Local UV Radiation Field Research Articles

ANOMALOUS DIFFUSE INTERSTELLAR BANDS IN THE SPECTRUM OF HERSCHEL 36. I. OBSERVATIONS OF ROTATIONALLY EXCITED CH AND CH+ABSORPTION AND STRONG, EXTENDED REDWARD WINGS ON SEVERAL DIBs

Anomalously broad diffuse interstellar bands (DIBs) at 5780.5, 5797.1, 6196.0, and 6613.6 A are found in absorption along the line of sight to Herschel 36, the star illuminating the bright Hourglass region of the H II region Messier 8. Interstellar absorption from excited CH+ in the J=1 level and from excited CH in the J=3/2 level is also seen. To our knowledge, neither those excited molecular lines nor such strongly extended DIBs have previously been seen in absorption from interstellar gas. These unusual features appear to arise in a small region near Herschel 36 which contains most of the neutral interstellar material in the sight line. The CH+ and CH in that region are radiatively excited by strong far-IR radiation from the adjacent infrared source Her 36 SE. Similarly, the broadening of the DIBs toward Herschel 36 may be due to radiative pumping of closely spaced high-J rotational levels of relatively small, polar carrier molecules. If this picture of excited rotational states for the DIB carriers is correct and applicable to most DIBs, the 2.7 degree cosmic microwave background may set the minimum widths (about 0.35 A) of known DIBs, with molecular processes and/or local radiation fields producing the larger widths found for the broader DIBs. Despite the intense local UV radiation field within the cluster NGC 6530, no previously undetected DIBs stronger than 10 mA in equivalent width are found in the optical spectrum of Herschel 36, suggesting that neither dissociation nor ionization of the carriers of the known DIBs by this intense field creates new carriers with easily detectable DIB-like features. Possibly related profile anomalies for several other DIBs are noted.

THE CIRCUMGALACTIC MEDIUM OF MASSIVE GALAXIES ATz∼ 3: A TEST FOR STELLAR FEEDBACK, GALACTIC OUTFLOWS, AND COLD STREAMS

We present new results on the kinematics, thermal and ionization state, and spatial distribution of metal-enriched gas in the circumgalactic medium (CGM) of massive galaxies at redshift ∼3, using the Eris suite of cosmological hydrodynamic "zoom-in" simulations. The reference run adopts a blastwave scheme for supernova feedback that produces large-scale galactic outflows, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, and a model for the diffusion of metals and thermal energy. The effect of the local UV radiation field is added in post-processing. The CGM (defined as all gas at R > 0.2 Rvir = 10 kpc, where Rvir is the virial radius) contains multiple phases having a wide range of physical conditions, with more than half of its heavy elements locked in a warm-hot component at T > 105 K. Synthetic spectra, generated by drawing sightlines through the CGM, produce interstellar absorption-line strengths of Lyα, C ii, C iv, Si ii, and Si iv as a function of the galactocentric impact parameter (scaled to the virial radius) that are in broad agreement with those observed at high redshift by Steidel et al. The covering factor of absorbing material declines less rapidly with impact parameter for Lyα and C iv compared to C ii, Si iv, and Si ii, with Lyα remaining strong (WLyα > 300 mÅ) to ≳ 5 Rvir = 250 kpc. Only about one third of all the gas within Rvir is outflowing. The fraction of sightlines within one virial radius that intercept optically thick, NH I>1017.2 cm-2 material is 27%, in agreement with recent observations by Rudie et al. Such optically thick absorption is shown to trace inflowing "cold" streams that penetrate deep inside the virial radius. The streams, enriched to metallicities above 0.01 solar by previous episodes of star formation in the main host and in nearby dwarfs, are the origin of strong (NC II>1013 cm-2) C ii absorption with a covering factor of 22% within Rvir and 10% within 2 Rvir. Galactic outflows do not cause any substantial suppression of the cold accretion mode. The central galaxy is surrounded by a large O vi halo, with a typical column density NO VI ≳ 1014 cm-2 and a near unity covering factor maintained all the way out to 150 kpc. This matches the trends recently observed in star-forming galaxies at low redshift by Tumlinson et al. Our zoom-in simulations of this single system appear then to reproduce quantitatively the complex baryonic processes that determine the exchange of matter, energy, and metals between galaxies and their surroundings.

