arXiv subjects

Matt J. Jarvis

Publications and source records attributed to Matt J. Jarvis.

At least 73 records · Page 4Linked to original sources

arXiv2020

A flexible method for estimating luminosity functions via Kernel Density Estimation

We propose a flexible method for estimating luminosity functions (LFs) based on kernel density estimation (KDE), the most popular nonparametric density estimation approach developed in modern statistics, to overcome issues surrounding binning of LFs. One challenge in applying KDE to LFs is how to treat the boundary bias problem, since astronomical surveys usually obtain truncated samples predominantly due to the flux-density limits of surveys. We use two solutions, the transformation KDE method ($\hatϕ_{\mathrm{t}}$), and the transformation-reflection KDE method ($\hatϕ_{\mathrm{tr}}$) to reduce the boundary bias. We develop a new likelihood cross-validation criterion for selecting optimal bandwidths, based on which, the posterior probability distribution of bandwidth and transformation parameters for $\hatϕ_{\mathrm{t}}$ and $\hatϕ_{\mathrm{tr}}$ are derived within a Markov chain Monte Carlo (MCMC) sampling procedure. The simulation result shows that $\hatϕ_{\mathrm{t}}$ and $\hatϕ_{\mathrm{tr}}$ perform better than the traditional binned method, especially in the sparse data regime around the flux-limit of a survey or at the bright-end of the LF. To further improve the performance of our KDE methods, we develop the transformation-reflection adaptive KDE approach ($\hatϕ_{\mathrm{tra}}$). Monte Carlo simulations suggest that it has a good stability and reliability in performance, and is around an order of magnitude more accurate than using the binned method. By applying our adaptive KDE method to a quasar sample, we find that it achieves estimates comparable to the rigorous determination by a previous work, while making far fewer assumptions about the LF. The KDE method we develop has the advantages of both parametric and non-parametric methods.

stat.ME↗

arXiv2020

The optically-selected 1.4-GHz quasar luminosity function below 1 mJy

We present the radio luminosity function (RLF) of optically-selected quasars below 1~mJy, constructed by applying a Bayesian-fitting stacking technique to objects well below the nominal radio flux-density limit. We test the technique using simulated data, confirming that we can reconstruct the RLF over three orders of magnitude below the typical $5σ$ detection threshold. We apply our method to 1.4-GHz flux-densities from the Faint Images of the Radio Sky at Twenty-cm survey (FIRST), extracted at the positions of optical quasars from the Sloan Digital Sky Survey (SDSS) over seven redshift bins up to $z=2.15$ {and measure the RLF down to two orders of magnitude below the FIRST detection threshold}. In the lowest redshift bin ($0.2<z<0.45$), we find that our measured RLF agrees well with deeper data from the literature. The RLF for the radio-loud quasars flattens below $\log_{10}[L_{1.4}/{\rm W\,Hz}^{-1}] \approx 25.5$ and becomes steeper again below $\log_{10}[L_{1.4}/{\rm W\,Hz}^{-1}] \approx 24.8$, where radio-quiet quasars start to emerge. The radio luminosity where radio-quiet quasars emerge coincides with the luminosity where star-forming galaxies are expected to start to dominate the radio source counts. This implies that there could be a significant contribution from star formation in the host galaxies, but additional data is required to investigate this further. The higher-redshift bins show a similar behaviour as for the lowest-$z$ bin, implying that the same physical process may be responsible.

astro-ph.GA↗

arXiv2019

Non-Gaussianity Constraints using Future Radio Continuum Surveys and the Multi-Tracer Technique

Tighter constraints on measurements of primordial non-Gaussianity will allow the differentiation of inflationary scenarios. The cosmic microwave background bispectrum-the standard method of measuring the local non-Gaussianity-is limited by cosmic variance. Therefore, it is sensible to investigate measurements of non-Gaussianity using the large-scale structure. This can be done by investigating the effects of non-Gaussianity on the power spectrum on large scales. In this study we forecast the constraints on the local primordial non-Gaussianity parameter $f_{\rm NL}$ that can be obtained with future radio surveys. We utilize the multi-tracer method which reduces the effect of cosmic variance and takes advantage of the multiple radio galaxy populations which are differently biased tracers of the same underlying dark matter distribution. Improvements on previous work include the use of observational bias and halo mass estimates, updated simulations and realistic photometric redshift expectations, thus producing more realistic forecasts. Combinations of SKA simulations and radio observations were used as well as different redshift ranges and redshift bin sizes. It was found that in the most realistic case the 1 - $σ$ error on $f_{\rm NL}$ falls within the range 4.07 and 6.58, rivalling the tightest constraints currently available.

