A Far-ultraviolet Fluorescent Molecular Hydrogen Emission Map of the Milky Way Galaxy

Transcription

1 A Far-ultraviolet Fluorescent Molecular Hydrogen Emission Map of the Milky Way Galaxy (The Astrophysical Journal Supplement Series, 231:21 (16pp), 2017 August) November 14, 2017 Young-Soo Jo Young-Soo Jo 1, Kwang-Il Seon 2,3, Kyoung-Wook Min 1, Jerry Edelstein 4 and Wonyong Han 2 1 Korea Advanced Institute of Science and Technology (KAIST), Korea 2 Korea Astronomy and Space Science Institute (KASI), Korea 3 Astronomy and Space Science Major, Korea University of Science and Technology, Korea 4 Space Science Laboratory, University of California, Berkeley, CA, USA

2 1. Introduction: Molecular Hydrogen (H 2 ) Interstellar Medium (ISM) - 99% gas à 75% hydrogen + 25% helium + some molecules - 1% dust à metals + graphites (C) + silicates (Si) Interstellar hydrogen - 60% atomic - 23% ionized - 17% molecular Molecular hydrogen (H 2 ) - the most abundant molecule in the universe - an important tracer of star-forming regions or molecular clouds - it is difficult to observe H 2 directly because of the absence of a permanent electric dipole moment How to infer the amount of H 2 1. gas-to-dust ratio (GDR) : án(h I + H 2 )/E(B-V )ñ = atoms cm -2 mag -1 (Bohlin et al ; Rachford et al ) à from the observations of a finite number of sight lines. 2. CO-to-H 2 conversion factor (X CO ) : X CO = (1.8 ± 0.3) cm -2 K -1 km -1 s. (Dame et al ) à X CO can vary depending on the local conditions of the ISM, e.g., the cloud density and excitation temperature. 3. Ideally, direct observations is the best way.

3 1. Introduction: FUV fluorescent H 2 emission FUV fluorescent H 2 emission - H 2 in the ground state absorb FUV photons of λ > 912 Å. à electronically excited states (B 1 Σ + u, C 1 Π u ) - ~10% of the excited H 2 dissociate - ~90% de-excite to the ground electronic state (X 1 Σ + g) à FUV emission lines at Å ~10% electron Bound-free continuum emission à dissociation (FUV emission) - Transitions within the vibrational-rotational energy levels of the ground electronic state à IR emission lines (quadrupole transition lines) - No NIR or FUV observational studies of fluorescent H 2 emission lines over the entire sky. ~90% electron Bound-bound emission lines (FUV emission) time-scale ~10-8 s Energy levels of molecular hydrogen (Pak et al. 2003) (Draine 2011)

4 1. Introduction: Objectives of the present study Main objectives - Construction of H 2 fluorescence emission map of a substantial fraction of the sky obtained with the FIMS - Construction of all-sky N(H 2 ) map by applying the photodissociation region (PDR) model - Estimation of X CO and GDR for the diffuse ISM in the Milky Way Galaxy

5 2. Observations and Construction of the Map: 2.1. Construction of 3-Dimensional Data Cube FIMS (Far-Ultraviolet Imaging Spectrograph) data - Main payload on the first Korean scientific satellite, STSAT-1 - Spectral imaging survey of diffuse FUV radiation from the ISM (spectral resolution: λ/δλ 550, imaging resolution 5 ) - Most of the L-band ( Å) data were recovered using an elaborate attitude correction procedure. - Finally, we constructed a three-dimensional (3D) L-band data cube covering ~86% of the sky. à spectra with 340 wavelength bins in each of 49,152 spatial pixels (N side = 64 corresponding to ~55 ) FIMS all-sky L-band FUV continuum map (~86% of the sky) Exposure time-weighted FIMS L-band spectrum.

6 2. Observations and Construction of the Map: 2.2. Construction of the Diffuse Fluorescent H 2 Emission Map FIMS (Far-Ultraviolet Imaging Spectrograph) data - The original data cube was rebinned to a larger wavelength bin size of 3 Å to increase the SNR. - The radius of the smoothing circle for each pixel was adaptively increased from 2 up to 15 in steps of 1 until the SNR per spectral bin was >15. - The continuum spectrum was defined as line segments that connect local minima in the coarser spectral bins of 20 Å and was linearly interpolated at the original spectral bins and then smoothed by a boxcar ten spectral bins wide. - The total intensity of the H 2 fluorescence emission was obtained by integrating the emission line spectrum over the two atomic-line-free regions ( and Å) An example of the measured spectrum and the constructed continuum spectrum for the Galactic coordinates (l, b)~(0, 15 ) FUV H 2 fluorescence emission map, in line units

7 2. Observations and Construction of the Map: 2.3. Comparison of the H 2 Fluorescent Emission Map with Other All-sky Maps (2/3) Other all-sky maps - Interstellar dust extinction, H I, and CO, which may be closely related to H 2 - Hα map ( the UV radiation field also ionizes H to produce Hα emission) - All four maps were smoothed using the smoothing radius map as was done for the H 2 fluorescence map. - Regions where the SNR < 5.0 were excluded from the Planck CO map.

8 2. Observations and Construction of the Map: 2.3. Comparison of the H 2 Fluorescent Emission Map with Other All-sky Maps (2/3) Before correction for the dust extinction - In general, the H 2 fluorescence intensity is proportional to the intensities of E(B V), N(H I), and Hα, but not that of CO, in low-intensity regions (i.e., optically thin, high latitudes) - It becomes saturated and shows large scatter in optically thick regions. - It also correlated with the Hα emission because they have common radiation sources of O- and B-type stars. - CO intensity does not correlate with FUV fluorescent H 2 intensity because the CO map shows that nonzero signals occur only near the Galactic plane where FUV photons are extinguished quickly.

