arXiv subjects

John L. Tonry

Publications and source records attributed to John L. Tonry.

At least 91 records · Page 5Linked to original sources

arXiv2006

The Discovery of Cepheids and a Distance to NGC 5128

We discuss a new distance to NGC 5128 (Centaurus A) based on Cepheid variables observed with the Hubble Space Telescope. Twelve F555W (V) and six F814W (I) epochs of cosmic-ray-split Wide Field Planetary Camera 2 observations were obtained. A total of 56 bona-fide Cepheids were discovered, with periods ranging from 5 to ~50 days; five of these are likely Population II Cepheids of the W Virginis class, associated with the bulge or halo of NGC 5128. Based on the period and V and I-band luminosities of a sub-sample of 42 classical (Pop I) Cepheids, and adopting a Large Magellanic Cloud distance modulus and extinction of 18.50 +/- 0.10 mag and E(B-V)=0.10 mag, respectively, the true reddening-corrected distance modulus to NGC 5128 is mu_0 = 27.67 +/- 0.12 (random)+/- 0.16 (systematic) mag, corresponding to a distance of 3.42 +/- 0.18 (random) +/- 0.25 (systematic) Mpc. The random uncertainty in the distance is dominated by the error on the assumed value for the ratio of total to selective absorption, R_V, in NGC 5128, and by the possible metallicity dependence of the Cepheid Period-Luminosity relation at V and I. This represent the first determination of a Cepheid distance to an early-type galaxy.

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arXiv2006

A Fundamental Relation Between Compact Stellar Nuclei, Supermassive Black Holes, and Their Host Galaxies

Imaging surveys with the Hubble Space Telescope (HST) have shown that roughly 50-80% of low- and intermediate-luminosity galaxies contain a compact stellar nucleus at their center, regardless of host galaxy morphological type. We combine HST imaging for early-type galaxies from the ACS Virgo Cluster Survey with ground-based long-slit spectra from KPNO to show that the masses of compact stellar nuclei in Virgo Cluster galaxies obey a tight correlation with the masses of the host galaxies. The same correlation is obeyed by the supermassive black holes (SBHs) found in predominantly massive galaxies. The compact stellar nuclei in the Local Group galaxies M33 and NGC205 are also found to fall along this same scaling relation. These results indicate that a generic by-product of galaxy formation is the creation of a Central Massive Object (CMO) -- either a SBH or a compact stellar nucleus -- that contains a mean fraction, ~ 0.2%, of the total galactic mass. In galaxies with masses greater than a few times 10^10 solar masses, SBHs appear to be the dominant mode of CMO formation.

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arXiv2006

Toward 1% Photometry: End-to-end Calibration of Astronomical Telescopes and Detectors

We review the systematic uncertainties that have plagued attempts to obtain high precision and high accuracy from ground-based photometric measurements using CCDs. We identify two main challenges in breaking through the 1% precision barrier: 1) fully characterizing atmospheric transmission, along the instrument's line of sight, and 2) properly identifying, measuring and removing instrumental artifacts. We discuss approximations and limitations inherent in the present methodology, and we estimate their contributions to systematic photometric uncertainties. We propose an alternative conceptual scheme for the relative calibration of astronomical apparatus: the availability of calibrated detectors whose relative spectral sensitivity is known to better than one part in $10^3$ opens up the possibility of in situ relative throughput measurements, normalized to a precision calibrated detector, using a stable but uncalibrated narrowband light source. An implementation scheme is outlined, which exploits the availability of tunable lasers to map out the relative wavelength response of an imaging system, using a flatfield screen and a calibrated reference photodiode. The merits and limitations of this scheme are discussed. In tandem with careful measurements of atmospheric transmission, this approach could potentially lead to reliable ground-based photometry with fractional uncertainties below the percent level.

