arXiv subjects

A. Bolton

Publications and source records attributed to A. Bolton.

At least 19 recordsLinked to original sources

arXiv2025

DESI Strong Lens Foundry II: DESI Spectroscopy for Strong Lens Candidates

We present the Dark Energy Spectroscopic Instrument (DESI) Strong Lensing Secondary Target Program. This is a spectroscopic follow-up program for strong gravitational lens candidates found in the DESI Legacy Imaging Surveys footprint. Spectroscopic redshifts for the lenses and lensed source are crucial for lens modeling to obtain physical parameters. The spectroscopic catalog in this paper consists of 73 candidate systems from the DESI Early Data Release (EDR). We have confirmed 20 strong lensing systems and determined four to not be lenses. For the remaining systems, more spectroscopic data from ongoing and future observations will be presented in future publications. We discuss the implications of our results for lens searches with neural networks in existing and future imaging surveys as well as for lens modeling. This Strong Lensing Secondary Target Program is part of the DESI Strong Lens Foundry project, and this is Paper II of a series on this project.

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arXiv2025

DESI Strong Lens Foundry III: Keck Spectroscopy for Strong Lenses Discovered Using Residual Neural Networks

We present spectroscopic data of strong lenses and their source galaxies using the Keck Near-Infrared Echellette Spectrometer (NIRES) and the Dark Energy Spectroscopic Instrument (DESI), providing redshifts necessary for nearly all strong-lensing applications with these systems, especially the extraction of physical parameters from lensing modeling. These strong lenses were found in the DESI Legacy Imaging Surveys using Residual Neural Networks (ResNet) and followed up by our Hubble Space Telescope program, with all systems displaying unambiguous lensed arcs. With NIRES, we target eight lensed sources at redshifts difficult to measure in the optical range and determine the source redshifts for six, between $z_s$ = 1.675 and 3.332. DESI observed one of the remaining source redshifts, as well as an additional source redshift within the six systems. The two systems with non-detections by NIRES were observed for a considerably shorter 600s at high airmass. Combining NIRES infrared spectroscopy with optical spectroscopy from our DESI Strong Lensing Secondary Target Program, these results provide the complete lens and source redshifts for six systems, a resource for refining automated strong lens searches in future deep- and wide-field imaging surveys and addressing a range of questions in astrophysics and cosmology.

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arXiv2025

ML-Driven Strong Lens Discoveries: Down to $\theta_E \sim 0.03''$ and $M_\mathrm{halo}< 10^{11} M_\odot$

We present results on extending the strong lens discovery space down to much smaller Einstein radii ($\theta_E\lesssim0.03''$) and much lower halo mass ($M_\mathrm{halo}<10^{11}M_\odot$) through the combination of JWST observations and machine learning (ML) techniques. First, we forecast detectable strong lenses with JWST using CosmoDC2 as the lens catalog, and a source catalog down to 29th magnitude. By further incorporating the VELA hydrodynamical simulations of high-redshift galaxies, we simulate strong lenses. We train a ResNet on these images, achieving near-100\% completeness and purity for ``conventional" strong lenses ($\theta_E\gtrsim 0.5''$), applicable to JWST, HST, the Roman Space Telescope and Euclid VIS. For the first time, we also search for very low halo mass strong lenses ($M_{halo}<10^{11}M_\odot$) in simulations, with $\theta_E\ll 0.5''$, down to the best resolution ($0.03''$) and depth (10,000~sec) limits of JWST using ResNet. A U-Net model is employed to pinpoint these small lenses in images, which are otherwise virtually impossible for human detection. Our results indicate that JWST can find $\sim 17$/deg$^2$ such low-halo-mass lenses, with the locations of $\sim 1.1$/deg$^2$ of these detectable by the U-Net at $\sim100$\% precision (and $\sim 7.0$/deg$^2$ at a 99.0\% precision). To validate our model for finding ``conventional" strong lenses, we apply it to HST images, discovering two new strong lens candidates previously missed by human classifiers in a crowdsourcing project (Garvin et al. 2022). This study demonstrates the (potentially ``superhuman") advantages of ML combined with current and future space telescopes for detecting conventional, and especially, low-halo-mass strong lenses, which are critical for testing CDM models.