9 2. Observations and Construction of the Map: 2.3. Comparison of the H 2 Fluorescent Emission Map with Other All-sky Maps (3/3) After correction for the dust extinction - We assumed that the emission sources and the interstellar dust are uniformly mixed. à The extinction corrected intensity: I ⅹτ / (1-e -τ ), where I is the observed intensity. - A strong correlation with the H 2 emission (E(B-V) and N(H I) especially strong) - The large scatter in (c) by the different mechanisms that produce the H 2 fluorescence emission and the Hα emission (Hα from ionized hydrogen ) (H 2 emission from cold and dense clouds) - A strong correlation between the extinctioncorrected H 2 fluorescence emission and CO emission (large scatter due to the sparsity of the CO map)

10 3. Discussion: 3.1. Photodissociation Region Modeling (1/3) Photodissociation Region (PDR) Modeling - We modeled the H 2 fluorescence-emitting regions as a PDR using the plane-parallel PDR code CLOUD (Black & van Dishoeck 1987; van Dishoeck & Black 1986) to investigate the spatial distribution of H 2 in the Milky Way. - The main input parameters of the CLOUD code were (1) the sticking probability and formation efficiency of H 2 on dust grains (y F ) (Black & Dalgarno 1973a,b, 1976) (2) hydrogen density (n H ) (3) cloud temperature (T) (4) strength of incident UV radiation (I UV ) in units of the average UV radiation field of Draine (1978) (5) N(H 2 ) - The three least-important parameters were fixed at typical values of y F =1, n H =10 cm -3, and T=100 K. (p/k = 1000 cm -3 K) - The TD1 star catalog (Thompson et al. 1978) and the Hipparcos star catalog (Perryman et al. 1997) were used to calculate stellar luminosities in 3D space. Radial profiles of (a) the intensity of the radiation field, I UV, and (b) gas density along the line of sight with (l, b)~(0, 15 ) All-sky map of the effective radiation field strength I UV

11 3. Discussion: 3.1. Photodissociation Region Modeling (2/3) An example of the model - Red dashed line is the best-fit PDR model spectrum with N(H 2 ) = cm -2, I UV =10 0.4, and a reduced χ 2 = The all-sky N(H 2 ) map - The all-sky N(H 2 ) map is very similar to the FUV fluorescent H 2 emission map. An example of the model fit for the line of sight of (l, b)~(0, 15 ) - However, the N(H 2 ) ranges from to cm -2 while the H 2 intensity ranges from to CU. à It is caused by the fraction of H 2 in the Milky Way (f H2 ).

12 3. Discussion: 3.1. Photodissociation Region Modeling (3/3) The fraction of H 2 in the Milky Way - The fraction of H 2 is defined as f H2 = 2N(H 2 ) / [2N(H 2 ) + N(H I))] - At optically thin regions where E(B V) < 0.1, f H2 < 1%. - f H2 gradually increases as E(B V) increases, i.e., f H2 is ~10% when E(B V) = 1 and ~50% when E(B V) = 5 in the Galactic plane - At optically thick regions, UV radiation is strongly attenuated à shielding the H 2 molecules from the photodissociation effect à allowing f H2 to increase with increasing E(B V) à The H 2 formation rate via dust grain catalysis will be more active in denser dust clouds. (a) All-sky map of the H 2 fraction f H2. (b) Scatterplot of f H2 as a function of E(B V). (c) N(H2) vs. E(B V).

13 3. Discussion: 3.2. Calculation of GDR gas-to-dust ratio (GDR) - Standard value of GDR án(h I+H 2 )/E(B-V )ñ= atoms cm -2 mag -1 (Bohlin et al ) à Copernicus observations of 75 stars (Rachford et al ) à FUSE observations of 38 lines of sight - GDR derived in the present study Median value = atoms cm -2 mag -1 average value = atoms cm -2 mag -1 - The scatter in the derived GDR à due to the simple modeling à due to the intrinsic variation: (Bohlin et al ) (Rachford et al ) The GDR depends on the local conditions in the ISM. The interstellar dust is more concentrated in the Galactic plane than is hydrogen gas. (a) All-sky map of GDR. (b) Scatterplot of N(H tot ) as a function of E(B V).

14 4. Summary We presented the first all-sky map of FUV fluorescent H 2 emission of the Milky Way Galaxy, covering ~76% of the sky. After correcting for dust extinction, the H 2 fluorescence emission clearly correlates with E(B V), N(H I), Hα, and CO. The spatial distribution of H 2 (N(H 2 )) from a simple plane-parallel PDR model with a uniform pressure of p/k = 1000 cm -3 K, and the interstellar radiation field from the luminosities of the UV stars. According to the model, N(H 2 ) ranges from to cm -2. The fraction of H 2 : f H2 = 2N(H 2 ) / [2N(H 2 ) + N(H I))] - f H2 < 1% at optically thin regions when E(B V) < f H2 ~ 50% when E(B V) = 5 in the Galactic plane (strong self-shielding by H 2 and active H 2 formation) The GDR : án(h I+H 2 )/E(B-V )ñ - average value = atoms cm -2 mag -1 à consistent with the standard value of atoms cm -2 mag -1 - However, GDR depends on the local conditions of the ISM and increases as E(B V) decreases. Thank you