Hubble Space Telescope WFPC2 Images Research Articles

Hubble Space TelescopePhotometry of the Dwarf Spheroidal Galaxy ESO 410‐G005

We present Hubble Space Telescope WFPC2 imaging of the nearby low surface brightness dwarf spheroidal galaxy ESO 410-G005, which has been resolved into stars for the first time. The resulting color-magnitude diagram for about 2500 stars shows a red giant branch with a tip at I = 22.4 ± 0.15 mag, which yields a distance of DMW = 1.9 ± 0.2 Mpc. ESO 410-G005 is found to be metal-poor with a mean metallicity of ⟨[Fe/H]⟩ = (-1.8 ± 0.4) dex estimated from its red giant branch. Upper asymptotic giant branch stars appear to be present near the center of the galaxy, indicative of a substantial, centrally concentrated intermediate-age population, unless these objects are artifacts of crowding. Previous studies did not detect ESO 410-G005 in Hα or in H I. Based on our distance estimate, ESO 410-G005 is a probable member of the Sculptor group of galaxies. Its linear separation from the nearest spiral, NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to 615 kpc depending on the assumed distance of NGC 55. The deprojected distance from the Sculptor group spiral NGC 300 is (385 ± 200) kpc. ESO 410-G005 appears to be a relatively isolated dSph within the Sculptor group. Its absolute magnitude, MV,0 = -12.1 ± 0.2 mag, its central surface brightness, μV,0 = 22.7 ± 0.1 mag arcsec-2, and its mean metallicity, [Fe/H] = (-1.8 ± 0.4) dex follow the trend observed for dwarf galaxies in the Local Group.

Hubble Space TelescopeNICMOS and WFPC2 Images of the HH 1 Jet: A Comparative Study

We present new Hubble Space Telescope (HST ) NICMOS images of the source region of the HH 1/2 —ow. The HH 1 jet is traced in the (Fe II) 1.64 km and the 2.12 km lines to about of the deeply H 2 2A.5 embedded VLA source. In general, the structure of the jet is similar in and (Fe II), with all the H 2 features having comparable brightness in both species. However, there is a gradient in the II) H 2 /(Fe ratio which increases with distance from the source. We also compare our infrared images with almost contemporary HST WFPC2 images in Ha and (S II). The ratio of (Fe II )t o (SII), both low-excitation tracers of weak shocks, is almost constant along the visible part of the jet but then increases by an order of magnitude where the optical jet disappears. If this is due to reddening, then the extinction, A V , increases by at least 4 mag in a space of corresponding to 420 AU. Beyond this obscuring ridge, we 0A.9, detect —ve more infrared knots. Less than 2A now separate the infrared jet and the tip of the compact 3.6 cm radio continuum jet centered on the VLA source. The heavy extinction which obscures the base of the IR jet is probably due to the compact cloud core recently detected in HCO'. The jet is observed to steadily increase in width, and we argue that this is either due to sideways ejection of shocked material from internal working surfaces or a sonic expansion of the hot jet beam. Surprisingly, the width of the jet is the same to within measurement errors in and (Fe II), contrary to the expectations of some H 2 models. Two small, previously detected oU-axis knots appear to form an independent HH —ow, which we call HH 501. An infrared cometary nebula (dubbed the X nebula) is found next to the HH 1 jet, along a line through the HH 501 knots, possibly supporting earlier speculations that yet one more source exists in the region. However, new proper motions of the HH 501 —ow suggest an origin of these knots near VLA 1, and it is therefore possible that the VLA 1 source itself may be a close binary, thus forming a triple system with the more distant VLA 2. Subject headings: infrared: ISM: lines and bandsISM: jets and out—owsstars: formation ¨ stars: mass loss

The Globular Cluster Systems in the Coma Ellipticals. I. The Luminosity Function in NGC 4874 and Implications for Hubble’s Constant

We have used deep Hubble Space Telescope WFPC2 images in V (F606W) and I (F814W) to measure the luminosity distribution of the globular clusters in NGC 4874, the central cD galaxy of the Coma cluster. We —nd the ii turnover ˇˇ point of the globular cluster luminosity function (GCLF) to lie at V 27.88 ^ 0.12, while the overall GCLF shape matches the standard Gaussian-like form with disper- sion We use the GCLF as a standard candle by matching the turnover points in NGC p V 1.49 ^ 0.12. 4874 and another Coma elliptical, IC 4051, with those of the giant ellipticals in the Virgo cluster (M87 and —ve others). The result is *(m ( M) (ComaVirgo) 4.06 ^ 0.11 mag, which converts to a Coma distance d 102 Mpc if the Virgo distance modulus is

