Allan Sandage

Studies of the Virgo cluster. VI - Morphological and kinematical structure of the Virgo cluster

The structure of the Virgo cluster is analyzed on the basis of the positions, Hubble types, and radial velocities of 1277 Virgo cluster galaxies. The surface distribution of galaxies is considered according to type, and is discussed using maps, isopleths, strip counts, and radial-density distributions. It is found that the Virgo cluster shows pronounced double structure. The main concentration has a large velocity dispersion and is made up predominantly of early-type galaxies, while the secondary concentration has a much smaller velocity dispersion and contains late types. There is a strong spatial segregation of the Hubble types, the early-type galaxies being more concentrated toward the cluster center. There is significant substructure in the cluster core. The irregularity of the Virgo cluster in both configuration and velocity space shows that the core and the envelope are still forming, and hence that the cluster is young. 85 references.

The Hubble Constant: A Summary of the Hubble Space Telescope Program for the Luminosity Calibration of Type Ia Supernovae by Means of Cepheids

This is the final summary paper of our 15 yr program using the Hubble Space Telescope (HST) to determine the Hubble constant from the Cepheid-calibrated luminosity of Type Ia supernovae. Several recent developments have made it necessary to put the summary on H0 on a broader basis than we originally thought (see the four papers cited in the text). The new Cepheid distances of the subset of 10 galaxies, which were hosts of normal SNe Ia, give weighted mean luminosities in B, V, and I at maximum light of -19.49, -19.46, and -19.22, respectively. These calibrate the adopted SNe Ia Hubble diagram from Paper III to give H0 = 62.3 ± 1.3(random) ± 5.0(systematic) in units of km s-1 Mpc-1. This is a global value because it uses the Hubble diagram between redshift limits of 3000 and 20,000 km s-1, beyond the effects of any local random and streaming motions. Local values of H0 between 4.4 and 30 Mpc from Cepheids, SNe Ia, 21 cm line widths, and the tip of the red giant branch (TRGB) all agree within 5% of our global value. This agreement of H0 on all scales from ~4 to 200 Mpc finds its most obvious explanation in the smoothing effect of vacuum energy on the otherwise lumpy gravitational field due to the nonuniform distribution of the local galaxies. The present value of H0 is consistent with physical methods relying on the time delay of gravitational lenses and the Sunyaev-Zeldovich effect, and it is not in contradiction with the CMB data. Our value of H0 is 14% smaller than the value of H0 found by Freedman et al. because our independent Cepheid distances to the six SNe Ia-calibrating galaxies used in that analysis average 0.35 mag larger than those used earlier.

OPTICAL IDENTIFICATION OF 3C 48, 3C 196, AND 3C 286 WITH STELLAR OBJECTS

Abstract : Radio positions of the three sources were deter mined with the two 90-ft antennas working as an interferometer with an rms accuracy in both coordinates better than 10 sec of arc. Direct photographs show that a star-like object exists within the error rectangle at each of the source positions. Exceedingly faint wisps of nebulosity are associated with the stars in 3C 48 and 3C 196. The observations are incomplete for 3C 286 in this regard. Photoelectric photometry of the stars shows each to have quite peculiar color indices, most closely resembling the colors of old novae, or possibly white dwarfs, but the authors are not suggesting identification with these types of stars. Optical spectra for 3C 48 show several very broad emission features. It is shown that the radiant flux in the optical region can be computed from the radio flux data and the theory of synchrotron radiation for 3C 48 and 3C 196, but not for 3C 286. This, together with other arguments, suggests that the optical as well as the radio flux could be due to the synchrotron mechanism, but the arguments are not conclusive.

