Photometric Zero Points Research Articles

AuvbyCaHβ ANALYSIS OF THE OLD OPEN CLUSTER, NGC 6819

NGC 6819 is a richly populated, older open cluster situated within the Kepler field. A CCD survey of the cluster on the uvbyCaHbeta system, coupled with proper-motion membership, has been used to isolate 382 highly probable, single-star unevolved main-sequence members over a 20-arcminute field centered on the cluster. From 278 F dwarfs with high precision photometry in all indices, a mean reddening of E(b-y) = 0.117 +/- 0.005 or E(B-V) = 0.160 +/- 0.007 is derived, where the standard errors of the mean include both internal errors and the photometric zero-point uncertainty. With the reddening fixed, the metallicity derived from the same 278 stars is [Fe/H] = -0.116 +/- 0.101 from m_1 and -0.055 +/- 0.033 from hk, for a weighted average of [Fe/H] = -0.06 +/- 0.04, where the quoted standard errors of the mean values include the internal errors from the photometric scatter plus the uncertainty in the photometric zero points. If metallicity is derived using individual reddening values for each star to account for potential reddening variation across the face of the cluster, the analogous result is unchanged. The cluster members at the turnoff of the color-magnitude diagram are used to test and confirm the recently discovered variation in reddening across the face of the cluster, with a probable range in the variation of Delta[E(B-V)] = 0.045 +/-0.015. With the slightly higher reddening and lower [Fe/H] compared to commonly adopted values, isochrone fitting leads to an age of 2.3 +/- 0.2 Gyr for an apparent modulus of (m-M) = 12.40 +/-0.12.

The ALHAMBRA Survey: Bayesian photometric redshifts with 23 bands for 3 deg2

The Advance Large Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) survey has observed eight different regions of the sky, including sections of the Cosmic Evolution Survey (COSMOS), DEEP2, European Large-Area Infrared Space Observatory Survey (ELAIS), Great Observatories Origins Deep Survey North (GOODS-N), Sloan Digital Sky Survey (SDSS) and Groth fields using a new photometric system with 20 optical, contiguous ∼300-Å filters plus the JHKs bands. The filter system is designed to optimize the effective photometric redshift depth of the survey, while having enough wavelength resolution for the identification of faint emission lines. The observations, carried out with the Calar Alto 3.5-m telescope using the wide-field optical camera Large Area Imager for Calar Alto (LAICA) and the near-infrared (NIR) instrument Omega-2000, represent a total of ∼700 h of on-target science images. Here we present multicolour point-spread function (PSF) corrected photometry and photometric redshifts for ∼438 000 galaxies, detected in synthetic F814W images. The catalogues are complete down to a magnitude I ∼ 24.5 AB and cover an effective area of 2.79 deg2. Photometric zero-points were calibrated using stellar transformation equations and refined internally, using a new technique based on the highly robust photometric redshifts measured for emission-line galaxies. We calculate Bayesian photometric redshifts with the Bayesian Photometric Redshift (bpz)2.0 code, obtaining a precision of δz/(1 + zs) = 1 per cent for I < 22.5 and δz/(1 + zs) = 1.4 per cent for 22.5 < I < 24.5. The global n(z) distribution shows a mean redshift 〈z〉 = 0.56 for I < 22.5 AB and 〈z〉 = 0.86 for I < 24.5 AB. Given its depth and small cosmic variance, ALHAMBRA is a unique data set for galaxy evolution studies.

THE ARAUCARIA PROJECT. A DISTANCE DETERMINATION TO THE LOCAL GROUP SPIRAL M33 FROM NEAR-INFRARED PHOTOMETRY OF CEPHEID VARIABLES

Motivated by an amazing range of reported distances to the nearby Local Group spiral galaxy M33, we have obtained deep near-infrared photometry for 26 long-period Cepheids in this galaxy with the ESO VLT. From the data we constructed period-luminosity relations in the J and K bands which together with previous optical VI photometry for the Cepheids by Macri et al. were used to determine the true distance modulus of M33, and the mean reddening affecting the Cepheid sample with the multiwavelength fit method developed in the Araucaria Project. We find a true distance modulus of 24.62 for M33, with a total uncertainty of +- 0.07 mag which is dominated by the uncertainty on the photometric zero points in our photometry. The reddening is determined as E(B-V)=0.19 +- 0.02, in agreement with the value used by the HST Key Project of Freedman et al. but in some discrepancy with other recent determinations based on blue supergiant spectroscopy and an O-type eclipsing binary which yielded lower reddening values. Our derived M33 distance modulus is extremely insensitive to the adopted reddening law. We show that the possible effects of metallicity and crowding on our present distance determination are both at the 1-2% level and therefore minor contributors to the total uncertainty of our distance result for M33.

