Evolution of Molecular and Atomic Gas Phases in the Milky Way

Abstract

We analyze radial and azimuthal variations of the phase balance between the molecular and atomic interstellar medium (ISM) in the Milky Way (MW) using archival CO( J=1-0) and HI 21 cm data. In particular, the azimuthal variations-between the spiral arm and interarm regions-are analyzed without any explicit definition of the spiral arm locations. We show that the molecular gas mass fraction, i.e.,, varies predominantly in the radial direction: starting from~100% at the center, remaining 50% toR 6 kpc and decreasing to ∼10%-20% at = R 8.5 kpc when averaged over the whole disk thickness (from ∼100% to ≳60%, then to ∼50% in the midplane). Azimuthal, arm-interarm variations are secondary: only~20% in the globally molecule-dominated inner MW, but becoming larger, ∼40%-50%, in the atom-dominated outskirts. This suggests that in the inner MW the gas remains highly molecular ( > f 50% mol ) as it moves from an interarm region into a spiral arm and back into the next interarm region. Stellar feedback does not dissociate molecules much, and the coagulation and fragmentation of molecular clouds dominate the evolution of the ISM at these radii. The trend differs in the outskirts where the gas phase is globally atomic ( < f 50% mol ). The HI and H 2 phases cycle through spiral arm passage there. These different regimes of ISM evolution are also seen in external galaxies (e.g., the LMC, M33, and M51). We explain the radial gradient of f mol using a simple flow continuity model. The effects of spiral arms on this analysis are illustrated in the Appendix.

The prevalence of star formation as a function of Galactocentric radius

“…the conversion of total gas mass to stars) to decrease within this radius, but it is not clear how this might relate to the rate of production of stars in dense clumps measured by the SFF. Koda et al (2016) show that the molecular gas fraction increases steadily with decreasing RGC, but we see in Figure 2 that the surface density of mass in dense clumps falls rapidly within 4 kpc (Bronfman et al 1988;Urquhart et al 2014a). The production of molecular clouds from neutral gas therefore is more efficient at small RGC where the H2/HI ratio is nearly 100% (Koda et al 2016).…”

“…Koda et al (2016) show that the molecular gas fraction increases steadily with decreasing RGC, but we see in Figure 2 that the surface density of mass in dense clumps falls rapidly within 4 kpc (Bronfman et al 1988;Urquhart et al 2014a). The production of molecular clouds from neutral gas therefore is more efficient at small RGC where the H2/HI ratio is nearly 100% (Koda et al 2016). The fraction of molecular gas in the form of dense clumps within these clouds, while more or less steady, on average outside ∼4 kpc, albeit with very large, apparently random variations from cloud to cloud (Eden et al 2012(Eden et al , 2013, falls sharply inside this radius.…”

Variation in GMC Association Properties across the Bars, Spiral Arms, Inter-arms, and Circumnuclear Region of M100 (NGC 4321) Extracted from ALMA Observations

“…There are no high-resolution HI data of M100 that allow us to distinguish the armto-inter-arm variation; these data are not available for most of the nearby galaxies also. Nonetheless, gas phase change between the arm and inter-arm regions has been observed in the Milky Way (Koda et al 2016). Interstellar gas becomes molecular as it enters spiral arms, but is dissociated back to the atomic phase upon leaving the arms.…”

Physical Properties of Molecular Clouds for the Entire Milky Way Disk

“…Recently Rice et al (2016) have shown that the brightest molecular clouds seem to be spatially correlated with the spiral structures of the Galaxy. On the other hand, Koda et al (2016) showed that there is rather modest variation of the fraction of molecular to atomic gas (about 20%) between the arm and the interarm regions of the Milky Way. These authors noticed that the main change in gas phase occurs at R gal > 6 kpc where the molecular fraction drops below 50% (see Figure 9, bottom left panel).…”

“…At larger R gal , the H i surface density falls exponentially with a scale length of R HI ≈ 3.75 kpc (Kalberla & Kerp 2009). Both Nakanishi & Sofue (2016) and Koda et al (2016) showed that the fraction of the gas in molecular form, f H2 = Σ H2 /(Σ HI +Σ H2 ), declines steadily with R gal . This is also shown in the bottom left panel of Figure 9 using our estimate of Σ H2 .…”