The Cepheid Distance to NGC 5236 (M83) with the ESO Very Large Telescope

Published in: Astrophys. J. 590 (2003) 256-270 citations recorded in [Science Citation Index] Abstract: Cepheids have been observed in NGC 5236 (M 83) using the ANTU (UT1) 8.2 meter telescope of the ESO VLT with FORS1. Observations over 34 epochs in V and I have been made between January 2000 and July 2001. Twelve Cepheid candidates have periods between 12 and 55 days. The true distance modulus is adopted to be (m-M)_0 = 28.25 +/- 0.15, which corresponds to a distance of 4.5 +/- 0.3 Mpc. The distance of NGC 5253 has been rediscussed. The mean distance of (m-M)_0 = 28.01 +/- 0.15 (based on SN 1972E) shows the galaxy to be a close neighbor of M 83 (separation = 0.5 +/- 0.4 Mpc), suggesting that the two galaxies may have interacted in the past and thus possibly explaining the amorphous morphology of NGC 5253. M 83 is the principal member of the nearby M 83 group containing also, besides NGC 5253, several dwarf members, for five of which TRGB distances are available (Karachentsev et al. 2002a, A&A, 385, 21). The adopted group distance of (m-M)_0 = 28.28 +/- 0.10 (4.5 +/- 0.2 Mpc) together with its mean recession velocity of v_LG = 249 +/- 42 km/s shows again the extreme quietness of the local (1 Mpc to 10 Mpc) expansion field. M 83 fits onto the local mean Hubble flow line of the velocity-distance relation (with H_0 ~ 60) with no significant deviation, supporting the earlier conclusion that the local velocity expansion field is remarkably cold on a scale of 10 Mpc, contrary to the predictions of the simplest cold dark matter model for large scale structure. The role of a cosmological constant has been invoked as a possible solution in providing a nearly uniform force field everywhere in the presence of a lumpy galaxy distribution. (abridged)Cepheids have been observed in NGC 5236 (M83) using the Antu (Unit Telescope 1) 8.2 m telescope of the ESO Very Large Telescope with the Focal Reducer/Low Dispersion Spectrograph 1. Repeated imaging observations have been made between 2000 January and 2001 July. Images were obtained in 34 epochs in the V band and in six epochs in the I band. The photometry was made with the ROMAFOT reduction package and checked independently with DoPHOT and a modified version of HSTPHOT. Twelve Cepheid candidates have periods ranging between 12 and 55 days. The dereddened distance modulus is adopted to be (m - M)0 = 28.25 ± 0.15, which corresponds to a distance of 4.5 ± 0.3 Mpc. The Cepheid distance of NGC 5253 has been rediscussed and strengthened by its SN 1972E. The mean distance of (m - M)0 = 28.01 ± 0.15 (based on SN 1972E) shows the galaxy to be a close neighbor of M83, suggesting that the two galaxies may have interacted in the past and thus possibly explaining the amorphous morphology of NGC 5253. The distance difference between M83 and NGC 5253 is only 0.5 ± 0.4 Mpc. The projected distance is only ~ 0.15 Mpc. M83 is the principal member of the nearby M83 group containing also, besides NGC 5253, several dwarf members, for five of which tip of the red giant branch distances are available. The adopted group distance of (m - M)0 = 28.28 ± 0.10 (4.5 ± 0.2 Mpc), together with its mean recession velocity of vLG = 249 ± 42 km s-1, shows again the extreme quietness of the local (1-10 Mpc) expansion field. M83 fits onto the local mean Hubble flow line of the velocity-distance relation (with H0~ 60) with no significant deviation, supporting the earlier conclusion that the local velocity expansion field is remarkably cold on a scale of 10 Mpc, contrary to the predictions of the simplest cold dark matter model for large-scale structure. The role of a cosmological constant has been invoked as a possible solution in providing a nearly uniform force field everywhere in the presence of a lumpy galaxy distribution.

New Period-Luminosity and Period-Color relations of classical Cepheids: I. Cepheids in the Galaxy

Galactic fundamental-mode Cepheids with good B, V, and (in most cases) I photometry by Berdnikov et al. (2000) and with homogenized color excesses E(B V) based on Fernie et al. (1995) are used to determine their period-color (P-C) relation in the range 0:4 1:4). The latter eect is enhanced by a suggestive break of the P-L relation of LMC and SMC at log P= 1: 0t owards still shallower values as shown in a forthcoming paper.

Cepheid Calibration of the Peak Brightness of Type Ia Supernovae. VIII. SN 1990N in NGC 4639

Repeated imaging observations have been made of NGC 3982 with the Hubble Space Telescope between 2000 March and May, over an interval of 53 days. Images were obtained on 12 epochs in the F555W band and on five epochs in the F814W band. The galaxy hosted the Type Ia supernova (SN Ia) SN 1998aq. A total of 26 Cepheid candidates were identified, with periods ranging from 10 to 45 days, using photometry with the DoPHOT program. The dereddened distance to NGC 3982 is estimated from these data using various criteria to maximize signal-to-noise ratio and reliability: the values lie between 31.71 and 31.82, with uncertainties in the mean of typically ±0.14 mag for each case. A parallel analysis using photometry with HSTphot discovered 13 variables, yielding a distance modulus of 31.85 ± 0.16. The final adopted modulus is (M-m)0 = 31.72 ± 0.14 (22 ± 1.5 Mpc). Photometry of SN 1998aq that is available in the literature is used in combination with the derived distance to NGC 3982 to obtain the peak absolute magnitude of this supernova. The lower limit (no extinction within the host galaxy) for MV is -19.47 ± 0.15 mag. Corrections for decline rate and intrinsic color to carry these to the reduced system of Parodi and collaborators have been performed. The derived luminosities at hand are fully consistent with the mean of the eight normal SNe Ia previously calibrated with Cepheids. Together they yield H0 ≈ 60 ± 2(internal) km s-1 Mpc-1 based on an assumed LMC distance modulus of 18.50. We point out that correcting some of the systematic errors and including uncertainty estimates due to them leads to H0 = 58.7 ± 6.3(internal) km s-1 Mpc-1.

