arXiv subjects

Colin Norman

Publications and source records attributed to Colin Norman.

At least 55 records · Page 3Linked to original sources

arXiv2020

The Chemical Evolution of Galaxy Clusters: Dissecting the Iron Mass Budget of the Intracluster Medium

We study the chemical evolution of galaxy clusters by measuring the iron mass in the ICM after dissecting the abundance profiles into different components. We use Chandra archival observations of 186 morphologically regular clusters in the redshift range [0.04, 1.07]. For each cluster we compute the iron abundance and gas density profiles. We aim at identifying in the iron distribution a central peak associated with the BCG, and an approximately constant plateau associated with early enrichment. We are able to firmly identify the two components in a significant fraction of the sample, simply relying on the fit of the abundance profile. We compute the iron mass included in the iron peak and plateau, and the gas mass-weighted iron abundance out to $r_{500}$. While the iron plateau shows no evolution, we find marginal decrease with redshift in the iron peak. We measure that the fraction of iron peak mass is typically a few percent (~1%) of the total iron mass within $r_{500}$. Therefore, since the total iron mass budget is dominated by the plateau, we find consistently that the global gas mass-weighted iron abundance does not evolve significantly. We are also able to reproduce past claims of evolution in the global iron abundance, which turn out to be due to the use of cluster samples with different selection methods combined to the use of emission-weighted instead of gas mass-weighted abundance values. Finally, while the intrinsic scatter in the iron plateau mass is consistent with zero, the iron peak mass exhibits a large scatter, in line with the fact that the peak is produced after the virialization of the halo and depends on the formation of the hosting cool core and the associated feedback processes. We conclude that only a spatially-resolved approach can resolve the issue of the ICM iron evolution, reconciling the contradictory results obtained in the last ten years.

astro-ph.GA↗

arXiv2019

QSO obscuration at high redshift ($z \gtrsim 7$): Predictions from the BlueTides simulation

High-$z$ AGNs hosted in gas rich galaxies are expected to grow through significantly obscured accretion phases. This may limit or bias their observability. In this work, we use \textsc{BlueTides}, a large volume cosmological simulation of galaxy formation to examine quasar obscuration for the highest-redshift ($z \geq 7$) supermassive black holes residing in the center of galaxies. We find that for the bright quasars, most of the high column density gas ($>90\%$) resides in the innermost regions of the host galaxy, (typically within $< 10$ ckpc), while the gas in the outskirts is a minor contributor to the $N_\mathrm H$. The brightest quasars can have large angular variations in galactic obscuration, over 2 orders of magnitude, where the lines of sight with the lowest obscuration are those formed via strong gas outflows driven by AGN feedback. We find that for the overall AGN population, the mean $N_\mathrm H$ is generally larger for high luminosity and BH mass, while the $N_\mathrm H$ distribution is significantly broadened, developing a low $N_\mathrm H $ wing due to the angular variations driven by the AGN outflows/feedback. The obscured fraction P($N_{\rm H} > 10^{23} {\rm cm}^{-2}$) typically range from 0.6 to 1.0 for increasing $L_{X}$ (with $L_X > 10^{43} \rm{ergs/s}$), with no clear trend of redshift evolution. With respect to the galaxy host property, we find a linear relation between $N_{\rm H}$, $M_*$ and $M_{\rm H_2}$ with $\log N_{\rm H} = (0.24 \pm 0.03) \log M_{*} + (20.7 \pm 0.3)$ and $\log N_{\rm H} = (0.47 \pm 0.03) \log M_{\rm H_2} + (18.4 \pm 0.3)$. The dust optical depth in the UV band $τ_{\mathrm UV}$ has tight positive correlation with $N_{\rm H}$. Our dust extincted UVLF is about 1.5 dex lower than the intrinsic UVLF, implying that more than 99\% of the $z \sim 7$ AGNs are heavily dust extincted and therefore would be missed by the UV band observation.

