arXiv subjects

Carl Grillmair

Publications and source records attributed to Carl Grillmair.

At least 19 recordsLinked to original sources

arXiv2021

Spitzer IRAC observations of JWST calibration stars

We present infrared photometry of all 36 potential JWST calibrators for which there is archival Spitzer IRAC data. This photometry can then be used to inform stellar models necessary to provide absolute calibration for all JWST instruments. We describe in detail the steps necessary to measure IRAC photometry from archive retrieval to photometric corrections. To validate our photometry we examine the distribution of uncertainties from all detections in all four IRAC channels as well as compare the photometry and its uncertainties to those from models, ALLWISE, and the literature. 75% of our detections have standard deviations per star of all observations within each channel of less than three percent. The median standard deviations are 1.2, 1.3, 1.1, and 1.9% in [3.6] - [8.0] respectively. We find less than 8% standard deviations in differences of our photometry with ALLWISE, and excellent agreement with literature values (less than 3% difference) lending credence to our measured fluxes. JWST is poised to do ground-breaking science, and accurate calibration and cross-calibration with other missions will be part of the underpinnings of that science.

astro-ph.SR↗

arXiv2019

The Detailed Science Case for the Maunakea Spectroscopic Explorer, 2019 edition

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more.

astro-ph.IM↗

arXiv2017

Science-Driven Optimization of the LSST Observing Strategy

The Large Synoptic Survey Telescope is designed to provide an unprecedented optical imaging dataset that will support investigations of our Solar System, Galaxy and Universe, across half the sky and over ten years of repeated observation. However, exactly how the LSST observations will be taken (the observing strategy or "cadence") is not yet finalized. In this dynamically-evolving community white paper, we explore how the detailed performance of the anticipated science investigations is expected to depend on small changes to the LSST observing strategy. Using realistic simulations of the LSST schedule and observation properties, we design and compute diagnostic metrics and Figures of Merit that provide quantitative evaluations of different observing strategies, analyzing their impact on a wide range of proposed science projects. This is work in progress: we are using this white paper to communicate to each other the relative merits of the observing strategy choices that could be made, in an effort to maximize the scientific value of the survey. The investigation of some science cases leads to suggestions for new strategies that could be simulated and potentially adopted. Notably, we find motivation for exploring departures from a spatially uniform annual tiling of the sky: focusing instead on different parts of the survey area in different years in a "rolling cadence" is likely to have significant benefits for a number of time domain and moving object astronomy projects. The communal assembly of a suite of quantified and homogeneously coded metrics is the vital first step towards an automated, systematic, science-based assessment of any given cadence simulation, that will enable the scheduling of the LSST to be as well-informed as possible.

astro-ph.IM↗

arXiv2016

The IPAC Image Subtraction and Discovery Pipeline for the intermediate Palomar Transient Factory

We describe the near real-time transient-source discovery engine for the intermediate Palomar Transient Factory (iPTF), currently in operations at the Infrared Processing and Analysis Center (IPAC), Caltech. We coin this system the IPAC/iPTF Discovery Engine (or IDE). We review the algorithms used for PSF-matching, image subtraction, detection, photometry, and machine-learned (ML) vetting of extracted transient candidates. We also review the performance of our ML classifier. For a limiting signal-to-noise ratio of 4 in relatively unconfused regions, "bogus" candidates from processing artifacts and imperfect image subtractions outnumber real transients by ~ 10:1. This can be considerably higher for image data with inaccurate astrometric and/or PSF-matching solutions. Despite this occasionally high contamination rate, the ML classifier is able to identify real transients with an efficiency (or completeness) of ~ 97% for a maximum tolerable false-positive rate of 1% when classifying raw candidates. All subtraction-image metrics, source features, ML probability-based real-bogus scores, contextual metadata from other surveys, and possible associations with known Solar System objects are stored in a relational database for retrieval by the various science working groups. We review our efforts in mitigating false-positives and our experience in optimizing the overall system in response to the multitude of science projects underway with iPTF.

