Nimisha G. Kantharia National Centre for Radio Astrophysics Tata Institute of FundamentalResearch Pune Galaxies
Electromagnetic Spectrum Radio frequencies
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
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
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 Milkyway 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.