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arXiv2019

An Exo-Kuiper Belt and An Extended Halo around HD 191089 in Scattered Light

We have obtained Hubble Space Telescope STIS and NICMOS, and Gemini/GPI scattered light images of the HD 191089 debris disk. We identify two spatial components: a ring resembling Kuiper Belt in radial extent (FWHM: ${\sim}$25 au, centered at ${\sim}$46 au), and a halo extending to ${\sim}$640 au. We find that the halo is significantly bluer than the ring, consistent with the scenario that the ring serves as the "birth ring" for the smaller dust in the halo. We measure the scattering phase functions in the 30°-150° scattering angle range and find the halo dust is both more forward- and backward-scattering than the ring dust. We measure a surface density power law index of -0.68${\pm}$0.04 for the halo, which indicates the slow-down of the radial outward motion of the dust. Using radiative transfer modeling, we attempt to simultaneously reproduce the (visible) total and (near-infrared) polarized intensity images of the birth ring. Our modeling leads to mutually inconsistent results, indicating that more complex models, such as the inclusion of more realistic aggregate particles, are needed.

astro-ph.EP↗

arXiv2018

The recoiling black hole candidate 3C 186: spatially-resolved quasar feedback and further evidence of a blue-shifted broad line region

We present the results of integral field spectroscopy of the gravitational wave (GW) recoiling black hole candidate 3C 186. The goal of the observations is to study the kinematics of the [OIII]5007 narrow emission line region (NLR) of the quasar, and investigate the origin of the velocity offsets originally measured for different UV lines. The results show that i) the spatial structure of the NLR is complex. The [OIII]5007 line shows significant velocity offsets with respect to the systemic redshift of the source. Different components at different velocities (-670, -100, + 75 km s^-1) are produced in different regions of the source. ii) we detect both the narrow and the broad components of the Hbeta line. The narrow component generally follows the kinematics of the [OIII] line, while the broad component is significantly blue-shifted. The peak of the broad line is near the blue end, or possibly outside of the sensitivity band of the instrument, implying a velocity offset of >~1800 km s^-1. This result is in agreement with the interpretation of the QSO as a GW recoiling black hole. The properties of the NLR show that the observed outflows are most likely the effect of radiation pressure on the (photoionized) gas in the interstellar medium of the host galaxy.

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arXiv2018

Extreme magnification of a star at redshift 1.5 by a galaxy-cluster lens

Galaxy-cluster gravitational lenses can magnify background galaxies by a total factor of up to ~50. Here we report an image of an individual star at redshift z=1.49 (dubbed "MACS J1149 Lensed Star 1 (LS1)") magnified by >2000. A separate image, detected briefly 0.26 arcseconds from LS1, is likely a counterimage of the first star demagnified for multiple years by a >~3 solar-mass object in the cluster. For reasonable assumptions about the lensing system, microlensing fluctuations in the stars' light curves can yield evidence about the mass function of intracluster stars and compact objects, including binary fractions and specific stellar evolution and supernova models. Dark-matter subhalos or massive compact objects may help to account for the two images' long-term brightness ratio.

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arXiv2017

Capabilities of ACAD-OSM, an active method for the correction of aperture discontinuities

The increasing complexity of the aperture geometry of the future space- and ground based-telescopes will limit the performance of the next generation of coronagraphic instruments for high contrast imaging of exoplanets. We propose here a new closed-loop optimization technique using two deformable mirrors to correct for the effects of complex apertures on coronagraph performance, alternative to the ACAD technique previously developed by our group. This technique, ACAD-OSM, allows the use of any coronagraphs designed for continuous apertures, with complex, segmented, apertures, maintaining high performance in contrast and throughput. We show the capabilities of this technique on several pupil geometries (segmented LUVOIR type aperture, WFIRST, ELTs) for which we obtained high contrast levels with several deformable mirror setups (size, number of actuators, separation between them), coronagraphs (apodized pupil Lyot and vortex coronagraphs) and spectral bandwidths, which will help us present recommendations for future coronagraphic instruments. We show that this active technique handles, without any revision to the algorithm, changing or unknown optical aberrations or discontinuities in the pupil, including optical design misalignments, missing segments and phase errors.