astro-ph.IM↗

arXiv2016

Velocity Variations in the Phoenix-Hermus Star Stream

Measurements of velocity and density perturbations along stellar streams in the Milky Way provide a time integrated measure of dark matter substructure at larger galactic radius than the complementary instantaneous inner halo strong lensing detections of dark matter sub-halos in distant galaxies. An interesting case to consider is the proposed Phoenix-Hermus star stream, which is long, thin and on a nearly circular orbit, making it a particular good target to study for velocity variations along its length. In the presence of dark matter sub-halos the distribution of velocities is significantly perturbed in a manner that is readily understood with the impulse approximation. A set of simulations show that only sub-halos above a few 10^7 M_sun lead to reasonably long lived observationally detectable velocity variations of amplitude of order 1 kms, with an average of about one visible hit per (two-armed) stream. An implication is that globular clusters themselves will not have a visible impact on the stream. Radial velocities have the benefit that they are completely insensitive to distance errors. Distance errors scatter individual star velocities perpendicular and tangential to the mean orbit but their mean values remain unbiased. Calculations like these help build the quantitative case to acquire large, fairly deep, velocity samples of stream stars.

astro-ph.GA↗

arXiv2013

Kinematic Discovery of a Stellar Stream located in Pisces

We report the kinematic discovery of the Pisces Stellar Stream (PSS), at Galactic longitude of roughly l=135 degrees and longitude between -39 < b < -36 deg. We originally identified this halo substructure from velocities of red giant branch stars in Sloan Digital Sky Survey (SDSS) Data Release 8, and confirmed its presence in turnoff stars from SDSS photometric data. The PSS is a narrow, kinematically cold tidal stream, with a velocity dispersion of 8 km/s. Its metallicity is [Fe/H]=-2.2, with ~0.3 dex dispersion. The color-magnitude signature of the stream turnoff, combined with our measured metallicity, places the PSS at a distance of 35+/-3 kpc. The Pisces Stellar Stream is the same as the previously announced "Triangulum stream" and part of the proposed "stream a". We rule out an association of the PSS with other previously known Milky Way substructures in the same region of the sky.

astro-ph.GA↗

arXiv2011

The Star Formation History of M32

We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5Gyr old stars contribute \som40%+/- 17% of M32's mass, while 55%+/-21% of M32's mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are =6.8+/-1.5 Gyr and <[M/H]>=-0.01+/-0.08 dex. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H]\sim-0.7) stars, suggesting that blue straggler stars contribute ~2% of the mass at F1; the remaining \sim3% of the mass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent parameters of 2.9+/-0.2 Gyr and [M/H]=0.02+/-0.01 dex at F1 (~2.7 re). This contradicts spectroscopic studies that show a steep age gradient from M32's center to 1re. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with \sim95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; \sim30%+/-7.5% of its mass is in a 5-8 Gyr old population, and \sim65%+/-9% of the mass is in a 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are =9.2+/-1.2 Gyr and <[M/H]>=-0.10+/-0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31's disk at F2 (\sim1 disk scale length) to be 5-9 Gyr, our results agree with an inside-out disk formation scenario for M31's disk.

astro-ph.CO↗

arXiv2010

The Deepest HST Color-Magnitude Diagram of M32: Evidence for Intermediate-Age Populations

We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometry based on HST ACS/HRC images for a region 110" from the center of M32 and a background field about 320" away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe/H] = -0.2 in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch bump in M32 which indicate that the mean age of M32's dominant population at ~2' from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of bright asymptotic giant branch stars. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or blue straggler stars in M32. M32's dominant population of 8--10 Gyr implies a formation redshift of 1 < z_f < 2, precisely when observations of the specific star formation rates and models of "downsizing" imply galaxies of M32's mass ought to be forming their stars. Our CMD therefore provides a "ground-truth" of downsizing scenarios at z=0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. [Abridged]

