分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Gas cooling and accretion in haloes delivers mass and angular momentum onto galaxies. In this work, we investigate the accuracy of the modelling of this important process in several different semi-analytic (SA) galaxy formation models (GALFORM, L-GALAXIES and MORGANA) through comparisons with a hydrodynamical simulation performed with the moving-mesh code AREPO. Both SA models and the simulation were run without any feedback or metal enrichment, in order to focus on the cooling and accretion process. All of the SA models considered here assume that gas cools from a spherical halo. We found that the assumption that the gas conserves its angular momentum when moving from the virial radius, r_{\rm vir}, to the central region of the halo, r\sim 0.1 r_{\rm vir}, is approximately consistent with the results from our simulation, in which gas typically retains 70-80\% of its angular momentum during this process. We also found that, compared to the simulation, the MORGANA model tends to overestimate the mean specific angular momentum of cooled-down gas, the L-GALAXIES model also tends to overestimate this in low-redshift massive haloes, while the two older GALFORM models tend to underestimate the angular momentum. In general, the predictions of the new GALFORM cooling model developed by Hou et al. agree the best with the simulation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The planar distributions of satellite galaxies around the Milky Way and Andromeda have been extensively studied as potential challenges to the standard cosmological model. Using the Sloan Digital Sky Survey and the Millennium simulation we extend such studies to the satellite galaxies of massive galaxy clusters. We find that both observations and simulations of galaxy clusters show an excess of anisotropic satellite distributions. On average, satellites in clusters have a higher degree of anisotropy than their counterparts in Milky-Way-mass hosts once we account for the difference in their radial distributions. The normal vector of the plane of satellites is strongly aligned with the host halo's minor axis, while the alignment with the large-scale structure is weak. At fixed cluster mass, the degree of anisotropy is higher at higher redshift. This reflects the highly anisotropic nature of satellites accretion points, a feature that is partly erased by the subsequent orbital evolution of the satellites. We also find that satellite galaxies are mostly accreted singly so group accretion is not the explanation for the high flattening of the planes of satellites.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Qiuhan He
Andrew Robertson
James Nightingale
Shaun Cole
Carlos S. Frenk
Richard Massey
Aristeidis Amvrosiadis
Ran Li
Xiaoyue Cao
Amy Etherington
摘要： A fundamental prediction of the cold dark matter (CDM) model of structure formation is the existence of a vast population of dark matter haloes extending to subsolar masses. By contrast, other dark matter models, such as a warm thermal relic (WDM), predict a cutoff in the mass function at a mass which, for popular models, lies approximately between 10^7 and 10^{10}~{\rm M}_\odot. We use mock observations to demonstrate the viability of a forward modelling approach to extract information about low-mass dark haloes lying along the line-of-sight to galaxy-galaxy strong lenses. This can be used to constrain the mass of a thermal relic dark matter particle, m_\mathrm{DM}. With 50 strong lenses at Hubble Space Telescope resolution and a maximum pixel signal-to-noise ratio of \sim50, the expected median 2\sigma constraint for a CDM-like model (with a halo mass cutoff at 10^{7}~{\rm M}_\odot) is m_\mathrm{DM} > 4.10 \, \mathrm{keV} (50% chance of constraining m_{\rm DM} to be better than 4.10 keV). If, however, the dark matter is a warm particle of m_\mathrm{DM}=2.2 \, \mathrm{keV}, our 'Approximate Bayesian Computation' method would result in a median estimate of m_\mathrm{DM} between 1.43 and 3.21 keV. Our method can be extended to the large samples of strong lenses that will be observed by future telescopes, and could potentially rule out the standard CDM model of cosmogony. To aid future survey design, we quantify how these constraints will depend on data quality (spatial resolution and integration time) as well as on the lensing geometry (source and lens redshifts).
