arXiv subjects

Garth D. Illingworth

Publications and source records attributed to Garth D. Illingworth.

At least 109 records · Page 6Linked to original sources

arXiv1999

The Evolution of Early-Type Galaxies in Distant Clusters II: Internal Kinematics of 55 Galaxies in the z=0.33 Cluster CL1358+62

We define a large sample of galaxies for use in a study of the fundamental plane in the intermediate redshift cluster CL1358+62 at $z=0.33$. We have analyzed high resolution spectra for 55 members of the cluster. The data were acquired with the Low Resolution Imaging Spectrograph on the Keck I 10m telescope. A new algorithm for measuring velocity dispersions is presented and used to measure the internal kinematics of the galaxies. This algorithm has been tested against the Fourier Fitting method so the data presented here can be compared with those measured previously in nearby galaxies. We have measured central velocity dispersions suitable for use in a fundamental plane analysis. The data have high $S/N$ and the resulting random errors on the dispersions are very low, typically $<5%$. Uncertainties due to mismatch of the stellar templates has been minimized through several tests and the total systematic error is of order $\about 5%$. Good seeing enabled us to measure velocity dispersion profiles and rotation curves for most of the sample and although a large fraction of the galaxies display a high level of rotation, the gradients of the total second moment of the kinematics are all very regular and similar to those in nearby galaxies. We conclude that the data therefore can be reliably corrected for aperture size in a manner consistent with nearby galaxy samples.

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arXiv1999

The Evolution of Early-Type Galaxies in Distant Clusters III.: M/L_V Ratios in the z=0.33 Cluster CL1358+62

Keck spectroscopy and Hubble Space Telescope WFPC2 imaging over a 1.5x1.5 Mpc field of CL1358+62 at z=0.33 are used to study the Fundamental Plane of galaxies based on a new, large sample of 53 galaxies. First, we have constructed the Fundamental Plane for the 30 E and S0 galaxies and find that it has the following shape: r_e = sigma**(1.31+-0.13) * _e**(-0.86+-0.10), similar to that found locally. The 1-sigma intrinsic scatter about this plane is 14% in M/L(V), comparable to that observed in Coma. We conclude that these E and S0 galaxies are structurally mature and homogeneous, like those observed in nearby clusters. The M/L(V) ratios of these early-type galaxies are offset from the Coma Fundamental Plane by delta log M/L(V) = -0.13+- 0.03 (q0=0.1), indicative of mild luminosity evolution. This evolution suggests a formation epoch for the stars of z > 1. We have also analyzed the M/L(V) ratios of galaxies of type S0/a and later. These early-type spirals follow a different plane from the E and S0 galaxies, with a scatter that is twice as large as the scatter for the E/S0s. The difference in the tilt between the plane of the spirals and the plane of the E/S0s is shown to be due to a systematic correlation of velocity dispersion with residual from the plane of the early-type galaxies. These residuals also correlate with the residuals from the Color-Magnitude relation. Thus for spirals in clusters, we see a systematic variation in the luminosity-weighted mean properties of the stellar populations with central velocity dispersion. If this is a relative age trend, then luminosity-weighted age is positively correlated with dispersion. [abridged version]

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arXiv1999

The HST Key Project on the Extragalactic Distance Scale XXV. A Recalibration of Cepheid Distances to Type Ia Supernovae and the Value of the Hubble Constant

Cepheid-based distances to seven Type Ia supernovae (SNe)-host galaxies have been derived using the standard HST Key Project on the Extragalactic Distance Scale pipeline. For the first time, this allows for a transparent comparison of data accumulated as part of three different HST projects, the Key Project, the Sandage et al. Type Ia SNe program, and the Tanvir et al. Leo I Group study. Re-analyzing the Tanvir et al. galaxy and six Sandage et al. galaxies we find a mean (weighted) offset in true distance moduli of 0.12+/-0.07 mag -- i.e., 6% in linear distance -- in the sense of reducing the distance scale, or increasing H0. Adopting the reddening-corrected Hubble relations of Suntzeff et al. (1999), tied to a zero point based upon SNe~1990N, 1981B, 1998bu, 1989B, 1972E and 1960F and the photometric calibration of Hill et al. (1998), leads to a Hubble constant of H0=68+/-2(random)+/-5(systematic) km/s/Mpc. Adopting the Kennicutt et al. (1998) Cepheid period-luminosity-metallicity dependency decreases the inferred H0 by 4%. The H0 result from Type Ia SNe is now in good agreement, to within their respective uncertainties, with that from the Tully-Fisher and surface brightness fluctuation relations.

