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Edward A. Baltz

Publications and source records attributed to Edward A. Baltz.

At least 19 recordsLinked to original sources
arXiv2017

A Hubble Space Telescope survey for novae in M87. III. Are novae good standard candles 15 days after maximum brightness?

Ten weeks of daily imaging of the giant elliptical galaxy M87 with the Hubble Space Telescope (HST) has yielded 41 nova light curves of unprecedented quality for extragalactic cataclysmic variables. We have recently used these light curves to demonstrate that the observational scatter in the so-called Maximum-Magnitude Rate of Decline (MMRD) relation for classical novae is so large as to render the nova-MMRD useless as a standard candle. Here we demonstrate that a modified Buscombe - de Vaucouleurs hypothesis, namely that novae with decline times t2 > 10 days converge to nearly the same absolute magnitude about two weeks after maximum light in a giant elliptical galaxy, is supported by our M87 nova data. For 13 novae with daily-sampled light curves, well determined times of maximum light in both the F606W and F814W filters, and decline times $t2 > 10 days we find that M87 novae display M(606W,15) = -6.37 +/- 0.46 and M(814W,15) = -6.11 +/- 0.43. If very fast novae with decline times t2 < 10 days are excluded, the distances to novae in elliptical galaxies with stellar binary populations similar to those of M87 should be determinable with 1 sigma accuracies of +/-20% with the above calibrations.

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arXiv2017

A Hubble Space Telescope Survey for Novae in M87. II. Snuffing out the Maximum Magnitude - Rate of Decline Relation for Novae as a Non-Standard Candle, and a Prediction of the Existence of Ultrafast Novae

The extensive grid of numerical simulations of nova eruptions of Yaron et al.(2005) first predicted that some classical novae might deviate significantly from the Maximum Magnitude - Rate of Decline (MMRD) relation, which purports to characterise novae as standard candles. Kasliwal et al. (2011) have announced the observational detection of an apparently new class of faint, fast classical novae in the Andromeda galaxy. These objects deviate strongly from the MMRD relationship, exactly as predicted by Yaron et al. (2005). Shara et al. (2016) recently reported the first detections of faint, fast novae in M87. These previously overlooked objects are as common in the giant elliptical galaxy M87 as they are in the giant spiral M31; they comprise about 40% of all classical nova eruptions and greatly increase the observational scatter in the MMRD relation. We use the extensive grid of nova simulations of Yaron et al. (2005) to identify the underlying causes of the existence of faint, fast novae. These are systems which have accreted, and can thus eject, only very low mass envelopes, of order 10^-7 - 10^-8 Msun, on massive white dwarfs. Such binaries include, but are not limited to, the recurrent novae. These same models predict the existence of ultrafast novae which display decline times t2 as short as five hours. We outline a strategy for their future detection.

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arXiv2016

A Hubble Space Telescope Survey for Novae in M87. I. Light and Color Curves, Spatial Distributions and the Nova Rate

The Hubble Space Telescope has imaged the central part of M87 over a 10 week span, leading to the discovery of 32 classical novae and nine fainter, likely very slow and/or symbiotic novae. In this first in a series of papers we present the M87 nova finder charts, and the light and color curves of the novae. We demonstrate that the rise and decline times, and the colors of M87 novae are uncorrelated with each other and with position in the galaxy. The spatial distribution of the M87 novae follows the light of the galaxy, suggesting that novae accreted by M87 during cannibalistic episodes are well-mixed. Conservatively using only the 32 brightest classical novae we derive a nova rate for M87: $363_{-45}^{+33}$ novae/yr. We also derive the luminosity-specific classical nova rate for this galaxy, which is $7.88_{-2.6}^{+2.3} /yr/ 10^{10}L_\odot,_{K}$. Both rates are 3-4 times higher higher than those reported for M87 in the past, and similarly higher than those reported for all other galaxies. We suggest that most previous ground-based surveys for novae in external galaxies, including M87, miss most faint, fast novae, and almost all slow novae near the centers of galaxies.