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arXiv2006

The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies

(Abridged) The ACS Virgo Cluster Survey is an HST program to obtain high-resolution, g and z-band images for 100 early-type members of the Virgo Cluster, spanning a range of ~460 in blue luminosity. Based on this large, homogeneous dataset, we present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than M_B ~ -15 (66 < f_n < 82%), and find no evidence for nucleated dwarfs to be more concentrated to the center of Virgo than their non-nucleated counterparts. Resolved stellar nuclei are not present in galaxies brighter than M_B ~ -20.5, however, there is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at M_B ~ -17.6, the traditional dividing point between dwarf and giant galaxies. On average, nuclei are ~3.5 mag brighter than a typical globular cluster and have a median half-light radius ~4.2 pc. Nuclear luminosities correlate with nuclear sizes and, in galaxies fainter than M_B ~ -17.6, nuclear colors. Comparing the nuclei to the "nuclear clusters" found in late-type spiral galaxies reveals a close match in terms of size, luminosity and overall frequency, pointing to a formation mechanism that is rather insensitive to the detailed properties of the host galaxy. The mean nuclear-to-galaxy luminosity ratio is indistinguishable from the mean SBH-to-bulge mass ratio, calculated in early-type galaxies with detected supermassive black holes (SBHs). We argue that compact stellar nuclei might be the low-mass counterparts of the SBHs detected in the bright galaxies, and that one should think in terms of "Central Massive Objects" -- either SBHs or compact stellar nuclei -- that accompany the formation of almost all early-type galaxies and contain a mean fraction ~0.3% of the total bulge mass.

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arXiv2006

The ACS Virgo Cluster Survey. VI. Isophotal Analysis and the Structure of Early-Type Galaxies

(Abridged) We present a detailed analysis of the morphology, isophotal parameters and surface brightness profiles for 100 early-type members of the Virgo Cluster, from dwarfs (M_B = -15.1 mag) to giants (M_B = -21.8 mag). Each galaxy has been imaged in two filters, closely resembling the Sloan g and z passbands, using the Advanced Camera for Surveys on board the Hubble Space Telescope. Dust and complex morphological structures are common, with kiloparsec-scale stellar disks, bars, and nuclear stellar disks seen in 60% of galaxies with intermediate luminosity (-20 < M_B < -17), and dust seen in 42% of galaxies brighter than M_B = -18.9 mag. Dust morphologies range from faint wisps and patches on tens of parsec scales, to regular, highly organized kpc-scale dust disks, often showing evidence of recent star formation. Surface brightness profiles and isophotal parameters are derived typically within 8 kpc from the center for the brightest galaxies, and 1.5 kpc for the faintest systems, with a resolution (FWHM) of 7 pc. Based on a parametrization of the surface brightness profiles in terms of a Sersic or core-Sersic model, we find that 1) there is no evidence of a bimodal behavior of the slope, gamma, of the profile in the innermost regions; 2) although the brightest galaxies have shallow inner profiles, the shallowest profiles (lowest gamma values) are found in faint dwarf systems; 3) the widely adopted separation of early-type galaxies between "core" and "power-law" types, which had originally been prompted by the claim of a clear bimodal distribution of gamma values, is untenable; and 4) there is no evidence of a structural dichothomy between dwarf and regular ellipticals.

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arXiv2005

The Rate of Type Ia Supernovae at High Redshift

We derive the rates of Type Ia supernovae (SNIa) over a wide range of redshifts using a complete sample from the IfA Deep Survey. This sample of more than 100 SNIa is the largest set ever collected from a single survey, and therefore uniquely powerful for a detailed supernova rate (SNR) calculation. Measurements of the SNR as a function of cosmological time offer a glimpse into the relationship between the star formation rate (SFR) and Type Ia SNR, and may provide evidence for the progenitor pathway. We observe a progressively increasing Type Ia SNR between redshifts z~0.3-0.8. The Type Ia SNR measurements are consistent with a short time delay (t~1 Gyr) with respect to the SFR, indicating a fairly prompt evolution of SNIa progenitor systems. We derive a best-fit value of SFR/SNR 580 h_70^(-2) M_solar/SNIa for the conversion factor between star formation and SNIa rates, as determined for a delay time of t~1 Gyr between the SFR and the Type Ia SNR. More complete measurements of the Type Ia SNR at z>1 are necessary to conclusively determine the SFR--SNR relationship and constrain SNIa evolutionary pathways.