NGC 6811: AN INTERMEDIATE-AGE CLUSTER IN THEKEPLERFIELD

NGC 6811 is one of the four open clusters located in the Kepler spacecraft field of view. We obtained UBVRI photometry of the cluster on six nights (four of them photometric) with the 1.08 m Hall and 1.83 m Perkins telescopes at Lowell Observatory. The mean photometric precision ranges from better than 0.01 mag in the V, B - V, V - R, and V - I indices among stars brighter than magnitude 15 to about 0.05 mag for U - B at magnitude 18. We followed a Bayesian statistical approach using the Markov Chain Monte Carlo algorithm to determine the cluster reddening, distance, age, and iron content from a maximum-likelihood fit to stellar isochrones. Using the Yale-Yonsei isochrones, we derived E(B - V) 0.066 {+-} 0.025, (m - M){sub V} = 10.13 {+-} 0.19, age = 1.06 {+-} 0.19 Gyr, and Z = 0.012 {+-} 0.004 ([Fe/H] = -0.18); with the Padova isochrones, we found E(B - V) = 0.081 {+-} 0.015, (m - M){sub V} = 10.31 {+-} 0.11, age = 0.94 {+-} 0.08 Gyr, and Z = 0.012 {+-} 0.002 ([Fe/H] = -0.20). The uncertainties include possible errors in the photometric zero points of {+-}0.01 mag, butmore » do not include uncertainties in the models. Taking into account the differences between the two sets of models, we find the best estimate for the cluster parameters to be E(B - V) = 0.074 {+-} 0.024, (m - M){sub V} = 10.22 {+-} 0.18, age = 1.00 {+-} 0.17 Gyr, and Z = 0.012 {+-} 0.004 ([Fe/H] = -0.19).« less

Adjusting the distance scales of old and intermediate-age tracers via statistical parallaxes

AbstractThe method of statistical-parallax determination is used to adjust the photometric distance-scale zero points and test the selection of several samples of old and intermediate-age tracer objects in the Milky Way, including RR Lyrae- and Mira-type variables, blue and red horizontal-branch stars, and red giants. We obtain reasonable and stable absolute-magnitude (and, hence, photometric distance-scale) corrections for all samples except for the red horizontal-branch stars, which appear to be significantly contaminated by main-sequence turn-off and subgiant stars.

CFHTLenS: improving the quality of photometric redshifts with precision photometry★

Here we present the results of various approaches to measure accurate colours and photometric redshifts (photo-z's) from wide-field imaging data. We use data from the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS) which have been re- processed by the CFHT Lensing Survey (CFHTLenS) team in order to carry out a number of weak gravitational lensing studies. An emphasis is put on the correction of systematic effects in the photo-z's arising from the different Point Spread Functions (PSF) in the five optical bands. Different ways of correcting these effects are discussed and the resulting photo-z accuracies are quantified by comparing the photo-z's to large spectroscopic redshift (spec-z) data sets. Careful homogenisation of the PSF between bands leads to increased overall accuracy of photo-z's. The gain is particularly pronounced at fainter magnitudes where galaxies are smaller and flux measurements are affected more by PSF-effects. We also study possible re- calibrations of the photometric zeropoints (ZPs) with the help of galaxies with known spec-z's. We find that if PSF-effects are properly taken into account, a re-calibration of the ZPs becomes much less important suggesting that previous such re-calibrations described in the literature could in fact be mostly corrections for PSF-effects rather than corrections for real inaccuracies in the ZPs. The implications of this finding for future surveys like KiDS, DES, LSST, or Euclid are mixed. On the one hand, ZP re-calibrations with spec-z's might not be as accurate as previously thought. On the other hand, careful PSF homogenisation might provide a way out and yield accurate, homogeneous photometry without the need for full spectroscopic coverage. This is the first paper in a series describing the technical aspects of CFHTLenS. (abridged)