THE VIRUS-P EXPLORATION OF NEARBY GALAXIES (VENGA): THEXCOGRADIENT IN NGC 628

We measure the radial profile of the 12CO(1-0) to H_2 conversion factor (Xco) in NGC 628. The H\alpha emission from the VENGA integral field spectroscopy is used to map the star formation rate surface density (\Sigma_{SFR}). We estimate the molecular gas surface density (\Sigma_{H2}) from \Sigma_{SFR} by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure Xco. We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (H\alpha vs. far-UV plus 24\mu m), and CO maps from different telescopes (single-dish and interferometers). The observed Xco profile is robust against these systematics, drops by a factor of 2 from R~7 kpc to the center of the galaxy, and is well fit by a gradient \Delta log(Xco)=0.06\pm0.02 dex kpc^-1. We study how changes in Xco follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in Xco can be explained by a combination of decreasing metallicity, and decreasing \Sigma_{H2} with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of Xco in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single Xco value when large changes in gas surface density or metallicity are present.

Rotationally Excited H2 in the Magellanic Clouds

AbstractWe have performed a survey study of rotational excited-state H2 Lyman-Werner absorption lines in the entire FUSE Magellanic Clouds Legacy archive. These lines reflect the UV pumping and formation conditions of H2, enabling a more comprehensive study of H2 gas properties, e.g. J-level populations N(J) and b-values (generally indicating the velocity dispersion). Combining with our previous measurements of N(Hi) and N(H2), we derived H2 excitation temperatures, gas volume density n(H), and local UV radiation field strength IUV for each sight line. The results indicate a weaker correlation between n(H) and IUV in Magellanic Clouds than the Galactic sight lines. We also obtained N(H)/E(B − V) ratios from the Spitzer-SAGE and previous CO J = 1 − 0 / Hi 21 cm surveys at sight line locations, using dust modeling and standard line brightness-column density conversion factors. They show a roughly linear correlation with absorption-based N(H)/E(B − V) values, and have a similar scatter (∼0.7 dex) across the LMC and SMC.

Evaporating very small grains as tracers of the UV radiation field in photo-dissociation regions

Context. In photo-dissociation regions (PDRs), Polycyclic Aromatic Hydrocarbons (PAHs) could be produced by evaporation of Very Small Grains (VSGs) by the impinging UV radiation field from a nearby star. Aims. We investigate quantitatively the transition zone between evaporating Very Small Grains (eVSGs) and PAHs in several PDRs. Methods. We study the relative contribution of PAHs and eVSGs to the mid-IR emission in a wide range of excitation conditions. We fit the observed mid-IR emission of PDRs by using a set of template band emission spectra of PAHs, eVSGs and gas lines. The fitting tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the community as an IDL routine. From the results of the fit, we derive the fraction of carbon f_eVSG locked in eVSGs and compare it to the intensity of the local UV radiation field. Results. We show a clear decrease of f_eVSG with increasing intensity of the local UV radiation field, which supports the scenario of photo-destruction of eVSGs. Conversely, this dependence can be used to quantify the intensity of the UV radiation field for different PDRs, including non resolved ones. Conclusions. PAHTAT can be used to trace the intensity of the local UV radiation field in regions where eVSGs evaporate, which correspond to relatively dense (nH = [100, 10^5 ] cm-3) and UV irradiated PDRs (G0 = [100, 5x10^4]) where H2 emits in rotational lines.

