High Redshift Css Radio Galaxies

We discuss CSS radio galaxies from 2 samples of radio sources. The rst sample is a complete sample of 28 3CR galaxies with redshifts in the range 0:6 < z < 1:8 for which we have HST, UKIRT and VLA imaging data. The 2 CSS galaxies contained in this sample, 3C49 and 3C241, appear to be less active than galaxies with radio sizes of order 50{100 kpc, but similar in activity to galaxies with radio sizes larger than 100 kpc. This is interpreted as being due to the rise and fall of a starburst that is induced by the radio jet as it passes through the galaxy. The underlying hosting galaxies of the two CSS sources seem not to be diierent from the other galaxies in the sample. Their dust content, stellar masses, de Vaucouleurs radii and location on the fundamental plane are all typical for the galaxies of the whole sample. We argue that the simplest interpretation is that the CSS sources are simply the youngest radio sources in the sample. The second sample is a representative sample of ultra-luminous radio galaxies with a narrow range in radio power, but a wide range in redshift (0 < z < 4:3). VLA imaging of this sample indicates that the percentage of CSS galaxies is 10 % and does not depend on redshift. The median angular size evolves as LAS median (1 + z) ?1:20:3. Assuming that the size of the radio sources is dictated by ram pressure, this is interpreted as due to the gas density within the galaxy being roughly constant over this large range of redshift, but the density outside the galaxy evolving as (1 + z) 3 .