astro-ph.CO↗

arXiv2019

Accretion and star formation in 'radio-quiet' quasars

Radio observations allow us to identify a wide range of active galactic nuclei (AGN), which play a significant role in the evolution of galaxies. Amongst AGN at low radio-luminosities is the 'radio-quiet' quasar (RQQ) population, but how they contribute to the total radio emission is under debate, with previous studies arguing that it is predominantly through star formation. In this talk, SVW summarised the results of recent papers on RQQs, including the use of far-infrared data to disentangle the radio emission from the AGN and that from star formation. This provides evidence that black-hole accretion, instead, dominates the radio emission in RQQs. In addition, we find that this accretion-related emission is correlated with the optical luminosity of the quasar, whilst a weaker luminosity-dependence is evident for the radio emission connected with star formation. What remains unclear is the process by which this accretion-related emission is produced. Understanding this for RQQs will then allow us to investigate how this type of AGN influences its surroundings. Such studies have important implications for modelling AGN feedback, and for determining the accretion and star-formation histories of the Universe.

astro-ph.GA↗

arXiv2019

Measuring the HI mass function below the detection threshold

We present a Bayesian Stacking technique to directly measure the HI mass function (HIMF) and its evolution with redshift using galaxies formally below the nominal detection threshold. We generate galaxy samples over several sky areas given an assumed HIMF described by a Schechter function and simulate the HI emission lines with different levels of background noise to test the technique. We use Multinest to constrain the parameters of the HIMF in a broad redshift bin, demonstrating that the HIMF can be accurately reconstructed, using the simulated spectral cube far below the HI mass limit determined by the $5σ$ flux-density limit, i.e. down to $M_{\rm HI} = 10^{7.5}$ M$_{\odot}$ over the redshift range $0 < z < 0.55$ for this particular simulation, with a noise level similar to that expected for the MIGHTEE survey. We also find that the constraints on the parameters of the Schechter function, $ϕ_{\star}$, $M_\star$ and $α$ can be reliably fit, becoming tighter as the background noise decreases as expected, although the constraints on the redshift evolution are not significantly affected. All the parameters become better constrained as the survey area increases. In summary, we provide an optimal method for estimating the HI mass at cosmological distances that allows us to constrain the HI mass function below the detection threshold in forthcoming HI surveys. This study is a first step towards the measurement of the HIMF at high ($z>0.1$) redshifts.

astro-ph.GA↗

arXiv2019

A new sample of southern radio galaxies: Host galaxy masses and star-formation rates

In this study we define a new sample of distant powerful radio galaxies in order to study their host-galaxy properties and provide targets for future observations of HI absorption with new radio telescopes and to understand the fuelling and feedback from such sources. We have cross-matched the Sydney University Molonglo Sky Survey (SUMSS) radio catalogue at 843 MHz with the VISTA Hemisphere Survey (VHS) near-infrared catalogue using the Likelihood Ratio technique. Photometric redshifts from the Dark Energy Survey are then used to assign redshifts to the radio source counterparts. We found a total of 249 radio sources with photometric redshifts over a 148 deg^2 region. By fitting the optical and near-infrared photometry with spectral synthesis models we determine the stellar mass and star-formation rates of the radio sources, finding typical stellar masses of 10^{11} - 10^{12}M$_{\odot}$ for the powerful high-redshift radio galaxies. We also find a population of low-mass blue galaxies. However, by comparing the derived star-formation rates to the radio luminosity, we suggest that these sources are false positives in our likelihood ratio analysis. Follow up, higher-resolution (<5 arcsec) radio imaging would help alleviate these mid-identifications, as the limiting factor in our cross-identifications is the low resolution (~45 arcsec) of the SUMSS radio imaging.

astro-ph.GA↗

arXiv2018

Cosmology with Phase 1 of the Square Kilometre Array; Red Book 2018: Technical specifications and performance forecasts

We present a detailed overview of the cosmological surveys that will be carried out with Phase 1 of the Square Kilometre Array (SKA1), and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5,000 sqdeg; a wide and deep continuum galaxy and HI intensity mapping survey over 20,000 sqdeg from z = 0.35 - 3; and a deep, high-redshift HI intensity mapping survey over 100 sqdeg from z = 3 - 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to z ~ 3 with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to z = 6. These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical surveys like LSST and Euclid, leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys, and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

astro-ph.CO↗

arXiv2018

The Stripe 82 1-2 GHz Very Large Array Snapshot Survey: host galaxy properties and accretion rates of radio galaxies

