arXiv subjects

Alex G. Kim

Publications and source records attributed to Alex G. Kim.

51 records · Page 3Linked to original sources

arXiv2017

Measuring the growth rate of structure with Type IA Supernovae from LSST

We investigate measuring the peculiar motions of galaxies up to $z=0.5$ using Type Ia supernovae (SNe Ia) from LSST, and predict the subsequent constraints on the growth rate of structure. We consider two cases. Our first is based on measurements of the volumetric SNe Ia rate and assumes we can obtain spectroscopic redshifts and light curves for varying fractions of objects that are detected pre-peak luminosity by LSST (some of which may be obtained by LSST itself and others which would require additional follow-up). We find that these measurements could produce growth rate constraints at $z<0.5$ that significantly outperform those using Redshift Space Distortions (RSD) with DESI or 4MOST, even though there are $\sim4\times$ fewer objects. For our second case, we use semi-analytic simulations and a prescription for the SNe Ia rate as a function of stellar mass and star formation rate to predict the number of LSST SNe IA whose host redshifts may already have been obtained with the Taipan+WALLABY surveys, or with a future multi-object spectroscopic survey. We find $\sim 18,000$ and $\sim 160,000$ SN Ia with host redshifts for these cases respectively. Whilst this is only a fraction of the total LSST-detected SNe Ia, they could be used to significantly augment and improve the growth rate constraints compared to only RSD. Ultimately, we find that combining LSST SNe Ia with large numbers of galaxy redshifts will provide the most powerful probe of large scale gravity in the $z<0.5$ regime over the coming decades.

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arXiv2016

Host Galaxy Identification for Supernova Surveys

Host galaxy identification is a crucial step for modern supernova (SN) surveys such as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST), which will discover SNe by the thousands. Spectroscopic resources are limited, so in the absence of real-time SN spectra these surveys must rely on host galaxy spectra to obtain accurate redshifts for the Hubble diagram and to improve photometric classification of SNe. In addition, SN luminosities are known to correlate with host-galaxy properties. Therefore, reliable identification of host galaxies is essential for cosmology and SN science. We simulate SN events and their locations within their host galaxies to develop and test methods for matching SNe to their hosts. We use both real and simulated galaxy catalog data from the Advanced Camera for Surveys General Catalog and MICECATv2.0, respectively. We also incorporate "hostless" SNe residing in undetected faint hosts into our analysis, with an assumed hostless rate of 5%. Our fully automated algorithm is run on catalog data and matches SNe to their hosts with 91% accuracy. We find that including a machine learning component, run after the initial matching algorithm, improves the accuracy (purity) of the matching to 97% with a 2% cost in efficiency (true positive rate). Although the exact results are dependent on the details of the survey and the galaxy catalogs used, the method of identifying host galaxies we outline here can be applied to any transient survey.

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arXiv2014

Giving Cosmic Redshift Drift a Whirl

Redshift drift provides a direct kinematic measurement of cosmic acceleration but it occurs with a characteristic time scale of a Hubble time. Thus redshift observations with a challenging precision of $10^{-9}$ require a 10 year time span to obtain a signal-to-noise of 1. We discuss theoretical and experimental approaches to address this challenge, potentially requiring less observer time and having greater immunity to common systematics. On the theoretical side we explore allowing the universe, rather than the observer, to provide long time spans; speculative methods include radial baryon acoustic oscillations, cosmic pulsars, and strongly lensed quasars. On the experimental side, we explore beating down the redshift precision using differential interferometric techniques, including externally dispersed interferometers and spatial heterodyne spectroscopy. Low-redshift emission line galaxies are identified as having high cosmology leverage and systematics control, with an 8 hour exposure on a 10-meter telescope (1000 hours of exposure on a 40-meter telescope) potentially capable of measuring the redshift of a galaxy to a precision of $10^{-8}$ (few $\times 10^{-10}$). Low-redshift redshift drift also has very strong complementarity with cosmic microwave background measurements, with the combination achieving a dark energy figure of merit of nearly 300 (1400) for 5% (1%) precision on drift.

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arXiv2013

Robust Strong Lensing Time Delay Estimation

Strong gravitational lensing of time variable sources such as quasars and supernovae creates observable time delays between the multiple images. Time delays can provide a powerful cosmographic probe through the "time delay distance" involving the ratio of lens, source, and lens-source distances. However, lightcurves of lensed images have measurement gaps, noise, systematics such as microlensing from substructure along an image line of sight, and no a priori functional model, making robust time delay estimation challenging. Using Gaussian process techniques, we demonstrate success in accurate blind reconstruction of time delays and reduction in uncertainties for real data.

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arXiv2013

Model Independent Tests of Cosmic Growth vs Expansion

We use Gaussian Processes to map the expansion history of the universe in a model independent manner from the Union2.1 supernovae data and then apply these reconstructed results to solve for the growth history. By comparing this to BOSS and WiggleZ large scale structure data we examine whether growth is determined wholly by expansion, with the measured gravitational growth index testing gravity without assuming a model for dark energy. A further model independent analysis looks for redshift dependent deviations of growth from the general relativity solution without assuming the growth index form. Both approaches give results consistent with general relativity.