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arXiv2005

The ACS Virgo Cluster Survey XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies

We use HST/ACS imaging of 100 early-type galaxies in the ACS Virgo Cluster Survey to investigate the nature of diffuse star clusters (DSCs). Compared to globular clusters (GCs), these star clusters have moderately low luminosities (M_V > -8) and a broad distribution of sizes (3 < r_h < 30 pc), but they are principally characterized by their low mean surface brightnesses which can be more than three magnitudes fainter than a typical GC (mu_g > 20 mag arcsec^-2). The median colors of diffuse star cluster systems are red, 1.1 < g-z < 1.6, which is redder than metal-rich GCs and often as red as the galaxy itself. Most DSC systems thus have mean ages older than 5 Gyr or else have super-solar metallicities. We find that 12 galaxies in our sample contain a significant excess of diffuse star cluster candidates -- nine are lenticulars (S0s), and five visibly contain dust. We also find a substantial population of DSCs in the halo of the giant elliptical M49, associated with the companion galaxy VCC 1199. Most DSC systems appear to be both aligned with the galaxy light and associated with galactic disks, but at the same time many lenticulars do not host substantial DSC populations. Diffuse star clusters in our sample share similar characteristics to those identified in other nearby lenticular, spiral, and dwarf galaxies, and we suggest that DSCs preferentially form, survive, and coevolve with galactic disks. Their properties are broadly consistent with those of merged star cluster complexes, and we note that despite being 3 - 5 magnitudes brighter than DSCs, ultra-compact dwarfs have similar surface brightnesses. The closest Galactic analogs to the DSCs are the old open clusters. We suggest that if a diffuse star cluster population did exist in the disk of the Milky Way, it would be very difficult to find. (Abridged)

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arXiv2005

Supernova Cosmology and the ESSENCE project

The proper usage of Type Ia supernovae (SNe Ia) as distance indicators has revolutionized cosmology, and added a new dominant component to the energy density of the Universe, dark energy. Following the discovery and confirmation era, the currently ongoing SNe Ia surveys aim to determine the properties of the dark energy. ESSENCE is a five year ground-based supernova survey aimed at finding and characterizing 200 SNe Ia in the redshift domain z=[0.2-0.8]. The goal of the project is to put constraints on the equation of state parameter, w, of the dark energy with an accuracy of <10%. This paper presents these ongoing efforts in the context of the current developments in observational cosmology.

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arXiv2005

The ACS Virgo Cluster Survey IX: The Color Distributions of Globular Cluster Systems in Early-Type Galaxies

We present the color distributions of globular cluster (GC) systems for 100 Virgo cluster early-type galaxies observed in the ACS Virgo Cluster Survey. The color distributions of individual GC systems are consistent with continuous trends across galaxy luminosity, color, and stellar mass. On average, almost all galaxies possess a component of metal-poor GCs, with the average fraction of metal-rich GCs ranging from 15 to 60%. The colors of both subpopulations correlate with host galaxy luminosity and color, with the red GCs having a steeper slope. To convert color to metallicity, we also introduce a preliminary (g-z)-[Fe/H] relation calibrated to Galactic, M49 and M87 GCs. This relation is nonlinear with a steeper slope for [Fe/H] < -0.8. As a result, the metallicities of the metal-poor and metal-rich GCs vary similarly with respect to galaxy luminosity and stellar mass, with relations of [Fe/H]_MP ~ L^0.16 ~ M_star^0.17 and [Fe/H]_MR ~ L^0.26 ~ M_star^0.22, respectively. Although these relations are shallower than the mass-metallicity relation predicted by wind models and observed for dwarf galaxies, they are very similar to the mass-metallicity relation for star forming galaxies in the same mass range. The offset between the two GC populations varies slowly (~ M_star^0.05) and is approximately 1 dex across three orders of magnitude in mass, suggesting a nearly universal amount of enrichment between the formation of the two populations of GCs. We also find that although the metal-rich GCs show a larger dispersion in color, it is the *metal-poor GCs* that have an equal or larger dispersion in metallicity. Like the color-magnitude relation, these relations derived from globular clusters present stringent constraints on the formation and evolution of early-type galaxies. (Abridged)

