8'2 The ISM in Disk Galaxies

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8.2 The ISM in Disk Galaxies

• Kim, Dae-Won

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Fig 8.15 21cm 25 channel maps of NGC 5033 (Bosma
1978)
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Data Reduction

• Choose line-free channels and average them.
• Subtract the averaged line-free channel from each
  line-radiation channels.
• Assign a typical velocity to each point in the
  galaxy by identifying the frequency where the
  line radiation has peak intensity. (see the
  figure in this page.)
• Integrate over frequency the total line emission
  from each point to determine, via equation 8.22
  for example, the column density at that point on
  the sky.

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Fig 8.17 HI velocity map, Fig 8.16 HI column
density (Bosma 1978)
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Furthermore

• Notice that a high-quality line profile contains
  much more information than the values of the
  total column density and central velocity.
• Data cube typically lack of the spatial
  resolution. ? usually interpreted by
  model-fitting procedure in which one attempts to
  optimize the fit between the observations and
  pseudo-data derived from a suitably parameterized
  model galaxy.

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8.2.1 Global measures

• HI and H2 in disk galaxies
• Can be derived from data of low spatial
  resolution, these masses have been determined for
  several hundred galaxies.
• M_dyn vs. M_gas M_HI M_H2

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Fig 8.20 Early-type spirals are less gas-rich
than late-type spirals.
Fig 8.21 The ISM of early type tends to be
predominantly molecular, while that of a late
type tends to be atomic.
Hence, M_HI/H_dyn increase quite strongly down
the Hubble sequence, M_H2/M_dyn is approximately
independent of Hubble type. because, 1) Late
type galaxies tend to have lower luminosities and
metallicities. 2) ration of CO-to-M_H2 could be
different in other galaxies with that of the
Milky Way.
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• Radio continuum and IR luminosities
• Show tight correlation between the
  radio-continuum and infrared luminosities of
  galaxies even if the galaxies lie on different
  Hubble type. ? generated by single process
  thought be the formation of massive stars.
• UV converted by dust grain into IR.
• Supernovae are responsible for the radio.

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Fig 8.22 Radio continuum and IR luminosity.
Fig 8.23 correlation between radio continuum and
the cold component of the ISM (1.5Ghz vs. 2.6mm
by 12CO)
Fig 22 suggest a close physical connection
between cool gas, star formation and supernovae
as explained previous pages. Is the correlation
of Fig 23 the same nature? In this case, some
galaxies might possess clouds that are rich in CO
by inconspicuous in the spectral lines of CO
because they are deficient in luminous stars to
head the CO.
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Fig 8.24 NGC891 at 1.49GHz
NGC891 at optical
Although, radio-continuum is tightly correlated
with the cold gas, it does not come from the thin
disk to which cold gas if confined. The actual
distribution of radio-continuum is naturally
explained if cosmic rays diffuse more than
kiloparsec from their places of birth.
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