A sample of 1161 radio galaxies with $0.01 < z < 0.7$ and $10^{21} < L_{1.4~\rm GHz} / \textrm{W Hz}^{-1} < 10^{27}$ is selected from the Stripe 82 1-2 GHz Karl G. Jansky Very Large Array Snapshot Survey, which covers 100 square degrees and has a 1$σ$ noise level of 88 $μ$Jy beam$^{-1}$. Optical spectra are used to classify these sources as high excitation and low excitation radio galaxies (HERGs and LERGs), resulting in 60 HERGs, 149 LERGs and 600 `probable LERGs'. The host galaxies of the LERGs have older stellar populations than those of the HERGs, in agreement with previous results in the literature. We find that the HERGs tend to have higher Eddington-scaled accretion rates than the LERGs but that there is some overlap between the two distributions. We show that the properties of the host galaxies vary continuously with accretion rate, with the most slowly accreting sources having the oldest stellar populations, consistent with the idea that these sources lack a supply of cold gas. We find that 84 per cent of our sample release more than 10 per cent of their accretion power in their jets, showing that mechanical AGN feedback is significantly underestimated in many hydrodynamical simulations. There is a scatter of $\sim 2$ dex in the fraction of the accreted AGN power deposited back into the ISM in mechanical form, showing that the assumption in many simulations that there is a direct scaling between accretion rate and radio-mode feedback does not necessarily hold. We also find that mechanical feedback is significant for many of the HERGs in our sample as well as the LERGs.

astro-ph.GA↗

arXiv2018

Photometric redshifts for the next generation of deep radio continuum surveys - II. Gaussian processes and hybrid estimates

Building on the first paper in this series (Duncan et al. 2018), we present a study investigating the performance of Gaussian process photometric redshift (photo-z) estimates for galaxies and active galactic nuclei detected in deep radio continuum surveys. A Gaussian process redshift code is used to produce photo-z estimates targeting specific subsets of both the AGN population - infrared, X-ray and optically selected AGN - and the general galaxy population. The new estimates for the AGN population are found to perform significantly better at z > 1 than the template-based photo-z estimates presented in our previous study. Our new photo-z estimates are then combined with template estimates through hierarchical Bayesian combination to produce a hybrid consensus estimate that outperforms either of the individual methods across all source types. Photo-z estimates for radio sources that are X-ray sources or optical/IR AGN are signficantly improved in comparison to previous template-only estimates, with outlier fractions and robust scatter reduced by up to a factor of ~4. The ability of our method to combine the strengths of the two input photo-z techniques and the large improvements we observe illustrate its potential for enabling future exploitation of deep radio continuum surveys for both the study of galaxy and black hole co-evolution and for cosmological studies.

astro-ph.GA↗

arXiv2018

LOFAR/H-ATLAS: The low-frequency radio luminosity - star-formation rate relation

Radio emission is a key indicator of star-formation activity in galaxies, but the radio luminosity-star formation relation has to date been studied almost exclusively at frequencies of 1.4 GHz or above. At lower radio frequencies the effects of thermal radio emission are greatly reduced, and so we would expect the radio emission observed to be completely dominated by synchrotron radiation from supernova-generated cosmic rays. As part of the LOFAR Surveys Key Science project, the Herschel-ATLAS NGP field has been surveyed with LOFAR at an effective frequency of 150 MHz. We select a sample from the MPA-JHU catalogue of SDSS galaxies in this area: the combination of Herschel, optical and mid-infrared data enable us to derive star-formation rates (SFRs) for our sources using spectral energy distribution fitting, allowing a detailed study of the low-frequency radio luminosity--star-formation relation in the nearby Universe. For those objects selected as star-forming galaxies (SFGs) using optical emission line diagnostics, we find a tight relationship between the 150 MHz radio luminosity ($L_{150}$) and SFR. Interestingly, we find that a single power-law relationship between $L_{150}$ and SFR is not a good description of all SFGs: a broken power law model provides a better fit. This may indicate an additional mechanism for the generation of radio-emitting cosmic rays. Also, at given SFR, the radio luminosity depends on the stellar mass of the galaxy. Objects which were not classified as SFGs have higher 150-MHz radio luminosity than would be expected given their SFR, implying an important role for low-level active galactic nucleus activity.

