arXiv subjects

Megan Donahue

Publications and source records attributed to Megan Donahue.

132 records · Page 8Linked to original sources

arXiv2000

HST Observations of Vibrationally-Excited Molecular Hydrogen in Cluster Cooling Flow Nebulae

We report new HST NICMOS and WFPC2 imaging of emission-line nebulae in the central galaxies of 3 clusters of galaxies purported to host massive cooling flows, NGC1275, A2597, and PKS0745. The spectral signature of vibrationally- excited molecular hydrogen (VEMH) has been seen in every galaxy searched so far in a cluster cooling flow with an optical emission line nebula. We have discovered that the VEMH gas extends several kpc from the centers of A2597 and PKS0745, while the vibrationally-excited molecular hydrogen in NGC1275 appears to be mostly confined to its nucleus, with some extended emission <1 kpc from the center. The VEMH in A2597 and PKS0745-191 seems to be nearly co-spatial with the optical emission-line filaments in those systems. Candidates for heating the nebulae are X-ray irradiation by the ICM, UV fluorescence by young stars, and shocks. UV heating by young stars provides the most satisfactory explanation for the H2 emission in A2597 and PKS0745; X-ray irradiation is energetically unlikely and strong shocks (v>40 km/s) are ruled out by the high H2/H-alpha ratios. If UV heating is the main energy input, a few billion solar masses of molecular gas is present in A2597 and PKS0745. UV irradiation models predict a significant amount of 1-2 micron emission from higher excitation H2 transitions and moderate far infrared luminosities (~1e44/h^2 erg/s) for A2597 and PKS0745. Even in the context of UV fluorescence models, the total amount of H2 gas and star formation inferred from these observations is too small to account for the cooling flow rates and longevities inferred from the X-rays. We note an interesting new constraint on cooling flow models: the radio sources do not provide a significant amount of shock heating, and therefore cannot counterbalance the cooling of the X-ray gas in the cluster cores.

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arXiv1999

The Second Most Distant Cluster of Galaxies in the EMSS

We report on our ASCA, Keck, and ROSAT observations of MS1137.5+6625, the second most distant cluster of galaxies in the Einstein Extended Medium Sensitivity Survey (EMSS), at redshift 0.78. We now have X-ray temperatures, velocity dispersions, and X-ray images for a complete, high-redshift sample of EMSS clusters. ASCA observations of MS1137.5+6625 yield a temperature of 5.7 +2.1, -1.1 keV and a metallicity of 0.43 +0.40,-0.37 solar (90% confidence). Keck-II spectroscopy of 22 cluster members reveals a velocity dispersion of 884 +185, -124 km/s. ROSAT observations show that MS1137.5+6625 is regular and centrally concentrated. A beta-model fit yields a core radius of only 71 h^{-1} kpc (q_0=0.1) with beta=0.70 +0.45,-0.15. The gas fraction is 0.06 +/-0.04 h^{-3/2}. This is the highest redshift EMSS cluster with a possible cooling flow (~20-400 solar masses/yr). The velocity dispersion, temperature, gas fraction, and iron abundance of MS1137 are statistically the same as those properties in low-z clusters of similar luminosity. We derive a high-z temperature function for EMSS clusters with 0.5 < z < 0.9 and compare it with temperature functions at lower redshifts, showing that evolution of the temperature function is relatively modest. Supplementing our high-z sample with other data, we demonstrate that neither the cluster L-T relation, nor cluster metallicities, nor the cluster gas fraction has detectably evolved with redshift. The very modest degree of evolution in the L-T relation inferred from these data is inconsistent with the absence of evolution in the X-ray luminosity functions derived from ROSAT cluster surveys if a critical-density structure formation model is assumed.

