ASTC22. Lecture L15

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ASTC22. Lecture L15 S0 and spiral galaxies -
observations
Measuring the distribution of starlight
CCDs Signal-to-Noise ratio from CCDs NGC 7331
as an example of a far-away disk galaxy
discovery, surface photometry, distance,
physical scale, surface brightness. (Another
example M104 Sombrero.) Ursa Major group -
colors and brightness distribution with
type Differences between the visible, IR, and UV
observations
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NGC 7331 - discovered in the constellation of
Perseus by W. Herschel in 1784. It is one of the
brighter galaxies which is not included in
Messier's catalog.
Technical information Telescope 1.0-m Jacobus
Kapteyn Telescope.Instrument JAG CCD
Camera.Detector SITe2.Filters and exposure
times 10100 second exposures in R, V and B.
Credit Daniel Bramich (ING) and Nik Szymanek.
Date 2001
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Two observers, two Largest Telescopes in the
World of their time
W. Herschel, a German musician and astronomer
built this 48-in (126-cm diameter, 12m focal
length) telescope in 1789. He discovered Uranus
and its 2 moons, nebulae like NGC7331, and
sketched the shape of the Galaxy (a pancake
flattened in geometrical ratio 15, with the sun
in the center). An Irishman W. Parsons first
observed spiral structure of 14 nebulae,
including NGC 7331, before 1850. His sketch of
M51
William Herschel (1738-1822)
Lord Rosse (William Parsons) (1800 -1867) and a
photograph taken before 1880 of his 1847
Leviathan 72-in (183-cm) telescope.
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Spiral and S0 galaxies produce most visible
light in the local universe
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Anatomy of a CCD (a chip you also have in your
electronic camera, webcam, video recorder)
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Practical issues with CCDs 1. Cosmic ray
problems -- false stars (need to be removed) 2.
Overexposure -- streaks in the image, ghost
images3. Flat fielding of a CCD removing
uneven sensitivity of pixels 4. Calibrating an
instrument -- obtaining flux or magnitude from

counts
(ghost images)
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Q To improve the S/N by a factor of two, how
much longer an exposure do you need, in the two
limiting regimes dominated by read-out noise and
statistical noise?
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m2 m1
How to measure an exponential length scale of the
disk Choose m1-m21.08 mag on the dashed line,
since it corresponds to the factor 1/e
in brightness e
exp(1)2.718
2.5 log e 2.50.4343 1.08 mag
read off the length
scale hR
55.
55
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Definition
Q how is it possible to see anything that
faint? What S/N do we need?
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Compare with suns absolute magnitude VT(0)
4.83 mag
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Sombrero, or Messier 104 is a prototype of a
large Sa galaxy with a huge bulge, at the
southern edge of the Virgo cluster of galaxies.
Its mV 8 mag, a bit too dim for seeing with
the naked eye, but small telescopes can depict it
nicely. M104 is seen 6 deg off edge-on viewing.
It has 2000 globular clusters, and an inner,
small, tilted gas disk seen in X-rays, which may
feed the central black hole with mass 1e9 Msun,
still small compared with the mass of M104
0.8e12 Msun.
In 1912
V.Slipher measured the radial
speed VR 1000 km/s, which was
the first hint that M104 is a
galaxy, not a gas nebula.
The HST/ACS, RGB-color, 6-image mosaic (2003) is
much less saturated than fig 5.5 from our
textbook (above).
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Brightness Color
type
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Visible vs. UV-bandpass observations