astro-ph.IM↗

arXiv2017

Active correction of aperture discontinuities (ACAD) for space telescope pupils: a parametic analysis

As the performance of coronagraphs improves, the achievable contrast is more and more dependent of the shape of the pupil. The future generation of space and ground based coronagraphic instruments will have to achieve high contrast levels on on-axis and/or segmented telescopes. To correct for the high amplitude aberrations introduced by secondary mirror structures and segmentation of the primary mirror, we explore a two deformable mirror (DM) method. The major difficulty of several DM methods is the non-linear relation linking actuator strokes to the point spread function in the coronagraph focal plane. The Active Compensation of Aperture Discontinuities (ACAD) method is achieving this minimization by solving a non linear differential Monge Ampere equation. Once this open loop method have reached the minimum, a close-loop stroke minimization method can be applied to correct for phase and amplitude aberrations to achieve the ultimate contrast. In this paper, I describe the results of the parametric analysis that that I have undertaken on this method. After recalling the principle of the method, I will described the explored parameter space (deformable mirror set-up, shape of the pupil, bandwidth, coronagraph designs). I will precisely described the way I simulated the Vortex coronagraph for this numerical simulation. Finally I will present the preliminary results of this parametric analysis for space telescope pupils only.

astro-ph.IM↗

arXiv2017

Correcting for the effects of pupil discontinuities with the ACAD method

The current generation of ground-based coronagraphic instruments uses deformable mirrors to correct for phase errors and to improve contrast levels at small angular separations. Improving these techniques, several space and ground based instruments are currently developed using two deformable mirrors to correct for both phase and amplitude errors. However, as wavefront control techniques improve, more complex telescope pupil geometries will soon be a limiting factor for these next generation coronagraphic instru- ments. The technique presented in this proceeding, the Active Correction of Aperture Discontinuities method, is taking advantage of the fact that most future coronagraphic instruments will include two deformable mirrors, and is proposing to find the shapes and actuator movements to correct for the effect introduced by these complex pupil geometries. We here present the results of the parametric analysis realized on the WFIRST pupil for which we obtained high contrast levels with several deformable mirror setups (size, separation between them), coronagraphs (Vortex charge 2, vortex charge 4, APLC) and spectral bandwidths. However, because contrast levels and separation are not the only metrics to maximize the scientific return of an instrument, we also included in this study the influence of these deformable mirror shapes on the throughput of the instrument and sensitivity to pointing jitters. Finally, we present results obtained on another potential space based telescope segmented aperture. The main result of this proceeding is that we now obtain comparable performance than the coronagraphs previously designed for WFIRST. First result from the parametric analysis strongly suggest that the 2 deformable mirror set up (size and distance between them) has a important impact on the performance in contrast and throughput of the final instrument.

astro-ph.IM↗

arXiv2017

Active correction of aperture discontinuities - optimized stroke minimization II: optimization for future missions

High-contrast imaging and spectroscopy provide unique constraints for exoplanet formation models as well as for planetary atmosphere models. Instrumentation techniques in this field have greatly improved over the last two decades, with the development of stellar coronagraphy, in parallel with specific methods of wavefront sensing and control. Next generation space- and ground-based telescopes will allow the characterization cold solar-system like planets for the first time and maybe even in situ detection of bio-markers. However, the growth of primary mirror diameters, necessary for these detection, comes with an increase of their complexity (segmentation, secondary mirror features). These discontinuities in the aperture can greatly limit the performance of coronagraphic instruments. In this context, we introduced a new technique, Active Correction of Aperture Discontinuities - Optimized Stroke Minimization (ACAD-OSM), to correct for the diffractive effects of aperture discontinuities in the final image plane of a coronagraph, using deformable mirrors. In this paper, we present several tools that can be used to optimize the performance of this technique for its application to future large missions. In particular, we analyze the influence of the deformable setup (size and separating distance) and found that there is an optimal point for this setup, optimizing the performance of the instrument in contrast and throughput while minimizing the strokes applied to the deformable mirrors. These results will help us design future coronagraphic instruments to obtain the best performance.