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arXiv2005

A Search for Variable Stars and Planetary Occultations in NGC 2301 II: Variability

We performed R-band time series observations of the young, metal rich open cluster NGC 2301 for 12 nights in Feb. 2004. B images were also obtained and color magnitude diagrams, having limits of R=19.5 and B=21.5, were produced. Only asmall effort was made to determine cluster membership as our magnitude limits are far deeper than previously published values. Our photometric precision, for the brightest 5 magnitudes of sources, is 1-2 mmag. We determine that for the $\sim$4000 stars which have time-series data, 56% are variable and of these, approximately 13% are observed to exhibit periodic light curves ranging from tens of minutes to days. We present some examples of the light curves obtained and produce cuts in variability space based on parameters such as color and amplitude. The percentage of variability is approximately equal across all colors with the majority of variables having amplitudes of 0.15 magnitudes or less. In general, redder stars show larger variability amplitudes. We find a smooth decline in the number of periodic variables toward longer period. This decline is probably due to a transition from intrinsic to extrinsic variability and, in part, to our limited observing period of 12 nights. Essentially all the A and F main sequence stars in our sample are variable ($\sim$2 mmag and larger) and most present complex light curves containing multiple periods suggestive of their inclusion in the $δ$Sct and $γ$Dor classes. A variable non-cluster member giant and two variable white dwarf candidates are discussed. Our equational description of variability is shown to be an excellent predictive tool for determining the cumulative fraction of variables that will be observed in a photometric survey. Our entire dataset is available electronically.

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arXiv2005

The ACS Virgo Cluster Survey X. Half-light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation

We have measured half-light radii, r_h, for globular clusters (GCs) belonging to the 100 early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC 4697. An analysis of the dependencies of the measured r_h on both the properties of the GCs themselves and their host galaxies reveals that the average r_h increases with increasing galactocentric distance or, alternatively, with decreasing galaxy surface brightness. For the first time, we find that the average r_h decreases with the host galaxy color. We also show that there is no evidence for a variation of r_h with the luminosity of the GCs. Finally, we find in agreement with previous observations that the average r_h depends on the color of GCs, with red GCs being ~17% smaller than their blue counterparts. We show that this difference is probably a consequence of an intrinsic mechanism, rather than projection effects, and that it is in good agreement with the mechanism proposed in Jordan (2004). We discuss these findings in light of two simple pictures for the origin of the r_h of GCs and show that both lead to a behavior in rough agreement with the observations. After accounting for the dependencies found we show that the average GC half-light radii can be successfully used as a standard ruler for distance estimation. We outline the methodology, and provide a calibration for its use. We find = 2.7 +- 0.35 pc for GCs with (g-z)=1.2 mag in a galaxy with color (g-z)_{gal}=1.5 mag and at an underlying surface z-band brightness of mu_z = 21 mag arcsec^{-2}. Using this technique, we place an upper limit of 3.4 Mpc on the 1-sigma line-of-sight depth of the Virgo Cluster. Finally, we examine the form of the r_h distribution for our sample galaxies and provide an analytic expression which successfully describes this distribution. (Abridged)

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arXiv2005

The NN2 Flux Difference Method for Constructing Variable Object Light Curves

We present a new method for optimally extracting point-source time variability information from a series of images. Differential photometry is generally best accomplished by subtracting two images separated in time, since this removes all constant objects in the field. By removing background sources such as the host galaxies of supernovae, such subtractions make possible the measurement of the proper flux of point-source objects superimposed on extended sources. In traditional difference photometry, a single image is designated as the ``template'' image and subtracted from all other observations. This procedure does not take all the available information into account and for sub-optimal template images may produce poor results. Given N total observations of an object, we show how to obtain an estimate of the vector of fluxes from the individual images using the antisymmetric matrix of flux differences formed from the N(N-1)/2 distinct possible subtractions and provide a prescription for estimating the associated uncertainties. We then demonstrate how this method improves results over the standard procedure of designating one image as a ``template'' and differencing against only that image.

