Claire Burke

Little change in the sizes of the most massive galaxies since z = 1

Recent reports suggest that elliptical galaxies have increased their size dramatically over the last ~8 Gyr. This result points to a major rethink of the processes dominating the late-time evolution of galaxies. In this paper we present the first estimates for the scale sizes of brightest cluster galaxies (BCGs) in the redshift range 0.8 <z <1.3 from an analysis of deep Hubble Space Telescope imaging, comparing to a well-matched local sample taken from the Local Cluster Substructure Survey at z~ 0.2. For a small sample of five high-redshift BCGs we measure half-light radii ranging from 14 to 53kpc using de Vaucuoleurs profile fits, with an average determined from stacking of 32.1 ± 2.5kpc compared to a value 43.2 ± 1.0kpc for the low-redshift comparison sample. This implies that the scale sizes of BCGs at z= 1 are ≃30 per cent smaller than at z= 0.25. Analyses comparing either Sersic or Petrosian radii also indicate little or no evolution between the two samples. The detection of only modest evolution at most out to z= 1 argues against BCGs having undergone the large increase in size reported for massive galaxies since z= 2 and in fact the scale-size evolution of BCGs appears closer to that reported for radio galaxies over a similar epoch. We conclude that this lack of size evolution, particularly when coupled with recent results on the lack of BCG stellar mass evolution, demonstrates that major merging is not an important process in the late-time evolution of these systems. The homogeneity and maturity of BCGs at z= 1 continues to challenge galaxy evolution models.

Growth of brightest cluster galaxies via mergers since z = 1

Hierarchical assembly within clusters of galaxies is tied directly to the evolution of the Brightest Cluster Galaxies (BCGs), which dominate the stellar light in the centres of rich clusters. In this paper we investigate the number of mergers onto BCGs in 14 X-ray selected clusters over the redshift range 0.8 < z < 1.4 using HST imaging data. We find significant differences in the numbers of companion galaxies to BCGs between the clusters in our sample indicating that BCGs in similar mass clusters can have very different merging histories. Within a 50 kpc radius around the BCGs we find an average of 6.45 \pm 1.15 companion galaxies with mass ratios (companion:BCG) between 1:1 and 1:20. The infalling companions show a 50/50 split between major (1:1 - 1:2) and minor (1:3 - 1:20) mergers. When compared to similar work using lower redshift clusters, these results demonstrate that both major and minor merging was more common in the past. Since the dynamical timescales for merging onto the BCG are relatively short compared with the look-back time to z ~ 1 our results suggest that the BCG stellar mass may increase by as much as 1.8 times since z = 1. However the growth rate of BCGs will be substantially less if stripped material from nearby companions ends up in the diffuse intracluster light.

Measurement of the intracluster light at z ∼ 1

A significant fraction of the total photospheric light in nearby galaxy clusters is thought to be contained within the diffuse intracluster light (ICL), which extends hundreds of kpc from cluster cores. The study of the ICL can reveal details of the evolutionary histories and processes occurring within galaxy clusters; however, since it has a very low surface brightness, it is often difficult to detect. We present here the first measurements of the ICL as a fraction of the total cluster light at z ∼ 1 using deep J-band (1.2 μm) imaging from a high-acuity wide-field K-band imager on the Very Large Telescope. We investigate the ICL in six X-ray selected galaxy clusters at 0.8 ≤ z ≤ 1.2 and find that the ICL below isophotes μ J = 22 magarcsec -2 constitutes 1-4 per cent of the total cluster light within a radius R 500. This is broadly consistent with simulations of the ICL at a similar redshift and when compared to nearby observations suggests that the fraction of the total cluster light that is in the ICL has increased by a factor of 2-4 since z ∼ 1. We also find the fraction of the total cluster light contained within the brightest cluster galaxy (BCG) to be 2.0-6.3 per cent at these redshifts, which in five out of six cases is larger than the fraction of the ICL component, in contrast to the results from nearby clusters. This suggests that the evolution in cluster cores involves substantial stripping activity at late times, in addition to the early build-up of the BCG stellar mass through merging. The presence of significant amount of stellar light at large radii from these BCGs may help towards solving the recent disagreement between the semi-analytic model predictions of BCG mass growth and the observed large masses and scale sizes reported for BCGs at high redshift.

