arXiv subjects

M. Franx

Publications and source records attributed to M. Franx.

120 records · Page 7Linked to original sources

arXiv1999

A High Fraction of Mergers in the cluster MS1054-03 at z=0.83

We used the Hubble Space Telescope to take a wide field, multicolor image of the high redshift cluster MS1054-03 at z=0.83. The Keck telescope was used to take 200 redshifts in the field. We have a total of 81 spectroscopically confirmed cluster members with deep WFPC2 imaging. A surprisingly large number of these cluster members are mergers: 13 out of 77 galaxies are classified as merger remnants, or mergers in progress. Most of these are red, and massive. Their restframe B luminosities are amongst the brightest in the cluster: we find that 5 out of the brightest 16 are mergers. No such bright mergers were found in a lower redshift cluster with similar wide field coverage. The mergers in MS1054-03 are preferentially found in the outskirts of the cluster. The distribution of pairs in the outskirts of the cluster shows an excess of galaxies with separation < 10 h^-1 kpc, which is independent confirmation of the enhanced interaction rate. The fraction of mergers (17 %) is comparable to the fraction of ellipticals (22 %). Hence up to 50 % of low redshift ellipticals may have formed through mergers since z=1. There is no evidence for strong star formation in most of the mergers. The mean stellar ages of the mergers may therefore be much older than the ``assembly age'', i.e., the age since the time the system was put together. These new results are qualitatively consistent with predictions from hierarchical galaxy formation, and are inconsistent with an early collapse for all early type galaxies.

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arXiv1999

Hubble Space Telescope weak lensing study of the z=0.83 cluster MS 1054-03

We have measured the weak gravitational lensing signal of MS 1054-03, a rich and X-ray luminous cluster of galaxies at a redshift of z=0.83, using a two-colour mosaic of deep WFPC2 images. The small corrections for the size of the PSF and the high number density of background galaxies obtained in these observations result in an accurate and well calibrated measurement of the lensing induced distortion. The strength of the lensing signal depends on the redshift distribution of the background galaxies. We used photometric redshift distributions from the Northern and Southern Hubble Deep Fields to relate the lensing signal to the mass. The predicted variations of the signal as a function of apparent source magnitude and colour agrees well with the observed lensing signal. We determine a mass of (1.2+-0.2)x10^15 Msun within an aperture of radius 1 Mpc. Under the assumption of an isothermal mass distribution, the corresponding velocity dispersion is 1311^{+83}_{-89} km/s. For the mass-to-light ratio we find 269+-37 Msun/Lsun. The errors in the mass and mass-to-light ratio include the contribution from the random intrinsic ellipticities of the source galaxies, but not the (systematic) error due to the uncertainty in the redshift distribution. However, the estimates for the mass and mass-to-light ratio of MS 1054-03 agree well with other estimators, suggesting that the mass calibration works well. The reconstruction of the projected mass surface density shows a complex mass distribution, consistent with the light distribution. The results indicate that MS 1054-03 is a young system. The timescale for relaxation is estimated to be at least 1 Gyr. Averaging the tangential shear around the cluster galaxies, we find that the velocity dispersion of an Lstar galaxy is 203+-33 km/s.

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arXiv1999

Luminous red mergers in the z=0.83 cluster MS1054-03: Direct evidence for hierarchical formation of massive early-type galaxies

We present results from a morphological study of the distant X-ray cluster MS1054-03 at z=0.83. We have obtained a large, two color HST WFPC2 mosaic of this cluster, and measured redshifts of 186 galaxies in the MS1054-03 field with the 10m Keck telescope. Of 81 spectroscopically confirmed cluster galaxies observerd with HST, 13 are merger systems. Most of these mergers will likely evolve into luminous elliptical galaxies, and some may evolve into S0 galaxies. If the galaxy population in MS1054-03 is typical for its redshift up to 50% of ellipticals may have formed in mergers at z<1. The mergers are generally red and have no detected OII 3727 emission. This result is consistent with the old stellar ages of ellipticals inferred from other studies. The mergers are preferentially found towards the outskirts of the cluster, indicating they probably occur in infalling clumps. A significant overabundance of close pairs of red galaxies detected in the outskirts of MS1054-03 confirms the large number of interacting galaxies in this cluster.