astro-ph.CO↗

arXiv2009

RR Lyrae variables in M32 and the disk of M31

We observed two fields near M32 with the Advanced Camera for Surveys/High Resolution Channel (ACS/HRC) on board the Hubble Space Telescope (HST). The main field, F1, is 1.8 arcmin from the center of M32; the second field, F2, constrains the M31 background, and is 5.4 arcmin distant. Each field was observed for 16-orbits in each of the F435W (narrow B) and F555W (narrow V) filters. The duration of the observations allowed RR Lyrae stars and other short-period variables to be detected. A population of RR Lyrae stars determined to belong to M32 would prove the existence of an ancient population in that galaxy, a subject of some debate. We detected 17 RR Lyrae variables in F1 and 14 in F2. A 1-sigma upper limit of 6 RR Lyrae variables belonging to M32 is inferred from these two fields alone. Use of our two ACS/WFC parallel fields provides better constraints on the M31 background, however, and implies that $7_{-3}^{+4}$ (68 % confidence interval) RR Lyrae variables in F1 belong to M32. We have therefore found evidence for an ancient population in M32. It seems to be nearly indistinguishable from the ancient population of M31. The RR Lyrae stars in the F1 and F2 fields have indistinguishable mean V-band magnitudes, mean periods, distributions in the Bailey diagram and ratios of RRc to RR(tot) types. However, the color distributions in the two fields are different, with a population of red RRab variables in F1 not seen in F2. We suggest that these might be identified with the detected M32 RR Lyrae population, but the small number of stars rules out a definitive claim.

astro-ph.SR↗

arXiv2009

RR Lyrae Variables in Two Fields in the Spheroid of M31

We present Hubble Space Telescope observations taken with the Advanced Camera for Surveys Wide Field Channel of two fields near M32 - between four and six kpc from the center of M31. The data cover a time baseline sufficient for the identification and characterization of 681 RR Lyrae variables of which 555 are ab-type and 126 are c-type. The mean magnitude of these stars is =25.29 +/- 0.05 where the uncertainty combines both the random and systematic errors. The location of the stars in the Bailey Diagram and the ratio of c-type RR Lyraes to all types are both closer to RR Lyraes in Oosterhoff type I globular clusters in the Milky Way as compared with Oosterhoff II clusters. The mean periods of the ab-type and c-type RR Lyraes are =0.557 +/- 0.003 and =0.327 +/- 0.003, respectively, where the uncertainties in each case represent the standard error of the mean. When the periods and amplitudes of the ab-type RR Lyraes in our sample are interpreted in terms of metallicity, we find the metallicity distribution function to be indistinguishable from a Gaussian with a peak at <[Fe/H]>=-1.50 +/- 0.02, where the quoted uncertainty is the standard error of the mean. Using a relation between RR Lyrae luminosity and metallicity along with a reddening of E(B-V) = 0.08 +/- 0.03, we find a distance modulus of (m-M)o=24.46 +/- 0.11 for M31. We examine the radial metallicity gradient in the environs of M31 using published values for the bulge and halo of M31 as well as the abundances of its dwarf spheroidal companions and globular clusters. In this context, we conclude that the RR Lyraes in our two fields are more likely to be halo objects rather than associated with the bulge or disk of M31, in spite of the fact that they are located at 4-6 kpc in projected distance from the center.

astro-ph.GA↗

arXiv2009

Properties of Dark Matter Revealed by Astrometric Measurements of the Milky Way and Local Galaxies

The fact that dark matter (DM), thus far, has revealed itself only on scales of galaxies and larger, again thrusts onto astrophysics the opportunity and the responsibility to confront the age old mystery "What is the nature of matter?" By deriving basic data on the nature of DM - e.g., mass of its particle(s), present mean temperature, distribution in galaxies and other structures in the universe, and capacity for dissipational collapse - we will be uncovering the properties of the dominant species of matter in the universe and significantly extending the standard models of particle physics. Determining the mass of the DM particle to an order of magnitude would help to sort out the particle family to which it (or they) belongs. Beyond mass, there are issues of stability. The DM particle may be unstable with a measurable half-life, or it may become unstable after absorbing a certain amount of energy from collisions. In both cases it would contribute to the present hot dark matter component. Some key parameters of DM can most accurately be measured in the very nearby universe because DM dominates the mass in the outer Milky Way (MW), in other galaxies in the Local Group, and in the Local Group in its entirety. The presence and amount of DM can be quantified by study of dynamical processes observable in fine detail within these entities. Precise measurements of 3-D velocities for stars, coherent star streams, and stars in satellite stellar systems out to the edge of the Galaxy can reveal "what is the shape, orientation, density law, and lumpiness of the dark matter halo" as well as "what is the total mass of the Galaxy?"