分类： 天文学 >> 星系和宇宙学 提交时间： 2023-02-21
Qiuhan He
Ran Li
Carlos S. Frenk
James Nightingale
Shaun Cole
Nicola C. Amorisco
Richard Massey
Andrew Robertson
Amy Etherington
Aristeidis Amvrosiadis
Xiaoyue Cao
摘要： We rederive the number density of intervening line-of-sight haloes relative to lens subhaloes in galaxy-galaxy strong lensing observations, where these perturbers can generate detectable image fluctuations. Previous studies have calculated the detection limit of a line-of-sight small-mass dark halo by comparing the lensing deflection angles it would cause, to those caused by a subhalo within the lens. However, this overly simplifies the difference in observational consequences between a subhalo and a line-of-sight halo. Furthermore, it does not take into account degeneracies between an extra subhalo and the uncertain properties of the main lens. More in keeping with analyses of real-world observations, we regard a line-of-sight halo as detectable only if adding it to a smooth model generates a statistically significant improvement in the reconstructed image. We find that the number density of detectable line-of-sight perturbers has been overestimated by as much as a factor of two in the previous literature. For typical lensing geometries and configurations, very deep imaging is sensitive to twice as many line-of-sight perturbers as subhaloes, but moderate depth imaging is sensitive to only slightly more line-of-sight perturbers than subhaloes.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Qiuhan He
Ran Li
Carlos S. Frenk
James Nightingale
Shaun Cole
Nicola C. Amorisco
Richard Massey
Andrew Robertson
Amy Etherington
Aristeidis Amvrosiadis
Xiaoyue Cao
摘要： We rederive the number density of intervening line-of-sight haloes relative to lens subhaloes in galaxy-galaxy strong lensing observations, where these perturbers can generate detectable image fluctuations. Previous studies have calculated the detection limit of a line-of-sight small-mass dark halo by comparing the lensing deflection angles it would cause, to those caused by a subhalo within the lens. However, this overly simplifies the difference in observational consequences between a subhalo and a line-of-sight halo. Furthermore, it does not take into account degeneracies between an extra subhalo and the uncertain properties of the main lens. More in keeping with analyses of real-world observations, we regard a line-of-sight halo as detectable only if adding it to a smooth model generates a statistically significant improvement in the reconstructed image. We find that the number density of detectable line-of-sight perturbers has been overestimated by as much as a factor of two in the previous literature. For typical lensing geometries and configurations, very deep imaging is sensitive to twice as many line-of-sight perturbers as subhaloes, but moderate depth imaging is sensitive to only slightly more line-of-sight perturbers than subhaloes.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
James W. Nightingale
Qiuhan He
Xiaoyue Cao
Aristeidis Amvrosiadis
Amy Etherington
Carlos S. Frenk
Richard G. Hayes
Andrew Robertson
Shaun Cole
Samuel Lange
Ran Li
Richard Massey
摘要： The cold dark matter (DM) model predicts that every galaxy contains thousands of DM subhalos; almost all other DM models include a physical process that smooths away the subhalos. The subhalos are invisible, but could be detected via strong gravitational lensing, if they lie on the line of sight to a multiply-imaged background source, and perturb its apparent shape. We present an automated strong lens analysis framework, and scan for DM subhalos in Hubble Space Telescope imaging of 54 strong lenses. We identify two compelling DM subhalo candidates (including one previously found in SLACS0946+1006), where a subhalo is favoured after every systematic test we perform. We find that the detectability of subhalos depends upon the assumed parametric form for the lens galaxy's mass distribution. Comparing fits which assume several more complex mass models reveals 5 additional (generally lower mass) DM subhalo candidates worthy of further study, and the removal of 11 false positives. We identify 44 non-detections, which are vital to building up enough statistical power to test DM models. Future work will apply even more flexible models to the results of this study, to constrain different DM models. Our full analysis results are available at https://github.com/Jammy2211/autolens_subhalo.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Qiuhan He
James Nightingale
Andrew Robertson
Aristeidis Amvrosiadis
Shaun Cole
Carlos S. Frenk
Richard Massey
Ran Li
Nicola C. Amorisco
R. Benton Metcalf
Xiaoyue Cao
Amy Etherington