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arXiv1999

Surface photometry of nearby field galaxies: the data

We have obtained integrated spectra and multi-filter photometry for a repre- sentative sample of ~200 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M_B = -14 to -22) and colors (B-R=0.4 to 1.8). Here we describe the sample selection criteria and the U, B, R surface photometry for these galaxies. The spectro- photometric results will be presented in a companion paper. Our goals for the project include measuring the current star formation rates and metallicity of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt (1992) to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high redshift. We discuss the observing, data reduction and calibration techniques, and show that our photometry agrees well with previous work in those cases where earlier data are available. We present an atlas of images, radial surface brightness profiles and color profiles, as well as tables of derived parameters. The atlas and tables of measurements will be made available electronically. We study the correlations of galaxy properties determined from the galaxy images. Our findings include: (1) colors determined within the effective radius correlate better with morphological type than with M_B and (2) 50 per cent of the low luminosity galaxies are bluest in their centers.

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arXiv1999

Spectrophotometry of nearby field galaxies: the data

We have obtained integrated and nuclear spectra, as well as U, B, R surface photometry, for a representative sample of 196 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M_B=-14 to -22). Here we present the spectrophotometry for these galaxies. The selection of the sample and the U, B, R surface photometry is described in a companion paper (Paper I). Our goals for the project include measuring the current star formation rates and metallicities of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt (1992a) to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high redshift. We describe the observing, data reduction and calibration techniques, and demonstrate that our spectrophotometry agrees well with that of Kennicutt. The spectra span the range 3550--7250 A at a resolution (FWHM) of ~6 A, and have an overall relative spectrophotometric accuracy of +/- 6 per cent. We present a spectrophotometric atlas of integrated and nuclear rest-frame spectra, as well as tables of equivalent widths and synthetic colors. We study the correlations of galaxy properties determined from the spectra and images. Our findings include: (1) galaxies of a given morphological class display a wide range of continuum shapes and emission line strengths if a broad range of luminosities are considered, (2) emission line strengths tend to in- crease and continua tend to get bluer as the luminosity decreases, and (3) the scatter on the general correlation between nuclear and integrated H_alpha emission line strengths is large.

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arXiv1999

VLT and NTT Observations of Two EIS Cluster Candidates. Detection of the Early-Type Galaxies Sequence at z~1

Optical data from the ESO VLT-UT1 Science Verification observations are combined with near-infrared data from SOFI at the NTT to obtain optical-infrared color-magnitude diagrams for the objects in the fields of two EIS cluster candidates. In both cases, evidence is found for a well-defined sequence of red galaxies that appear to be significantly more clustered than the background population. These results suggest that the two systems are real physical associations. The (R-Ks), (I-Ks) and (J-Ks) colors of the red sequences are used, in conjunction with similar data for spectroscopically confirmed clusters, to obtain redshift estimates of z ~ 0.9 and z ~ 1.0 for these two systems. These results make these EIS cluster candidates prime targets for follow-up spectroscopic observations to confirm their reality and to measure more accurately their redshift.

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arXiv1998

Near Infrared Observations of a Redshift 4.92 Galaxy: Evidence for Significant Dust Absorption

Near-infrared imaging and spectroscopy have been obtained of the gravitationally lensed galaxy at z=4.92 discovered in HST images by Franx et al. (1997). Images at 1.2, 1.6 and 2.2 microns show the same arc morphology as the HST images. The spectrum with resolution λ/ Δλ~ 70 shows no emission lines with equivalent width stronger than 100 A in the rest frame wavelength range 0.34 to 0.40 microns. In particular, [OII]3727 A and [NeIII]3869 A are not seen. The energy distribution is quite blue, as expected for a young stellar population with the observed Ly alpha flux. The spectral energy distribution can be fit satisfactorily for such a young stellar population when absorption by dust is included. The models imply a reddening 0.1 mag < E(B-V) < 0.4 mag. The stellar mass of the lensed galaxy lies in the range of 2 to 16 x 10^9 Msun. This is significantly higher than estimates based on the HST data alone. Our data imply that absorption by dust is important to redshifts of ~5.

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arXiv1998

Weak lensing analysis of Cl 1358+62 using Hubble Space Telescope observations

We report on the detection of weak gravitational lensing of faint, distant background galaxies by Cl 1358+62, a cluster of galaxies at a redshift of z=0.33. The observations were made using the HST. The measured shear is consistent with a velocity dispersion of 780+-50 km/s. The weak lensing mass is slightly lower than dynamical estimates and agrees well with X-ray mass estimates. The mass distribution is elongated similar to the light. The axis ratio of 0.30+-0.15 and position angle of -21+-8 degrees were measured directly from the observed shear and agree very well with a previous strong lensing analysis. We estimate the mass-to-light ratio to be 90+-13 h50 M_sun/L_Vsun. The HST point spread function 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.