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arXiv2012

Gaussian Process Cosmography

Gaussian processes provide a method for extracting cosmological information from observations without assuming a cosmological model. We carry out cosmography -- mapping the time evolution of the cosmic expansion -- in a model-independent manner using kinematic variables and a geometric probe of cosmology. Using the state of the art supernova distance data from the Union2.1 compilation, we constrain, without any assumptions about dark energy parametrization or matter density, the Hubble parameter and deceleration parameter as a function of redshift. Extraction of these relations is tested successfully against models with features on various coherence scales, subject to certain statistical cautions.

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arXiv2012

Dithering Strategies and Point-Source Photometry

The accuracy in the photometry of a point source depends on the point-spread function (PSF), detector pixelization, and observing strategy. The PSF and pixel response describe the spatial blurring of the source, the pixel scale describes the spatial sampling of a single exposure, and the observing strategy determines the set of dithered exposures with pointing offsets from which the source flux is inferred. In a wide-field imaging survey, sources of interest are randomly distributed within the field of view and hence are centered randomly within a pixel. A given hardware configuration and observing strategy therefore have a distribution of photometric uncertainty for sources of fixed flux that fall in the field. In this article we explore the ensemble behavior of photometric and position accuracies for different PSFs, pixel scales, and dithering patterns. We find that the average uncertainty in the flux determination depends slightly on dither strategy, whereas the position determination can be strongly dependent on the dithering. For cases with pixels much larger than the PSF, the uncertainty distributions can be non-Gaussian, with rms values that are particularly sensitive to the dither strategy. We also find that for these configurations with large pixels, pointings dithered by a fractional pixel amount do not always give minimal average uncertainties; this is in contrast to image reconstruction for which fractional dithers are optimal. When fractional pixel dithering is favored, a pointing accuracy of better than $\sim 0.15$ pixel width is required to maintain half the advantage over random dithers.

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arXiv2011

Science Yield of an Improved Wide Field Infrared Survey Telescope (WFIRST)

The Astronomy and Astrophysics Decadal Survey's highest recommended space mission was a Wide-Field Infrared Survey Telescope (WFIRST) to efficiently conduct three kinds of studies: dark energy surveys, exoplanet surveys, and guest surveys. In this paper we illustrate four potential WFIRST payloads that accomplish these objectives and that fully utilize optical and technical advances made since the community input to the Decadal Survey. These improvements, developed by our group, are: unobscured 1.3 or 1.5 m apertures; simultaneous dual focal lengths delivering pixel scales of 0.18" for imaging and 0.38" or 0.45" for slitless spectroscopy; the use of a prism in converging light for slitless spectroscopy; and payload features that allow up to 270 days/year observing the Galactic Bulge. These factors combine to allow WFIRST payloads that provide improved survey rates compared to previous mission concepts. In this report we perform direct comparisons of survey speeds for constant survey depth using our optical and exposure-time tools previously developed for JDEM. We further compare these four alternative WFIRST configurations to JDEM-Omega and to the European Space Agency's Euclid mission, and to an alternative Euclid configuration making use of the lessons learned here that delivers performance approaching that of WFIRST. We find that the unobstructed pupil is a major benefit to weak lensing owing to its tighter point spread function, improved signal to noise, and higher resolved galaxy count. Using two simultaneous plate scales in a fully focal system is practical and simplifies the optical train, and the use of a prism in converging light offers improved throughput compared to a grism. We find that a 45 degree outer baffle cutoff angle, combined with fully articulated solar panels and K-band antenna, substantially increase the exoplanet yield. Presented at the 217th AAS conference.

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arXiv2011

Correlated Supernova Systematics and Ground Based Surveys

Supernova distances provide a direct probe of cosmic acceleration, constraining dark energy. This leverage increases with survey redshift depth at a rate bounded by the systematic uncertainties. We investigate the impact of a wavelength-dependent, global correlation model of systematics in comparison to the standard local-redshift correlation model. This can arise from subclass uncertainties as features in the supernova spectrum redshift out of the observer photometric filters or spectral range. We explore the impact of such a systematic on ground-based supernova surveys such as Dark Energy Survey and LSST, finding distinctive implications. Extending the wavelength sensitivity to 1.05 microns through "extreme red" CCDs can improve the dark energy figure of merit by up to a factor 2.

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arXiv2007

Measuring Type Ia Supernova Distances and Redshifts From Their Multi-band Light Curves

The distance and redshift of a type Ia supernova can be determined simultaneously through its multi-band light curves. This fact may be used for imaging surveys that discover and obtain photometry for large numbers of supernovae; so many that it would be difficult to obtain a spectroscopic redshift for each. Using available supernova-analysis tools we find that there are several conditions in which a viable distance-redshift can be determined. Uncertainties in the effective distance at z~0.3 are dominated by redshift uncertainties coupled with the steepness of the Hubble law. By z~0.5 the Hubble law flattens out and distance-modulus uncertainties dominate. Observations that give S/N=50 at peak brightness and a four-day observer cadence in each of griz-bands are necessary to match the intrinsic supernova magnitude dispersion out to z=1.0. Lower S/N can be tolerated with the addition of redshift priors (e.g. from a host-galaxy photometric redshift), observations in an additional redder band, or by focusing on supernova redshifts that have particular leverage for this measurement. More stringent S/N requirements are anticipated as improved systematics control over intrinsic color, metallicity, and dust is attempted to be drawn from light curves.