Studies of the Virgo Cluster. I - Photometry of 109 galaxies near the cluster center to serve as standards

Attention is given to the technical aspects of photometric measurements of 109 galaxies near the center of the Virgo Cluster, noting various types of radii and surface brightness for about 50 E and dE galaxies in the sample that range in absolute magnitude from -20 to -12. These data are combined with data from the literature for giant E and dwarf E galaxies in the Local Group to study the systematic properties of E galaxies over a range of one million luminosities. The radial intensity profiles derived are fitted to the manifold of King (1978) models to derive model-dependent central surface brightness, core radii, and cutoff radii.

CEPHEID DISTANCES TO SNe Ia HOST GALAXIES BASED ON A REVISED PHOTOMETRIC ZERO POINT OF THE HST WFPC2 AND NEW PL RELATIONS AND METALLICITY CORRECTIONS

With this paper we continue the preparation for a forthcoming summary report of our experiment with the HST to determine the Hubble constant using Type Ia supernovae as standard candles. Two problems are addressed. (1) We examine the need for, and determine the value of, the corrections to the apparent magnitudes of our program Cepheids in the 11 previous calibration papers due to sensitivity drifts and charge transfer effects of the HST WFPC2 camera over the life time of the experiment from 1992 to 2001. (2) The corrected apparent magnitudes are applied to all our previous photometric data from which revised distance moduli are calculated for the eight program galaxies that are parents to the calibrator Ia supernovae. Two different Cepheid P-L relations are used; one for the Galaxy and one for the LMC. These differ both in slope and zero point at a fixed period. The procedures for determining the absorption and reddening corrections for each Cepheid are discussed. Corrections for the effects of metallicity differences between the program galaxies and the two adopted P-L relations are derived and applied. The distance moduli derived here for the eight supernovae program galaxies, and for 29 others, average 0.20 mag fainter (more distant) than those derived by Gibson et al. and Freedman et al. in their 2000 and 2001 summary papers for reasons discussed in this paper. The effect on the Hubble constant is the subject of our forthcoming summary paper.

Discovery of Cepheids in NGC 5253: Absolute peak brightness of SN Ia 1895B and SN Ia 1972E and the value of H(sub 0)

Observations of the Hubble Space Telescope (HST) between 1993 May 31 and 1993 July 19 in 20 epochs in the F555W passband and five epochs in the F785LP passband have led to the discovery of 14 Cepheids in the Amorphous galaxy NGC 5253. The apparent V distance modulus is (m-M)(sub AV) = 28.08 +/- 0.10 determined from the 12 Cepheids with normal amplitudes. The distance modulus using the F785LP data is consistent with the V value to within the errors. Five methods used to determine the internal reddening are consistent with zero differential reddening, accurate to a level of E(B-V) less than 0.05 mag, over the region occupied by Cepheids and the two supernovae (SNe) produced by NGC 5253. The apparent magnitudes at maximum for the two SNe in NGC 5253 are adopted as B(sub max) = 8.33 +/- 0.2 mag for SN 1895B, and B(sub max) = 8.56 +/- 0.1 and V(sub max) = 8.60 +/- 0.1 for SN 1972E which is a prototype SN of Type Ia. The apparent magnitude system used by Walker (1923) for SN 1859B has been corrected to the modern B scale and zero point to determine its adopted B(sub max) value.

Supernova Type Ia Luminosities, Their Dependence on Second Parameters, and the Value of H0

A sample of 35 Type Ia supernovae (SNe Ia) with good to excellent photometry in B and V, minimum internal absorption, and 1200 < v 30,000 km s-1 is compiled from the literature. As far as their spectra are known, they are all Branch-normal. For 29 of the SNe Ia, peak magnitudes in I are also known. The SNe Ia have uniform colors at maximum, i.e., = -0.012 mag (σ = 0.051) and = -0.276 mag (σ = 0.078). In the Hubble diagram, they define a Hubble line with a scatter of σM = 0.21-0.16 mag, decreasing with wavelength. The scatter is further reduced if the SNe Ia are corrected for differences in decline rate, Δm15, or color (B-V). A combined correction reduces the scatter to σ 0.13 mag. After the correction, no significant dependence remains on Hubble type or Galactocentric distance. The Hubble line suggests some curvature that can be differently interpreted. A consistent solution is obtained for a cosmological model with ΩM = 0.3, ΩΛ = 0.7, which is also indicated by much more distant SNe Ia. Absolute magnitudes are available for eight equally blue (Branch-normal) SNe Ia in spirals whose Cepheid distances are known. If their well-defined mean values of MB, MV, and MI are used to fit the Hubble line to the above sample of SNe Ia, one obtains H0 = 58.3 km s-1 Mpc-1, or, after adjusting all SNe Ia to the average values of Δm15 and (B-V), H0 = 60.9 km s-1 Mpc-1. Various systematic errors are discussed whose elimination tends to decrease H0. The value finally adopted at the 90% level, including random and systematic errors, is H0 = 58.5 ± 6.3 km s-1 Mpc-1. Several higher values of H0 from SNe Ia, as suggested in the literature, are found to depend on large corrections for variations of the light-curve parameter and/or on an unwarranted reduction of the Cepheid distances of the calibrating SNe Ia.