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arXiv1998

A Spectroscopic Survey of the Galaxy Cluster CL 1358+62 at z=0.328

We present a spectroscopic survey of the rich, X-ray selected, galaxy cluster CL 1358+6245 at z=0.328. When our 173 new multi-slit spectra of cluster galaxies are combined with data from the literature, we produce a catalog of 232 cluster members in a region 10'x11' (3.5 Mpc x 3.8 Mpc) surrounding the brightest cluster galaxy. These data are used to study the structure and dynamics of the cluster and to examine the radial and velocity distributions as a function of spectral type. We classify the spectral types of the cluster members according to the strengths of the Balmer absorption lines (Hdelta, Hgamma, and Hbeta) and the [OII] 3727 Ang emission line.

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arXiv1997

Measuring non-axisymmetry in spiral galaxies

We present a method for measuring small deviations from axisymmetry of the potential of a filled gas disk. The method is based on a higher order harmonic expansion of the full velocity field of the disk. This expansion is made by first fitting a tilted-ring model to the velocity field of the gas disk and subsequently expanding the velocity field along each ring into its harmonic terms. We use epicycle theory to derive equations for the harmonic terms in a distorted potential. The phase of each component of the distortion can vary with radius. We show that if the potential has a distortion of harmonic number m, the velocity field as seen on the sky exhibits an m-1 and m+1 distortion. As is to be expected, the effects of a global elongation of the halo are similar to an m=2 spiral arm. The main difference is that the phase of the spiral arm can vary with radius. Our method allows a measurement of epsilon_pot sin(2 phi_2), where epsilon_pot is the elongation of the potential and phi_2 is one of the viewing angles. Using \hi data, one can probe the potential at radii beyond the stellar disk, into the regime where dark matter is thought to be the dominant dynamical component. The method is applied the spiral galaxies NGC 2403 and NGC 3198 and the harmonic terms are measured up to ninth order. We find epsilon_pot sin(2 phi_2) to be 0.064 +/- 0.003 for NGC 2403 and 0.019 +/- 0.003 for NGC 3198. More galaxies should be examined to separate viewing angle from elongation in a statistical way.

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arXiv1997

Ellipticals with Kinematically-Distinct Cores: (V-I) Color Images with WFPC2

We have analysed HST/WFPC2 F555W and F814W images for fifteen elliptical galaxies with kinematically-distinct cores. For each of them we have derived surface brightness and isophotal parameter profiles in the two bands, color maps, and radial profiles in (V-I). We have detected photometric evidence for faint stellar disks, on scales of a few tens to a few arcseconds, in seven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494 and 5322. In NGC 1700, the isophotes are slightly boxy at the scale of the counter-rotating component, and disky at larger radii. We find no difference in (V-I) color greater than 0.02 mag between these disks and the surrounding galactic regions. Hence the stellar populations in the kinematically distinct cores are not strongly deviant from the population of the main body. For one galaxy, NGC 4365, the innermost region is bluer than the surrounding regions. This area extends to about 15pc, and contains a luminosity of 2.5x10^6 L$_\odot$. If interpreted as a stellar population effect, an age difference of $\sim$ 3-4 Gyrs, or an $[Fe/H]$ variation of about 0.2 dex, is derived. The nuclear intensity profiles show a large variety: some galaxies have steep cusp profiles, others have shallow cusps and a ``break radius''. The nuclear cusps of galaxies with kinematically-distinct cores follow the same trends as the nuclei of normal galaxies. We have not been able to identify a unique, qualifying feature in the WFPC2 images which distinguish the galaxies with kinematically distinct cores from the kinematically normal cores. [shortened]

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arXiv1995

Dust in the Cores of Early-Type Galaxies

The dust properties of all early-type galaxies imaged with the Planetary Camera of the HST in cycles I-III are examined. Dust is detected in 31 out of 64 galaxies, although the sample is biased against the detection of dust. From the distribution of observed axis ratios of the dust features it is inferred that 78 \pm 16% of early-type galaxies contain nuclear dust. From the dependence of the angle between dust major axis and galaxy major axis on size and appearance of the dust, it is found that dust features with semi-major axis size a_d > 250 pc are generally not settled. If it is assumed that dust lanes with a_d < 250 pc are in equilibrium in the galaxy potential, the observed distribution of misalignment angles can only be explained if elliptical galaxies have a mean triaxiality >= 0.4. The best fit is obtained for = 0.8, higher than values derived through other methods. This difference cannot be explained by a dependence of $\langle T \rangle$ on radius. Therefore, it seems dust is not always completely relaxed, even on the smallest scales. Furthermore, the rotation axis of the dust does not generally coincide with that of the stars. This seems to suggest that the dust is acquired from outside, and has some way of losing its angular momentum efficiently.

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