The Large Magellanic Cloud: diffuse interstellar bands, atomic lines and the local environmental conditions

The Large Magellanic Cloud (LMC) offers a unique laboratory to study the diffuse interstellar bands (DIBs) under conditions that are profoundly different from those in the Galaxy. DIB carrier abundances depend on several environmental factors, in particular the local UV radiation field. In this paper we present measurements of twelve DIBs in five lines of sight to early-type stars in the LMC, including the 30 Doradus region. From the high resolution spectra obtained with VLT/UVES we also derive environmental parameters that characterise the local interstellar medium (ISM) in the probed LMC clouds. These include the column density components (including total column density) for the atomic resonance lines of Na I, Ca II, Ti II, K I. In addition, we derive the H I column density from 21 cm line profiles, the total-to-selective visual extinction RV and the gas-to-dust ratio N(H I)/A_V. Furthermore, from atomic line ratios we derive the ionisation balance and relative UV field strength in these environments. We discuss the properties of the LMC ISM in the context of DIB carrier formation. The behaviour of DIBs in the LMC is compared to that of DIBs in different local environmental conditions in the Milky Way. A key result is that in most cases the diffuse band strengths are weak (up to factor 5) with respect to Galactic lines of sight of comparable reddening, EB-V. In the line of sight towards Sk -69 223 the 5780 and 5797 �DIBs are very similar in strength and profile to those observed towards HD 144217 , which is typical of an environment exposed to a strong

Molecular Hydrogen in the Damped Lyα Absorber of Q1331+170

We used HST STIS to obtain the spectrum of molecular hydrogen associated with the damped Lyα system at zabs = 1.7765 toward the quasar Q1331+170 at zem = 2.084. Strong H2 absorption was detected, with a total H2 column density of N(H2) = (4.45 ± 0.36) × 1019 cm-2. The molecular hydrogen fraction is f = 2N/(N + 2N) = 5.6% ± 0.7%, which is the greatest value reported so far in any redshifted damped Lyα system. This results from the combined effect of a relatively high dust-to-gas ratio, a low gas temperature, and an extremely low ambient UV radiation field. Based on the observed population of J states, we estimate the photoabsorption rate to be Rabs = (7.6 ± 2.4) × 10-13 s-1, corresponding to a local UV radiation field of J(1000 A) ≈ 2.1 × 10-3J1000 A, ☉, where J1000 A, ☉ is the UV intensity at 1000 A in the solar neighborhood. This is comparable to the metagalactic UV background intensity at this redshift and implies an extremely low star formation rate in the absorber's environment. We construct a simple model to describe the structure of the H2 absorber, with a best-fit total hydrogen number density of n(H) ≈ 0.2 cm-3 and an electron temperature of Te ≈ 140 K. Assuming spherical symmetry, the mass of the H2 cloud is estimated to be ≈6.5 × 107 M☉, larger than the masses of most giant molecular clouds in the Milky Way and nearby galaxies. The extinction of Q1331+170 due to the intervening DLA is EB-V ≈ 0.037, and we also find that the extinction by DLAs with firm H2 detections is significantly greater than those for which only upper limits of f have been determined. The observed CO-to-H2 column density ratio is NCO/N < 2.5 × 10-7, which is similar to the value measured for diffuse molecular clouds in the Galactic ISM. Finally, applying the inferred physical conditions to the observed C I fine structure excitation, we estimate the cosmic microwave background temperature to be TCMB = (7.2 ± 0.8) K at z = 1.77654, consistent with the predicted value of 7.566 K from the standard cosmology.

