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ASTC22. Lecture L15 S0 and spiral galaxies - observations

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,

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ASTC22. Lecture L15 S0 and spiral galaxies - observations

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  1. 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

  2. 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: 10*100 second exposures in R, V and B. Credit: Daniel Bramich (ING) and Nik Szymanek. Date: 2001

  3. 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 1:5, 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.

  4. Spiral and S0 galaxies: produce most visible light in the local universe

  5. Anatomy of a CCD (a chip you also have in your electronic camera, webcam, video recorder)

  6. 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)

  7. 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?

  8. m2 m1 How to measure an exponential length scale of the disk: Choose m1-m2=1.08 mag on the dashed line, since it corresponds to the factor 1/e in brightness: e = exp(1)=2.718 2.5 log e = 2.5*0.4343 = 1.08 mag read off the length scale: hR = 55”. 55”

  9. Definition Q: how is it possible to see anything that faint? What S/N do we need?

  10. Compare with sun’s absolute magnitude VT(0) = 4.83 mag

  11. 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. It’smV = +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 theradial 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).

  12. Brightness Color type

  13. Visible vs. UV-bandpass observations

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