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Internal kinematics, length scales, and surface brightnesses have been determined for a large sample of 53 galaxies in the cluster Cl 1358+62 at z = 0.33 from Keck spectroscopy and Hubble Space Telescope WFPC2 imaging over a 1.5 h-1 × 1.5 h-1 Mpc2 field of view. These data have been used to constrain the evolution of early-type galaxies in the cluster environment. We have constructed the fundamental plane using 30 E and S0 galaxies and draw the following conclusions. The fundamental plane at z = 0.33 has the form re ∝ σ1.31±0.13I, similar to that found locally. The 1 σ intrinsic scatter about this plane is ±14% in M/LV, comparable to that observed in Coma. We conclude that, even at intermediate redshifts, E and S0 galaxies are structurally mature and homogeneous, like those observed in nearby clusters. The M/LV ratios of these early-type galaxies are offset from the Coma fundamental plane by Δ log(M/LV) = -0.13 ± 0.03 (q0 = 0.1), indicative of mild luminosity evolution in the stellar populations in the Cl 1358+62 E and S0 galaxies. This level of evolution suggests a luminosity-weighted formation epoch for the stars of z > 1. The precise redshift depends on the initial mass function and on the cosmology. The scatter about the fundamental plane is consistent with that in the color-magnitude relation, indicating that the E/S0's have a scatter in luminosity-weighted ages of 15%. We have also analyzed the M/LV ratios of galaxies of type S0/a and later. These early-type spirals follow a different plane from the E and S0 galaxies: re ∝ σ0.66±0.29I, with a scatter that is twice as large as the scatter for the E/S0's. The difference in the tilt between the plane of the spirals and the plane of the E/S0's 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 the spirals in this cluster, as well as for the three E+A galaxies in the sample, 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 σ; i.e., more massive spiral galaxies have older stars on average. The residuals from the color-magnitude relation were used to correct the surface brightnesses of the early-type spirals. After this correction for age effects, these spirals fall on the fundamental plane of E/S0's. We conclude that the early-type spirals may well evolve onto the scaling relations of the old cluster members. After correcting the spirals for the systematic trend with color, their scatter about the fundamental plane does not decrease and remains twice as large as that for the E/S0's. This large scatter should be seen in a subsample of present-day cluster galaxies, if these spirals evolve into contemporary S0's, unless some unknown process reduces their scatter. The colors and M/LV ratios imply that many cluster galaxies were forming stars at least up until z ~ 0.5, but that this activity was specific to the spiral population. The E+A's will likely evolve into low-mass present-day S0 and Sa galaxies but be slightly bluer than the present-day (B-V) color-magnitude relation by ~0.08 mag, while fading to ~1 mag below L*.

Radial Color Gradient and Main-Sequence Mass Segregation in M30 (NGC 7099)

It has long been known that the post-core-collapse globular cluster M30 has a bluer-inward color gradient, and recent work suggests that the central deficiency of bright red giants does not fully account for this gradient. This study uses Hubble Space Telescope WFPC2 images in the F439W and F555W bands, along with ground-based CCD images with a wider field of view for normalization of the non-cluster background contribution, and finds \Delta(B-V) ~ +0.3 mag for the overall cluster starlight over the range r = 2 to >~ 1'. The slope of the color profile is: \Delta(B-V)/\Delta\log(r) = 0.20 +/- 0.07 mag dex^{-1}. We explore various algorithms for artificially redistributing the light of bright red giants and horizontal branch stars uniformly across the cluster. The traditional method of redistribution in proportion to the cluster brightness profile is shown to be inaccurate. There is no significant residual color gradient in M30 after proper uniform redistribution of all bright evolved stars; thus the color gradient in M30's central region appears to be due entirely to post-main-sequence stars. Two classes of plausible dynamical models, Fokker-Planck and multimass King models, are combined with theoretical stellar isochrones from Bergbusch & VandenBerg (1992) and from D'Antona et al. to quantify the effect of mass segregation of main sequence stars. In all cases, mass segregation of main sequence stars results in \Delta(B-V) ~ -0.06 to +0.02 mag over the range r = 20 - 80; this is consistent with M30's residual color gradient within measurement error. The observed fraction of evolved star light in the B and V bands agrees with the corresponding model predictions at small radii but drops below it for r >~ 20.

