibrium between heating and cooling processes. To account for excited (J >2) H2 levels column densities, an additional component of warm (~ 250K) and dense (nH>10^4 cm-3) gas within 0.03 pc of the star would be required. This solution reproduces the observations only if the ortho-to-para H2 ratio at formation is 1. In view of the extreme physical conditions and the unsupported requirement on the ortho-to-para ratio, we conclude that H2 excitation is most likely to be accounted for by the presence of warm molecular gas within the diffuse cloud heated by the local dissipation of turbulent kinetic energy. This warm H2 is required to account for the CH+ column density. It could also contribute to the CH abundance and explain the inhomogeneity of the CO abundance indicated by the comparison of absorption and emission spectra.

astro-ph↗

arXiv2006

The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wavebands

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S3MC), which imaged the star-forming body of the Small Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to compile a photometric catalog of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: 1) bright 5.8 um sources with very faint optical counterparts and very red mid-infrared colors ([5.8]-[8.0]>1.2), which we identify as YSOs. 2) Bright mid-infrared sources with mildly red colors (0.16<[5.8]-[8.0]<0.6), identified as carbon stars. 3) Bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars. And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This excess is reminiscent of debris disks, and is detected in only a small fraction of these stars (<5%). The majority of the brightest infrared point sources in the SMC fall into groups one to three. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (~7%).

astro-ph↗

arXiv2006

Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) I: Overview and Initial Results

We are performing a uniform and unbiased, ~7x7 degrees imaging survey of the Large Magellanic Cloud (LMC), using the IRAC and MIPS instruments on board the Spitzer Space Telescope in order to survey the agents of a galaxy's evolution (SAGE), the interstellar medium (ISM) and stars in the LMC. The detection of diffuse ISM with column densities >1.2x10^21 H cm^-2 permits detailed studies of dust processes in the ISM. SAGE's point source sensitivity enables a complete census of newly formed stars with masses >3 solar masses that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass loss rates >1x10^-8 solar masses per year will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by three months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are non-proprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data with a special focus on the N79 and N83 region. The SAGE epoch 1 point source catalog has ~4 million sources. The point source counts are highest for the IRAC 3.6 microns band and decrease dramatically towards longer wavelengths consistent with the fact that stars dominate the point source catalogs and that the dusty objects, e.g. young stellar objects and dusty evolved stars that detected at the longer wavelengths, are rare in comparison. We outline a strategy for identifying foreground MW stars, that may comprise as much as 18% of the source list, and background galaxies, that may comprise ~12% of the source list.

astro-ph↗

arXiv2002

H2 formation and excitation in the diffuse interstellar medium

We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and excitation of H2 in the diffuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photon-illuminated gas. The data constrain well the nR product between gas density and H2 formation rate on dust grains: nR = 1 to 2.2 e-15 s-1. For each line of sight the mean effective H2 density n, assumed uniform, is obtained by the best fit of the model to the observed N(J=1)/N(J=0) ratio, since the radiation field is known. Combining n with the nR values, we find similar H2 formation rates for the three stars of about R = 4 e-17 cm3/s. Because the target stars do not interact with the absorbing matter we can show that the H2 excitation in the J>2 levels cannot be accounted for by the UV pumping of the cold H2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H2 is corroborated by the fact that the star with the largest column density of CH+ has the largest amount of warm H2.

astro-ph↗

arXiv2000

Detection of mid-infrared Aromatic Hydrocarbon Emission Features from the Small Magellanic Cloud

The mid-infrared (5-16 micron) spectral energy distribution for an individual quiescent molecular cloud in the Small Magellanic Cloud (SMC) was observed using ISOCAM. The spectrum is dominated by broad emission bands at 6.2, 7.7, and 11.3 microns, with weaker bands at 8.6 and 12.7 microns. As these are the same bands, with similar shape and relative strengths, as observed in the ISM of our Galaxy, the same carriers must exist in both galaxies. The carriers are widely consider to be large molecules or clusters of aromatic hydrocarbons, which absorb ultraviolet and visible photons and emit mid-infrared photons during high-temperature pulses. Based on the brightness of the mid-infrared emission and the estimated strength of the radiation field in the SMC, the absorption by aromatic hydrocarbons is of order 10% of total dust absorption, comparable to the case for Galactic dust. Ultraviolet observations of extinction of most SMC stars have shown that dust in the SMC does not absorb in the 2175 Angstrom feature that is so prominent in Milky Way extinction. If aromatic hydrocarbons and featureless extinction curves were ubiquitous in the SMC, then we would conclude that aromatic hydrocarbons are not the carriers of the 2175 Angstrom feature. However, SMC extinction curve measurements are biased toward hot, luminous stars, where aromatic hydrocarbons are destroyed, so that the absence of the 2175 Angstrom bump may not be typical of SMC dust. The presence of aromatic hydrocarbons in the SMC further demonstrate that these molecules exist even in an interstellar medium with an order-of-magnitude lower metallicity than in the disk of the Milky Way.

astro-ph↗

arXiv1996

Survey of Fine-Scale Structure in the Far-Infrared Milky Way

Using the IRAS Infrared Sky Survey Atlas, we have made 60 x 60 deg mosaics of the far-infrared emission in the Milky Way. By applying a median normalizing spatial filter, we were able to eliminate the strong gradient in brightness towards the Galactic midplane. The resulting images reveal a "froth" of superposed filaments, voids, and shells. This fine-scale structure extends all the way down to the Galactic midplane. Moreover, it scales in intensity with the smoothly varying background, independent of latitude, thus indicating that the fine-scale residual emission is co-extensive with the smooth background. We conclude that the fine-scale structure is not merely of local origin, but consists of both nearby and more distant features in the disk. Although we had expected to find morphological evidence for supernova-driven "worms" or "chimneys" rooted in the Galactic plane, our processing shows the FIR fine-scale structure to be more complex (e.g. less coherent and less rooted) as viewed in projection. Analysis of the spatial statistics shows that the FIR fine-scale structure is self-similar with a spatial power-law exponent of -3 and a fractal dimension of 2.5 --- similar behavior to that found in isolated cirrus and molecular clouds. On scales larger than 1.5 deg, the power-law exponent flattens to -2.5, perhaps indicating a change in the characteristic structure. This could be due to different dynamical inputs organizing the small and large-scale structures (e.g. turbulence and diffusion on small scales vs. macroscopic winds and shock fronts on larger scales).

astro-ph↗