astro-ph.GA↗

arXiv2017

Improving Photometric Redshift Estimation using GPz: size information, post processing and improved photometry

The next generation of large scale imaging surveys (such as those conducted with the Large Synoptic Survey Telescope and Euclid) will require accurate photometric redshifts in order to optimally extract cosmological information. Gaussian Processes for photometric redshift estimation (GPz) is a promising new method that has been proven to provide efficient, accurate photometric redshift estimations with reliable variance predictions. In this paper, we investigate a number of methods for improving the photometric redshift estimations obtained using GPz (but which are also applicable to others). We use spectroscopy from the Galaxy and Mass Assembly Data Release 2 with a limiting magnitude of r<19.4 along with corresponding Sloan Digital Sky Survey visible (ugriz) photometry and the UKIRT Infrared Deep Sky Survey Large Area Survey near-IR (YJHK) photometry. We evaluate the effects of adding near-IR magnitudes and angular size as features for the training, validation and testing of GPz and find that these improve the accuracy of the results by ~15-20 per cent. In addition, we explore a post-processing method of shifting the probability distributions of the estimated redshifts based on their Quantile-Quantile plots and find that it improves the bias by ~40 per cent. Finally, we investigate the effects of using more precise photometry obtained from the Hyper Suprime-Cam Subaru Strategic Program Data Release 1 and find that it produces significant improvements in accuracy, similar to the effect of including additional features.

astro-ph.GA↗

arXiv2017

The New Galaxy Evolution Paradigm Revealed by the Herschel Surveys

The Herschel Space Observatory has revealed a very different galaxyscape from that shown by optical surveys which presents a challenge for galaxy-evolution models. The Herschel surveys reveal (1) that there was rapid galaxy evolution in the very recent past and (2) that galaxies lie on a a single Galaxy Sequence (GS) rather than a star-forming `main sequence' and a separate region of `passive' or `red-and-dead' galaxies. The form of the GS is now clearer because far-infrared surveys such as the Herschel ATLAS pick up a population of optically-red star-forming galaxies that would have been classified as passive using most optical criteria. The space-density of this population is at least as high as the traditional star-forming population. By stacking spectra of H-ATLAS galaxies over the redshift range 0.001 < z < 0.4, we show that the galaxies responsible for the rapid low-redshift evolution have high stellar masses, high star-formation rates but, even several billion years in the past, old stellar populations - they are thus likely to be relatively recent ancestors of early-type galaxies in the Universe today. The form of the GS is inconsistent with rapid quenching models and neither the analytic bathtub model nor the hydrodynamical EAGLE simulation can reproduce the rapid cosmic evolution. We propose a new gentler model of galaxy evolution that can explain the new Herschel results and other key properties of the galaxy population.

astro-ph.GA↗

arXiv2017

Photometric redshifts for the next generation of deep radio continuum surveys - I: Template fitting

We present a study of photometric redshift performance for galaxies and active galactic nuclei detected in deep radio continuum surveys. Using two multi-wavelength datasets, over the NOAO Deep Wide Field Survey Boötes and COSMOS fields, we assess photometric redshift (photo-z) performance for a sample of $\sim 4,500$ radio continuum sources with spectroscopic redshifts relative to those of $\sim 63,000$ non radio-detected sources in the same fields. We investigate the performance of three photometric redshift template sets as a function of redshift, radio luminosity and infrared/X-ray properties. We find that no single template library is able to provide the best performance across all subsets of the radio detected population, with variation in the optimum template set both between subsets and between fields. Through a hierarchical Bayesian combination of the photo-z estimates from all three template sets, we are able to produce a consensus photo-z estimate which equals or improves upon the performance of any individual template set.

astro-ph.GA↗

arXiv2017

The MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey

The MIGHTEE large survey project will survey four of the most well-studied extragalactic deep fields, totalling 20 square degrees to $μ$Jy sensitivity at Giga-Hertz frequencies, as well as an ultra-deep image of a single ~1 square degree MeerKAT pointing. The observations will provide radio continuum, spectral line and polarisation information. As such, MIGHTEE, along with the excellent multi-wavelength data already available in these deep fields, will allow a range of science to be achieved. Specifically, MIGHTEE is designed to significantly enhance our understanding of, (i) the evolution of AGN and star-formation activity over cosmic time, as a function of stellar mass and environment, free of dust obscuration; (ii) the evolution of neutral hydrogen in the Universe and how this neutral gas eventually turns into stars after moving through the molecular phase, and how efficiently this can fuel AGN activity; (iii) the properties of cosmic magnetic fields and how they evolve in clusters, filaments and galaxies. MIGHTEE will reach similar depth to the planned SKA all-sky survey, and thus will provide a pilot to the cosmology experiments that will be carried out by the SKA over a much larger survey volume.