Young Stellar Populations around SN 1987A

We present the first results of a study of the stellar population in a region of 30 pc radius around SN 1987A, based on an analysis of multiband Hubble Space Telescope (HST) WFPC2 images. The effective temperature, radius and, possibly, reddening of each star were determined by fitting the measured broadband magnitudes to the ones calculated with model atmospheres. In particular, we have determined effective temperatures and bolometric luminosities for 21,995 stars, and for a subsample of 2510 stars we also determined individual reddening corrections. In addition, we have identified all stars with H? equivalent widths in excess of 8 ?, a total of 492 stars. An inspection of the H-R diagram reveals the presence of several generations of young stars, with ages between 1 and 150 Myr, superposed on a much older field population (0.6-6 Gyr). A substantial fraction of young stars with ages around 12 Myr make up the stellar generation coeval to SN 1987A progenitor. The youngest stars in the field appear to be strong-line T Tauri stars, identified on the basis of their conspicuous (Weq > 8 ?) H? excesses. This constitute the first positive detection of low-mass (about 1-2 M?) pre-main-sequence (PMS) stars outside the Milky Way. Their positions in the H-R diagram appear to require that star formation in the LMC occurs with accretion rates about 10 times higher than in the Milky Way, i.e., ~10-4 M? yr-1. SN 1987A appears to belong to a loose, young cluster 12 ? 2 Myr old, in which the slope of the present mass function is almost identical to Salpeter's, i.e., ? = d log N/d log M -1.25 for masses above 3 M?, but becomes much flatter for lower masses, i.e., ? -0.5. On a large scale, we find that the spatial distributions of massive stars and low-mass PMS stars are conclusively different, indicating that different star formation processes operate for high- and low-mass stars. This results casts doubts on the validity of an initial mass function (IMF) concept on a small scale (say, less than 10 pc). Moreover, it appears that a determination of the low-mass end IMF in the LMC requires an explicit identification of PMS stars. A preliminary analysis, done for the whole field as a single entity, shows that the IMF slope for the young population present over the entire region is steeper than ? -1.7.

Young Stellar Nuclei in the Lenticular Galaxies. II. NGC 7280

We have undertaken bidimensional spectroscopy of the central part of the nearby lenticular galaxy NGC 7280 with the Multi-Pupil Fiber Spectrograph of the 6 m telescope of the Special Astrophysical Observatory. We find a rather young stellar nucleus, with a mean population age of 1.5 ± 0.5 Gyr, which is more metal-rich than the bulge at R ≈ 1 kpc by an order of magnitude. The chemically and age-decoupled nucleus seems to be spatially resolved: the circumnuclear absorption index isolines represent ellipses elongated in P.A. ≈ 100°–110°. The same orientation, P.A. = 103°, is found for the elongated circumnuclear stellar structure, revealed from the morphological analysis of the Hubble Space Telescope (HST) WFPC2 and NICMOS images of NGC 7280 and seen best of all at R = 1''. Taking into account the stellar kinematics inside R ≈ 2'', we conclude that this structure is a circumnuclear stellar disk inclined with respect to the global plane of the galaxy. Meanwhile, both photometric and kinematical data in the radius range 2''–8'' imply the existence of an intermediate-scale bar elongated in P.A. ≈ 60°. The circumnuclear ionized gas is distributed and rotates in the plane orthogonal to the plane of the circumnuclear stellar disk.

A Determination ofH0with the CLASS Gravitational Lens B1608+656. II. Mass Models and the Hubble Constant from Lensing

