Galaxies Nimisha G. Kantharia National Centre for Radio Astrophysics Tata Institute of...

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.