astro-ph.IM↗

arXiv2017

Proper Motions of Jets on the Kiloparsec Scale: New Results with HST

The Hubble Space Telescope recently celebrated 25 years of operation. Some of the first images of extragalactic optical jets were taken by HST in the mid-1990s; with time baselines on the order of 20 years and state-of-the-art astrometry techniques, we are now able to reach accuracies in proper-motion measurements on the order of a tenth of a milliarcsecond per year. We present the results of a recent HST program to measure the kiloparsec-scale proper motions of eleven nearby optical jets with Hubble, the first sample of its kind. When paired with VLBI proper-motion measurements on the parsec scale, we are now able to map the full velocity profile of these jets from near the black hole to the final deceleration as they extend out into and beyond the host galaxy. We see convincing evidence that weak-flavor jets (i.e., FR Is) have a slowly increasing jet speed up to 100 pc from the core, where superluminal components are first seen.

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arXiv2016

An HST Proper-Motion Study of the Large-scale Jet of 3C273

The radio galaxy 3C 273 hosts one of the nearest and best-studied powerful quasar jets. Having been imaged repeatedly by the Hubble Space Telescope (HST) over the past twenty years, it was chosen for an HST program to measure proper motions in the kiloparsec-scale resolved jets of nearby radio-loud active galaxies. The jet in 3C 273 is highly relativistic on sub-parsec scales, with apparent proper motions up to 15$c$ observed by VLBI (Lister et al., 2013). In contrast, we find that the kpc-scale knots are compatible with being stationary, with a mean speed of $-$0.2$\pm$0.5$c$ over the whole jet. Assuming the knots are packets of moving plasma, an upper limit of 1c implies a bulk Lorentz factor $Γ<$2.9. This suggests that the jet has either decelerated significantly by the time it reaches the kpc scale, or that the knots in the jet are standing shock features. The second scenario is incompatible with the inverse Compton off the Cosmic Microwave Background (IC/CMB) model for the X-ray emission of these knots, which requires the knots to be in motion, but IC/CMB is also disfavored in the first scenario due to energetic considerations, in agreement with the recent finding of Meyer & Georganopoulos (2014) which ruled out the IC/CMB model for the X-ray emission of 3C 273 via gamma-ray upper limits.

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arXiv2016

Active compensation of aperture discontinuities for WFIRST-AFTA: analytical and numerical comparison of propagation methods and preliminary results with a WFIRST-AFTA-like pupil

The new frontier in the quest for the highest contrast levels in the focal plane of a coronagraph is now the correction of the large diffractive artifacts effects introduced at the science camera by apertures of increasing complexity. The coronagraph for the WFIRST/AFTA mission will be the first of such instruments in space with a two Deformable Mirrors wavefront control system. Regardless of the control algorithm for these multi Deformable Mirrors, they will have to rely on quick and accurate simulation of the propagation effects introduced by the out-of-pupil surface. In the first part of this paper, we present the analytical description of the different approximations to simulate these propagation effects. In Annex A, we prove analytically that, in the special case of surfaces inducing a converging beam, the Fresnel method yields high fidelity for simulations of these effects. We provide numerical simulations showing this effect. In the second part, we use these tools in the framework of the Active Compensation of Aperture Discontinuities technique (ACAD) applied to pupil geometries similar to WFIRST-AFTA. We present these simulations in the context of the optical layout of the High-contrast imager for Complex Aperture Telescopes, which will test ACAD on a optical bench. The results of this analysis show that using the ACAD method, an apodized pupil lyot coronagraph and the performance of our current deformable mirrors, we are able to obtain, in numerically simulations, a dark hole with an AFTA-like pupil. Our numerical simulation shows that we can obtain contrast better than $2.10^{-9}$ in monochromatic light and better than 3.e-8 with 10% bandwidth between 5 and 14 lambda/D.