We present mass models of the four-image gravitational lens system B1608+656, based on information obtained through VLBA imaging, VLA monitoring, and Hubble Space Telescope (HST) WFPC2 and NICMOS imaging. We have determined a mass model for the lens galaxies that reproduces (1) all image positions within the observational errors, (2) two out of three flux-density ratios within about 10% from the observed ratios, and (3) the model time delays within 1% from their observed values, given our best estimate of the Hubble parameter. Using the time delays determined in a companion paper, we also find that the best isothermal mass model gives H0 = 59 km s-1 Mpc-1 for Ωm = 1 and ΩΛ = 0.0, or H0 = (65-63) km s-1 Mpc-1 for Ωm = 0.3 and ΩΛ = 0.0-0.7. The statistical errors indicate the 95.4% (2 σ) confidence interval. A systematic error of ±15 km s-1 Mpc-1 is estimated from a 20% (1 σ) uncertainty in the steepness of radial mass profile. This cosmological determination of H0 agrees well with determinations from three other gravitational lens systems (i.e., B0218+357, Q0957+561, and PKS 1830-211), Type Ia supernovae, the Sunyaev-Zeldovich effect and local determinations. The current agreement on H0—within the 1 σ statistical errors—from four of five gravitational lens systems (1) emphasizes the reliability of its determination from isolated gravitational lens systems and (2) suggests that a close-to-isothermal mass profile can describe disk galaxies (e.g., B0218+357 and possibly PKS 1830-211), ellipticals (e.g., B1608+656), and central cluster ellipticals (e.g., Q0957+561). The average of H0 from B0218+357, Q0957+561, B1608+656, and PKS 1830-211, gives H = 69 ± 7 km s-1 Mpc-1 for a flat universe with Ωm = 1 or H = 74 ± 8 km s-1 Mpc-1 for Ωm = 0.3 and ΩΛ = 0.0-0.7. When including PG 1115+080, these values decrease to 64 ± 11 km s-1 Mpc-1 and 68 ± 13 km s-1 Mpc-1, respectively. The errors are the estimated 2 σ errors on the average. The Hubble parameter from gravitational lenses seems to agree best with local determinations of H0 for a low-density universe, under the assumption that all lenses are nearly isothermal.

The Globular Cluster Systems in the Coma Ellipticals. II. Metallicity Distribution and Radial Structure in NGC 4874 and Implications for Galaxy Formation

Deep Hubble Space Telescope WFPC2 images in V and I are used to investigate the globular cluster system (GCS) in NGC 4874, the central cD galaxy of the Coma cluster. Although the luminosity function of the clusters displays its normal Gaussian-like shape and turnover level, other features of the system are surprising. We find the GCS to be (a) spatially extended, with core radius rc ~ 22 kpc, (b) entirely metal poor (a narrow, unimodal metallicity distribution with [Fe/H] ~ -1.5), and (c) modestly populated for a cD-type galaxy, with specific frequency SN = 3.7 ± 0.5. Model interpretations suggest to us that as much as half of this galaxy might have accreted from low-mass satellites, but no single one of the three classic modes of galaxy formation (accretion, disk mergers, in situ formation) can supply a fully satisfactory model for the formation of NGC 4874. Even when they are used in combination, strong challenges to these models remain. We suggest that the principal anomaly in this GCS is essentially the complete lack of metal-rich clusters. If these were present in normal (M87-like) numbers in addition to the metal-poor ones that are already there, then the GCS in total would more closely resemble what we see in many other giant E galaxies. This supergiant galaxy appears to have avoided forming globular clusters during the main metal-rich stage of star formation that built the bulk of the galaxy.

The Age Difference between the Globular Cluster Subpopulations in NGC 4472

The age difference between the two main globular cluster subpopulations in the Virgo giant elliptical galaxy NGC 4472 (M49) has been determined using Hubble Space Telescope WFPC2 images in the F555W and F814W filters. Accurate photometry has been obtained for several hundred globular clusters in each of the two main subpopulations, down to more than 1 mag below the turnover of their luminosity functions. This allows precise determinations of both the mean colors and the turnover magnitudes of the two main subpopulations. By comparing the data with various population synthesis models, the age-metallicity pairs that fit both the observed colors and magnitudes have been identified. The metal-poor and the metal-rich globular clusters are found to be coeval within the errors (~3 Gyr). If one accepts the validity of our assumptions, these errors are dominated by model uncertainties. A systematic error of up to 4 Gyr could affect this result if the blue and the red clusters have significantly different mass distributions. However, that one subpopulation is half as old as the other is excluded at the 99% confidence level. The different globular cluster populations are assumed to trace the galaxy's major star formation episodes. Consequently, the vast majority of globular clusters (and by implication the majority of stars) in NGC 4472 formed at high redshifts but by two distinct mechanisms or in two episodes. The distance to NGC 4472 is determined to be 15.8 ± 0.8 Mpc, which is in excellent agreement with six of the seven Cepheid distances to Virgo Cluster spiral galaxies. This implies that the spiral and elliptical galaxies in the main body of Virgo are at the same distance.