On a New Parameter to Estimate the Helium Content in Old Stellar Systems11This article makes use of data obtained from the ESO/ST-ECF Science Archive Facility; from the Isaac Newton Group Archive, which is maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge; and from the Canadian Astronomy Data Centre operated by the National Research

ABSTRACTWe introduce a new parameter Δξ—the difference in magnitude between the red giant branch (RGB) bump and the point on the main sequence (MS) at the same color as the bump (which we call the “benchmark”)—to estimate the helium content in old stellar systems. The Δξ parameter is a helium indicator, since an increase in helium makes, at fixed age and iron abundance, the RGB bump brighter and the MS benchmark fainter. Moreover, its sensitivity to helium is linear over the entire metallicity range. Δξ is also minimally affected by changes of a few gigayears in cluster age, by uncertainties in the photometric zero point, by the amount of reddening, or by the effects of evolution on the horizontal branch. The two main drawbacks of the Δξ parameter include the need for precise and large photometric data sets and a strong dependence of the ΔY/Δξ slope on metallicity. We selected almost two dozen relatively bright Galactic globular clusters (GGCs) with low foreground reddening and a broad range of iron abundance (-2.45 ≤ [Fe/H] ≤ -0.70 dex), together with precise and homogeneous multiband (BVI) photometry. The observed Δξ parameters and those predicted from α-enhanced evolutionary models agree reasonably well if we assume a primordial helium content of Y = 0.20 (abundance by mass). The discrepancy becomes ∼4σ (ΔB = 0.20 mag) only in the metal-poor regime. Comparison with prescriptions based on a canonical primordial helium content (Y = 0.245 indicates that the observed Δξ values are smaller than predicted. These findings support previous results suggesting that current evolutionary models overestimate the luminosity of the RGB bump. The inclusion of envelope overshooting can eliminate the discrepancy. The Δξ parameter of GGCs provides an independent detection of prestellar helium, at least at the 5σ level.

Calibration of BVRI Photometry for the Wide Field Channel of the HST Advanced Camera for Surveys

ABSTRACTWe present new observations of two Galactic globular clusters, PAL4 and PAL14, using the Wide Field Channel of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and reanalyze archival data from a third, NGC2419. We matched our photometry of hundreds of stars in these fields from the ACS images to existing ground-based photometry of faint sequences that were calibrated on the standard BVRI system of Landolt. These stars are significantly fainter than those generally used for HST calibration purposes and therefore are much better matched to supporting precision photometry of ACS science targets. We were able to derive more accurate photometric transformation coefficients for the commonly used ACS broadband filters, compared with those published by Sirianni et al., due to the use of a factor of several more calibration stars that span a greater range of color. We find that the inferred transformations from each cluster individually do not vary significantly from the average, except for a small offset of the photometric zero point in the F850LP filter. Our results suggest that the published prescriptions for the time-dependent correction of CCD charge transfer efficiency appear to work very well over the ∼3.5 yr interval that spans our observations of PAL4 and PAL14 and the archived images of NGC2419.

THE ALHAMBRA PHOTOMETRIC SYSTEM

This paper presents the characterization of the optical range of the ALHAMBRA photometric system, a 20 contiguous, equal-width, medium-band CCD system with wavelength coverage from 3500A to 9700A. The photometric description of the system is done by presenting the full response curve as a product of the filters, CCD and atmospheric transmission curves, and using some first and second order moments of this response function. We also introduce the set of standard stars that defines the system, formed by 31 classic spectrophotometric standard stars which have been used in the calibration of other known photometric systems, and 288 stars, flux calibrated homogeneously, from the Next Generation Spectral Library (NGSL). Based on the NGSL, we determine the transformation equations between Sloan Digital Sky Survey (SDSS) ugriz photometry and the ALHAMBRA photometric system, in order to establish some relations between both systems. Finally we develop and discuss a strategy to calculate the photometric zero points of the different pointings in the ALHAMBRA project.