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arXiv1999

The Hubble Space Telescope Extragalactic Distance Scale Key Project XXIII. The Discovery of Cepheids In NGC 3319

The distance to NGC 3319 has been determined from Cepheid variable stars as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen and four epochs of observations, using filters F555W (V) and F814W (I) respectively, were made with the Wide Field Planetary Camera 2. Thirty-three Cepheid variables between periods of 8 and 47 days were discovered. Adopting a Large Magellanic Cloud distance modulus of 18.50 +- 0.10 mag and extinction of E(V-I)=0.13 mag, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 30.78 +- 0.14 (random) +- 0.10 (systematic) mag and the extinction of E(V-I) = 0.06 mag were determined for NGC 3319. This galaxy is the last galaxy observed for the HST H0 Key Project.

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arXiv1999

The HST Key Project on the Extragalactic Distance Scale. XXII. The Discovery of Cepheids in NGC 1326-A

We report on the detection of Cepheids and the first distance measurement to the spiral galaxy NGC 1326-A, a member of the Fornax cluster of galaxies. We have employed data obtained with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. Over a 49 day interval, a total of twelve V-band (F555W) and eight I-band (F814W) epochs of observation were obtained. Two photometric reduction packages, ALLFRAME and DoPHOT, have been employed to obtain photometry measures from the three Wide Field CCDs. Variability analysis yields a total of 17 Cepheids in common with both photometry datasets, with periods ranging between 10 and 50 days. Of these 14 Cepheids with high-quality lightcurves are used to fit the V and I period-luminosity relations and derive apparent distance moduli, assuming a Large Magellanic Cloud distance modulus (m-M) (LMC) = 18.50 +- 0.10 mag and color excess E(B-V) = 0.10 mag. Assuming A(V)/E(V-I) = 2.45, the DoPHOT data yield a true distance modulus to NGC 1326-A of (m-M)_0 = 31.36 +- 0.17 (random) +- 0.13 (systematic) mag, corresponding to a distance of 18.7 \pm 1.5 (random) \pm 1.2 (systematic) Mpc. The derived distance to NGC 1326-A is in good agreement with the distance derived previously to NGC 1365, another spiral galaxy member of the Fornax cluster. However the distances to both galaxies are significantly lower than to NGC 1425, a third Cepheid calibrator in the outer parts of the cluster.

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arXiv1999

A High Merger Fraction in the Rich Cluster MS1054-03 at z=0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies

We present a morphological study of the galaxy population of the luminous X-ray cluster MS1054-03 at z=0.83. The sample consists of 81 spectroscopically confirmed cluster members in a 3 x 2 Mpc area imaged in F606W and F814W with WFPC2. We find thirteen ongoing mergers in MS1054-03, comprising 17% of the L > L* cluster population. Most of these mergers will likely evolve into luminous (\sim 2 L*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming the galaxy population in MS1054-03 is typical for its redshift it is estimated that \sim 50% of present-day cluster ellipticals experienced a major merger at z < 1. The mergers are preferentially found in the outskirts of the cluster, and probably occur in small infalling clumps. Morphologies, spectra, and colors of the mergers show that their progenitors were typically E/S0s or early-type spirals with mean stellar formation redshifts z* \gtrsim 1.7. The red colors of the merger remnants are consistent with the low scatter in the color-magnitude relation in rich clusters at lower redshift. The discovery of a high fraction of mergers in this young cluster is direct evidence against formation of ellipticals in a single ``monolithic'' collapse at high redshift, and in qualitative agreement with predictions of hierarchical models for structure formation.