Nuclear Bar, Star Formation, and Gas Fueling in the Active Galaxy NGC 4303

A combination of Hubble Space Telescope (HST) WFPC2 and NICMOS images are used to investigate the gas/dust and stellar structure inside the central 300 pc of the nearby active galaxy NGC 4303. The NICMOS H-band (F160W) image reveals a bright core and a nuclear elongated bar-like structure of 250 pc in diameter. The bar is centered on the bright core, and its major axis is oriented in proyection along the spin axis of the nuclear gaseous rotating disk recently detected (Colina & Arribas 1999). The V-H (F606W - F160W) image reveals a complex gas/dust distribution with a two-arm spiral structure of about 225 pc in radius. The southwestern arm is traced by young star-forming knots while the northeastern arm is detected by the presence of dust lanes. These spirals do not have a smooth structure but rather they are made of smaller flocculent spirals or filament-like structures. The magnitudes and colors of the star-forming knots are typical of clusters of young stars with masses of 0.5 to 1 x $10^5 M_{solar}, and ages of 5 to 25 million years. The overall structure of the nuclear spirals as well as the size, number and masses of the star-forming knots are explained in the context of a massive gaseous nuclear disk subject to self-gravitational instabilities and to the gravitational field created by the nuclear bar. According to the model, the gaseous disk has a mass of about 5 x 10^7 M_{solar} inside a radius of 400 pc, the bar has a radius of 150 pc and a pattern speed of about 0.5 Myr^{-1}, and the average mass accretion rate into the core (R < 8 pc) is about 0.01 M_{solar}$ yr^{-1} for about 80 Myr.

The pattern speed of the bar in NGC 4596

The pattern speed is a defining parameter of any barred galaxy. A large number of model-dependent techniques have therefore been developed to derive the pattern speed. However, the only model-independent technique for measuring this quantity—the Tremaine-Weinberg method—has hitherto been applied to just one case, the SB0 galaxy NGC 936. In this paper, we apply the technique to a second system, the SBa galaxy NGC 4596. The resulting estimate for the pattern speed is Ωp=52±13 km s−1 kpc−1. This result is corroborated by a spectrum obtained along the major axis of the bar in this system. The corotation radius associated with this pattern speed lies just beyond the end of the bar indicating a fast bar. Combining the bar major-axis spectra with data obtained from a Hubble Space Telescope WFPC2 image, we also find strong evidence for a nuclear disc.

A new radio double lens from CLASS: B1127+385

We present the discovery of a new gravitational lens system with two compact radio images separated by 0.701 +/- 0.001 arcsec, The lens system was discovered in the Cosmic Lens All Sky Survey (CLASS) as a flat-spectrum radio source. Both radio components show structure in a Very Long Baseline Array (VLBA) 8.4-GHz radio image. No further extended structure is seen in Very Large Array (VLA), Multi-Element Radio Linked Interferometer Network (MERLIN) or VLBA images, Hubble Space Telescope (HST) WFPC2 images in F555W and F814W show two extended objects close to the radio components, which we identify as two lens galaxies, Their colours and mass-to-light ratios seem to favour two late-type spiral galaxies at relatively high redshifts (z(d) greater than or similar to 0.5). Faint emission is also detected at positions corresponding to the radio images, A two-lens mass model can explain the observed VLBA structure. The best-fitting model has a reduced chi(2) of 1.1. The relative positions of the VLBA subcomponents are reproduced within 0.08 mas, and the flux density ratios within 20 per cent. We also reproduce the position angle and separation of the two VLBA subcomponents in A and B within the observational errors, which we consider strong evidence for the validity of the lens model. Moreover, we find a surface density axis ratio of 0.74(-0.12)(+0.10) for the primary lens (G1), consistent with the surface brightness axis ratio of 0.69 +/- 0.15. Also, the surface density position angle of (4.9(-22.4)(+28.2))degrees of G1 compares well with the (-6 +/- 13)degrees position angle of the surface brightness distribution. The errors indicate the 99 per cent confidence interval.

