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Register a new userProbing Galactic reddening with the 8620 A diffuse interstellar band
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Using the correlation between the equivalent width of the diffuse interstellar band (DIB) at 8620 A and the interstellar reddening reported by Munari (2000) GAIA could directly trace the interstellar extinction throughout the Galaxy. We checked for the magnitude and distance limitations of this method in 42 Galactic directions by simulating the RVS data on the stellar sample provided by the model of stellar population synthesis of the Galaxy (Robin et al. 2003). The simulation indicates that the imprint of the 8620 A DIB will be detected in the RVS spectra of stars with magnitudes up to V~16 with sufficient accuracy to trace not only the distribution of the interstellar medium but also the radial component of its kinematic motion, i.e. the radial velocity of the mass center of the dust cloud in the line of sight.
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We aim to make use of the measurements from the Giraffe Inner Bulge Survey (GIBS) and the Gaia$-$ESO survey (GES) to study the kinematics and distance of the carrier of DIB$,lambda$8620, as well as other properties. We successfully detected and measured DIB$,lambda$8620 in 760 of 4117 GES spectra. Combined with the DIBs measured in GIBS spectra, we confirmed a tight relation between EW and $E(J-K_{rm S})$ as well as $A_{rm V}$, with similar fitting coefficients to those found by previous works. With a more accurate sample and the consideration of the solar motion, the rest-frame wavelength of DIB$,lambda$8620 was redetermined as 8620.83 r{A}, with a mean fit error of 0.36 r{A}. We studied the kinematics of the DIB carriers by tracing their median radial velocities in each field in the local standard of rest ($V_{rm LSR}$) and into the galactocentric frame ($V_{rm GC}$), respectively, as a function of the Galactic longitudes. Based on the median $V_{rm LSR}$ and two Galactic rotation models, we obtained valid kinematic distances of the DIB carriers for nine GIBS and ten GES fields. We also found a linear relation between the DIB$,lambda$8620 measured in this work and the near-infrared DIB in APOGEE spectra at $1.5273,mu m$. We demonstrate that the DIB carriers can be located much closer to the observer than the background stars based on the following arguments: (i) qualitatively, the carriers occupy in the Galactic longitude$-$velocity diagram typical rotation velocities of stars in the local Galactic disk, while the background stars in the GIBS survey are mainly located in the Galactic bulge; (ii) quantitatively, all the derived kinematic distances of the DIB carriers are smaller than the median distances to background stars in each field.
We developed a set of procedures to automatically detect and measure the DIB around 8620 {AA} (the Gaia DIB) for a wide range of temperatures. The DIB profile is fit with a Gaussian function. Specifically, the DIB feature is extracted from the spectra of late-type stars by subtracting the corresponding synthetic spectra. For early-type stars we applied a specific model based on the Gaussian process that needs no prior knowledge of the stellar parameters. The method was tested on $sim$5000 spectra from the Giraffe Inner Bulge Survey (GIBS). After validation, we obtained 4194 reasonable fitting results from the GIBS database. An EW versus $E(J,{-},K_{rm S})$ relation is derived as $E(J,{-},K_{rm S}),{=},1.875,({pm},0.152),{times},{rm EW},{-},0.011,({pm},0.048)$, according to $E(B,{-},V)/{rm EW},{=},2.721$, which is highly consistent with previous results toward similar sightlines. After a correction based on the VVV database for both EW and reddening, the coefficient derived from individual GIBS fields, $E(J,{-},K_{rm S})/{rm EW},{=},1.884,{pm},0.225$, is also in perfect agreement with literature values. Based on a subsample of 1015 stars toward the Galactic center within $-3^{circ},{<},b,{<},3^{circ}$ and $-6^{circ},{<},l,{<},3^{circ}$, we determined a rest-frame wavelength of the Gaia DIB as 8620.55 {AA}. A Gaussian profile is proved to be a proper and stable assumption for the Gaia DIB as no intrinsic asymmetry is found.
The Sun lies in the middle of an enormous cavity of a million degree gas, known as the Local Bubble. The Local Bubble is surrounded by a wall of denser neutral and ionized gas. The Local Bubble extends around 100 pc in the plane of Galaxy and hundreds of parsecs vertically, but absorption-line surveys of neutral sodium and singly-ionized calcium have revealed a highly irregular structure and the presence of neutral clouds within an otherwise tenuous and hot gas. We have undertaken an all-sky, European-Iranian survey of the Local Bubble in the absorption of a number of diffuse interstellar bands (DIBs) to offer a novel view of our neighbourhood. Our dedicated campaigns with ESOs New Technology Telescope and the INGs Isaac Newton Telescope comprise high signal-to-noise, medium-resolution spectra, concentrating on the 5780 and 5797 AA bands which trace ionized/irradiated and neutral/shielded environments, respectively; their carriers are unknown but likely to be large carbonaceous molecules. With about 660 sightlines towards early-type stars distributed over distances up to about 200 pc, our data allow us to reconstruct the first ever 3D DIB map of the Local Bubble, which we present here. While we confirm our expectations that the 5780 AA DIB is relatively strong compared to the 5797 AA DIB in hot/irradiated regions such as which prevail within the Local Bubble and its walls, and the opposite is true for cooler/shielded regions beyond the confines of the Local Bubble, we unexpectedly also detect DIB cloudlets inside of the Local Bubble. These results reveal new insight into the structure of the Local Bubble, as well as helping constrain our understanding of the carriers of the DIBs.
We present new three-dimensional (3D) interstellar dust reddening maps of the Galactic plane in three colours, E(G-Ks), E(Bp-Rp) and E(H-Ks). The maps have a spatial angular resolution of 6 arcmin and covers over 7000 deg$^2$ of the Galactic plane for Galactic longitude 0 deg $<$ $l$ $<$ 360 deg and latitude $|b|$ $<$ $10$ deg. The maps are constructed from robust parallax estimates from the Gaia Data Release 2 (Gaia DR2) combined with the high-quality optical photometry from the Gaia DR2 and the infrared photometry from the 2MASS and WISE surveys. We estimate the colour excesses, E(G-Ks), E(Bp-Rp) and E(H-Ks), of over 56 million stars with the machine learning algorithm Random Forest regression, using a training data set constructed from the large-scale spectroscopic surveys LAMOST, SEGUE and APOGEE. The results reveal the large-scale dust distribution in the Galactic disk, showing a number of features consistent with the earlier studies. The Galactic dust disk is clearly warped and show complex structures possibly spatially associated with the Sagittarius, Local and Perseus arms. We also provide the empirical extinction coefficients for the Gaia photometry that can be used to convert the colour excesses presented here to the line-of-sight extinction values in the Gaia photometric bands.
The spectrum of any star viewed through a sufficient quantity of diffuse interstellar material reveals a number of absorption features collectively called diffuse interstellar bands (DIBs). The first DIBs were reported 90 years ago, and currently well over 500 are known. None of them has been convincingly identified with any specific element or molecule, although recent studies suggest that the DIB carriers are polyatomic molecules containing carbon. Most of the DIBs currently known are at visible and very near-infrared wavelengths, with only two previously known at wavelengths beyond one micron (10,000 Angstroms), the longer of which is at 1.318 microns. Here we report the discovery of thirteen diffuse interstellar bands in the 1.5-1.8 micron interval on high extinction sightlines toward stars in the Galactic centre. We argue that they originate almost entirely in the Galactic Centre region, a considerably warmer and harsher environment than where DIBs have been observed previously. The relative strengths of these DIBs toward the Galactic Centre and the Cygnus OB2 diffuse cloud are consistent with their strengths scaling mainly with extinction by diffuse material.
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