Establishing an analogue population for the most distant galaxies

Lyman break analogues (LBAs) are local galaxies selected to match a more distant (usually z~3) galaxy population in luminosity, UV-spectral slope and physical characteristics, and so provide an accessible laboratory for exploring their properties. However, as the Lyman break technique is extended to higher redshifts, it has become clear that the Lyman break galaxies (LBGs) at z~3 are more massive, luminous, redder, more extended and at higher metallicities than their z~5 counterparts. Thus extrapolations from the existing LBA samples (which match z=3 properties) have limited value for characterising z>5 galaxies, or inferring properties unobservable at high redshift. We present a new pilot sample of twenty-one compact star forming galaxies in the local (0.05<z<0.25) Universe, which are tuned to match the luminosities and star formation volume densities observed in z>~5 LBGs. Analysis of optical emission line indices suggests that these sources have typical metallicities of a few tenths Solar (again, consistent with the distant population). We also present radio continuum observations of a subset of this sample (13 sources) and determine that their radio fluxes are consistent with those inferred from the ultraviolet, precluding the presence of a heavily obscured AGN or significant dusty star formation.Comment: 13 pages, MNRAS accepte

Spatially Aware Dictionary Learning and Coding for Fossil Pollen Identification

We propose a robust approach for performing automatic species-level recognition of fossil pollen grains in microscopy images that exploits both global shape and local texture characteristics in a patch-based matching methodology. We introduce a novel criteria for selecting meaningful and discriminative exemplar patches. We optimize this function during training using a greedy submodular function optimization framework that gives a near-optimal solution with bounded approximation error. We use these selected exemplars as a dictionary basis and propose a spatially-aware sparse coding method to match testing images for identification while maintaining global shape correspondence. To accelerate the coding process for fast matching, we introduce a relaxed form that uses spatially-aware soft-thresholding during coding. Finally, we carry out an experimental study that demonstrates the effectiveness and efficiency of our exemplar selection and classification mechanisms, achieving $86.13\%$ accuracy on a difficult fine-grained species classification task distinguishing three types of fossil spruce pollen.Comment: CVMI 201

Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies: - I. New constraints on cosmic reionisation from the luminosity and redshift-dependent fraction of Lyman-alpha emission

We present results from a new Keck spectroscopic survey of UV-faint LBGs in the redshift range 3<z<7. Combined with earlier Keck and published ESO VLT data, our sample contains more than 600 dropouts, offering new insight into the nature of sub-L* sources typical of those likely to dominate the cosmic reionisation process. Here we use this sample to characterise the fraction of strong Lya emitters within the continuum-selected dropouts. By quantifying how the "Lya fraction" varies with redshift, we seek to constrain changes in Lya transmission associated with reionisation. In order to distinguish the effects of reionisation from other factors which affect the Lya fraction (e.g. dust, ISM kinematics), we study the luminosity and redshift-dependence of the Lya fraction over 3<z<6, when the IGM is known to be ionised. These results reveal that low luminosity galaxies show strong Lya emission much more frequently than luminous systems, and that at fixed luminosity, the prevalence of strong Lya emission increases moderately with redshift over 3 < z < 6. Based on the correlation between blue UV slopes and strong Lya emitting galaxies in our dataset, we argue that the Lya fraction trends are governed by redshift and luminosity-dependent variations in the dust obscuration, with likely additional contributions from trends in the kinematics and covering fraction of neutral hydrogen. We find a tentative decrease in the Lya fraction at z~7 based on the limited IR spectra for candidate z~7 lensed LBGs, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the supply of z and Y-drops now available from Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Lya fraction using optical and near-IR spectrographs, thereby extending the study conducted in this paper to 7<z<8.Comment: 23 pages, 15 figures, submitted to MNRA