Searching for Wolf‐Rayet Stars in I Zw 18: the Origin of HeiiEmission

I Zw 18 is the most metal-poor star-forming galaxy known and is an ideal laboratory to probe stellar evolution theory at low metallicities. Using archival Hubble Space Telescope WFPC2 imaging and Faint Object Spectrograph spectroscopy, we were able to improve previous studies. We constructed a continuum-free He II λ4686 map, which was used to identify Wolf-Rayet (W-R) stars recently found by ground-based spectroscopy and to locate diffuse nebular emission. Most of the He II λ4686 emission is associated with the northwest stellar cluster clearly displaced from the surrounding shell-like [O III] and Hα emission. We found evidence for He II sources, compatible with five to nine WNL stars and/or compact nebular He II λ4686 emission, as well as residual diffuse emission. Only one of them is outside the northwest cluster. We have done an extensive comparison between our results and the recent ground-based data used by Izotov et al. and Legrand et al. to identify WN and WC stars in I Zw 18. The differences between the various data may be understood in terms of varying slit locations, continuum fits, and contamination by nebular lines. We have calculated evolutionary tracks for massive stars and synthesis models at the appropriate metallicity (Z ≈ 0.02 Z☉). These single-star models predict a mass limit MWR ≈ 90 M☉ for W-R stars to become WN and WC/WO. For an instantaneous burst model with a Salpeter initial mass function extending up to Mup ≈ 120-150 M☉, our model predictions are in reasonable agreement with the observed equivalent widths. Our model is also able to fully reproduce the observed equivalent widths of nebular He II λ4686 emission due to the presence of WC/WO stars. This quantitative agreement and the spatial correlation of nebular He II λ4686 with the stellar cluster and the position of W-R stars shown from the ground-based spectra further supports the hypothesis that W-R stars are responsible for nebular He II emission in extragalactic H II regions.

Infrared Observations of Ongoing Star Formation in the 30 Doradus Nebula and a Comparison with [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]T[/ITAL][ITAL]elescope[/ITAL] WFPC2 Images

Intercomparisons of ground-based IR continuum and H2 images with Hubble Space Telescope WFPC2 images of the 30 Dor Nebula reveal detailed structural relationships, which provide new information about current star formation there. Numerous stellar IR sources have been discovered in or near the bright nebular filaments west and northeast of R136; their locations are intimately connected with the nebular microstructures, as well as with early O stars in dense nebular knots whose optical spectral classifications indicate extreme youth. The H2 emission predominates in the dust clouds beyond the bright nebulosity and IR sources with respect to R136. The emerging picture suggests that a new stellar generation is being triggered by the energetic activity of the massive central cluster in the remanent interstellar material around its periphery. 30 Dor will likely evolve into a giant shell H II region similar to N11 in the LMC, containing an older association inside an evacuated central cavity, which is surrounded by H II regions ionized by a younger population. Such two-stage starbursts may be characteristic of massive-star formation on this scale.

Deep [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]T[/ITAL][ITAL]elescope[/ITAL] Galaxy and Pair Counts as Tests of Merger History

We consider the use of counts of faint galaxy pairs as tests of the role of merging in galaxy evolution, using both simulations and Hubble Space Telescope (HST) data in comparison with models incorporating a range in the evolution of merger rate. Artificial data, including simulation of the surface-brightness selection effects implicit in HST imagery, indicate that simple pairing statistics are robust enough for this test. However, the differences between pair fractions for various plausible scenarios of merger-rate evolution are small enough to be easily hidden by statistical errors or environmental differences. With a Poisson-based algorithm, we identify pairs with physical distances between components 60 h-1 kpc. Deep HST WFPC2 images of the field around the radio galaxy 53W002 at z = 2.4 in F814W (I) and F606W (V) show that 12%?20% (~10 pairs) of galaxies at mI 24 or mV 24.6 are paired. This supports a mild enhancement of pair fraction toward high redshifts (1?2), as concluded in other recent studies of faint galaxy pairs. The magnitude limits eliminate most possible members of the grouping associated with 53W002 itself. We compare the counts of galaxies and galaxy pairs in the 53W002 field and other recent studies to a detailed model incorporating overall traditional galaxy spectral evolution and various merger rates, in conjunction with our previous evaluation of the local merger rate. The data support evolution of the merger rate, such that the comoving density of merging systems varies as (1 + z)?, with 0 ? ? ? 2. This is consistent with the constraint ? < 3, beyond which local elliptical galaxies (E/S0 types) are overproduced from merging, and includes the case ? ? 2 required by our model to support the merger hypothesis, whereby almost all elliptical galaxies are merger remnants. The ? ? 2 case also expects a large fraction of early galaxies undergoing merging. Therefore, we support the merger-rate evolution required to support the merger hypothesis and the expectation that early merging is important in the overall evolution of galaxies. Assuming merger remnants to be elliptical galaxies subsequently undergoing passive evolution, our model can reproduce at once counts of faint galaxies and galaxy pairs, as well as the present-day luminosity function of ellipticals, for the Hubble constant H0 ? 50 km s-1 Mpc-1 and a cosmological constant ? = 0. These lines of evidence do not appear to offer compelling arguments for more exotic cosmologies or star formation histories. A preliminary study shows the fraction of paired galaxies in the 53W002 field to be much larger than that in the Hubble Deep Field (5%?6% at most in the HDF), suggesting physical field-to-field fluctuations in pair statistics. Further progress to constrain merging history requires not only larger samples, preferably coupled to at least partial redshift information, but better indications as to which faint objects in multiple systems should be considered as independent galaxies and which are parts of single dynamical units.

