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Galaxies
Nimisha G. Kantharia
National Centre for Radio Astrophysics Tata Institute of FundamentalResearch
Pune
Electromagnetic Spectrum Radio frequencies
Accessible em bands on earth
Milky way in Infrared
• Stellar distribution is traced in this map.
Some Terminology• Distance - light year: (3 x 10^5 ) km/s * (365 * 24
* 60 * 60) sec ~ 9.5 x 10^12 km ! compare with earth radius ~ 6400 km compare with galactic radius ~ 100,000 light yrs!
• Distance in Solar system: Astronomical unit = 1.5 x 10^8 km (earth-sun distance)
• Mass – solar mass: 1.9 x 10^30 kg !!• Luminosity – solar luminosity: 3.8 x 10^26 Watts
• GMRT Khodad
Galaxies• Collection of stars and gas. ~10^11 stars.
Gas ~ 2-3% of stellar mass. Many types.
• http://imagine.gsfc.nasa.gov/
Types of Galaxies• Hubble Tuning Fork diagram – evolutionary
sequence? spirals – gas; ellipticals – less gas
Spirals edge-on view• Andromeda galaxy• M31 www.noao.edu/image_gallery/galaxies.html
Spirals face-on view• Whirlpool galaxy• M51 www.noao.edu/image_gallery/galaxies.html
Types of Galaxies
• Can you classify these galaxies?
Galaxy Catalogues -Messier• Charles Messier gave Messier catalogue:
M31, M33, M51• Identified 'nebulous objects' while looking
for comets in the 18th century• 110 objects catalogued – still widely used
by astronomers
Messier catalogue
• 110 nebulous objects - supernova remnants, galaxies, HII regions.
• http://www.seds.org/messier/
New General Catalogue (NGC)• Compiled by J.L.E.
Dreyer in 1888.• Galaxies, open
clusters, globular clusters of stars, HII regions, supernova remnants
• 47 Tucanae = NGC104• http://www.seds.org
NGC 253 – starburst http://www.seds.org
Other catalogues
• Catalogues have since been created by specific optical observatories e.g. UGC (using Palomar data), ESO and for specific objects e.g. Markarian (galaxies bright in uv), Arp (interacting galaxies).
• What information is required to catalogue galaxies?
How to locate galaxies in the sky?
• To locate galaxies, stars etc on sky
• Celestial sphere• Right ascension• Declination• Visible stars above
horizon: ~ 3000 - all in Milky way
http://inkido.indiana.edu/a100/celestialsphere.html
Rotation in Galaxies• Familiar with earth's revolution around sun • Stars, gas in galaxies differentially rotate around
the centre.
Rotation curves
Rotation curve of a typical spiral galaxy: predicted (A) and
observed (B). Dark matter can explain the velocity curve
having a "flat" appearance out to large radii. (from
wikipedia)
Dark Matter in galaxies• Gravity balanced by centrifugal force• G Mm / R^2 = mv^2 / R• v = sqrt ( GM / R)• At R_max using observed v km/s, an
estimate of mass of galaxy can be obtained.
• ~ 96% mass in universe is dark
GMRT – low radio frequencies• GMRT consists of 30 disk antennas over 25 km
region and observes the universe at radio frequencies of 150, 240, 325, 610 and 1420 MHz.