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arXiv2025

DESI Strong Lens Foundry I: HST Observations and Modeling with GIGA-Lens

We present the Dark Energy Spectroscopic Instrument (DESI) Strong Lens Foundry. We discovered $\sim 3500$ new strong gravitational lens candidates in the DESI Legacy Imaging Surveys using residual neural networks (ResNet). We observed a subset (51) of our candidates using the Hubble Space Telescope (HST). All of them were confirmed to be strong lenses. We also briefly describe spectroscopic follow-up observations by DESI and Keck NIRES programs. From this very rich dataset, a number of studies will be carried out, including evaluating the quality of the ResNet search candidates and lens modeling. In this paper, we present our initial effort in these directions. In particular, as a demonstration, we present the lens model for DESI-165.4754-06.0423, with imaging data from HST, and lens and source redshifts from DESI and Keck NIRES, respectively. In this effort, we have applied a \emph{fully} forward-modeling Bayesian approach (GIGA-Lens), using \emph{multiple} GPUs, for the first time in both regards, to a strong lens with HST data, or any high resolution imaging.

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arXiv2024

Archetype-Based Redshift Estimation for the Dark Energy Spectroscopic Instrument Survey

We present a computationally efficient galaxy archetype-based redshift estimation and spectral classification method for the Dark Energy Survey Instrument (DESI) survey. The DESI survey currently relies on a redshift fitter and spectral classifier using a linear combination of PCA-derived templates, which is very efficient in processing large volumes of DESI spectra within a short time frame. However, this method occasionally yields unphysical model fits for galaxies and fails to adequately absorb calibration errors that may still be occasionally visible in the reduced spectra. Our proposed approach improves upon this existing method by refitting the spectra with carefully generated physical galaxy archetypes combined with additional terms designed to absorb data reduction defects and provide more physical models to the DESI spectra. We test our method on an extensive dataset derived from the survey validation (SV) and Year 1 (Y1) data of DESI. Our findings indicate that the new method delivers marginally better redshift success for SV tiles while reducing catastrophic redshift failure by $10-30\%$. At the same time, results from millions of targets from the main survey show that our model has relatively higher redshift success and purity rates ($0.5-0.8\%$ higher) for galaxy targets while having similar success for QSOs. These improvements also demonstrate that the main DESI redshift pipeline is generally robust. Additionally, it reduces the false positive redshift estimation by $5-40\%$ for sky fibers. We also discuss the generic nature of our method and how it can be extended to other large spectroscopic surveys, along with possible future improvements.

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arXiv2022

Overview of the Instrumentation for the Dark Energy Spectroscopic Instrument

The Dark Energy Spectroscopic Instrument (DESI) has embarked on an ambitious five-year survey to explore the nature of dark energy with spectroscopy of 40 million galaxies and quasars. DESI will determine precise redshifts and employ the Baryon Acoustic Oscillation method to measure distances from the nearby universe to z > 3.5, as well as measure the growth of structure and probe potential modifications to general relativity. In this paper we describe the significant instrumentation we developed for the DESI survey. The new instrumentation includes a wide-field, 3.2-deg diameter prime-focus corrector that focuses the light onto 5020 robotic fiber positioners on the 0.812 m diameter, aspheric focal surface. The positioners and their fibers are divided among ten wedge-shaped petals. Each petal is connected to one of ten spectrographs via a contiguous, high-efficiency, nearly 50 m fiber cable bundle. The ten spectrographs each use a pair of dichroics to split the light into three channels that together record the light from 360 - 980 nm with a resolution of 2000 to 5000. We describe the science requirements, technical requirements on the instrumentation, and management of the project. DESI was installed at the 4-m Mayall telescope at Kitt Peak, and we also describe the facility upgrades to prepare for DESI and the installation and functional verification process. DESI has achieved all of its performance goals, and the DESI survey began in May 2021. Some performance highlights include RMS positioner accuracy better than 0.1", SNR per \sqrtÅ > 0.5 for a z > 2 quasar with flux 0.28e-17 erg/s/cm^2/A at 380 nm in 4000s, and median SNR = 7 of the [OII] doublet at 8e-17 erg/s/cm^2 in a 1000s exposure for emission line galaxies at z = 1.4 - 1.6. We conclude with highlights from the on-sky validation and commissioning of the instrument, key successes, and lessons learned. (abridged)

