Galaxies - Galaxies - Basic Catalogs and Data Collections...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Galaxies - Basic Catalogs and Data Collections From my (biased?) observer's point of view, data on galaxies is where we must start in understanding them. With electronic collections, getting the numbers is easier than ever before. However, it still helps to know where the numbers come from, and what the initial selection of various catalogs was. In particular, just how a catalog sample was compiled may have a great influence on what one can learn from it. Surface-brightness and flux selection criteria are especially important in this regard. Furthermore, there is a big difference between an object list and a catalog including uniform physical data for each entry. To catalog something, we must see that (1) an object is there and (2) it is not something else. For galaxies, we must detect them at some wavelength and distinguish them from stars, planetary nebulae, galactic cirrus, and navigation satellites. Most surveys to this point have been done in the optical, until recently from examination of photographic plates. This implies certain detection limits: (1) Anything less than about 1 arcsecond in size looks like a star on normal ground-based images, and can be distinguished only by nonstellar colors or spectrum (QSOs, compact galaxies; this worked fine for the SDSS, for example). HST imagery shows that there are lots of faint galaxies at high redshift which could masquerade as faint stars from the ground (in practice this enters in significant numbers only for magnitudes fainter than V=20, although such rarities as "peas" may be somewhat brighter). (2) Any object with surface brightness too low (much less than about 1% of the night-sky brightness) would only be found when near enough for individual stars to be seen (this is how the Sculptor dwarf galaxy was discovered). Surface brightness (received flux per unit solid angle) is often measured in the definitely non-SI units of magnitude per square arcsecond for optical and near-IR passbands; one also encounters MJy/steradian, S(10) (the light of a V=10 star per square degree), and very occasionally the SI unit of a nit (candela per square meter). Major components in the optical are airglow (much in isolated atomic and molecular emission lines) and scattered starlight from Solar System dust (zodiaal light) and interstellar dust. From a dark site, the dust/airglow contributions are about equal in the V band. In the near-IR, airglow molecular emission (particularly OH bands, the "OH forest") become extremely bright; gains of many thousands in sensitivity are possible by going to space. Details on typical sky brightnesses may be found in
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 4

Galaxies - Galaxies - Basic Catalogs and Data Collections...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online