摘要： Strong gravitational lensing offers a compelling test of the cold dark matter paradigm, as it allows for subhaloes with masses of \sim10^{9} M_\odot and below to be detected. We test commonly-used techniques for detecting subhaloes superposed in images of strongly lensed galaxies. For the lens we take a simulated galaxy in a \sim10^{13} M_\odot halo grown in a high-resolution cosmological hydrodynamical simulation, which we view from two different directions. Though the resolution is high, we note the simulated galaxy still has an artificial core which adds additional complexity to the baryon dominated region. To remove particle noise, we represent the projected galaxy mass distribution by a series of Gaussian profiles which precisely capture the features of the projected galaxy. We first model the lens mass as a (broken) power-law density profile and then search for small haloes. Of the two projections, one has a regular elliptical shape, while the other has distinct deviations from an elliptical shape. For the former, the broken power-law model gives no false positives and correctly recovers the mass of the superposed small halo, but for the latter we find false positives and the inferred halo mass is overestimated by \sim4-5 times. We then use a more complex model in which the lens mass is decomposed into stellar and dark matter components. In this case, we show that we can capture the simulated galaxy's complex projected structures and correctly infer the input small halo.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Xiaoyue Cao
Ran Li
J. W. Nightingale
Richard Massey
Andrew Robertson
Carlos S. Frenk
Aristeidis Amvrosiadis
Nicola C. Amorisco
Qiuhan He
Amy Etherington
Shaun Cole
Kai Zhu
摘要： The elliptical power-law (EPL) model of the mass in a galaxy is widely used in strong gravitational lensing analyses. However, the distribution of mass in real galaxies is more complex. We quantify the biases due to this model mismatch by simulating and then analysing mock {\it Hubble Space Telescope} imaging of lenses with mass distributions inferred from SDSS-MaNGA stellar dynamics data. We find accurate recovery of source galaxy morphology, except for a slight tendency to infer sources to be more compact than their true size. The Einstein radius of the lens is also robustly recovered with 0.1% accuracy, as is the global density slope, with 2.5% relative systematic error, compared to the 3.4% intrinsic dispersion. However, asymmetry in real lenses also leads to a spurious fitted `external shear' with typical strength, \gamma_{\rm ext}=0.015. Furthermore, time delays inferred from lens modelling without measurements of stellar dynamics are typically underestimated by \sim5%. Using such measurements from a sub-sample of 37 lenses would bias measurements of the Hubble constant H_0 by \sim9%. Although this work is based on a particular set of MaNGA galaxies, and the specific value of the detected biases may change for another set of strong lenses, our results strongly suggest the next generation cosmography needs to use more complex lens mass models.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Amy Etherington
James W. Nightingale
Richard Massey
Andrew Robertson
XiaoYue Cao
Aristeidis Amvrosiadis
Shaun Cole
Carlos S. Frenk
Qiuhan He
David J. Lagattuta
Samuel Lange
Ran Li
摘要： Observations suggest that the dark matter and stars in early-type galaxies `conspire' to produce a surprisingly simple distribution of total mass, \rho(r)\propto\rho^{-\gamma}, with \gamma\approx2. We measure the distribution of mass in 48 early-type galaxies that gravitationally lens a resolved background source. By fitting the source light in every pixel of images from the Hubble Space Telescope, we find a mean \langle\gamma\rangle=2.075_{-0.024}^{+0.023} with intrinsic scatter between galaxies of \sigma_\gamma=0.172^{+0.022}_{-0.032} for the overall sample. This is consistent with, and has similar precision to traditional techniques that employ spectroscopic observations to supplement lensing with mass estimates from stellar dynamics. Comparing measurements of \gamma for individual lenses using both techniques, we find a statistically insignificant correlation of -0.150^{+0.223}_{-0.217} between the two, indicating a lack of statistical power or deviations from a power-law density in certain lenses. At fixed surface mass density, we measure a redshift dependence, \partial\langle\gamma\rangle/\partial z=0.345^{+0.322}_{-0.296}, that is consistent with traditional techniques for the same sample of SLACS and GALLERY lenses. Interestingly, the consistency breaks down when we measure the dependence of \gamma on the surface mass density of a lens galaxy. We argue that this is tentative evidence for an inflection point in the total-mass density profile at a few times the galaxy effective radius -- breaking the conspiracy.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Amy Etherington
James W. Nightingale
Richard Massey
XiaoYue Cao
Andrew Robertson
Nicola C. Amorisco
Aristeidis Amvrosiadis
Shaun Cole
Carlos S. Frenk
Qiuhan He
Ran Li
Sut-Ieng Tam