astro-ph.CO↗

arXiv2007

Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest

Precision astrometry at microarcsecond accuracy has application to a wide range of astrophysical problems. This paper is a study of the science questions that can be addressed using an instrument that delivers parallaxes at about 4 microarcsec on targets as faint as V = 20, differential accuracy of 0.6 microarcsec on bright targets, and with flexible scheduling. The science topics are drawn primarily from the Team Key Projects, selected in 2000, for the Space Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities of this mission to illustrate the importance of the next level of astrometric precision in modern astrophysics. SIM PlanetQuest is currently in the detailed design phase, having completed all of the enabling technologies needed for the flight instrument in 2005. It will be the first space-based long baseline Michelson interferometer designed for precision astrometry. SIM will contribute strongly to many astronomical fields including stellar and galactic astrophysics, planetary systems around nearby stars, and the study of quasar and AGN nuclei. SIM will search for planets with masses as small as an Earth orbiting in the `habitable zone' around the nearest stars using differential astrometry, and could discover many dozen if Earth-like planets are common. It will be the most capable instrument for detecting planets around young stars, thereby providing insights into how planetary systems are born and how they evolve with time. SIM will observe significant numbers of very high- and low-mass stars, providing stellar masses to 1%, the accuracy needed to challenge physical models. Using precision proper motion measurements, SIM will probe the galactic mass distribution and the formation and evolution of the Galactic halo. (abridged)

astro-ph↗

arXiv2002

Axisymmetric Dynamical Models of the Central Regions of Galaxies

We present axisymmetric, orbit superposition models for 12 galaxies using data taken with the Hubble Space Telescope (HST) and ground-based observatories. In each galaxy, we detect a central black hole (BH) and measure its mass to accuracies ranging from 10% to 70%. We demonstrate that in most cases the BH detection requires both the HST and ground-based data. Using the ground-based data alone does provide an unbiased measure of the BH mass (provided they are fit with fully general models), but at a greatly reduced significance. The most significant correlation with host galaxy properties is the relation between the BH mass and the velocity dispersion of the host galaxy; we find no other equally strong correlation, and no second parameter that improves the quality of the mass-dispersion relation. We are also able to measure the stellar orbital properties from these general models. The most massive galaxies are strongly biased to tangential orbits near the BH, consistent with binary BH models, while lower-mass galaxies have a range of anisotropies, consistent with an adiabatic growth of the BH.

astro-ph↗

arXiv2002

The slope of the black-hole mass versus velocity dispersion correlation

Observations of nearby galaxies reveal a strong correlation between the mass of the central dark object M and the velocity dispersion sigma of the host galaxy, of the form log(M/M_sun) = a + b*log(sigma/sigma_0); however, published estimates of the slope b span a wide range (3.75 to 5.3). Merritt & Ferrarese have argued that low slopes (<4) arise because of neglect of random measurement errors in the dispersions and an incorrect choice for the dispersion of the Milky Way Galaxy. We show that these explanations account for at most a small part of the slope range. Instead, the range of slopes arises mostly because of systematic differences in the velocity dispersions used by different groups for the same galaxies. The origin of these differences remains unclear, but we suggest that one significant component of the difference results from Ferrarese & Merritt's extrapolation of central velocity dispersions to r_e/8 (r_e is the effective radius) using an empirical formula. Another component may arise from dispersion-dependent systematic errors in the measurements. A new determination of the slope using 31 galaxies yields b=4.02 +/- 0.32, a=8.13 +/- 0.06, for sigma_0=200 km/s. The M-sigma relation has an intrinsic dispersion in log M that is no larger than 0.3 dex. In an Appendix, we present a simple model for the velocity-dispersion profile of the Galactic bulge.

