The Interstellar Medium (ISM)

To be a bit specific, the ISM varies widely in number density (i.e., particles per unit volume), temperature, ionization state, metalliticity, interstellar dust (kind and amount), and cosmic ray behavior. Regions of various contents also vary in size, shape, velocity field, electromagnetic field, gravitational field, and other behaviors.

To understand the ISM in general and for analysis, it has been found to be useful to divide it into 6 components which are detailed in Table: Interstellar Medium Components of Milky Way below.

Of course, there is really a continuum of behavior, but the 6 components are identifiable poles behavior.

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Table:  Interstellar Medium Components of Milky Way
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Component      Fractional  Scale height  Temperature  Number density  State of   Primary observational
               volume      above disk    (K)          (particles      Hydrogen   signature
               (%)         (pc)                       /cm**3) 
molecular      < 1         80            10--20       10**2           molecular  radio & infrared
cloud                                                   --10**6         (H_2)      molecular lines
                                                                                   (Not H_2)

Cold neutral   1--5        100--300      50--100      20--50          neutral    H I 21-cm line 
medium (CNM)                                                            atomic     absorption
                                                                        (H I) 

Warm neutral   10--20      300--400      6000-10000   0.2--0.5        neutral    H I 21-cm line
medium (WNM)                                                            atomic     emission
                                                                        (H I) 

Warm ionized   20--50      1000          8000         0.2--0.5        ionized    Hα line
medium (WIM)                                                            (H II)     emission

H II regions   < 1         70            8000         10**2           ionized    Hα line
                                                        --10**4         (H II)     emission

galactic       30--70      1000-3000     10**6        10**(-4)        ionized    X-ray emission;
halo corona                                --10**7      --10**(-2)      (H II)     absorption lines
(hot ionized                                                                       highly ionized metals,
medium (HIM))                                                                      primarily in UV
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References: Wikipedia: Interstellar medium: Interstellar matter.
Keywords: atomic hydrogen (H I), atomic hydrogen Hα line, galactic halo corona, galactic scale height, H II regions, hydrogen 21-centimeter line (21.1061140542 cm, 1420.4057 5176 67(9) MHz ≅ 1420 MHz) infrared band (fiducial range 0.7 μm -- 0.1 cm), intergalactic medium (IGM), interstellar matter, interstellar medium (ISM), Milky Way, molecular cloud, molecular hydrogen (H_2), number density, radio band (fiducial range 3 Hz -- 300 GHz = 0.3 THz, 0.1 cm -- 10**5 km), ultraviolet band (fiducial range 0.01--0.4 μm), visible band (fiducial range 0.4--0.7 μm = 4000--7000 Å), X-ray band (fiducial range 0.1--100 Å), etc.


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Features:

1. Note that in this insert, we are discussing baryonic matter (i.e., ordinary matter made of protons, neutrons, and electrons).

   Recall that galaxies have of order 10 times more dark matter than baryonic matter. The dark matter is probably an exotic fundamental particle that interacts with itself and other mass-energy forms very weakly, except through gravity.

   The mass fractions discussed below do NOT count dark matter.

2. As the Table shows, the ISM consists of mainly of hydrogen (H) gas: atomic (i.e., H I) gas, molecular gas (i.e., molecular hydrogen H_2 gas), and ionized (i.e., H II) gas. In fact, from the cosmic composition, hydrogen is about 73 % of the ISM by mass fraction.

   There is also helium (He), atomic or ionized. As a noble gas helium forms NO molecules at least under ordinary space conditions. In fact, helium emits and absorbs little, and so is often rather unnoticeable even though is about 25 % of the ISM by mass fraction according to the cosmic composition.

   Metals are at most about 2--4 ??? % of the baryonic matter of the observable universe by mass fraction (see David Weinberg 2016, "On the Deuterium-to-Hydrogen Ratio of the Interstellar Medium", p. 3, but this is NOT the best reference). However, they can emit and absorb significant electromagnetic radiation (EMR) depending on the local conditions.

   There is interstellar dust (which we discuss in more detail in IAL 21: Star Formation: Interstellar Dust). The interstellar dust is only a few percent by mass fraction, but because it is highly opaque, it is very noticeable at least where concentrated in molecular clouds. Note interstellar dust is almost entirely made of metals: there can be some hydrogen in compounds. Thus, the interstellar dust can be at most about 4 % ??? by mass fraction.

3. The values given in the Table are representative values for the Milky Way valid circa 2020s and are subject to revision.

4. Although formally, the values are for the Milky Way, but they probably roughly apply to all large spiral galaxies of the modern observable universe.

5. Collectively, the ISM in the Milky Way is of order 10 % of the stellar matter. Both ISM and stellar matter are baryonic matter, of course.

6. The amount of ISM in other spiral galaxies is comparable (Tacconi et al. 2018, p. 31): i.e., about 10 % of the stellar matter.

   Note the 10 % value is for modern/local observable universe. The amount increases with lookback time and was of order 50 % circa 10 Gyr ago: i.e, about 4 Gyr after the Big Bang: i.e., cosmic time ∼ 4 Gyr. This epoch is, in fact, called cosmic noon (z≅2, cosmic time ∼ 4 Gyr) and the star formation rate (SFR) was then at its highest value: it is now at cosmic present is only ∼ 10 % of its value then.

7. Elliptical galaxies have far less noticeable ISM than spiral galaxies and other galaxy types (i.e., irregular galaxies and lenticular galaxies). They usually have very little interstellar dust and the gas, though perhaps as abundant as in spiral galaxies, is very hot and radiates primarily in the X-ray band (fiducial range 0.1--100 Å) which electromagnetic radiation (EMR) is realtive hard to observe. Note there are NO emission lines for fully ionized hydrogen (H) and helium (He). Highly ionized metals do have X-ray band emission lines.

8. The galactic scale heights in the Table are just the characteristic heights of the component reach above the symmetry plane of the galactic disk.

9. To help understand the ISM, Table: Comparison of Number Densities compares the number density of the ISM to the number densities of other representative gases.

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     Table:  Comparison of Number Densities
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     System          Order of Number     Comment
                     Density
                     (particles/cm**3)
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     Room air        2*10**19            The most abundant molecule is nitrogen N_2 which
                                         is a very inert gas.  The 2nd most is O_2 which
                                         a very reactive gas and key ingredient for respiration.
                                         Ref: HI-334,370.
   
     Best Labora-    < 10**4             Ref:  Wikipedia:  Vacuum:  Measurement.
       tory vacuum
   
     Typical ISM     1                   This mostly atomic H and He with
                                         temperatures of order 50-10000 K.
                                         Ref:  Wikipedia:  Interstellar medium:  Interstellar matter.
   
     Molecular       10**2--10**6        This mostly H and He with temperatures
      clouds                             of order 50--100 K.
                                         The gas is mainly molecular H (i.e., H_2).
                                         Ref:  Wikipedia:  Interstellar medium:  Interstellar matter.
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Local file: local link: interstellar_medium_ism_table.html.
File: Galaxies file: interstellar_medium_ism_table.html.


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