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arXiv1999

The Velocity Dispersion of MS1054-03: A Massive Galaxy Cluster at High Redshift

We present results from a dynamical study of the high redshift, massive, X-ray luminous galaxy cluster MS1054--03. We significantly increase the number of confirmed cluster members by adding 20 to an existing set of twelve; using the confirmed members, we estimate MS1054--03's redshift, velocity dispersion, and mass. We find that z=0.8329 +/- 0.0017, sigma = 1170 +/- 150 km/s, and the central mass is approximately 1.9 +/- 0.5 x 10^{15} h^{-1} M_{odot} (within R=1 h^{-1} Mpc; H_0 =100h km s^{-1} Mpc^{-1}, q_0=0.5). MS1054--03 is one of a handful of high redshift (z>0.5) clusters known that also has X-ray and weak-lensing observations (Donahue et al. 1998; Luppino & Kaiser 1997); we find our dynamical mass agrees with mass estimates from both studies. The confirmation of MS1054--03 as a massive cluster at z~0.8 is consistent with an open (Omega_M~0.3) or flat, Lambda-dominated (Omega_M+Omega_{Lambda}=1) universe. In addition, we compare MS1054--03's velocity dispersion and X-ray temperature to a sample of low and intermediate redshift galaxy clusters to test for evolution in the sigma - T_x relation; we find no evidence for evolution in this relation to z~0.8.

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arXiv1998

The HST Key Project on the Extragalactic Distance Scale XVII. The Cepheid Distance to NGC 4725

The distance to NGC 4725 has been derived from Cepheid variables, as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen F555W (V) and four F814W (I) epochs of cosmic-ray-split Wide Field and Planetary Camera 2 observations were obtained. Twenty Cepheids were discovered, with periods ranging from 12 to 49 days. Adopting a Large Magellanic Cloud distance modulus and extinction of 18.50+/-0.10 mag and E(V-I)=0.13 mag, respectively, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 30.50 +/- 0.16 (random) +/- 0.17 (systematic) mag was determined for NGC 4725. The corresponding of distance of 12.6 +/- 1.0 (random) +/- 1.0 (systematic) Mpc is in excellent agreement with that found with an independent analysis based upon the DoPHOT photometry package. With a foreground reddening of only E(V-I)=0.02, the inferred intrinsic reddening of this field in NGC 4725, E(V-I)=0.19, makes it one of the most highly-reddened, encountered by the HST Key Project, to date.

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arXiv1998

Luminosity Evolution of Early-type Galaxies to z=0.83: Constraints on Formation Epoch and Omega

We present deep spectroscopy with the Keck telescope of eight galaxies in the luminous X-ray cluster MS1054-03 at z=0.83. The data are combined with imaging observations from the Hubble Space Telescope (HST). The spectroscopic data are used to measure the internal kinematics of the galaxies, and the HST data to measure their structural parameters. Six galaxies have early-type spectra, and two have "E+A" spectra. The galaxies with early-type spectra define a tight Fundamental Plane (FP) relation. The evolution of the mass-to-light ratio is derived from the FP. The M/L ratio evolves as Δlog M/L_B \propto -0.40 z (Omega_m=0.3, Omega_Lambda=0). The observed evolution of the M/L ratio provides a combined constraint on the formation redshift of the stars, the IMF, and cosmological parameters. For a Salpeter IMF (x=2.35) we find that z_form>2.8 and Omega_m<0.86 with 95% confidence. The constraint on the formation redshift is weaker if Omega_Lambda>0: z_form>1.7 if Omega_m=0.3 and Omega_Lambda=0.7. At present the limiting factor in constraining z_form and Omega from the observed luminosity evolution of early-type galaxies is the poor understanding of the IMF. We find that if Omega_m=1 the IMF must be significantly steeper than the Salpeter IMF (x>2.6).