Detectable Anthropogenic Shift toward Heavy Precipitation over Eastern China

AbstractChanges in precipitation characteristics directly affect society through their impacts on drought and floods, hydro-dams, and urban drainage systems. Global warming increases the water holding capacity of the atmosphere and thus the risk of heavy precipitation. Here, daily precipitation records from over 700 Chinese stations from 1956 to 2005 are analyzed. The results show a significant shift from light to heavy precipitation over eastern China. An optimal fingerprinting analysis of simulations from 11 climate models driven by different combinations of historical anthropogenic (greenhouse gases, aerosols, land use, and ozone) and natural (volcanic and solar) forcings indicates that anthropogenic forcing on climate, including increases in greenhouse gases (GHGs), has had a detectable contribution to the observed shift toward heavy precipitation. Some evidence is found that anthropogenic aerosols (AAs) partially offset the effect of the GHG forcing, resulting in a weaker shift toward heavy precipita...

SALT spectroscopic observations of galaxy clusters detected by ACT and a type II quasar hosted by a brightest cluster galaxy

We present Southern African Large Telescope (SALT) follow-up observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equator using the Sunyaev-Zel’dovich (SZ) effect. We conducted multi-object spectroscopic observations with the Robert Stobie Spectrograph in order to measure galaxy redshifts in each cluster field, determine the cluster line-of-sight velocity dispersions, and infer the cluster dynamical masses. We find that the clusters, which span the redshift range 0:3 < z < 0:55, range in mass from (5 20) 10 14 M (M200c). Their masses, given their SZ signals, are similar to those of southern hemisphere ACT clusters previously observed using Gemini and the VLT. We note that the brightest cluster galaxy in one of the systems studied, ACT-CL J0320.4+0032 at z = 0:38, hosts a Type II quasar. To our knowledge, this is only the third such system discovered, and therefore may be a rare example of a very massive halo in which quasar-mode feedback is actively taking place.

Impact of Anthropogenic Climate Change on the East Asian Summer Monsoon

AbstractThe East Asian summer monsoon (EASM) is important for bringing rainfall to large areas of China. Historically, variations in the EASM have had major impacts including flooding and drought. The authors present an analysis of the impact of anthropogenic climate change on EASM rainfall in eastern China using a newly updated attribution system. The results suggest that anthropogenic climate change has led to an overall decrease in total monsoon rainfall over the past 65 years and an increased number of dry days. However, the model also predicts that anthropogenic forcings have caused the most extreme heavy rainfall events to become shorter in duration and more intense. With the potential for future changes in aerosol and greenhouse gas emissions, historical trends in monsoon rainfall may not be indicative of future changes, although extreme rainfall is projected to increase over East Asia with continued warming in the region.

Attribution of Extreme Rainfall in Southeast China During May 2015

LJMU has developed LJMU Research Online for users to access the research output of the University more effectively. Copyright

Human influence on the record-breaking cold event in January of 2016 in Eastern China

Anthropogenic influences are estimated to have reduced the likelihood of an extreme cold event in midwinter with the intensity equal to or stronger than the record of 2016 in eastern China by about two‐thirds.

Adapting thermal-infrared technology and astronomical techniques for use in conservation biology

Astro-Ecology couples ‘off the shelf’ infrared imaging technology and astronomy instrumentation techniques for application in the field of conservation biology. Microbolometers are uncooled, infrared systems that image in the thermal-infrared range (8-15μm). These cameras are potentially ideal to use for the detection and monitoring of vulnerable species and are readily available as ’off the shelf’ systems. However to optimise the quality of the data for this purpose requires thorough detector calibration to account for the systematics that limit readout accuracy. In this paper we apply three analogous, standard astronomical instrumentation techniques to characterise the random and spatial noise present in a FLIR Tau 2 Core thermal-infrared camera. We use flat fielding, stacking and binning to determine that microbolometer FPAs are dominated by large structure noise and demonstrate how this can be corrected by subtracting median stacks of flat field exposures.

Addressing environmental and atmospheric challenges for capturing high-precision thermal infrared data in the field of astro-ecology

Using thermal infrared detectors mounted on drones, and applying techniques from astrophysics, we hope to support the field of conservation ecology by creating an automated pipeline for the detection and identification of certain endangered species and poachers from thermal infrared data. We test part of our system by attempting to detect simulated poachers in the field. Whilst we find that we can detect humans hiding in the field in some types of terrain, we also find several environmental factors that prevent accurate detection, such as ambient heat from the ground, absorption of infrared emission by the atmosphere, obscuring vegetation and spurious sources from the terrain. We discuss the effect of these issues, and potential solutions which will be required for our future vision for a fully automated drone-based global conservation monitoring system.