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arXiv2019

Rest-frame UV Properties of Luminous Strong Gravitationally Lensed Ly$α$ Emitters from the BELLS GALLERY Survey

We present deep rest-frame UV spectroscopic observations using the Gran Telescopio Canarias of six gravitationally lensed Lya emitters (LAEs) at $2.36<z<2.82$ selected from the BELLS GALLERY survey. By taking the magnifications into account, we show that LAEs can be as luminous as L(Lya) = 30x10$^{42}$ erg s-1 and M(UV) = -23 (AB) without invoking an AGN component, in contrast with previous findings. We measure Lya rest-frame equivalent widths, EW(Lya), ranging from 16Åto 50Åand Lya escape fractions, fesc(Lya), from 10% to 40%. Large EW(Lya) and fesc(Lya) are found predominantly in LAEs showing weak low-ionization ISM absorption (EW < 1Å) and narrow Lya profiles (< 300 km s-1 FWHM) with their peak close (< 80 km s-1) to their systemic redshifts, suggestive of less scatter from low HI column densities that favours the escape of Lya photons. We infer stellar metallicities of Z/Zsun ~ 0.2 in almost all LAEs by comparing the P-Cygni profiles of the wind lines NV1240Åand CIV1549Åwith those from stellar synthesis models. We also find a trend between M(UV) and the velocity offset of ISM absorption lines, such as the most luminous LAEs experience stronger outflows. The most luminous LAEs show star formation rates up to 180 Msun yr-1, yet they appear relatively blue ($β$(UV) ~ -1.8 to -2.0) showing evidence of little dust attenuation (E(B-V) = 0.10-0.14). These luminous LAEs may be particular cases of young starburst galaxies that have had no time to form large amounts of dust. If so, they are ideal laboratories to study the early phase of massive star formation, stellar and dust mass growth, and chemical enrichment histories of starburst galaxies at high-z.

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arXiv2019

Infrastructure and Strategies for Time Domain and MMA and Follow-Up

Time domain and multi-messenger astrophysics are growing and important modes of observational astronomy that will help define astrophysics in the 2020s. Significant effort is being put into developing the components of a follow-up system for dynamically turning survey alerts into data. This system consists of: 1) brokers that will aggregate, classify, and filter alerts; 2) Target Observation Managers (TOMs) for prioritizing targets and managing observations and data; and 3) observatory interfaces, schedulers, and facilities along with data reduction software and science archives. These efforts need continued community support and funding in order to complete and maintain them. Many of the efforts can be community open-source software projects but they will benefit from the leadership of professional software developers. The coordination should be done by institutions that are involved in the follow-up system such as the national observatories (e.g. LSST/Gemini/NOAO Mid-scale/Community Science and Data Center) or a new MMA institute. These tools will help the community to produce the most science from new facilities and will provide new capabilities for all users of the facilities that adopt them.

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arXiv2019

Mini-survey of the northern sky to Dec <+30

We propose an extension of the LSST survey to cover the northern sky to DEC < +30 (accessible at airmass <1.8). This survey will increase the LSST sky coverage by ~9,600 square degrees from 18,900 to 28,500 square degrees (a 50% increase) but use only 0.6-2.5% of the time depending on the synergies with other surveys. This increased area addresses a wide range of science cases that enhance all of the primary LSST science goals by significant amounts. The science enabled includes: increasing the area of the sky accessible for follow-up of multi-messenger transients including gravitational waves, mapping the milky way halo and halo dwarfs including discovery of RR Lyrae stars in the outer galactic halo, discovery of z>7 quasars in combination Euclid, enabling a second generation DESI and other spectroscopic surveys, and enhancing all areas of science by improving synergies with Euclid, WFIRST, and unique northern survey facilities. This white paper is the result of the Tri-Agency Working Group (TAG) appointed to develop synergies between missions and presents a unified plan for northern coverage. The range of time estimates reflects synergies with other surveys. If the modified DESC WFD survey, the ecliptic plane mini survey, and the north galactic spur mini survey are executed this plan would only need 0.6% of the LSST time, however if none of these are included the overall request is 2.5% of the 10 year survey life. In other words, the majority of these observations are already suggested as part of these other surveys and the intent of this white paper is to propose a unified baseline plan to carry out a broad range of objectives to facilitate a combination of multiple science objectives. A companion white paper gives Euclid specific science goals, and we support the white papers for southern extensions of the LSST survey.