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arXiv2006

Snapping Supernovae at z>1.7

We examine the utility of very high redshift Type Ia supernovae for cosmology and systematic uncertainty control. Next generation space surveys such as the Supernova/Acceleration Probe (SNAP) will obtain thousands of supernovae at z>1.7, beyond the design redshift for which the supernovae will be exquisitely characterized. We find that any z\gtrsim2 standard candles' use for cosmological parameter estimation is quite modest and subject to pitfalls; we examine gravitational lensing, redshift calibration, and contamination effects in some detail. The very high redshift supernovae - both thermonuclear and core collapse - will provide copious interesting information on star formation, environment, and evolution. However, the new observational systematics that must be faced, as well as the limited expansion of SN-parameter space afforded, does not point to high value for 1.7<z<3 SNe Ia in controlling evolutionary systematics relative to what SNAP can already achieve at z<1.7. Synergy with observations from JWST and thirty meter class telescopes afford rich opportunities for advances throughout astrophysics.

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arXiv2006

Spectral diversity of Type Ia Supernovae

We use published spectroscopic and photometric data for 8 Type Ia supernovae to construct a dispersion spectrum for this class of object, showing their diversity over the wavelength range 3700A to 7100A. We find that the B and V bands are the spectral regions with the least dispersion, while the U band below 4100A is more diverse. Some spectral features such as the Si line at 6150A are also highly diverse. We then construct two objective measures of 'peculiarity' by (i) using the deviation of individual objects from the average SN Ia spectrum compared to the typical dispersion and (ii) applying principle component analysis. We demonstrate these methods on several SNe Ia that have previously been classified as peculiar.

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arXiv2005

Optimal Extraction of Cosmological Information from Supernova Data in the Presence of Calibration Uncertainties

We present a new technique to extract the cosmological information from high-redshift supernova data in the presence of calibration errors and extinction due to dust. While in the traditional technique the distance modulus of each supernova is determined separately, in our approach we determine all distance moduli at once, in a process that achieves a significant degree of self-calibration. The result is a much reduced sensitivity of the cosmological parameters to the calibration uncertainties. As an example, for a strawman mission similar to that outlined in the SNAP satellite proposal, the increased precision obtained with the new approach is roughly equivalent to a factor of five decrease in the calibration uncertainty.

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arXiv2005

Ideal bandpasses for type Ia supernova cosmology

To use type Ia supernovae as standard candles for cosmology we need accurate broadband magnitudes. In practice the observed magnitude may differ from the ideal magnitude-redshift relationship either through intrinsic inhomogeneities in the type Ia supernova population or through observational error. Here we investigate how we can choose filter bandpasses to reduce the error caused by both these effects. We find that bandpasses with large integral fluxes and sloping wings are best able to minimise several sources of observational error, and are also least sensitive to intrinsic differences in type Ia supernovae. The most important feature of a complete filter set for type Ia supernova cosmology is that each bandpass be a redshifted copy of the first. We design practical sets of redshifted bandpasses that are matched to typical high resistivity CCD and HgCdTe infra-red detector sensitivities. These are designed to minimise systematic error in well observed supernovae, final designs for specific missions should also consider signal-to-noise requirements and observing strategy. In addition we calculate how accurately filters need to be calibrated in order to achieve the required photometric accuracy of future supernova cosmology experiments such as the SuperNova-Acceleration-Probe (SNAP), which is one possible realisation of the Joint Dark-Energy mission (JDEM). We consider the effect of possible periodic miscalibrations that may arise from the construction of an interference filter.

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arXiv2003

Effects of Systematic Uncertainties on the Supernova Determination of Cosmological Parameters

Mapping the recent expansion history of the universe offers the best hope for uncovering the characteristics of the dark energy believed to be responsible for the acceleration of the expansion. In determining cosmological and dark-energy parameters to the percent level, systematic uncertainties impose a floor on the accuracy more severe than the statistical measurement precision. We delineate the categorization, simulation, and understanding required to bound systematics for the specific case of the Type Ia supernova method. Using simulated data of forthcoming ground-based surveys and the proposed space-based SNAP mission we present Monte Carlo results for the residual uncertainties on the cosmological parameter determination. The tight systematics control with optical and near-infrared observations and the extended redshift reach allow a space survey to bound the systematics below 0.02 magnitudes at z=1.7. For a typical SNAP-like supernova survey, this keeps total errors within 15% of the statistical values and provides estimation of Omega_m to 0.03, w_0 to 0.07, and w' to 0.3; these can be further improved by incorporating complementary data.

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