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 Jo1, Kwang-Il Seon2,3, Kyoung-Wook Min1, Jerry Edelstein4 and Wonyong Han2
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

1. Introduction: Molecular Hydrogen (H2)▶ 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 (H2) ◀
- the most abundant molecule in the universe- an important tracer of star-forming regions or molecular clouds- it is difficult to observe H2 directly
because of the absence of a permanent electric dipole moment▶ How to infer the amount of H2 ◀
1. gas-to-dust ratio (GDR) :áN(H I + H2)/E(B-V )ñ = 5.8 ´ 1021 atoms cm-2 mag-1(Bohlin et al. 1978 ; Rachford et al. 2009 )à from the observations of a finite number of sight lines.
2. CO-to-H2 conversion factor (XCO) :XCO = (1.8 ± 0.3) × 1020 cm-2 K-1 km-1 s.
(Dame et al. 2001 )à XCO 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.

1. Introduction: FUV fluorescent H2 emission▶ FUV fluorescent H2 emission ◀
- H2 in the ground state absorb FUV photons of λ > 912 Å.
à electronically excited states (B1Σ+u, C1Πu)- ~10% of the excited H2 dissociate- ~90% de-excite to the ground electronic state (X1Σ+g)
à FUV emission lines at 912 – 1700 Å~10% electron
Bound-freecontinuum emission
à dissociation(FUV emission)- Transitions within the vibrational-rotational energy levels of the groundelectronic stateà IR emission lines (quadrupole transition lines)- No NIR or FUV observational studies of fluorescent H2 emission lines overthe entire sky.

~90% electron
Bound-boundemission lines(FUV emission)
time-scale ~10-8 s

Energy levels of molecular hydrogen(Pak et al. 2003)(Draine 2011)

1. Introduction: Objectives of the present study▶ Main objectives ◀
- Construction of H2 fluorescence emission mapof a substantial fraction of the sky obtained with the FIMS
- Construction of all-sky N(H2) map
by applying the photodissociation region (PDR) model- Estimation of XCO and GDR
for the diffuse ISM in the Milky Way Galaxy

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 (1370–1710 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 (Nside = 64 corresponding to ~55’)

FIMS all-sky L-band FUV continuum map(~86% of the sky)

Exposure time-weighted FIMS L-band spectrum.

2. Observations and Construction of the Map:

2.2. Construction of the Diffuse Fluorescent H2 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 Åand was linearly interpolated at the original spectral bins and then smoothed by a boxcar ten spectral bins wide.
- The total intensity of the H2 fluorescence emission was obtained by integrating the emission line spectrum over thetwo atomic-line-free regions (1450–1525 and 1560–1630 Å)

An example of the measured spectrum and the constructed
continuum spectrum for the Galactic coordinates (l, b)~(0°, 15°)

FUV H2 fluorescence emission map, in line units

2. Observations and Construction of the Map:

2.3. Comparison of the H2 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 H2- 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 H2 fluorescence map.
- Regions where the SNR < 5.0 were excluded from the Planck CO map.

2. Observations and Construction of the Map:

2.3. Comparison of the H2 Fluorescent Emission Map with Other All-sky Maps (2/3)▶ Before correction for the dust extinction ◀
- In general, the H2 fluorescence intensity
is proportional to the intensities ofE(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 scatterin 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 H2 intensity because the CO map
shows that nonzero signals occur only near
the Galactic plane where FUV photons areextinguished quickly.