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arXiv2016

SDSS-IV eBOSS emission-line galaxy pilot survey

The Sloan Digital Sky Survey IV extended Baryonic Oscillation Spectroscopic Survey (SDSS-IV/eBOSS) will observe 195,000 emission-line galaxies (ELGs) to measure the Baryonic Acoustic Oscillation standard ruler (BAO) at redshift 0.9. To test different ELG selection algorithms, 9,000 spectra were observed with the SDSS spectrograph as a pilot survey based on data from several imaging surveys. First, using visual inspection and redshift quality flags, we show that the automated spectroscopic redshifts assigned by the pipeline meet the quality requirements for a reliable BAO measurement. We also show the correlations between sky emission, signal-to-noise ratio in the emission lines, and redshift error. Then we provide a detailed description of each target selection algorithm we tested and compare them with the requirements of the eBOSS experiment. As a result, we provide reliable redshift distributions for the different target selection schemes we tested. Finally, we determine an target selection algorithms that is best suited to be applied on DECam photometry because they fulfill the eBOSS survey efficiency requirements.

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arXiv2015

The X-shooter Lens Survey - II. Sample presentation and spatially resolved kinematics

We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is to disentangle the stellar and dark matter content of massive early-type galaxies (ETGs), through combined strong gravitational lensing, dynamics and spectroscopic stellar population studies. The sample consists of 11 lens galaxies covering the redshift range from $0.1$ to $0.45$ and having stellar velocity dispersions between $250$ and $380\,\mathrm{km}\,\mathrm{s}^{-1}$. All galaxies have multi-band, high-quality HST imaging. We have obtained long-slit spectra of the lens galaxies with X-shooter on the VLT. We are able to disentangle the dark and luminous mass components by combining lensing and extended kinematics data-sets, and we are also able to precisely constrain stellar mass-to-light ratios and infer the value of the low-mass cut-off of the IMF, by adding spectroscopic stellar population information. Our goal is to correlate these IMF parameters with ETG masses and investigate the relation between baryonic and non-baryonic matter during the mass assembly and structure formation processes. In this paper we provide an overview of the survey, highlighting its scientific motivations, main goals and techniques. We present the current sample, briefly describing the data reduction and analysis process, and we present the first results on spatially resolved kinematics.

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arXiv2013

Stellar velocity dispersions and emission line properties of SDSS-III/BOSS galaxies

We perform a spectroscopic analysis of 492,450 galaxy spectra from the first two years of observations of the Sloan Digital Sky Survey-III/Baryonic Oscillation Spectroscopic Survey (BOSS) collaboration. This data set has been released in the ninth SDSS data release, the first public data release of BOSS spectra. We show that the typical signal-to-noise ratio of BOSS spectra is sufficient to measure stellar velocity dispersion and emission line fluxes for individual objects. The typical velocity dispersion of a BOSS galaxy is 240 km/s, with an accuracy of better than 30 per cent for 93 per cent of BOSS galaxies. The distribution in velocity dispersion is redshift independent between redshifts 0.15 and 0.7, which reflects the survey design targeting massive galaxies with an approximately uniform mass distribution in this redshift interval. The majority of BOSS galaxies lack detectable emission lines. We analyse the emission line properties and present diagnostic diagrams using the emission lines [OII], Hbeta, [OIII], Halpha, and [NII] (detected in about 4 per cent of the galaxies). We show that the emission line properties are strongly redshift dependent and that there is a clear correlation between observed frame colours and emission line properties. Within in the low-z sample around 0.15<z<0.3, half of the emission-line galaxies have LINER-like emission line ratios, followed by Seyfert-AGN dominated spectra, and only a small fraction of a few per cent are purely star forming galaxies. AGN and LINER-like objects, instead, are less prevalent in the high-z sample around 0.4<z<0.7, where more than half of the emission line objects are star forming. This is a pure selection effect caused by the non-detection of weak Hbeta emission lines in the BOSS spectra. Finally, we show that star forming, AGN and emission line free galaxies are well separated in the g-r vs r-i target selection diagram.

