Extragalactic Astronomy

• What are Quasars ?
• What are the different methods to calculate the masses of galaxies of all types ? How does dark matter affect the results ?

What are Quasars ? (Sumit)

Quasars are the extremely luminous nuclei of galaxies at large red shifts. It is believed that a super massive black hole at the centres of these galaxies is accreting the surrounding matter (consisting of stars, gas, dust etc.,). This owing to the high gravitational potential energy around these holes results in the release of prodigous amounts of energy. An accretion rate of about one solar mass per year, results in a quasar being several hundred times more luminous than an average galaxy, not only in the visible regions of the spectrum but also over x-rays, gamma rays etc. Infact,some quasars emit quadrillion times more energy than the sun in gamma rays alone !   Top

What are the different methods to calculate the masses of galaxies of all types ? How does dark matter affect the results ?

(Sumit)

The motion of the sun around the galactic centre, about eight kiloparsecs away, at a velocity of around 220 km/sec, enables the total mass contained within this distance to be estimated. This is just an application of the familiar Kepler's law, (GM_tot x T² = 4 x pi x R³ ) or (V² x R = GM_tot).

The observed rotational velocities of spiral galaxies (from the Doppler shift of say the 21 cm line etc.) also enables their mass to be estimated in this manner. In the case of elliptical galaxies, the velocity dispersion squared <V²> is used to estimate M_tot. (This also is applied to giant clusters of galaxies like the coma cluster, where the angular size and distance of the cluster enables its linear size R to be determined). Then the velocity dispersion squared <V²> is related to the total cluster mass M_c through <V²
> x R = G x M_c or M_c = <V²> x R / G.

The velocity and velocity dispersions can also be estimated spiral and elliptical galaxies through the Tully-Fischer and Faber-Jackson relations respectively. These relate V to luminosity L (measurable !) of the galaxy, thus L proportional to V⁴ for spiral and L proportional to sigma⁴ for elliptical galaxies. M can then be estimated (by knowing V or sigma, see previous expressions), OR by the typical M / L(mass-luminosity ratio) for the two types. The presence of dark matter is deduced from the fact that the dynamical mass considerably exceeds the luminous mass.

In the case of many large spiral galaxies (including our own),the rotational velocity does not fall off with distance (that is V² inversely proportional to 1/R) as what one would expect if most of the mass is concentrated in a central region. V is instead found to remain more or less constant with r (that is 'flattens out'). This suggests that M increases with R (from Kepler's law V² / R = G x M / R² , So V is constant, if M proportional to R). So the galaxy is surrounded by a dark matter halo extending to hundred kilo parsecs or more, with density falling of as 1 / r², so that M is proportional to r.This is also true for ellipticals, clusters of galaxies, binary galaxies, dwarf galaxies etc. Dark matter increases the M / L ratio, affecting only the dynamics, not generating light.   Top