Using NIR spectral features in an SDSS sample of early-type galaxies to constrain the low-mass Initial Mass Function

The stellar initial mass function (IMF) is a major component in galaxy formation theory, it describes the original distribution of stars as a function of mass at the epoch of formation of the population. To investigate the form and possible variation of the IMF at low stellar-masses, features are measured in spectra from the Sloan Digital Sky Survey DR7 for a sample of ∼2000, low-redshift (0.010 - 0.057) red sequence galaxies and in simple stellar population models that cover a range of elemental abundances and IMFs. Particular attention is paid to the calcium triplet (at ∼8600 Å) and the sodium doublet (at ∼8200 Å) which are characteristic of high-mass ( 8 M ) and low-mass ( 0.5 M ) stars respectively and the NaD (at 5895 Å) and CaI (at 4227 Å) features which show a strong response to their respective elemental abundances. The combination of these spectral features is a useful technique for separating the effects of elemental abundance and IMF on the size of the measured spectral features, allowing the form and variation of the IMF to be investigated individually. The sodium abundance is constrained from the NaD index, popularly disregarded as an abundance indicator due to the effect of interstellar absorption on this feature. In this work there is no strong evidence to suggest that interstellar absorption has a detrimental effect on the abundance constraints from the NaD index. Evidence is found for an increasing sodium abundance ([Na/Fe] ∝ σ 1.654±0.050 ) and marginally decreasing calcium abundance ([Ca/Fe] ∝ σ −0.19±0.12 ) with increasing galaxy mass. With these abundances taken into consideration, the IMF variation found is slightly shallower at higher masses but also consistent with zero; ∆x = -0.111±0.071 ∆ log σ (where x is the power-law index describing the slope of the IMF). Taking into consideration the average abundances, the best-fitting IMF is found to be approximately Chabrier-like, with outlying galaxies that correspond to more extreme environments including high sodium abundances and bottom-heavy IMFs

MOPTOP: A multi-colour optimised optical polarimeter

We present the design and science case for the Liverpool Telescope's fourth-generation polarimeter; MOPTOP: a Multicolour OPTimised Optical Polarimeter which is optimised for sensitivity and bi-colour observations. We introduce an optimised polarimeter which is as far as possible limited only by the photon counting efficiency of the detectors. Using a combination of CMOS cameras, a continuously rotating half-wave plate and a wire grid polarising beamsplitter, we predict we can accurately measure the polarisation of sources to ∼ 1% at ∼19th magnitude in 10 minutes on a 2 metre telescope. For brighter sources we anticipate much low systematics (0.1%) than our current polarimeter. The design also gives the ability to measure polarization and photometric variability on timescales as short as a few seconds. Overall the instrument will allow accurate measurements of the intra-nightly variability of the polarisation of sources such as gamma-ray bursts and blazars (AGN orientated with the jet pointing toward the observer), allowing the constraint of magnetic field models revealing more information about the formation, ejection and collimation of jets. © 2016 SPIE

Robotic Polarimetry of Blazars

The motivation of this thesis was the study of radio-loud, active galaxies. These galaxies house relativistic jets at their centres, powered by accretion onto a super massive black hole. The focus was on the optical flux and polarised emission produced by these powerful jets. An automated pipeline was developed to reduce data from the Liverpool Telescope Ringo2 and Ringo3 polarimeters. As part of this work, the Ringo3 instrumental polarisation and depolarisation were characterised by repeated observations of standard stars. The Ringo2 and Ringo3 optical polarimetry and photometry of a sample of 20 gamma-ray bright blazars were combined with Fermi gamma-ray space telescope data and were used to explore possible correlations and thus probe the emission sites in the jet. We found that optical and gamma-ray fluxes had strong, positive correlations. This suggests that the dominant source of optical and gamma-ray emission is from shared emission regions. If the Inverse Compton model is adopted to explain the gamma-ray emission (i.e. upscattering of photons by relativistic electrons), this correlation suggests that synchrotron self-Compton emission processes are occurring in the jet, along with inverse Compton upscattering from nearby electrons (rather than those outside the jet). The gamma-ray flux and optical degree of polarisation were not significantly correlated. The optical flux and degree of polarisation were weakly positively correlated (with correlations that did not improve with an introduced lag). Both of these results imply that there is no large scale highly ordered magnetic field in the region where the gamma-ray emission originates. We found that the maximum degree of polarisation differs depending on the location of the source's synchrotron-peak. This may be a result of the viewing angle of the observer with respect to the jet. This suggests that the majority of optical polarisation is produced in shocked regions within the jet, downstream of the main emission region. We found that the degree of polarisation was lower during a period of polarisation angle rotation compared with a period of non-rotation. This implies that the downstream magnetic field structure is either helical or compressed in a direction transverse to that of the jet. Consistent with other work, our Ringo3 colour analysis showed that, with the exception of one source, flat spectrum radio quasars had a `redder' when brighter property. This suggests that when the source is more luminous, the jet (i.e. non-thermal) emission dominates over the thermal emission from the accretion disk (which is powerful in FSRQs). We found that BL~Lacs had a `bluer' when brighter behaviour, suggesting that the brighter emission may come from more energetic photons within the jet. We presented data from our long-term, multi-colour, blazar monitoring campaign. We found that all but one source had a `redder' polarisation when the polarisation was higher. This implies that the highest polarisation is associated with higher densities of lower energy particles in the jet. Well-sampled, regular cadence data is very important for the effective study and interpretation of blazars. This is particularly crucial for the interpretation of the position angle rotations, which can afford information about the electric vector angle (and hence the magnetic field angle). In this work, we presented the design of a new multicolour polarimeter, MOPTOP. The optical components in MOPTOP allow as much of the light from the source to be exploited as possible by replacing the rotating Polaroid (from the Ringo polarimeter design) with a rotating half-wave plate and beam splitter. MOPTOP's design minimises exposure times, allowing more frequent observations and a better sampling of data. A densely sampled monitoring program that is not interrupted by periods of sunlight would be highly desirable for the study of blazar jets

