The Sun, stellar-population models, and the age estimation of high-redshift galaxies

Given sufficiently deep optical spectroscopy, the age estimation of high-redshif t ($z > 1$) galaxies has been claimed to be a relatively robust process (e.g. Dunlop et al. 1996) due to the fact that, for ages $< 5$Gyr, the near-ultraviolet light of a stellar population is expected to be dominated by `well-understood' main-sequence (MS) stars. Recently, however, the reliability of this process has been called into question by Yi et al (2000), who claim to have developed models in which the spectrum produced by the main sequence reddens much more rapidly than in the models of Jimenez et al (2000a), leading to much younger age estimates for the reddest known high-redshift ellipticals. In support of their revised age estimates, Yi et al cite the fact that their models can reproduce the spectrum of the Sun at an age of 5 Gyr, whereas the solar spectrum is not reproduced by the Jimenez et al models until $\simeq 10$ Gyr. Here we confirm this discrepancy, but point out that this is in fact a {\it strength} of the Jimenez et al models and indicative of some flaw in the models of Yi et al (which, in effect, imply that the Sun will turn into a red giant any minute now). We have also explored the models of Worthey (1994) (which are known to differ greatly from those of Jimenez et al in the treatment of post-MS evolution) and find that the main-sequence component of Worthey's models also cannot reproduce the solar spectrum until an age of 9-10 Gyr. We conclude that either the models of Yi et al are not as main-sequence dominated at 4-5 Gyr as claimed, or that the stellar evolutionary timescale in these models is in error by a factor possibly as high as two. (abridged)Comment: Submitted to MNRAS, final versio

Simulating the assembly of galaxies at redshifts z = 6 - 12

We use state-of-the-art simulations to explore the physical evolution of galaxies in the first billion years of cosmic time. First, we demonstrate that our model reproduces the basic statistical properties of the observed Lyman-break galaxy (LBG) population at z = 6 - 8, including the evolving ultra-violet (UV) luminosity function (LF), the stellar-mass density (SMD), and the average specific star-formation rates (sSFR) of LBGs with M_{UV} < -18 (AB mag). Encouraged by this success we present predictions for the behaviour of fainter LBGs extending down to M_{UV} <= -15 (as will be probed with the James Webb Space Telescope) and have interrogated our simulations to try to gain insight into the physical drivers of the observed population evolution. We find that mass growth due to star formation in the mass-dominant progenitor builds up about 90% of the total z ~ 6 LBG stellar mass, dominating over the mass contributed by merging throughout this era. Our simulation suggests that the apparent "luminosity evolution" depends on the luminosity range probed: the steady brightening of the bright end of the LF is driven primarily by genuine physical luminosity evolution and arises due to a fairly steady increase in the UV luminosity (and hence star-formation rates) in the most massive LBGs. However, at fainter luminosities the situation is more complex, due in part to the more stochastic star-formation histories of lower-mass objects; at this end, the evolution of the UV LF involves a mix of positive and negative luminosity evolution (as low-mass galaxies temporarily brighten then fade) coupled with both positive and negative density evolution (as new low-mass galaxies form, and other low-mass galaxies are consumed by merging). We also predict the average sSFR of LBGs should rise from sSFR = 4.5 Gyr^-1 at z = 6 to about 11 Gyr^-1 by z = 9.Comment: Accepted for publication in MNRA

Optical off-nuclear spectra of quasar hosts and radio galaxies

We present optical (~3200A to ~9000A) off-nuclear spectra of 26 powerful active galaxies in the redshift range 0.1 < z < 0.3, obtained with the Mayall and William Herschel 4-meter class telescopes. The sample consists of radio-quiet quasars, radio-loud quasars (all with -23 > M_V > -26) and radio galaxies of Fanaroff & Riley Type II (with extended radio luminosities and spectral indices comparable to those of the radio-loud quasars). The spectra were all taken approximately 5 arcseconds off-nucleus, with offsets carefully selected so as to maximise the amount of galaxy light falling into the slit, whilst simultaneously minimising the amount of scattered nuclear light. The majority of the resulting spectra appear to be dominated by the integrated stellar continuum of the underlying galaxies rather than by light from the non-stellar processes occurring in the active nuclei, and in many cases a 4000A break feature can be identified. The individual spectra are described in detail, and the importance of the various spectral components is discussed. Stellar population synthesis modelling of the spectra will follow in a subsequent paper (Nolan et al. 2000).Comment: 23 pages, LaTeX, uses MNRAS style file, incorporates 71 postscript figures, to be published in MNRAS. Contact author: [email protected]

Coherent control and feedback cooling in a remotely-coupled hybrid atom-optomechanical system

Cooling to the motional ground state is an important first step in the preparation of nonclassical states of mesoscopic mechanical oscillators. Light-mediated coupling to a remote atomic ensemble has been proposed as a method to reach the ground state for low frequency oscillators. The ground state can also be reached using optical measurement followed by feedback control. Here we investigate the possibility of enhanced cooling by combining these two approaches. The combination, in general, outperforms either individual technique, though atomic ensemble-based cooling and feedback cooling each individually dominate over large regions of parameter space.Comment: 28 pages, 5 figures, 2 tables. Updated to include exemplary experimental parameters and expanded discussion of noise source

Can't get no learning: the Brexit fiasco through the lens of policy learning

It seems paradoxical to suggest that theories of learning might be used to explain policy failure. Yet the Brexit fiasco connects with recent approaches linking varieties of policy learning to policy pathologies. This article sets out to explain the UK government’s (mis)management of the Brexit process from June 2016 to May 2019 from a policy learning perspective. Drawing on interviews with UK policy-makers and stakeholders, we ask how did the UK government seek to learn during the Brexit negotiations? We consider four modes of learning: reflexivity, epistemic, hierarchical, and bargaining. By empirically tracing the policy process and scope conditions for each of these, we argue that learning through the first three modes proved highly dysfunctional. This forced the government to rely on bargaining between competing factions, producing a highly political form of learning which stymied the development of a coherent Brexit strategy. We argue that the analysis of Brexit as a policy process – rather than a political event – reveals how policy dynamics play an important role in shaping the political context within which they are located. The article concludes that public policy analysis can, therefore, add significant value to our understanding of Brexit by endogenising accounts of macro political developments

The redshifts of bright sub-mm sources

One of the key goals in observational cosmology over the next few years will be to establish the redshift distribution of the recently-discovered sub-mm source population. In this brief review I discuss and summarize the redshift information which has been gleaned to date for the ~ 50 bright sub-mm sources which have been uncovered via the six main classes of survey performed with SCUBA on the JCMT over the last 2-3 years. Despite the biases inherent in some of these surveys, and the crudeness of the redshift information available in others, I conclude that all current information suggests that only 10-15 % of luminous sub-mm sources lie at z < 2, and that the median redshift of this population is z ~ 3. I suggest that such a high median redshift is arguably not unexpected given current theories designed to explain the correlation between black-hole mass and spheroid mass found at low redshift. In such scenarios, peak AGN emission is expected to correspond to, or even to cause termination of major star-formation activity in the host spheroid. In contrast, maximum dust emission is expected to occur roughly half-way through the star-formation process. Given that optical emission from bright quasars peaks at z = 2.5, dust-emission from massive ellipticals might be reasonably expected to peak at some point in the preceding ~ 1 Gyr, at z ~ 3. Confirmation or refutation of this picture requires significantly-improved redshift information on bright samples of SCUBA sources.Comment: 11 pages, 5 figures, FIRSED2000 conference proceedings, eds. I.M. van Bemmel, B. Wilkes, & P. Barthel Elsevier New Astronomy Review