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arXiv1999

Omega-Matter from the Temperature-Redshift Distribution of EMSS Clusters of Galaxies

We constrain Omega_m through a maximum likelihood analysis of temperatures and redshifts of the high-redshift clusters from the EMSS. We simultaneously fit the low-redshift Markevitch (1998) sample (an all-sky sample from ROSAT with z=0.04- 0.09), a moderate redshift EMSS sample from Henry (1997) (9 clusters with z=0.3- 0.4), and a more distant EMSS sample (5 clusters with z=0.5-0.83 from Donahue et al. 1999) finding best-fit values of Omega_m = 0.45+/-0.1 for an open universe and Omega_m=0.27+/-0.1 for a flat universe. We analyze the effects of our governing assumptions, including the evolution and dispersion of the cluster L-T relation, the evolution and dispersion of the cluster M-T relation, the choice of low-redshift cluster sample, and the accuracy of the standard Press-Schechter formalism. We examine whether the existence of the massive distant cluster MS1054-0321 skews our results and find its effect to be small. From our maximum likelihood analysis we conclude that our results are not very sensitive to our assumptions, and bootstrap analysis shows that our results are not sensitive to the current temperature measurement uncertainties. The systematic uncertainties are ~+/- 0.1, and Omega_m=1 universes are ruled out at greater than 99.7% (3 sigma confidence.

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arXiv1998

On the Evolution of the Temperature-Viral Mass Relation for Clusters of Galaxies

X-ray temperature measurements of clusters of galaxies are now reaching to redshifts high enough to constrain Omega_0. A redshift-dependent relation that maps these X-ray temperatures to the virial masses of clusters is an essential ingredient when one is trying to determine cosmological parameters from cluster evolution. Most such relations assume that clusters form from top-hat perturbations that virialized just before the time we are observing them. The smaller Omega_0 is, the less accurate these relations become. Here we derive a relation between virial mass and cluster temperature that allows for the fact that clusters form gradually and cease forming when the density of the universe drops well below the critical value. We show how sensitively the expected redshift distribution of clusters depends on the mass-temperature relation used and argue that one needs to use a relation that yields no evolution in the low-Ω_0 limit.

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arXiv1998

A Very Hot, High Redshift Cluster of Galaxies: More Trouble for Omega_0 = 1

We have observed the most distant (z=0.829) cluster of galaxies in the Einstein Extended Medium Sensitivity Survey, with the ASCA and ROSAT satellites. We find an X-ray temperature of 12.3 +3.1/-2.2 keV for this cluster, and the ROSAT map reveals significant substructure. The high temperature of MS1054-0321 is consistent with both its approximate velocity dispersion, based on the redshifts of 12 cluster members we have obtained at the Keck and the Canada-France-Hawaii telescopes, and with its weak lensing signature. The X-ray temperature of this cluster implies a virial mass ~ 7.4 x 10^14 h^-1 solar masses, if the mean matter density in the universe equals the critical value, or larger if Omega_0 < 1. Finding such a hot, massive cluster in the EMSS is extremely improbable if clusters grew from Gaussian perturbations in an Omega_0 = 1 universe. Combining the assumptions that Omega_0 = 1 and that the intial perturbations were Gaussian with the observed X-ray temperature function at low redshift, we show that the probability of this cluster occurring in the volume sampled by the EMSS is less than a few times 10^{-5}. Nor is MS1054-0321 the only hot cluster at high redshift; the only two other $z > 0.5$ EMSS clusters already observed with ASCA also have temperatures exceeding 8 keV. Assuming again that the initial perturbations were Gaussian and Omega_0 = 1, we find that each one is improbable at the < 10^{-2} level. These observations, along with the fact that these luminosities and temperatures of the high-$z$ clusters all agree with the low-z L_X-T_X relation, argue strongly that Omega_0 < 1. Otherwise, the initial perturbations must be non-Gaussian, if these clusters' temperatures do indeed reflect their gravitational potentials.

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arXiv1997

Infall Regions of Galaxy Clusters

In hierarchical clustering, galaxy clusters accrete mass through the aggregation of smaller systems. Thus, the velocity field of the infall regions of clusters contains significant random motion superimposed on radial infall. Because the purely spherical infall model does not predict the amplitude of the velocity field correctly, methods estimating the cosmological density parameter Omega_0 based on this model yield unreliable biased results. In fact, the amplitude of the velocity field depends on local dynamics and only very weakly on the global properties of the universe. We use N-body simulations of flat and open universes to show that the amplitude of the velocity field of the infall regions of dark matter halos is a direct measure of the escape velocity within these regions. We can use this amplitude to estimate the mass of dark matter halos within a few megaparsecs from the halo center. In this region dynamical equilibrium assumptions do not hold. The method yields a mass estimate with better than 30% accuracy. If galaxies trace the velocity field of the infall regions of clusters reliably, this method provides a straightforward way to estimate the amount of mass surrounding rich galaxy clusters from redshift data alone.

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