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arXiv2005

The ACS Virgo Cluster Survey V: SBF Calibration for Giant and Dwarf Early-type Galaxies

As part of the Advanced Camera for Survey (ACS) Virgo Cluster Survey, we have measured Surface Brightness Fluctuations (SBF) in a sample of 100 early-type Virgo galaxies. Distances derived from these measurements are needed to explore the three-dimensional structure of the Virgo Cluster, study the intrinsic parameters of globular clusters associated with the program galaxies, and compare with the galaxy distances derived from globular cluster luminosity functions. Our SBF measurements have been performed in the F850LP bandpass of the Wide Field Channel of the ACS on the Hubble Space Telescope. These are the first measurements of this kind, and we present the first SBF calibration for this bandpass. The measured fluctuations depend on galaxy stellar population variations, which we quantify by galaxy color (g_475-z_850)_0, where g_475 and z_850 are the galaxy magnitudes, respectively, in the F475W and F850LP ACS filters. We derive the following calibration for the absolute SBF magnitude M_850: M_850 = -2.06 +/- 0.04 + (2.0 +/ 0.2) [ (g_475-z_850)_0-1.3 ] in the range 1.3 <(g_475-z_850)_0 <= 1.6, and M_850 = -2.06 +/- 0.04 + (0.9 +/- 0.2) [ (g_475-z_850)_0-1.3 ] in the range 1.0 <=(g_475-z_850)_0 <= 1.3. Physically, the two different color regimes correspond to different galaxy types: giant ellipticals and S0s at the red end, and early-type dwarfs at the blue end. For the first time in SBF studies, we are able to provide a firm empirical calibration of SBF in early-type dwarf galaxies. Our results agree with stellar population model predictions from Bruzual & Charlot (2003).

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arXiv2005

The ACS Virgo Cluster Survey. VII. Resolving the Connection Between Globular Clusters and Ultra-Compact Dwarf Galaxies

We investigate the connection between globular clusters and ultra-compact dwarf galaxies (UCDs) by examining the properties of several compact objects associated with M87, all of which were previously classified as globular clusters. Combining imaging from the Hubble Space Telescope with ground-based Keck spectroscopy, we find two objects to have half-light radii, velocity dispersions and mass-to-light ratios that are consistent with the predictions of population synthesis models for old, metal-rich, luminous globular clusters. Three other objects are much larger, with half-light radii of approximately 20pc, and have V-band mass-to-light ratios in the range 6-9. These objects, which we consider to be UCDs, resemble the nuclei of nucleated dwarf elliptical galaxies in Virgo, having similar mass-to-light ratios, luminosities and colors. These UCDs are found to obey the extrapolated scaling relations of galaxies more closely than those of Galactic globular clusters. There appears to be a transition between the two types of stellar systems at a mass of about two million solar masses. If the UCDs are gravitationally bound, then we suggest that the presence of dark matter is the fundamental property distinguishing globular clusters from UCDs. More than half of the UCD candidates uncovered in the ACS Virgo Cluster Survey are associated with a single galaxy -- M87 -- which suggests that proximity to the Virgo center may be of critical importance for the formation of these objects. These results show that distinguishing bonafide UCDs from bright globular clusters requires a careful analysis of their detailed structural and dynamical properties, particularly their mass-to-light ratios. (ABRIDGED)