CCD PHOTOMETRY OF STANDARD STARS AT MAIDANAK ASTRONOMICAL OBSERVATORY IN UZBEKSTAN: TRANSFORMATIONS AND COMPARISONS

Observation of standard stars is of crucial importance in stellar photometry. We have studied the standard transformation relations of the UBVRI CCD photometric system at the Maidanak Astronomical Observatory in Uzbekistan. All observations were made with the AZT-22 1.5m telescope, SITe 2k CCD or Fairchild 486 CCD, and standard Bessell UBVRI filters from 2003 August to 2007 September. We observed many standard stars around the celestial equator observed by SAAO astronomers. The atmospheric extinction coefficients, photometric zero points, and time variation of photometric zero points of each night were determined. Secondary extinction coefficients and photometric zero points were very stable, while primary extinction coefficients showed a distinct seasonal variation. We also determined the transformation coefficients for each filter. For B, V, R, and I filters, the transformation to the SAAO standard system could be achieved with a straight line or a combination of two straight lines. However, in the case of the U filter and Fairchild 486 CCD combination, a significant non-linear correction term - related to the size of Balmer jump or the strength of the Balmer lines - of up to 0.08 mags was required. We found that our data matched well the SAAO photometry in V, B-V, V-I, and R-I. But in U-B, the difference in zero point was about 3.6 mmag and the scatter was about 0.02 mag. We attribute the relatively large scatter in U-B to the larger error in U of the SAAO photometry. We confirm the mostly small differences between the SAAO standard UBVRI system and the Landolt standard system. We also attempted to interpret the seasonal variation of the atmospheric extinction coefficients in the context of scattering sources in the earth's atmosphere.

Photometric calibration of the Supernova Legacy Survey fields

We present the photometric calibration of the Supernova Legacy Survey (SNLS) fields. The SNLS aims at measuring the distances to SNe Ia at (0.3<z<1) using MegaCam, the 1 deg^2 imager on the Canada-France-Hawaii Telescope (CFHT). The uncertainty affecting the photometric calibration of the survey dominates the systematic uncertainty of the key measurement of the survey, namely the dark energy equation of state. The photometric calibration of the SNLS requires obtaining a uniform response across the imager, calibrating the science field stars in each survey band (SDSS-like ugriz bands) with respect to standards with known flux in the same bands, and binding the calibration to the UBVRI Landolt standards used to calibrate the nearby SNe from the literature necessary to produce cosmological constraints. The spatial non-uniformities of the imager photometric response are mapped using dithered observations of dense stellar fields. Photometric zero-points against Landolt standards are obtained. The linearity of the instrument is studied. We show that the imager filters and photometric response are not uniform and publish correction maps. We present models of the effective passbands of the instrument as a function of the position on the focal plane. We define a natural magnitude system for MegaCam. We show that the systematics affecting the magnitude-to-flux relations can be reduced if we use the spectrophotometric standard star BD +17 4708 instead of Vega as a fundamental flux standard. We publish ugriz catalogs of tertiary standards for all the SNLS fields.

Light and motion in SDSS Stripe 82: the catalogues

We present a new public archive of light-motion curves in Sloan Digital Sky Survey (SDSS) Stripe 82, covering 99° in right ascension from $\alpha = 20^{\mathrm h}\!\!.7$ to $3^{\mathrm h}\!\!.3$ and spanning $2^{\circ}\!\!.52$ in declination from $\delta = -1^{\circ}\!\!.26$ to $1^{\circ}\!\!.26$ , for a total sky area of ∼249 deg 2 . Stripe 82 has been repeatedly monitored in the u, g, r, i and z bands over a seven-year baseline. Objects are cross-matched between runs, taking into account the effects of any proper motion. The resulting catalogue contains almost 4 million light-motion curves of stellar objects and galaxies. The photometry are recalibrated to correct for varying photometric zero-points, achieving ∼20 and 30 mmag rms accuracy down to 18 mag in the g, r, i and z bands for point sources and extended sources, respectively. The astrometry are recalibrated to correct for inherent systematic errors in the SDSS astrometric solutions, achieving ∼32 and 35 mas rms accuracy down to 18 mag for point sources and extended sources, respectively. For each light-motion curve, 229 photometric and astrometric quantities are derived and stored in a higher level catalogue. On the photometric side, these include mean exponential and point spread function (PSF) magnitudes along with uncertainties, rms scatter, χ 2 per degree of freedom, various magnitude distribution percentiles, object type (stellar or galaxy), and eclipse, Stetson and Vidrih variability indices. On the astrometric side, these quantities include mean positions, proper motions as well as their uncertainties and χ 2 per degree of freedom. The light-motion curve catalogue presented here is complete down to r∼ 21.5 and is at present the deepest large-area photometric and astrometric variability catalogue available.