摘要： The distribution of dark and luminous matter can be mapped around galaxies that gravitationally lens background objects into arcs or Einstein rings. New surveys will soon observe hundreds of thousands of galaxy lenses, and current, labour-intensive analysis methods will not scale up to this challenge. We instead develop a fully automatic, Bayesian method which we use to fit a sample of 59 lenses imaged by the Hubble Space Telescope in uniform conditions. We set out to \textit{leave no lens behind} and focus on ways in which automated fits fail in a small handful of lenses, describing adjustments to the pipeline that allows us to infer accurate lens models. Our pipeline ultimately fits {\em all} 59 lenses in our sample, with a high success rate key because catastrophic outliers would bias large samples with small statistical errors. Machine Learning techniques might further improve the two most difficult steps: subtracting foreground lens light and initialising a first, approximate lens model. After that, increasing model complexity is straightforward. We find a mean \sim1\% measurement precision on the measurement of the Einstein radius across the lens sample which {\em does not degrade with redshift} up to at least z=0.7 -- in stark contrast to other techniques used to study galaxy evolution, like stellar dynamics. Our \texttt{PyAutoLens} software is open source, and is also installed in the Science Data Centres of the ESA Euclid mission.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Amy Etherington
James W. Nightingale
Richard Massey
Sut-Ieng Tam
XiaoYue Cao
Anna Niemiec
Qiuhan He
Andrew Robertson
Ran Li
Aristeidis Amvrosiadis
Shaun Cole
Jose M. Diego
Carlos S. Frenk
Brenda L. Frye
David Harvey
Mathilde Jauzac
Anton M. Koekemoer
David J. Lagattuta
Marceau Limousin
Guillaume Mahler
摘要： The distribution of mass in galaxy-scale strong gravitational lenses is often modelled as an elliptical power law plus `external shear', which notionally accounts for neighbouring galaxies and cosmic shear. We show that it does not. Except in a handful of rare systems, the best-fit values of external shear do not correlate with independent measurements of shear: from weak lensing in 45 Hubble Space Telescope images, or in 50 mock images of lenses with complex distributions of mass. Instead, the best-fit shear is aligned with the major or minor axis of 88% of lens galaxies; and the amplitude of the external shear increases if that galaxy is disky. We conclude that `external shear' attached to a power law model is not physically meaningful, but a fudge to compensate for lack of model complexity. Since it biases other model parameters that are interpreted as physically meaningful in several science analyses (e.g. measuring galaxy evolution, dark matter physics or cosmological parameters), we recommend that future studies of galaxy-scale strong lensing should employ more flexible mass models.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
David Martinez-Delgado
Andrew P. Cooper
Javier Roman
Annalisa Pillepich
Denis Erkal
Sarah Pearson
John Moustakas
Chervin F. P. Laporte
Seppo Laine
Mohammad Akhlaghi
Dustin Lang
Dmitry Makarov
Alejandro S. Borlaff
Giuseppe Donatiello
William J. Pearson
Juan Miro-Carretero
Jean-Charles Cuillandre
Helena Dominguez
Santi Roca-Fabrega
Carlos S. Frenk
摘要： Mergers and tidal interactions between massive galaxies and their dwarf satellites are a fundamental prediction of the Lambda-Cold Dark Matter cosmology. These events are thought to provide important observational diagnostics of nonlinear structure formation. Stellar streams in the Milky Way and Andromeda are spectacular evidence for ongoing satellite disruption. However, constructing a statistically meaningful sample of tidal streams beyond the Local Group has proven a daunting observational challenge, and the full potential for deepening our understanding of galaxy assembly using stellar streams has yet to be realised. Here we introduce the Stellar Stream Legacy Survey, a systematic imaging survey of tidal features associated with dwarf galaxy accretion around a sample of ~3100 nearby galaxies within z~0.02, including about 940 Milky Way analogues. Our survey exploits public deep imaging data from the DESI Legacy Imaging Surveys, which reach surface brightness as faint as ~29 mag/arcsec^2 in the r band. As a proof of concept of our survey, we report the detection and broad-band photometry of 24 new stellar streams in the local Universe. We discuss how these observations can yield new constraints on galaxy formation theory through comparison to mock observations from cosmological galaxy simulations. These tests will probe the present-day mass assembly rate of galaxies, the stellar populations and orbits of satellites, the growth of stellar halos and the resilience of stellar disks to satellite bombardment.
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