Detection of molecular hydrogen in a near Solar-metallicity damped Lyman-α system at $z_\mathsf{abs}$ ≈ 2 toward Q 0551-366

We report the detection of H_2, CI, CI*, CI** and Cl I lines in a near Solar-metallicity ([Zn/H]=-0.13) damped Lyman-alpha (DLA) system at z_abs=1.962 observed on the line of sight to the quasar Q 0551-366. The iron-peak elements, X=Fe, Cr and Mn are depleted compared to zinc, [X/Zn] -0.8, probably because they are tied up onto dust grains. Among the three detected H_2-bearing clouds, spanning 55 km s^-1 in velocity space, we derive a total molecular hydrogen column density N(H_2)=2.6x10^17 cm^-2 and a mean molecular fraction f=2N(H_2)/(2N(H_2)+N(HI))=1.7x10^-3. The depletion of heavy elements (S, Si, Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observed in the diffuse neutral gas of the Galactic halo. This depletion is approximately the same in the six CI-detected components independently of the presence or absence of H_2. The gas clouds in which H_2 is detected always have large densities, n_H>30 cm^-3, and low temperatures, T_01<~100 K. This shows that presence of dust, high particle density and/or low temperature are required for molecules to be present. The photo-dissociation rate derived in the components where H_2 is detected suggests the existence of a local UV radiation field similar in strength to the one in the Galaxy. Star formation therefore probably occurs near these H_2-bearing clouds.

Detection of molecular hydrogen in a near Solar-metallicity damped Lyman-α system at $z_\mathsf{abs}$ ≈ 2 toward Q 0551-366

We report the detection of H 2 , C I , C I * , C I * * and Cl 1 lines in a near Solar-metallicity ([Zn/H] = -0.13) damped Lyman-a (DLA) system at z a b s = 1.962 observed on the line of sight to the quasar Q0551-366. The iron-peak elements, X = Fe, Cr and Mn are depleted compared to zinc, [X/Zn] ∼ -0.8, probably because they are tied up onto dust grains. Among the three detected H 2 -bearing clouds, spanning 55 km s - 1 in velocity space, we derive a total molecular hydrogen column density N(H 2 ) = 2.6 × 10 1 7 cm - 2 and a mean molecular fraction f = 2N(H 2 )/(2N(H 2 ) + N(H I )) = 1.7 x 10 - 3 . The depletion of heavy elements (S, Si, Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observed in the diffuse neutral gas of the Galactic halo. This depletion is approximately the same in the six C i-detected components independently of the presence or absence of H 2 . The gas clouds in which H 2 is detected always have large densities, n H > 30 cm - 3 , and low temperatures, T 0 1 < 100 K. This shows that presence of dust, high particle density and/or low temperature is required for molecules to be present. The photo-dissociation rate derived in the components where H 2 is detected suggests the existence of a local UV radiation field similar in strength to the one in the Galaxy. Star formation therefore probably occurs near these H 2 -bearing clouds.

Observations of [Cii] 158 Micron Line and Far‐Infrared Continuum Emission toward the High‐Latitude Molecular Clouds in Ursa Major

We report the results of a rocket-borne observation of the [C II] 158 μm line and far-infrared continuum emission at 152.5 μm toward the high-latitude molecular clouds in Ursa Major. We also present the results of a follow-up observation of the millimeter 12CO J = 1 → 0 line over a selected region observed by the rocket-borne experiment. We have discovered three small CO cloudlets from the follow-up 12CO observations. We show that these molecular cloudlets, as well as the MBM clouds (MBM 27, 28, 29, and 30), are not gravitationally bound. Magnetic pressure and turbulent pressure dominate the dynamic balance of the clouds. After removing the H I-correlated and background contributions, we find that the [C II] emission peak is displaced from the 152.5 μm and CO peaks, while the 152.5 μm continuum emission is spatially correlated with the CO emission. We interpret this behavior by attributing the origin of the [C II] emission to the photodissociation regions around the molecular clouds illuminated by the local UV radiation field. We also find that the ratio of the molecular hydrogen column density to the velocity-integrated CO intensity is 1.19 ± 0.29 × 1020 cm-2 (K km s-1)-1, from the FIR continuum and the CO data. The average [C II]/FIR intensity ratio over the MBM clouds is 0.0071, which is close to the all-sky average of 0.0082 reported by FIRAS on the COBE satellite. The average [C II]/CO ratio over the same regions is 420, significantly lower than in molecular clouds in the Galactic plane.