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arXiv2015

Shocking Signals of Dark Matter Annihilation

We examine whether charged particles injected by self-annihilating Dark Matter into regions undergoing Diffuse Shock Acceleration (DSA) can be accelerated to high energies. We consider three astrophysical sites where shock acceleration is supposed to occur, namely the Galactic Centre, galaxy clusters and Active Galactic Nuclei (AGN). For the Milky Way, we find that the acceleration of cosmic rays injected by dark matter could lead to a bump in the cosmic ray spectrum provided that the product of the efficiency of the acceleration mechanism and the concentration of DM particles is high enough. Among the various acceleration sources that we consider (namely supernova remnants (SNRs), Fermi bubbles and AGN jets), we find that the Fermi bubbles are a potentially more efficient accelerator than SNRs. However both could in principle accelerate electrons and protons injected by dark matter to very high energies. At the extragalactic level, the acceleration of dark matter annihilation products could be responsible for enhanced radio emission from colliding clusters and prediction of an increase of the anti-deuteron flux generated near AGNs.

hep-ph↗

arXiv2015

Radio Loud AGNs are Mergers

We measure the merger fraction of Type 2 radio-loud and radio-quiet active galactic nuclei at z>1 using new samples. The objects have HST images taken with WFC3 in the IR channel. These samples are compared to the 3CR sample of radio galaxies at z>1 and to a sample of non-active galaxies. We also consider lower redshift radio galaxies with HST observations and previous generation instruments (NICMOS and WFPC2). The full sample spans an unprecedented range in both redshift and AGN luminosity. We perform statistical tests to determine whether the different samples are differently associated with mergers. We find that all (92%) radio-loud galaxies at z>1 are associated with recent or ongoing merger events. Among the radio-loud population there is no evidence for any dependence of the merger fraction on either redshift or AGN power. For the matched radio-quiet samples, only 38% are merging systems. The merger fraction for the sample of non-active galaxies at z>1 is indistinguishable from radio-quiet objects. This is strong evidence that mergers are the triggering mechanism for the radio-loud AGN phenomenon and the launching of relativistic jets from supermassive black holes. We speculate that major BH-BH mergers play a major role in spinning up the central supermassive black holes in these objects.

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arXiv2014

A possible link between the GeV excess and the 511 keV emission line in the Galactic Centre

The morphology and characteristics of the so-called GeV gamma-ray excess detected in the Milky Way lead us to speculate about a possible common origin with the 511 keV line mapped by the SPI experiment about ten years ago. In the previous version of our paper, we assumed 30 GeV dark matter particles annihilating into $b \bar{b}$ and obtained both a morphology and a 511 keV flux (ϕ_{511 keV} ~ 10^{-3} ph/cm^2/s) in agreement with SPI observation. However our estimates assumed a negligible number density of electrons in the bulge which lead to an artificial increase in the flux (mostly due to negligible Coulomb losses in this configuration). Assuming a number density greater than $n_e > 10^{-3} cm^{-3}$, we now obtain a flux of 511 keV photons that is smaller than ϕ_{511 keV} ~ 10^{-6} ph/cm^2/s and is essentially in agreement with the 511 keV flux that one can infer from the total number of positrons injected by dark matter annihilations into $b \bar{b}$. We thus conclude that -- even if 30 GeV dark matter particles were to exist-- it is impossible to establish a connexion between the two types of signals, even though they are located within the same 10 deg region in the galactic centre.