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arXiv1998

The HST Key Project on the Extragalactic Distance Scale XII. The Discovery of Cepheids and a New Distance to NGC 2541

We report the detection of Cepheids and a new distance to the spiral galaxy NGC 2541, based on data obtained with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). A total of 25 exposures (divided into 13 epochs) are obtained using the F555W filter (transformed to Johnson V), and nine exposures (divided into five epochs) using the F814W filter (transformed to Cousins I). Photometric reduction of the data is performed using two independent packages, DoPHOT and DAOPHOT II/ALLFRAME, which give very good agreement in the measured magnitudes. A total of 34 bona fide Cepheids, with periods ranging from 12 to over 60 days, are identified based on both sets of photometry. By fitting V and I period-luminosity relations, apparent distance moduli are derived assuming a Large Magellanic Cloud distance modulus and mean color excess of 18.50 +/- 0.10 mag and E(B-V) = 0.10 mag respectively. Adopting A(V)/E(V-I)=2.45, we obtain a true distance modulus to NGC 2541 of 30.47 +/- 0.11 (random) +/- 0.12 (systematic) mag (D = 12.4 +/- 0.6 (random) +/- 0.7 (systematic) Mpc), and a total (Galactic plus internal) mean color excess E(B-V) = 0.08 +/- 0.05 (internal error) mag.

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arXiv1998

The Hubble Space Telescope Extragalactic Distance Scale Key Project. X. The Cepheid Distance to NGC 7331

The distance to NGC 7331 has been derived from Cepheid variables observed with HST/WFPC2, as part of the Extragalactic Distance Scale Key Project. Multi-epoch exposures in F555W (V) and F814W (I), with photometry derived independently from DoPHOT and DAOPHOT/ALLFRAME programs, were used to detect a total of 13 reliable Cepheids, with periods between 11 and 42 days. The relative distance moduli between NGC 7331 and the LMC, imply an extinction to NGC 7331 of A_V = 0.47+-0.15 mag, and an extinction-corrected distance modulus to NGC 7331 of 30.89+-0.14(random) mag, equivalent to a distance of 15.1 Mpc. There are additional systematic uncertainties in the distance modulus of +-0.12 mag due to the calibration of the Cepheid Period-Luminosity relation, and a systematic offset of +0.05+-0.04 mag if we applied the metallicity correction inferred from the M101 results of Kennicutt et al 1998.

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arXiv1998

The Color-Magnitude Relation in CL 1358+62 at z=0.33: Evidence for Significant Evolution in the S0 Population

We use a large mosaic of HST WFPC2 images to measure the colors and morphologies of 194 spectroscopically confirmed members of the rich galaxy cluster CL1358+62 at z=0.33. We study the color-magnitude (CM) relation as a function of radius in the cluster. The intrinsic scatter in the restframe B-V CM relation of the elliptical galaxies is very small: ~0.022 magnitudes. The CM relation of the ellipticals does not depend significantly on the distance from the cluster center. In contrast, the CM relation for the S0 galaxies does depend on radius: the S0s in the core follow a CM relation similar to the ellipticals, but at large radii (R>0.7Mpc) the S0s are systematically bluer and the scatter in the CM relation approximately doubles to ~0.043 magnitudes. The blueing of the S0s is significant at the 95% confidence level. These results imply that the S0 galaxies in the outer parts of the cluster have formed stars more recently than the S0s in the inner parts. A likely explanation is that clusters at z=0.33 continue to accrete galaxies and groups from the field and that infall extinguishes star formation. The apparent homogeneity of the elliptical galaxy population implies that star formation in recently accreted ellipticals was terminated well before accretion occurred. We have constructed models to explore the constraints that these observations place on the star formation history of cluster galaxies. We conclude that the population of S0s in clusters is likely to evolve as star forming galaxies are converted into passively evolving galaxies. Assuming a constant accretion rate after z=0.33, we estimate ~15% of the present day early-type galaxy population in rich clusters was accreted between z=0.33 and z=0. The ellipticals (and the brightest S0s) are probably a more stable population, at least since z=0.6.