Equality, diversity and inclusion perspectives

An overview of the developments arising from equality, diversity and inclusion events at this year’s National Astronomy Meeting, by Vinesh Maguire-Rajpaul on behalf of the organizers

Characterization of a dual-beam, dual-camera optical imaging polarimeter

Polarization plays an important role in various time-domain astrophysics to understand the magnetic fields, geometry, and environments of spatially unresolved variable sources. In this paper we present the results of laboratory and on-sky testing of a novel dual-beam, dual-camera optical imaging polarimeter (MOPTOP) exploiting high sensitivity, low-noise CMOS technology and designed to monitor variable and transient sources with low systematic errors and high sensitivity. We present a data reduction algorithm that corrects for sensitivity variations between the cameras on a source-by-source basis. Using our data reduction algorithm, we show that our dual-beam, dual-camera technique delivers the benefits of low and stable instrumental polarization ($<0.05$\% for lab data and $<0.25$\% for on sky data) and high throughput while avoiding the additional sky brightness and image overlap problems associated with dual-beam, single-camera polarimeters

IO:I, a near-infrared camera for the Liverpool Telescope

IO:I is a new instrument that has recently been commissioned for the Liverpool Telescope, extending current imaging capabilities beyond the optical and into the near-infrared. Cost has been minimized by the use of a previously decommissioned instrument's cryostat as the base for a prototype and retrofitting it with Teledyne's 1.7-μm cutoff Hawaii-2RG HgCdTe detector, SIDECAR ASIC controller, and JADE2 interface card. The mechanical, electronic, and cryogenic aspects of the cryostat retrofitting process will be reviewed together with a description of the software/hardware setup. This is followed by a discussion of the results derived from characterization tests, including measurements of read noise, conversion gain, full well depth, and linearity. The paper closes with a brief overview of the autonomous data reduction process and the presentation of results from photometric testing conducted on on-sky, pipeline processed data. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)

Equity and diversity in astronomy

`Why is there a black hole where women should be?' asked Member of Parliament Chi Onwurah during her plenary talk on women in science at EWASS 2018. Gender equity was among a variety of topics discussed in a day-long Special Session

Experiments with synchronized sCMOS cameras

Scientific-CMOS (sCMOS) cameras can combine low noise with high readout speeds and do not suffer the charge multiplication noise that effectively reduces the quantum efficiency of electron multiplying CCDs by a factor 2. As such they have strong potential in fast photometry and polarimetry instrumentation. In this paper we describe the results of laboratory experiments using a pair of commercial off the shelf sCMOS cameras based around a 4 transistor per pixel architecture. In particular using a both stable and a pulsed light sources we evaluate the timing precision that may be obtained when the cameras readouts are synchronized either in software or electronically. We find that software synchronization can introduce an error of ~ 200-msec. With electronic synchronization any error is below the limit (~ 50-msec) of our simple measurement technique. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

A study of Be stars in the time domain - I. Spectral data and polarimetry

We present the first part of a spectroscopic and polarimetric study on a sample of 58 Be stars that have been measured since 1998. The aim of the study is to understand the time-scales of disc variability, formation, and dissipation as a function of the properties (mass, luminosity, and rotational velocity) of the underlying B star. In this paper, we classified the sample based on the presence of emission or absorption of the Hα line, and the shape of the peak as single or double peak, as well as noting changes between emission and non-emission states. We find a probability of ∼0.75 per cent per year that an object in the sample will undergo such a change. We also present re-derived values of the projected rotational velocities for the sample. When we compare our polarization values with those from the literature, we find that most of the stars do not show a change in the value of the polarization angle; however, a small number show significant changes which could be attributed to either disc strength (optical depth) or geometry changes. Finally, we show how, by combining the (interstellar corrected) degree of polarization and the projected rotational velocity, we can construct an inclination angle-free parameter that includes the true equatorial velocity. Using this inclination angle-independent parameter, we show that the populations of single and double peak stars are indistinguishable, giving further evidence that Be star line profiles are essentially inclination angle driven

Air Gauge Characteristics Linearity Improvement

This paper discusses calibration uncertainty and linearity issues of the typical back-pressure air gauge. In this sort of air gauge, the correlation between the measured dimension (represented by the slot width) and the air pressure in the measuring chamber is used in a proportional range. However, when high linearity is required (e.g., nonlinearity less than 1%), the measuring range should be shortened. In the proposed method, based on knowledge of the static characteristics of air gauges, the measuring range is kept unchanged but the nonlinearity is decreased. The static characteristics may be separated into two sections, each of them approximated with a different linear function. As a result, the nonlinearity is reduced from 5% down to 1% and even below