Photometric calibration of the Swift ultraviolet/optical telescope

We present the photometric calibration of the Swift UltraViolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux, and the photometric zero points (which are accurate to better than 4 per cent) for each of the seven UVOT broadband filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position dependent uniformity, and instrument response over the 1600-8000A operational range. Because the UVOT is a photon counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results.

Sloan Digital Sky Survey Standard Star Catalog for Stripe 82: The Dawn of Industrial 1% Optical Photometry

We describe a standard star catalog constructed using multiple SDSS photometric observations (at least four per band, with a median of 10) in the ugriz system. The catalog includes 1.01 million nonvariable unresolved objects from the equatorial stripe 82 (|δJ2000.0| < 1.266°) in the right ascension range 20h34m-4h00m and with the corresponding r-band (approximately Johnson V-band) magnitudes in the range 14-22. The distributions of measurements for individual sources demonstrate that the photometric pipeline correctly estimates random photometric errors, which are below 0.01 mag for stars brighter than 19.5, 20.5, 20.5, 20, and 18.5 in ugriz, respectively (about twice as good as for individual SDSS runs). Several independent tests of the internal consistency suggest that the spatial variation of photometric zero points is not larger than ∼0.01 mag (rms). In addition to being the largest available data set with optical photometry internally consistent at the ∼1% level, this catalog provides a practical definition of the SDSS photometric system. Using this catalog, we show that photometric zero points for SDSS observing runs can be calibrated within a nominal uncertainty of 2% even for data obtained through 1 mag thick clouds, and we demonstrate the existence of He and H white dwarf sequences using photometric data alone. Based on the properties of this catalog, we conclude that upcoming large-scale optical surveys such as the Large Synoptic Survey Telescope will be capable of delivering robust 1% photometry for billions of sources.

Systematic effects in the sound horizon scale measurements

We investigate three potential sources of bias in distance estimations made assuming that a very simple estimator of the baryon acoustic oscillation (BAO) scale provides a standard ruler. These are the effects of the non-linear evolution of structure, scale-dependent bias and errors in the survey window function estimation. The simple estimator used is the peak of the smoothed correlation function, which provides a variance in the BAO scale that is close to optimal, if appropriate low-pass filtering is applied to the density field. While maximum-likelihood estimators can eliminate biases if the form of the systematic error is fully modelled, we estimate the potential effects of unmodelled or mis-modelled systematic errors. Non-linear structure growth using the Smith et al. prescription biases the acoustic scale by <0.3 per cent at z ≥ 1 under the correlation-function estimator. The biases due to representative but simplistic models of scale-dependent galaxy bias are below 1 per cent at z ≥ 1 for bias behaviour in the realms suggested by halo model calculations, which is expected to be below the statistical errors for a 1000-deg 2 spectroscopic survey. The distance bias due to a survey window function error is given in a simple closed form and it is shown that it has to be kept below 2 per cent so as not to bias acoustic scale more than 1 per cent at z = 1, although the actual tolerance can be larger depending upon galaxy bias. These biases are comparable to statistical errors for ambitious surveys if no correction is made for them. We show that rms photometric zero-point errors (at limiting magnitude 25 mag) below 0.14 and 0.01 mag for redshift z = 1 (red galaxies) and z = 3 (Lyman-break galaxies), respectively, are required in order to keep the distance estimator bias below 1 per cent.

The Araucaria Project: Distance to the Local Group Galaxy NGC 3109 from Near‐Infrared Photometry of Cepheids

We present near-infrared J- and K-band photometry of 77 Cepheid variables in the Local Group galaxy NGC 3109. Combining our data with the previously published optical V- and I-band photometry of Cepheids in this galaxy we derive an accurate distance and interstellar reddening to NGC 3109. Adopting a distance modulus of 18.5 mag for the Large Magellanic Cloud, we obtain a true distance modulus to NGC 3109 of (m-M)_0 = 25.571 +- 0.024 mag (random error), corresponding to a distance of 1.30 +- 0.02 Mpc. The systematic uncertainty on this value (apart from the adopted LMC distance) is of the order of +-3%, the main contributors to this value being the uncertainty on the photometric zero points, and the effect of blending with unresolved companion stars. The total reddening determined from our multiwavelength solution is E(B-V)=0.087 +- 0.012 mag. About half of the reddening is produced internal to NGC 3109. Our distance result is consistent with previous determinations of the distance to NGC 3109, but has significantly reduced error bars.