astro-ph.GA↗

arXiv2017

Large Synoptic Survey Telescope Galaxies Science Roadmap

The Large Synoptic Survey Telescope (LSST) will enable revolutionary studies of galaxies, dark matter, and black holes over cosmic time. The LSST Galaxies Science Collaboration has identified a host of preparatory research tasks required to leverage fully the LSST dataset for extragalactic science beyond the study of dark energy. This Galaxies Science Roadmap provides a brief introduction to critical extragalactic science to be conducted ahead of LSST operations, and a detailed list of preparatory science tasks including the motivation, activities, and deliverables associated with each. The Galaxies Science Roadmap will serve as a guiding document for researchers interested in conducting extragalactic science in anticipation of the forthcoming LSST era.

astro-ph.GA↗

arXiv2017

The prevalence of core emission in faint radio galaxies in the SKA Simulated Skies

Empirical simulations based on extrapolations from well-established low-frequency ($< 5$ GHz) surveys fail to accurately model the faint, high frequency ($>10$~GHz) source population; they under-predict the number of observed sources by a factor of two below $S_{18~\rm GHz} = 10$ mJy and fail to reproduce the observed spectral index distribution. We suggest that this is because the faint radio galaxies are not modelled correctly in the simulations and show that by adding a flat-spectrum core component to the FRI sources in the SKA Simulated Skies, the observed 15-GHz source counts can be reproduced. We find that the observations are best matched by assuming that the fraction of the total 1.4-GHz flux density which originates from the core varies with 1.4-GHz luminosity; sources with 1.4-GHz luminosities $< 10^{25} \rm W \, Hz^{-1}$ require a core fraction $\sim 0.3$, while the more luminous sources require a much smaller core fraction of $5 \times 10^{-4}$. The low luminosity FRI sources with high core fractions which were not included in the original simulation may be equivalent to the compact `FR0' sources found in recent studies.

astro-ph.GA↗

arXiv2017

Investigating the Unification of LOFAR-detected powerful AGN in the Boötes Field

Low radio frequency surveys are important for testing unified models of radio-loud quasars and radio galaxies. Intrinsically similar sources that are randomly oriented on the sky will have different projected linear sizes. Measuring the projected linear sizes of these sources provides an indication of their orientation. Steep-spectrum isotropic radio emission allows for orientation-free sample selection at low radio frequencies. We use a new radio survey of the Boötes field at 150 MHz made with the Low Frequency Array (LOFAR) to select a sample of radio sources. We identify 44 radio galaxies and 16 quasars with powers $P>10^{25.5}$ W Hz$^{-1}$ at 150 MHz using cross-matched multi-wavelength information from the AGN and Galaxy Evolution Survey (AGES), which provides spectroscopic redshifts. We find that LOFAR-detected radio sources with steep spectra have projected linear sizes that are on average 4.4$\pm$1.4 larger than those with flat spectra. The projected linear sizes of radio galaxies are on average 3.1$\pm$1.0 larger than those of quasars (2.0$\pm$0.3 after correcting for redshift evolution). Combining these results with three previous surveys, we find that the projected linear sizes of radio galaxies and quasars depend on redshift but not on power. The projected linear size ratio does not correlate with either parameter. The LOFAR data is consistent within the uncertainties with theoretical predictions of the correlation between the quasar fraction and linear size ratio, based on an orientation-based unification scheme.

astro-ph.GA↗

arXiv2017

Calibrating photometric redshifts with intensity mapping observations

Imaging surveys of galaxies will have a high number density and angular resolution yet a poor redshift precision. Intensity maps of neutral hydrogen (HI) will have accurate redshift resolution yet will not resolve individual sources. Using this complementarity, we show how the clustering redshifts approach, proposed for spectroscopic surveys can also be used in combination with intensity mapping observations to calibrate the redshift distribution of galaxies in an imaging survey and, as a result, reduce uncertainties in photometric redshift measurements. We show how the intensity mapping surveys to be carried out with the MeerKAT, HIRAX and SKA instruments can improve photometric redshift uncertainties to well below the requirements of DES and LSST. The effectiveness of this method as a function of instrumental parameters, foreground subtraction and other potential systematic errors is discussed in detail.

astro-ph.CO↗