2. Observations and Construction of the Map:

2.3. Comparison of the H2 Fluorescent Emission Map with Other All-sky Maps (3/3)▶ After correction for the dust extinction ◀
- We assumed that the emission sources andthe 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 H2 emission(E(B-V) and N(H I) especially strong)- The large scatter in (c) by the different
mechanisms that produce the H2fluorescence emission and the Hα emission(Hα from ionized hydrogen )(H2 emission from cold and dense clouds)- A strong correlation between the extinctioncorrected H2 fluorescence emission and COemission(large scatter due to the sparsity of the COmap)

3. Discussion: 3.1. Photodissociation Region Modeling (1/3)▶ Photodissociation Region (PDR) Modeling ◀
- We modeled the H2 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 H2 in the Milky Way.
- The main input parameters of the CLOUD code were
(1) the sticking probability and formation efficiency of H2 on dust grains (yF) (Black & Dalgarno 1973a,b, 1976)(2) hydrogen density (nH)(3) cloud temperature (T)(4) strength of incident UV radiation (IUV) in units of the average UV radiation field of Draine (1978)(5) N(H2)- The three least-important parameters were fixed at typical values of yF =1, nH=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 tocalculate stellar luminosities in 3D space.

Radial profiles of (a) the intensity of the radiation field, IUV,and (b) gas density along the line of sight with (l, b)~(0°, 15°)

All-sky map of the effective radiation field strength IUV3. Discussion: 3.1. Photodissociation Region Modeling (2/3)▶ An example of the model ◀
- Red dashed line is the best-fit PDR modelspectrum with N(H2) = 1020.1 cm-2, IUV=100.4, and a reduced χ2 = 1.58.
▶ The all-sky N(H2) map ◀
- The all-sky N(H2) map is very similar to theFUV fluorescent H2 emission map.

An example of the model fit for the line of sight of (l, b)~(0°, 15°)

- However, the N(H2) ranges from 1016 to1023 cm-2 while the H2 intensity ranges
from 103.5 to 106.4 CU.
à It is caused by the fraction of H2 in theMilky Way (fH2 ).

3. Discussion: 3.1. Photodissociation Region Modeling (3/3)▶ The fraction of H2 in the Milky Way ◀
- The fraction of H2 is defined as fH2 = 2N(H2) / [2N(H2) + N(H I))]
- At optically thin regions where E(B–V) < 0.1, fH2 < 1%.
- fH2 gradually increases as E(B–V) increases,i.e., fH2 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 attenuatedà shielding the H2 molecules from the photodissociation effectà allowing fH2 to increase with increasing E(B–V)
à The H2 formation rate via dust grain catalysis will be more active in denser dust clouds.

(a) All-sky map of the H2 fraction fH2. (b) Scatterplot of fH2 as a function of E(B–V). (c) N(H2) vs. E(B–V).

3. Discussion: 3.2. Calculation of GDR▶ gas-to-dust ratio (GDR) ◀
- Standard value of GDR
áN(H I+H2)/E(B-V )ñ=5.8´1021 atoms cm-2 mag-1(Bohlin et al. 1978 )à Copernicus observations of 75 stars(Rachford et al. 2009 )à FUSE observations of 38 lines of sight- GDR derived in the present study
Median value = 7.5´1021 atoms cm-2 mag-1
average value = 5.1´1021 atoms cm-2 mag-1- The scatter in the derived GDR
à due to the simple modelingà due to the intrinsic variation:(Bohlin et al. 1978 )(Rachford et al. 2009 )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(Htot) as a function of E(B–V).

4. Summary▶ We presented the first all-sky map of FUV fluorescent H2 emission of the Milky Way Galaxy, covering ~76% of the sky.
▶ After correcting for dust extinction, the H2 fluorescence emission clearly correlates with E(B–V), N(H I), Hα, and CO.
▶ The spatial distribution of H2 (N(H2)) from a simple plane-parallel PDR modelwith 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(H2) ranges from 1016 to 1023 cm-2.
▶ The fraction of H2 : fH2 = 2N(H2) / [2N(H2) + N(H I))]
- fH2 < 1% at optically thin regions when E(B–V) < 0.1- fH2 ~ 50% when E(B–V) = 5 in the Galactic plane (strong self-shielding by H2 and active H2 formation)▶ The GDR : áN(H I+H2)/E(B-V )ñ
- average value = 5.1´1021 atoms cm-2 mag-1 à consistent with the standard value of 5.8´1021 atoms cm-2 mag-1- However, GDR depends on the local conditions of the ISM and increases as E(B–V) decreases.

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