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arXiv2008

Analytic models of plausible gravitational lens potentials

Gravitational lenses on galaxy scales are plausibly modelled as having ellipsoidal symmetry and a universal dark matter density profile, with a Sersic profile to describe the distribution of baryonic matter. Predicting all lensing effects requires knowledge of the total lens potential: in this work we give analytic forms for that of the above hybrid model. Emphasising that complex lens potentials can be constructed from simpler components in linear combination, we provide a recipe for attaining elliptical symmetry in either projected mass or lens potential. We also provide analytic formulae for the lens potentials of Sersic profiles for integer and half-integer index. We then present formulae describing the gravitational lensing effects due to smoothly-truncated universal density profiles in cold dark matter model. For our isolated haloes the density profile falls off as radius to the minus fifth or seventh power beyond the tidal radius, functional forms that allow all orders of lens potential derivatives to be calculated analytically, while ensuring a non-divergent total mass. We show how the observables predicted by this profile differ from that of the original infinite-mass NFW profile. Expressions for the gravitational flexion are highlighted. We show how decreasing the tidal radius allows stripped haloes to be modelled, providing a framework for a fuller investigation of dark matter substructure in galaxies and clusters. Finally we remark on the need for finite mass halo profiles when doing cosmological ray-tracing simulations, and the need for readily-calculable higher order derivatives of the lens potential when studying catastrophes in strong lenses.

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arXiv2008

Color Bimodality in M87 Globular Clusters

We present an analysis of a 50 orbit HST ACS observation of the M87 globular cluster system. We use the extraordinary depth of this dataset to test whether the colors and magnitudes show evidence for a mass-metallicity relation in globular cluster populations. We find only a weak or absent relation between the colors and magnitudes of the metal poor subpopulation of globular clusters. The weakness or absence of a color-magnitude relation is established over a wide range in luminosity from $M_V=-11$ to $M_V=-6$, encompassing most of the M87 globular clusters. The constancy of the colors of the metal-poor subpopulation seen in our 50 orbit observation is in contrast to suggestions from single orbit ACS data that the metal-poor globular clusters in M87 and several other galaxies show a "blue tilt." The formal best fit for the mass-metallicity relation for the metal-poor subpopulation in our much deeper data is $Z\propto M^{0.08\pm0.05}$. Our analysis of these data also shows a possible small "red tilt" in the metal-rich globular cluster subpopulation. While either of these small tilts may be real, they may also illustrate the limit to which mass-metallicity relations can be determined, even in such extraordinarily deep data. We specifically test for a wide range of systematic effects and find that while small tilts cannot be confirmed or rejected, the data place a strong upper limit to any tilt of $|0.20|\pm0.05$. This upper limit is much smaller than some earlier claims from single orbit data, and strongly limits self-enrichment within globular clusters. This mass-metallicity relation for globular clusters is also shallower than the relation for galaxies, suggesting that the formation mechanisms for these two types of objects are different.

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arXiv2008

Strategies for Determining the Nature of Dark Matter

In this review, we discuss the role of the various experimental programs taking part in the broader effort to identify the particle nature of dark matter. In particular, we focus on electroweak scale dark matter particles and discuss a wide range of search strategies being carried out and developed to detect them. These efforts include direct detection experiments, which attempt to observe the elastic scattering of dark matter particles with nuclei, indirect detection experiments, which search for photons, antimatter and neutrinos produced as a result of dark matter annihilations, and collider searches for new TeV-scale physics. Each of these techniques could potentially provide a different and complementary set of information related to the mass, interactions and distribution of dark matter. Ultimately, it is hoped that these many different tools will be used together to conclusively identify the particle or particles that constitute the dark matter of our universe.

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arXiv2007

Can Astrophysical Gamma Ray Sources Mimic Dark Matter Annihilation in Galactic Satellites?

The nature of the cosmic dark matter is unknown. The most compelling hypothesis is that dark matter consists of weakly interacting massive particles (WIMPs) in the 100 GeV mass range. Such particles would annihilate in the galactic halo, producing high-energy gamma rays which might be detectable in gamma ray telescopes such as the GLAST satellite. We investigate the ability of GLAST to distinguish between WIMP annihilation sources and astrophysical sources. Focusing on the galactic satellite halos predicted by the cold dark matter model, we find that the WIMP gamma-ray spectrum is nearly unique; separation of the brightest WIMP sources from known source classes can be done in a convincing way by including spectral and spatial information. Candidate WIMP sources can be further studied with Imaging Atmospheric Cerenkov Telescopes. Finally, Large Hadron Collider data might have a crucial impact on the study of galactic dark matter.