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arXiv1997

A Pair of Lensed Galaxies at z=4.92 in the Field of CL1358+62

The cluster CL1358+62 displays a prominent red arc in WFPC2 images obtained with the Hubble Space Telescope. Keck spectra of the arc show Ly alpha emission at 7204 Angstrom, a continuum drop blueward of the line, and several absorption lines to the red. We identify the arc as a gravitationally lensed galaxy at a redshift of z=4.92. It is the highest redshift object currently known. A gravitational lens model was used to reconstruct images of the high-redshift galaxy. The reconstructed image is asymmetric, containing a bright knot and a patch of extended emission 0.4 arcsec from the knot. The effective radius of the bright knot is 0.022 arcsec or 130 h^-1 pc. The extended patch is partially resolved into compact regions of star formation. The reconstructed galaxy has I_AB= 24, giving a bolometric luminosity of about 3e11 Lsun. This can be produced by a star formation rate of 36 h^-2 Msun /yr (q0=0.5), or by an instantaneous star burst of 3e8 Msun. The spectral lines show velocity variations on the order of 300 km/s along the arc. The Si II line is blue shifted with respect to the Ly alpha emission, and the Ly alpha emission line is asymmetric with a red tail. These spectral features are naturally explained by an outflow model, in which the blue side of the Ly alpha line has been absorbed by outflowing neutral H I. Evidence from other sources indicates that outflows are common in starburst galaxies at high and low redshift. We have discovered a companion galaxy with a radial velocity only 450 km/s different than the arc's. The serendipitous discovery of these two galaxies suggests that systematic searches may uncover galaxies at even higher redshifts.

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arXiv1997

The Extragalactic Distance Scale Key Project VIII. The Discovery of Cepheids and a New Distance to NGC 3621 Using the Hubble Space Telescope

We report on the discovery of Cepheids in the field spiral galaxy NGC 3621, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). NGC 3621 is one of 18 galaxies observed as a part of The HST Key Project on the Extragalactic Distance Scale, which aims to measure the Hubble constant to 10% accuracy. Sixty-nine Cepheids with periods in the range 9--60 days were observed over 12 epochs using the F555W filter, and 4 epochs using the F814W filter. The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent packages, DAOPHOT II/ALLFRAME and DoPHOT. Period-luminosity relations in the V and I bands were constructed using 36 fairly isolated Cepheids present in our set of 69 variables. Extinction-corrected distance moduli relative to the LMC of 10.63 +/- 0.07 mag and 10.56 +/- 0.10 mag were obtained using the ALLFRAME and DoPHOT data, respectively. True distance moduli of 29.13 +/- 0.18 mag and 29.06 +/- 0.18 mag, corresponding to distances of 6.3 Mpc and 6.1 Mpc, were obtained by assuming values of 18.50 +/- 0.10 mag for the distance modulus of the LMC and E(V-I) = 0.13 mag for the reddening of the LMC.

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arXiv1997

Optical Rotation Curves of Distant Field Galaxies: Sub-L* Systems

Moderate-resolution spectroscopic observations from the Keck 10m telescope are used to derive internal kinematics for eight faint disk galaxies in the fields flanking the Hubble Deep Field. The spectroscopic data are combined with high-resolution F814W WFPC2 images from the Hubble Space Telescope which provide morphologies and scale-lengths, inclinations and orientations. The eight galaxies have redshifts 0.15 < z < 0.75, magnitudes 18.6 < I_814 < 22.1 and luminosities -21.8 < M_B < -19.0 (H_0 = 75 and q_0 = 0.05). Terminal disk velocities are derived from the spatially-resolved velocity profiles by modeling the effects of seeing, slit width, slit misalignment with galaxy major axis, and inclination for each source. These data are combined with the sample of Vogt et al. (1996) to provide a high-redshift Tully-Fisher relation that spans three magnitudes. This sample was selected primarily by morphology and magnitude, rather than color or spectral features. We find no obvious change in the shape or slope of the relation with respect to the local Tully-Fisher relation. The small offset of < 0.4 B mag with respect to the local relation is presumably caused by luminosity evolution in the field galaxy population, and does not correlate with galaxy mass. A comparison of disk surface brightness between local and high-redshift samples yields a similar offset, ~0.6 mag. These results provide further evidence for only a modest increase in luminosity with lookback time.

