arXiv subjects

James Dunlop

Publications and source records attributed to James Dunlop.

39 records · Page 3Linked to original sources

arXiv1997

Old Galaxies at High Redshift

The most passive galaxies at high redshift are unlikely to be identified by either narrow-band emission-line searches, or by Lyman limit searches (both techniques which have been highlighted at this meeting) simply because such selection methods rely on the presence of a strong ultraviolet component. Selection on the basis of extreme radio power has also proved to yield optically active objects with the majority of high-redshift objects studied to date displaying complex elongated optical/UV morphologies, relatively blue optical-ultraviolet continuum colours, and strong emission lines. These features, coupled with the failure to detect any spectral signatures of old stars at $z > 1$, has led to the suggestion that these galaxies are being observed close to or even during a general epoch of formation. However, we have recently demonstrated that radio selection at significantly fainter (mJy) flux densities can be used to identify apparently passively evolving elliptical galaxies at high redshift. Deep Keck spectra have now been obtained for two such objects yielding absorption line redshifts $z \simeq 1.5$; 53W091 at $z = 1.552$ (Dunlop et al. 1996) and most recently 53W069 at $z = 1.432$. The ultraviolet SEDs of these galaxies indicate minimum ages $> 3$ Gyr while, as stressed in this article, the strength of the reddenning-independent ultraviolet spectral breaks actually indicate a greater minimum age of 5 Gyr for both objects assuming solar metallicity. Since the spectra comprise the integrated light of each galaxy to radii greater than $r_e$, I argue that it is difficult to justify the adoption of significantly super-solar metallicity in interpreting these data. It thus seems hard to escape the conclusion that $Ω_0 < 1$ and that, irrespective of the adopted cosmology, at least some massive ellipitical galaxies were formed at high redshift ($z> 5$).

astro-ph↗

arXiv1997

Cosmic Star-Formation History, as traced by Radio Source Evolution

I briefly review our current knowledge of the cosmological evolution of radio sources, and show that the redshift distributions of new complete samples of radio sources confirm the existence of the high-redshift decline in comoving number density (or `cutoff') beyond $z \simeq 2.5$ first deduced by Dunlop & Peacock (1990). Taken at face value these new data favour a luminosity dependent `cutoff', in which the decline is least drastic for the most luminous radio sources. I demonstrate, however, that regardless of the precise form of radio source evolution, the evolution of radio luminosity density is well determined, and appears uncannily similar to the evolution of ultra-violet luminosity density from star-forming galaxies in the universe (e.g. Madau 1997). I convert radio luminosity density into an estimate of black hole fueling rate per Mpc$^3$, and conclude that radio source evolution is a good tracer of the star-formation history of the Universe; at any epoch, for every 10$^7$ M$_{\odot}$ of material converted into stars, approximately 1 M$_{\odot}$ appears to be consumed by radio-loud active galactic nuclei. If this is indeed true at all epochs, then this would imply that star-formation activity in the Universe peaked at $z \simeq 2 - 2.5$, and that the values of star-formation rate density currently derived from Lyman-limit galaxies at $z \simeq 2.8$ and $z \simeq 4$ are under-estimates by a factor $\simeq 4$. Finally I briefly speculate as to why radio source evolution might trace global star-formation activity as manifested primarily in the disc/spiral population, whereas the hosts of the radio sources themselves generally appear to be well-evolved elliptical galaxies.

astro-ph↗

arXiv1997

LBDS 53W091: An Old red Galaxy at z=1.552

The weak radio source LBDS 53W091 is associated with a very faint (R approx. 24.5) red (R-K approx. 5.8) galaxy. Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption-line redshift, z=1.552 +/- 0.002. The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6V star, and a single-burst old stellar population that matches the IR colors, the optical energy distribution and the spectral discontinuities has a minimum age of 3.5 Gyr. We present detailed population synthesis analyses of the observed spectrum in order to estimate the time since the last major epoch of star formation. We discuss the discrepancies in these estimates resulting from using different models, subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these techniques. The models most consistent with the data tend to yield ages at z=1.55 of >3.5Gyr, similar to that inferred for the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of Omega_0, only certain cosmological expansion times are consistent with the age of LBDS 53W091; in particular, for Omega_0=1, only models with H_0 < 45 km/s/Mpc are permitted. For H_0=50 km/s/Mpc and Omega_0=0.2, we derive a formation redshift, z_f >= 5.

astro-ph↗