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.