Weak Lensing Analysis of Cl 1358+62 UsingHubble Space TelescopeObservations

We report on the detection of weak gravitational lensing of faint, distant background objects by Cl 1358+62, a rich cluster of galaxies at a redshift of z = 0.33. The observations consist of a large, multicolor mosaic of Hubble Space Telescope WFPC2 images. The number density of approximately 50 background objects arcmin-2 allows us to do a detailed weak lensing analysis of this cluster. We detect a weak lensing signal out to~1.5 Mpc from the cluster center. The observed distortion is consistent with a singular isothermal sphere model with a velocity dispersion of 780 ± 50 km s-1. The total projected mass within a radius of 1 Mpc corresponding to this model is (4.4 ± 0.6) × 1014 M☉. The errors given here represent the random error due to the ellipticities of the background galaxies. The uncertainty in the redshift distribution introduces an additional systematic error of ~10% in the weak lensing mass. The weak lensing mass is slightly lower than dynamical estimates and agrees well with X-ray mass estimates. The mass distribution is elongated in a similar way as the light. The axis ratio of 0.30 ± 0.15 and position angle of -21° ± 7° were measured directly from the observations and agree very well with a previous strong lensing determination. A two-dimensional reconstruction of the cluster mass surface density shows that the peak of the mass distribution coincides with the peak of the light distribution. We find a value of (90 ± 13)h50 M☉ L for the mass-to-light ratio, consistent with being constant with radius. The point-spread function of HST is highly anisotropic at the edges of the individual chips. This systematically perturbs the shapes of objects, and we present a method for applying the appropriate correction.

Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228

We develop the simulated extinction to measure average foreground Galactic extinction from field galaxy number counts and colors. The method comprises simulating extinction in suitable reference fields by changing the isophotal detection limit. This procedure takes into account selection effects; in particular, the change in the isophotal detection limit (and hence in the isophotal magnitude completeness limit) with extinction, and the galaxy color-magnitude relation. We present a first application of the method to the Hubble Space Telescope WFPC2 images of the gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields, including the Hubble Deep Field, are used as reference to measure the average extinction toward the gamma-ray burst (GRB) in the F606W passband. From the counts, we derive an average extinction of AV=0.5 mag, but the dispersion of 0.4 mag between the estimates from the different reference fields is significantly larger than can be accounted for by Poisson plus clustering uncertainties. Although the counts differ, the average F606W-F814W colors of the field galaxies agree well. The extinction implied by the average color difference between the GRB field and the reference galaxies is AV=0.6 mag, with a dispersion in the estimated extinction from the four reference fields of only 0.1 mag. All of our estimates are in good agreement with the value of 0.81±0.27 mag obtained by Burstein & Heiles, and with the extinction of 0.78±0.12 measured by Schlegel et al. from maps of dust IR emission. However, the discrepancy between the widely varying counts and the very stable colors in these high-latitude fields is worth investigating.

The stellar content of a spiral arm of NGC 4321

The effects of the passage of a spiral arm through the disc of the giant Virgo Sc galaxy NGC 4321 are investigated with Hubble Space Telescope WFPC2 images in two colours. Concentrating on a portion of the southern spiral arm of NGC 4321, we have applied a new program to solve for the star formation histories in the arm and interarm regions separately. The observational uncertainties and the variable crowding across the spiral arm are taken into account using the results of artificial star tests. In the interarm regions the data are consistent with a constant star formation rate for the last 50 Myr while the stars in the arm region show a star formation rate four times larger than in the interarm regions in the last 5 Myr.

HD 98800: A Unique Stellar System of Post–T Tauri Stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.