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arXiv2006

Determination of Dark Matter Properties at High-Energy Colliders

If the cosmic dark matter consists of weakly-interacting massive particles, these particles should be produced in reactions at the next generation of high-energy accelerators. Measurements at these accelerators can then be used to determine the microscopic properties of the dark matter. From this, we can predict the cosmic density, the annihilation cross sections, and the cross sections relevant to direct detection. In this paper, we present studies in supersymmetry models with neutralino dark matter that give quantitative estimates of the accuracy that can be expected. We show that these are well matched to the requirements of anticipated astrophysical observations of dark matter. The capabilities of the proposed International Linear Collider (ILC) are expected to play a particularly important role in this study.

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arXiv2006

Luminosity Function of Faint Globular Clusters in M87

We present the luminosity function to very faint magnitudes for the globular clusters in M87, based on a 30 orbit \textit{Hubble Space Telescope (HST)} WFPC2 imaging program. The very deep images and corresponding improved false source rejection allow us to probe the mass function further beyond the turnover than has been done before. We compare our luminosity function to those that have been observed in the past, and confirm the similarity of the turnover luminosity between M87 and the Milky Way. We also find with high statistical significance that the M87 luminosity function is broader than that of the Milky Way. We discuss how determining the mass function of the cluster system to low masses can constrain theoretical models of the dynamical evolution of globular cluster systems. Our mass function is consistent with the dependence of mass loss on the initial cluster mass given by classical evaporation, and somewhat inconsistent with newer proposals that have a shallower mass dependence. In addition, the rate of mass loss is consistent with standard evaporation models, and not with the much higher rates proposed by some recent studies of very young cluster systems. We also find that the mass-size relation has very little slope, indicating that there is almost no increase in the size of a cluster with increasing mass.

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arXiv2005

MACHOs in M31? Absence of evidence but not evidence of absence

We present results of a microlensing survey toward the Andromeda Galaxy (M31) carried out during four observing seasons at the Isaac Newton Telescope (INT). This survey is part of the larger microlensing survey toward M31 performed by the Microlensing Exploration of the Galaxy and Andromeda (MEGA) collaboration. Using a fully automated search algorithm, we indentify 14 candidate microlensing events, three of which are reported here for the first time. Observations obtained at the Mayall telescope are combined with the INT data to produce composite lightcurves for these candidates. The results from the survey are compared with theoretical predictions for the number and distribution of events. These predictions are based on a Monte Carlo calculation of the detection efficiency and disk-bulge-halo models for M31. The models provide the full phase-space distribution functions for the lens and source populations and are motivated by dynamical and observational considerations. They include differential extinction and span a wide range of parameter space characterised primarily by the mass-to-light ratios for the disk and bulge. For most models, the observed event rate is consistent with the rate predicted for self-lensing -- a MACHO halo fraction of 30% or higher can be ruled at the 95% confidence level. The event distribution does show a large near-far asymmetry hinting at a halo contribution to the microlensing signal. Two candidate events are located at particularly large projected radii on the far side of the disk. These events are difficult to explain by self lensing and only somewhat easier to explain by MACHO lensing. A possibility is that one of these is due to a lens in a giant stellar stream.

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arXiv2005

HST Imaging of MEGA Microlensing Candidates in M31

We investigate $HST$/ACS and WFPC2 images at the positions of five candidate microlensing events from a large survey of variability in M31 (MEGA). Three closely match unresolved sources, and two produce only flux upper limits. All are confined to regions of the color-magnitude diagram where stellar variability is unlikely to be easily confused with microlensing. Red variable stars cannot explain these events (although background supernova are possible for two). If these lenses arise in M31's halo, they are due to masses $0.15 < m / M_\odot < 0.49$ (95% certainty, for a $δ$-function mass distribution), brown dwarfs for disk lenses, and stellar masses for bulge lenses.

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arXiv2004

Dark Matter Candidates

It is now widely accepted that most of mass--energy in the universe is unobserved except by its gravitational effects. Baryons make only about 4% of the total, with "dark matter" making up about 23% and the "dark energy" responsible for the accelerated expansion of the universe making up the remainder. We focus on the dark matter, which is the primary constituent of galaxies. We outline the observed properties of this material, enumerating some candidates covering 90 orders of magnitude in mass. Finally, we argue that the weak scale (100 GeV) is relevant to new physics, including the dark matter problem.