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arXiv2013

Luminosity Function from dedicated SDSS-III and MMT data of quasars in 0.7<z<4.0 selected with a new approach

We present a measurement of the quasar luminosity function in the range 0.68 2.15. Using pure luminosity evolution models, we fitted our LF measurements, and predicted quasar number counts as a function of redshift and observed magnitude. These predictions are useful inputs for future cosmology surveys such as those relying on the observation of quasars to measure baryon acoustic oscillations.

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arXiv2011

The BigBOSS Experiment

BigBOSS is a Stage IV ground-based dark energy experiment to study baryon acoustic oscillations (BAO) and the growth of structure with a wide-area galaxy and quasar redshift survey over 14,000 square degrees. It has been conditionally accepted by NOAO in response to a call for major new instrumentation and a high-impact science program for the 4-m Mayall telescope at Kitt Peak. The BigBOSS instrument is a robotically-actuated, fiber-fed spectrograph capable of taking 5000 simultaneous spectra over a wavelength range from 340 nm to 1060 nm, with a resolution R = 3000-4800. Using data from imaging surveys that are already underway, spectroscopic targets are selected that trace the underlying dark matter distribution. In particular, targets include luminous red galaxies (LRGs) up to z = 1.0, extending the BOSS LRG survey in both redshift and survey area. To probe the universe out to even higher redshift, BigBOSS will target bright [OII] emission line galaxies (ELGs) up to z = 1.7. In total, 20 million galaxy redshifts are obtained to measure the BAO feature, trace the matter power spectrum at smaller scales, and detect redshift space distortions. BigBOSS will provide additional constraints on early dark energy and on the curvature of the universe by measuring the Ly-alpha forest in the spectra of over 600,000 2.2 < z < 3.5 quasars. BigBOSS galaxy BAO measurements combined with an analysis of the broadband power, including the Ly-alpha forest in BigBOSS quasar spectra, achieves a FOM of 395 with Planck plus Stage III priors. This FOM is based on conservative assumptions for the analysis of broad band power (kmax = 0.15), and could grow to over 600 if current work allows us to push the analysis to higher wave numbers (kmax = 0.3). BigBOSS will also place constraints on theories of modified gravity and inflation, and will measure the sum of neutrino masses to 0.024 eV accuracy.

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arXiv2010

The clustering of massive galaxies at z~0.5 from the first semester of BOSS data

We calculate the real- and redshift-space clustering of massive galaxies at z~0.5 using the first semester of data by the Baryon Oscillation Spectroscopic Survey (BOSS). We study the correlation functions of a sample of 44,000 massive galaxies in the redshift range 0.4<z<0.7. We present a halo-occupation distribution modeling of the clustering results and discuss the implications for the manner in which massive galaxies at z~0.5 occupy dark matter halos. The majority of our galaxies are central galaxies living in halos of mass 10^{13}Msun/h, but 10% are satellites living in halos 10 times more massive. These results are broadly in agreement with earlier investigations of massive galaxies at z~0.5. The inferred large-scale bias (b~2) and relatively high number density (nbar=3e-4 h^3 Mpc^{-3}) imply that BOSS galaxies are excellent tracers of large-scale structure, suggesting BOSS will enable a wide range of investigations on the distance scale, the growth of large-scale structure, massive galaxy evolution and other topics.