astro-ph.HE↗

arXiv2014

Cluster candidates around low power radio-galaxies at z~1-2 in COSMOS

We search for high redshift ($z\sim$1-2) galaxy clusters using low luminosity radio galaxies (FR~I) as beacons and our newly developed Poisson Probability Method (PPM) based on photometric redshift information and galaxy number counts. We use a sample of 32 FR~Is within the Cosmic Evolution Survey (COSMOS) field from Chiaberge et al. (2009) catalog. We derive a reliable subsample of 21 {\it bona fide} Low Luminosity Radio Galaxies (LLRGs) and a subsample of 11 High Luminosity Radio Galaxies (HLRGs), on the basis of photometric redshift information and NRAO VLA Sky Survey (NVSS) radio fluxes. The LLRGs are selected to have 1.4~GHz rest frame luminosities lower than the fiducial FR~I/FR~II divide. This also allows us to estimate the comoving space density of sources with $L_{1.4}\simeq 10^{32.3}\,\hbox{erg}\,\hbox{s}^{-1}\,\hbox{Hz}^{-1}$ at $z\simeq 1.1$, which strengthens the case for a strong cosmological evolution of these sources. In the fields of the LLRGs and HLRGs we find evidence that 14 and 8 of them reside in rich groups or galaxy clusters, respectively. Thus, overdensities are found around $\sim70\%$ of the FR~Is, independently of the considered subsample. This rate is in agreement with the fraction found for low redshift FR~Is and it is significantly higher than that of FR~IIs at all redshifts. Although our method is primarily introduced for the COSMOS survey, it may be applied to both present and future wide field surveys such as SDSS Stripe 82, LSST, and Euclid. Furthermore, cluster candidates found with our method are excellent targets for next generation space telescopes such as JWST.

astro-ph.GA↗

arXiv2014

A new method to search for high redshift clusters using photometric redshifts

We describe a new method (Poisson Probability Method, PPM) to search for high redshift galaxy clusters and groups by using photometric redshift information and galaxy number counts. The method relies on Poisson statistics and is primarily introduced to search for Mpc-scale environments around a specific beacon. The PPM is tailored to both the properties of the FR I radio galaxies in the Chiaberge et al. (2009) sample, that are selected within the COSMOS survey, and on the specific dataset used. We test the efficiency of our method of searching for cluster candidates against simulations. Two different approaches are adopted. i) We use two z~1 X-ray detected cluster candidates found in the COSMOS survey and we shift them to higher redshift up to z=2. We find that the PPM detects the cluster candidates up to z=1.5, and it correctly estimates both the redshift and size of the two clusters. ii) We simulate spherically symmetric clusters of different size and richness, and we locate them at different redshifts (i.e. z = 1.0,1.5, and 2.0) in the COSMOS field. We find that the PPM detects the simulated clusters within the considered redshift range with a statistical 1-sigma redshift accuracy of ~0.05. The PPM is an efficient alternative method for high-redshift cluster searches that may also be applied to both present and future wide field surveys such as SDSS Stripe 82, LSST, and Euclid. Accurate photometric redshifts and a survey depth similar or better than that of COSMOS (e.g. I<25) are required.

astro-ph.GA↗

arXiv2013

Optical Proper Motion Measurements of the M87 Jet: New Results from the Hubble Space Telescope

We report new results from an HST archival program to study proper motions in the optical jet of the nearby radio galaxy M87. Using over 13 years of archival imaging, we reach accuracies below 0.1c in measuring the apparent velocities of individual knots in the jet. We confirm previous findings of speeds up to 4.5c in the inner 6" of the jet, and report new speeds for optical components in the outer part of the jet. We find evidence of significant motion transverse to the jet axis on the order of 0.6c in the inner jet features, and superluminal velocities parallel and transverse to the jet in the outer knot components, with an apparent ordering of velocity vectors possibly consistent with a helical jet pattern. Previous results suggested a global deceleration over the length of the jet in the form of decreasing maximum speeds of knot components from HST-1 outward, but our results suggest that superluminal speeds persist out to knot C, with large differentials in very nearby features all along the jet. We find significant apparent accelerations in directions parallel and transverse to the jet axis, along with evidence for stationary features in knots D, E, and I. These results are expected to place important constraints on detailed models of kpc-scale relativistic jets.

astro-ph.CO↗