The Calibration of the Swift UVOT Optical Observations: A Recipe for Photometry

ABSTRACTThe Ultraviolet/Optical Telescope (UVOT) on board Swift has the capability to provide critical insight into the physics of the early afterglows of gamma‐ray bursts (GRBs). But without precise calibration of the UVOT to standard photometric systems, it is impossible to leverage late‐time, ground‐based follow‐up data to the early‐time UVOT observations. In this paper, we present a calibration of the Swift UVOT photometry to the standard Johnson UBV system for the UVOT U, B, and V filters, and a step‐by‐step photometry recipe for analyzing these data. We base our analysis on aperture photometry performed on the ground‐based and UVOT observations of the local standard stars in the fields of supernovae (SNe) 2005am and 2005cf and a number of Landolt standard stars. We find that the optimal photometry aperture radius for UVOT data is small (2 .″5 for unbinned data, 3 .″0 for 2 × 2 binned data), and we show that the coincidence‐loss (C‐loss) correction is important even for relatively faint magnitudes (16–19 mag). Based on a theoretically motivated model, we fit the C‐loss correction with two parameters, the photometric zero point (ZP) and the saturation magnitude (m∞), and derive tight constraints for both parameters [σ(ZP) = 0.01 mag and σ(m∞) = 0.02 mag)]. We find that the color‐term correction is not necessary for the UVOT B and V filters but that it is necessary for the U filter for blue objects [(U - V)<0.4 mag]. We analyze the UVOT UBV photometry of SN 2005am and find that the UVOT photometry is generally consistent with the ground‐based observations but that a difference of up to 0.5 mag is found when the SN became faint. We also apply our calibration results to the UVOT observations of GRB 050603. There is a scatter of ∼0.04–0.08 mag in our final UVOT photometry, the cause of which is unclear, but it may be partly due to the spatial variation in the pixel sensitivity of the UVOT detector.

Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High‐Luminosity Clusters

We present new (B, I) photometry for the globular cluster systems in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B - I) index. Furthermore, the mean colors and internal color range of the blue subpopulation are remarkably similar from one galaxy to the next, to well within the ±0.02-0.03 mag uncertainties in the foreground reddenings and photometric zero points. By contrast, the mean color and internal color range for the red subpopulation differ from one galaxy to the next by twice as much as the blue population. All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (MI < -10.5) the color distribution becomes broad and less obviously bimodal. This effect was first noticed by Ostrov et al. and Dirsch et al. for the Fornax giant NGC 1399; our data suggest that it may be a characteristic of many BCGs and perhaps other large galaxies. Our data indicate that the blue (metal-poor) clusters brighter than MI -10 become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M0.55. A basically similar relation has been found for M87 by Strader et al. (2005). We argue that these GCS characteristics are consistent with a hierarchical-merging galaxy formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 107-1010 M☉, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more preenrichment could occur.

TheuBVIPhotometric System. II. Standard Stars

Paper I of this series described the design of a CCD-based photometric system that is optimized for ground-based measurements of the size of the Balmer discontinuity in stellar spectra. This "uBVI" system combines the Thuan-Gunn u filter with the standard Johnson-Kron-Cousins BVI filters and can be used to discover luminous yellow supergiants in extragalactic systems and post–asymptotic giant branch stars in globular clusters and galactic halos. In the present paper we use uBVI observations obtained on 54 nights with 0.9 m telescopes at Kitt Peak and Cerro Tololo to construct a catalog of standardized u magnitudes for standard stars taken from the 1992 catalog of Landolt. We describe the selection of our 14 Landolt fields and give details of the photometric reductions, including red leak and extinction corrections, transformation of all the observations onto a common magnitude system, and the establishment of the photometric zero point. We present a catalog of u magnitudes of 103 stars suitable for use as standards. We show that data obtained with other telescopes can be transformed to our standard system with better than 1% accuracy.

A Cross‐Calibration between Tycho‐2 Photometry and Hubble Space Telescope Spectrophotometry

ABSTRACTBy analyzing a well‐calibrated sample of 256 stars observed with both Hipparcos and HST, I show that Tycho‐2 photometry and HST spectrophotometry are accurate and stable enough to obtain a precise cross‐calibration. Based on this analysis, I obtain the following photometric zero points with respect to Vega for Tycho‐2: 0.020 ± 0.001 (BT - VT), 0.078 ± 0.009 (BT), and 0.058 ± 0.009 (VT).