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arXiv2004

Kaluza-Klein Dark Matter, Electrons and Gamma Ray Telescopes

Kaluza-Klein dark matter particles can annihilate efficiently into electron-positron pairs, providing a discrete feature (a sharp edge) in the cosmic $e^+ e^-$ spectrum at an energy equal to the particle's mass (typically several hundred GeV to one TeV). Although this feature is probably beyond the reach of satellite or balloon-based cosmic ray experiments (those that distinguish the charge and mass of the primary particle), gamma ray telescopes may provide an alternative detection method. Designed to observe very high-energy gamma-rays, ACTs also observe the diffuse flux of electron-induced electromagnetic showers. The GLAST satellite, designed for gamma ray astronomy, will also observe any high energy showers (several hundred GeV and above) in its calorimeter. We show that high-significance detections of an electron-positron feature from Kaluza-Klein dark matter annihilations are possible with GLAST, and also with ACTs such as HESS, VERITAS or MAGIC.

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arXiv2004

Microlensing Surveys of M31 in the Wide Field Imaging Era

The Andromeda Galaxy (M31) is the closest large galaxy to the Milky Way, thus it is an important laboratory for studying massive dark objects in galactic halos (MACHOs) by gravitational microlensing. Such studies strongly complement the studies of the Milky Way halo using the the Large and Small Magellanic Clouds. We consider the possibilities for microlensing surveys of M31 using the next generation of wide field imaging telescopes with fields of view in the square degree range. We consider proposals for such imagers both on the ground and in space. For concreteness, we specialize to the SNAP proposal for a space telescope and the LSST proposal for a ground based telescope. We find that a modest space-based survey of 50 visits of one hour each is considerably better than current ground based surveys covering 5 years. Crucially, systematic effects can be considerably better controlled with a space telescope because of both the infrared sensitivity and the angular resolution. To be competitive, 8 meter class wide-field ground based imagers must take exposures of several hundred seconds with several day cadence.

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arXiv2004

Markov Chain Monte Carlo Exploration of Minimal Supergravity with Implications for Dark Matter

We explore the full parameter space of Minimal Supergravity (mSUGRA), allowing all four continuous parameters (the scalar mass m_0, the gaugino mass m_1/2, the trilinear coupling A_0, and the ratio of Higgs vacuum expectation values tan beta) to vary freely. We apply current accelerator constraints on sparticle and Higgs masses, and on the b -> s gamma branching ratio, and discuss the impact of the constraints on g_mu-2. To study dark matter, we apply the WMAP constraint on the cold dark matter density. We develop Markov Chain Monte Carlo (MCMC) techniques to explore the parameter regions consistent with WMAP, finding them to be considerably superior to previously used methods for exploring supersymmetric parameter spaces. Finally, we study the reach of current and future direct detection experiments in light of the WMAP constraint.

hep-ph↗
arXiv2004

Evidence of Halo Microlensing in M31

We report results from intensive monitoring of two fields on either side of M31,emphasizing microlensing involving stars and masses in M31. These results stem from the three-year VATT/Columbia survey of variability on 3d to 2m timescales. Observations were conducted intensively from 1997-1999, with baselines 1995-now,at the Vatican Advanced Technology Telescope, MDM 1.3-meter, and Isaac Newton telescopes, totaling about 200 epochs. The two fields cover 560 square arcmin total, along the minor axis on either side of M31. Candidate microlensing events are subject to tests to distinguish microlenses from variable stars. Fourprobable microlensing events total, compared to carefully computed event rate and efficiency models, indicate a marginally significant microlensing activity above expectations for the stellar lenses alone in M31 (and the Galaxy) acting as lenses. Maximum likelihood analyses of the event distribution in timescale and across the face of M31 indicates a microlensing dark matter halo fraction consistent with that seen in our Galaxy towards the LMC (Alcock et al. 2000). For a nearly singular isothermal sphere model, we find a microlensing halo mass fraction f=0.29 +0.30/-0.13 of total dark matter, and a poorly constrained lensing component mass 0.02 to 1.5 solar mass, 1 sigma limits). This study is the prototype for a larger one approaching completion; between them there is significant evidence for asymmetry in the distribution of microlensing events across the face of M31, and a large population of halo microlensing objects. (Abridged)

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