arXiv subjects

Pieter G. van Dokkum

Publications and source records attributed to Pieter G. van Dokkum.

148 records · Page 9Linked to original sources

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 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

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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