1.1 Molecular Clouds and Star Formation in Spiral Galaxies
1.1.2 The Interstellar Medium Components: an overview
The interstellar medium (ISM) can be understood both in terms of its chemical composition as well as its thermal phases (e. g. Lequeux 2005; Osterbrock & Ferland 2005). In terms of chemical composition, the ISM is a mixture of gas and and dust. The most abundant element is hydrogen, followed by helium. Hydrogen can be neutral as well as ionised depending on the local temperature and radiation conditions (e. g. Osterbrock & Ferland 2005). It is also composed of several molecules: molecular hydrogen (H2), carbon monoxide (CO), carbon-
hydrogen (CH), carbon monosulfide (CS), cyanide (CN) compunds, hydrogen cyanide (HCN), hydroxyl (OH) compounds, ammonia (NH3) (e. g. De Becker 2013).
The ISM can be broadly divided in the following thermal phases: the cold neutral medium (CNM) and dense molecular, which has the densest gas (n∼ 101−106 cm−3) and typical
temperatures aroundT ≈10−100 K; the warm neutral medium (WNM) and warm ionised
medium (WIM), which has lower densities (n∼10−1−104 cm−3) and higher temperatures
10−3) cm−3 and highest temperatures aroundT≈106K (e. g. Goldsmith et al. 1969; McKee
& Ostriker 1977; Cox 2005). The different thermal phases of the ISM are summarised in Table 1.1.
Phase T [K] n[cm−3] Properties
Hot Ionised (HIM) !106 10−3 Heated by shocks and collisional ionisation
Cooled by adiabatic expansion; X-ray emission Traced with UV, X-ray and radio observations Warm Ionised
(WIM) ∼
104 10−1−104 Heated by photoionisation
Cooled by line, free-free, and fine structure emis- sion
Traced by optical (lines) and radio observations Warm Atomic
(WNM)
5000−104 0.6 Warm neutral medium (HI)
Heated from dust photoelectric effect
Ionisation by background radiation and cosmic rays Cooling by line emission and fine structure lines Traced by HI 21-cm line and Optical, UV absorption lines
Cold Atomic (CNM) 50−100 20−50 Heated by dust photoelectric effect
Ionisation by background radiation and cosmic rays Cooled by fine structure lines
Traced by HI 21-cm emission, optical and UV ab- sorption lines
Diffuse Molecular ∼50 102 Cooled by fine structure lines
Observed with HI 21-cm emission; absoprtion, op- tical and UV lines
Dense Molecular
(MM) 10−50 10
3−106 Cooled by CO, CI fine structure line
Traced by CO 2.6-mm and dust FIR emission Table 1.1: Phases of the Interstellar Medium. Based on information from Draine (2011); Lequeux (2005); Pettitt (2015).
The phases exist in pressure equilibrium and the thermal energy is regulated by cooling and heating mechanisms. Cooling results mainly from fine structure lines, molecular transitions, and collisional lines. These processes operate at different temperatures depending on the excitation temperature of the transition (e. g. Dyson & Williams 1997; Wolfire et al. 1995; Koyama & Inutsuka 2000). Heating results from the photoelectric effect, ionisation by cosmic rays and strong radiation, and shock heating (e. g. Goldsmith et al. 1969; Wolfire et al. 1995; Koyama & Inutsuka 2000). Supernova feedback can drive gas to the hottest phase as well (e. g. Cox & Smith 1974; McKee & Ostriker 1977). The net energy gained or lost is quantified by the function (e. g. Lequeux 2005):
1.1. Molecular Clouds and Star Formation in Spiral Galaxies
where Lis the general loss function,nis the number density,Λis the cooling function, andΓ
is the heating function. IfL>0, cooling dominates; if L=0, the medium is in equilibrium; if
L<0, heating dominates. The behaviour of this function is shown in the temperature-density
and pressure-density diagrams of Figure 1.2 assuming the cooling and heating functions in Koyama & Inutsuka (2000). In the bottom panel of Figure 1.2, the pressure decreases with density between 100and 101 cm−3and this is a region of an unstable gas phase as any slight
perturbation out of equilibrium would quickly cool or heat and move to the parts of the curves with positive slope.
The neutral hydrogen is usually traced by the 21-cm transition, which provides informa- tion of its spatial distribution and a galaxy’s rotation curve (e. g. Walter et al. 2008; Kalberla & Kerp 2009; Sofue 2017). The ionised hydrogen can be found either in HIIregions surround-
ing young O and B stars that emit sufficiently energetic radiation to ionise the nearby gas (Strömgren, 1939). Additionally, it may be found in a hot diffuse distribution surrounding the galaxy known as the corona. This gas may be the result of gas heated by supernova explosions (e g Lequeux 2005; Draine 2011).
Molecular hydrogen (H2) is an abundant molecule, but its direct detection is very difficult
because the high symmetry of the molecule does not allow∆J=±1 transitions. Higher transi-
tions such as∆J=±2 are possible, but have an excitation temperature of 500 K (e g. Rybicki
& Lightman 2004). The temperatures in molecular clouds are too low to excite this transi- tion (Timmermann et al., 1996). The formation of H2 is usually explained with the following
mechanism: dust grains function as a catalyst for H atoms to bond and form the molecule (Hollenbach & Salpeter, 1971; Gould & Salpeter, 1963), however the entire process is not fully understood yet. This requires high density environments that can shield the molecule from strong UV radiation. H2 is one of the main components of molecular clouds and it is ex-
pected to play an important role in the star formation process as it exists in the densest parts of star forming clouds (e. g. Habart et al. 2005; Dobbs et al. 2014; Klessen & Glover 2016).
Carbon monoxide (CO) is also an abundant molecule that plays an important role in tracing molecular clouds. It is mainly observed atλ=2.6 mm, which corresponds to theJ =1→0
transition. Other transitions such as the J = 2 → 1 are also used. The CO is assumed to
trace the H2distribution, but the information derived about it depends the conversion factor
Figure 1.2: Top panel: temperature-density map of the general loss function of equation (1.2) using the cooling functionΛof Koyama & Inutsuka (2000) and a constantΓ term. The red and blue regions correspond to where the heating and cooling terms dominate and the colour scales with the magnitude of the function. The line where the regions meet corresponds to the equilibrium point (n2Λ= nΓ).
1.1. Molecular Clouds and Star Formation in Spiral Galaxies
Figure 1.3: Example of an l-vlos map made from CO observations in the plane of the Milky Way.
Molecular clouds are expected to be associated with the peaks in this map. Image taken from Miville- Deschênes et al. (2017).
by Smith et al. (2014) also show that there may be a significant fraction of diffuse CO that is not necessarily associated with H2 clouds. Nevertheless, this is still the most common tracer of molecular clouds and the dense gas phase in both Milky Way and extragalactic surveys.