astro-ph↗

arXiv2000

Black Hole Mass Estimates from Reverberation Mapping and from Spatially Resolved Kinematics

Black hole (BH) masses that have been measured by reverberation mapping in active galaxies fall significantly below the correlation between bulge luminosity and BH mass determined from spatially resolved kinematics of nearby normal galaxies. This discrepancy has created concern that one or both techniques suffer from systematic errors. We show that BH masses from reverberation mapping are consistent with the recently discovered relationship between BH mass and galaxy velocity dispersion. Therefore the bulge luminosities are the probable source of the disagreement, not problems with either method of mass measurement. This result underscores the utility of the BH mass -- velocity dispersion relationship. Reverberation mapping can now be applied with increased confidence to galaxies whose active nuclei are too bright or whose distances are too large for BH searches based on spatially resolved kinematics.

astro-ph↗

arXiv2000

Stellar Populations in the Phoenix Dwarf (dIrr/dSph) Galaxy as Observed by HST/WFPC2

We present HST/WFPC2 photometry of the central regions of the Phoenix dwarf. Accurate photometry allows us to: 1) confirm the existence of the horizontal branch previously detected by ground-based observations, and use it to determine a distance to Phoenix, 2) clearly detect the existence of multiple ages in the stellar population of Phoenix, 3) determine a mean metallicity of the old red giant branch stars in Phoenix, and suggest that Phoenix has evolved chemically over its lifetime, 4) extract a rough star formation history for the central regions which suggests that Phoenix has been forming stars roughly continuously over its entire lifetime.

astro-ph↗

arXiv2000

A Relationship Between Nuclear Black Hole Mass and Galaxy Velocity Dispersion

We describe a correlation between the mass M_BH of a galaxy's central black hole and the luminosity-weighted line-of-sight velocity dispersion sigma_e within the half-light radius. The result is based on a sample of 26 galaxies, including 13 galaxies with new determinations of black hole masses from Hubble Space Telescope measurements of stellar kinematics. The best-fit correlation is M_BH = 1.2 (+-0.2) x 10^8 M_sun (sigma_e/200 km/s)^(3.75 (+-0.3))over almost three orders of magnitude in M_BH; the scatter in M_BH at fixed sigma_e is only 0.30 dex and most of this is due to observational errors. The M_BH-sigma_e relation is of interest not only for its strong predictive power but also because it implies that central black hole mass is constrained by and closely related to properties of the host galaxy's bulge.

astro-ph↗

arXiv1999

Axisymmetric, 3-Integral Models of Galaxies: A Massive Black Hole in NGC3379

We fit axisymmetric 3-integral dynamical models to NGC3379 using the line-of-sight velocity distribution obtained from HST/FOS spectra of the galaxy center and ground-based long-slit spectroscopy along four position angles, with the light distribution constrained by WFPC2 and ground-based images. We have fitted models with inclinations from 29 (intrinsic galaxy type E5) to 90 degrees (intrinsic E1) and black hole masses from 0 to 1e9 M_solar. The best-fit black hole masses range from 6e7 to 2e8 M_solar, depending on inclination. The velocity ellipsoid of the best model is not consistent with either isotropy or a two-integral distribution function. Along the major axis, the velocity ellipsoid becomes tangential at the innermost bin, radial in the mid-range radii, and tangential again at the outermost bins. For the acceptable models, the radial to tangential dispersion in the mid-range radii ranges from 1.1 < sigma_r / sigma_t < 1.7. Compared with these 3-integral models, 2-integral isotropic models overestimate the black hole mass since they cannot provide adequate radial motion. However, the models presented in this paper still contain restrictive assumptions-namely assumptions of constant M/L and spheroidal symmetry-requiring yet more models to study black hole properties in complete generality.

astro-ph↗