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arXiv2009

Detection of a Dark Substructure through Gravitational Imaging

We report the detection of a dark substructure through direct gravitational imaging - undetected in the HST-ACS F814W image - in the gravitational lens galaxy of SLACS SDSSJ0946+1006 (the "Double Einstein Ring"). The detection is based on a Bayesian grid reconstruction of the two-dimensional surface density of the galaxy inside an annulus around its Einstein radius (few kpc). [...] We confirm this detection by modeling the system including a parametric mass model with a tidally truncated pseudo-Jaffe density profile; in that case the substructure mass is M_sub=(3.51+-0.15)x10^9 Msun, located at (-0.651+-0.038,1.040+-0.034)'', precisely where also the surface density map shows a strong convergence peak. [...] We set a lower limit of (M/L)_V}>=120 (Msun/L}_V,sun (3-sigma) inside a sphere of 0.3 kpc centred on the substructure (r_tidal=1.1kpc). The result is robust under substantial changes in the model and the data-set (e.g. PSF, pixel number and scale, source and potential regularization, rotations and galaxy subtraction). Despite being at the limits of detectability, it can therefore not be attributed to obvious systematic effects. Our detection implies a dark matter mass fraction at the radius of the inner Einstein ring of f_CDM=2.15^{+2.05}_{-1.25} percent (68 percent C.L) in the mass range 4x10^6 Msun to 4x10^9 Msun assuming alpha=1.9+-0.1 (with dN/dm ~ m^-alpha). Assuming a flat prior on alpha, between 1.0 and 3.0, increases this to f_CDM=2.56^{+3.26}_{-1.50} percent (68 percent C.L). The likelihood ratio is 0.51 between our best value (f_CDM=0.0215) and that from simulations (f_sim=0.003). Hence the inferred mass fraction, admittedly based on a single lens system, is large but still consistent with predictions. [...]

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arXiv2009

The Structure & Dynamics of Massive Early-type Galaxies: On Homology, Isothermality and Isotropy inside one Effective Radius

Based on 58 SLACS strong-lens early-type galaxies with direct total-mass and stellar-velocity dispersion measurements, we find that inside one effective radius massive elliptical galaxies with M_eff >= 3x10^10 M_sun are well-approximated by a power-law ellipsoid with an average logaritmic density slope of = -dlog(rho_tot)/dlog(r)=2.085^{+0.025}_{-0.018} (random error on mean) for isotropic orbits with beta_r=0, +-0.1 (syst.) and sigma_gamma' <= 0.20^{+0.04}_{-0.02} intrinsic scatter (all errors indicate the 68 percent CL). We find no correlation of gamma'_LD with galaxy mass (M_eff), rescaled radius (i.e. R_einst/R_eff) or redshift, despite intrinsic differences in density-slope between galaxies. Based on scaling relations, the average logarithmic density slope can be derived in an alternative manner, fully independent from dynamics, yielding =1.959 +- 0.077. Agreement between the two values is reached for =0.45 +- 0.25, consistent with mild radial anisotropy. This agreement supports the robustness of our results, despite the increase in mass-to-light ratio with total galaxy mass: M_eff ~ L_{V,eff}^(1.363+-0.056). We conclude that massive early-type galaxies are structurally close-to homologous with close-to isothermal total density profiles (<=10 percent intrinsic scatter) and have at most some mild radial anisotropy. Our results provide new observational limits on galaxy formation and evolution scenarios, covering four Gyr look-back time.

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arXiv2009

Strong Gravitational Lensing as a Probe of Gravity, Dark-Matter and Super-Massive Black Holes

Whereas considerable effort has been afforded in understanding the properties of galaxies, a full physical picture, connecting their baryonic and dark-matter content, super-massive black holes, and (metric) theories of gravity, is still ill-defined. Strong gravitational lensing furnishes a powerful method to probe gravity in the central regions of galaxies. It can (1) provide a unique detection-channel of dark-matter substructure beyond the local galaxy group, (2) constrain dark-matter physics, complementary to direct-detection experiments, as well as metric theories of gravity, (3) probe central super-massive black holes, and (4) provide crucial insight into galaxy formation processes from the dark matter point of view, independently of the nature and state of dark matter. To seriously address the above questions, a considerable increase in the number of strong gravitational-lens systems is required. In the timeframe 2010-2020, a staged approach with radio (e.g. EVLA, e-MERLIN, LOFAR, SKA phase-I) and optical (e.g. LSST and JDEM) instruments can provide 10^(2-4) new lenses, and up to 10^(4-6) new lens systems from SKA/LSST/JDEM all-sky surveys around ~2020. Follow-up imaging of (radio) lenses is necessary with moderate ground/space-based optical-IR telescopes and with 30-50m telescopes for spectroscopy (e.g. TMT, GMT, ELT). To answer these fundamental questions through strong gravitational lensing, a strong investment in large radio and optical-IR facilities is therefore critical in the coming decade. In particular, only large-scale radio lens surveys (e.g. with SKA) provide the large numbers of high-resolution and high-fidelity images of lenses needed for SMBH and flux-ratio anomaly studies.

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