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arXiv2005

The ACS Virgo Cluster Survey IV: Data Reduction Procedures for Surface Brightness Fluctuation Measurements with the Advanced Camera for Surveys

The Advanced Camera for Surveys (ACS) Virgo Cluster Survey is a large program to image 100 early-type Virgo galaxies using the F475W and F850LP bandpasses of the Wide Field Channel of the ACS instrument on the Hubble Space Telescope (HST). The scientific goals of this survey include an exploration of the three-dimensional structure of the Virgo Cluster and a critical examination of the usefulness of the globular cluster luminosity function as a distance indicator. Both of these issues require accurate distances for the full sample of 100 program galaxies. In this paper, we describe our data reduction procedures and examine the feasibility of accurate distance measurements using the method of surface brightness fluctuations (SBF) applied to the ACS Virgo Cluster Survey F850LP imaging. The ACS exhibits significant geometrical distortions due to its off-axis location in the HST focal plane; correcting for these distortions by resampling the pixel values onto an undistorted frame results in pixel correlations that depend on the nature of the interpolation kernel used for the resampling. This poses a major challenge for the SBF technique, which normally assumes a flat power spectrum for the noise. We investigate a number of different interpolation kernels and show through an analysis of simulated galaxy images having realistic noise properties that it is possible, depending on the kernel, to measure SBF distances using distortion-corrected ACS images without introducing significant additional error from the resampling. We conclude by showing examples of real image power spectra from our survey.

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arXiv2005

A Search for Variable Stars and Planetary Occultations in NGC2301 I: Techniques

We observed the young open cluster NGC 2301 for 14 nights in Feb. 2004 using the orthogonal transfer CCD camera (OPTIC). We used PSF shaping techniques ("square stars") during the observations allowing a larger dynamic range (4.5 magnitudes) of high photometric precision results ($\le$2 mmag) to be obtained. These results are better than similar observing campaigns using standard CCD imagers. This paper discusses our observational techniques and presents initial results for the variability statistics found in NGC 2301. Details of the variability statistics as functions of color, variability type, stellar type, and cluster location will appear in paper II.

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arXiv2004

Redshift-Independent Distances to Type Ia Supernovae

We describe a procedure for accurately determining luminosity distances to Type Ia supernovae (SNe Ia) without knowledge of redshift. This procedure, which may be used as an extension of any of the various distance determination methods currently in use, is based on marginalizing over redshift, removing the requirement of knowing $z$ a priori. We demonstrate that the Hubble diagram scatter of distances measured with this technique is approximately equal to that of distances derived from conventional redshift-specific methods for a set of 60 nearby SNe Ia. This indicates that accurate distances for cosmological SNe Ia may be determined without the requirement of spectroscopic redshifts, which are typically the limiting factor for the number of SNe that modern surveys can collect. Removing this limitation would greatly increase the number of SNe for which current and future SN surveys will be able to accurately measure distance. The method may also be able to be used for high-$z$ SNe Ia to determine cosmological density parameters without redshift information.

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arXiv2004

High Redshift Supernova Rates

We use a sample of 42 supernovae detected with the Advanced Camera for Surveys on-board the Hubble Space Telescope as part of the Great Observatories Origins Deep Survey to measure the rate of core collapse supernovae to z~0.7 and type Ia supernovae to z~1.6. This significantly increases the redshift range where supernova rates have been estimated from observations. The rate of core collapse supernovae can be used as an independent probe of the cosmic star formation rate. Based on the observations of 17 core collapse supernovae, we measure an increase in the core collapse supernova rate by a factor of 1.6 in the range 0.3 1 compared to low redshift. At higher redshift (z>1), we find a suggested decrease in the type Ia rate with redshift. This evolution of the Ia rate with redshift is consistent with a type Ia progenitor model where there is a substantial delay between the formation of the progenitor star and the explosion of the supernova. Assuming that the type Ia progenitor stars have initial main sequence masses 3-8 M_Sun, we find that 5-7% of the available progenitors explode as type Ia supernovae.

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