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arXiv1997

Evolution of Early-Type Galaxies in Distant Clusters: The Fundamental Plane using HST Imaging and Keck Spectroscopy

We present new results on the Fundamental Plane of galaxies in two rich clusters, Cl1358+62 at $z=0.33$ and MS2053--04 at $z=0.58$, based on Keck and HST observations. Our new data triple the sample of galaxies with measured Fundamental Plane parameters at intermediate redshift. The early-type galaxies in these clusters define very clear Fundamental Plane relations, confirming an earlier result for Cl0024+16 at $z=0.39$. This large sample allows us to estimate the scatter reliably. We find it to be low, at 0.067 dex in $\log r_e$, or 17% in re, similar to that observed in comparable low redshift clusters. This suggests that the structure of the older galaxies has changed little since $z=0.58$. The $M/L_V$ ratios of early-type galaxies clearly evolve with redshift; the evolution is consistent with $Δ\log M/L_V \sim -0.3z$. The $M/L_V$ ratios of two E+A galaxies in Cl1358+62 are also lower by a factor of $\sim 3$, consistent with the hypothesis that they underwent a starburst 1 Gyr previously. We conclude that the Fundamental Plane can therefore be used as a sensitive diagnostic of the evolutionary history of galaxies. Our data, when compared to the predictions of simple stellar population models, imply that the oldest cluster galaxies formed at high redshift ($z > 2$). We infer a different evolutionary history for the E+A galaxies, in which a large fraction of stars formed at $z<1$. Larger samples spanning a larger redshift range are needed to determine the influence of starbursts on the general cluster population.

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arXiv1996

Keck Spectroscopy of Redshift z~3 Galaxies in the Hubble Deep Field

We have obtained spectra with the 10-m Keck telescope of a sample of 24 galaxies having colors consistent with star-forming galaxies at redshifts 2 2. Combined with 5 previously confirmed high-redshift galaxies in the HDF, the 16 confirmed sources yield a comoving volume density of n>2.5^-4 h50^3 Mpc^-3 for q0=0.05, or n>1.2^-3 h50^3 Mpc^-3 for q0=0.5. These densities are comparable to estimates of the local volume density of galaxies brighter than L*, and could be almost three times higher still if all 29 of the unconfirmed candidates in our original sample are indeed also at high redshift. The galaxies are small but luminous, with half-light radii 1.8 < r(1/2) < 6.5 h50^-1 kpc and absolute magnitudes -21.5 > M_B > -23. The HST images show a wide range of morphologies, including several with very close, small knots of emission embedded in wispy extended structures. Using rest-frame UV continuum fluxes with no dust correction, we calculate star formation rates in the range 7 - 24 or 3 - 9 h50^-2 Msun/yr for q0=0.05 and q0=0.5, respectively. The variety of morphologies and the high number density of z=3 galaxies in the HDF suggest that they represent a range of physical processes and stages of galaxy formation and evolution, rather than any one class of object, such as massive ellipticals. A key issue remains the measurement of masses. These high-redshift objects are likely to be the low-mass, starbursting building blocks of more massive galaxies seen today.

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arXiv1996

Ellipticals with Kinematically--Distinct Cores: WFPC2 Imaging of Globular Clusters

Here we present HST WFPC2 imaging of 14 kinematically--distinct core ellipticals to examine their globular cluster systems. In particular, we probe the galaxy central regions, for which we might expect to see the strongest signatures of some formation and destruction processes. These data substantially increase the number of extragalactic globular cluster systems studied to date. We find that the globular cluster luminosity, and color, vary only weakly, if at all, with galactocentric distance. The mean color of globular clusters are constant with globular cluster magnitude. Several clear trends are also present. Firstly, globular cluster colors are bluer (more metal poor by ~ 0.5 dex) than the underlying galaxy starlight at any given galactocentric distance. Second, we find a correlation over roughly ten magnitudes between the mean globular cluster metallicity and parent galaxy luminosity of the form Z goes as L^0.4. This relationship includes dwarf ellipticals, spiral galaxy bulges and giant ellipticals. Third, we find that globular cluster surface density distribution can be described by a core model, for which the core radius correlates with galaxy luminosity. Our results favor scenarios in which ellipticals form from massive, gas rich progenitors at early epochs. Detailed simulations of the formation of globular cluster systems would be valuable to draw firmer conclusions.

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