The Importance of Rotational Support for the Hot Gas in Elliptical Galaxies

Transcription

1 The Importance of Rotational Support for the Hot Gas in Elliptical Galaxies Thesis prospectus by Steven Diehl Department of Physics and Astronomy Ohio University Athens, OH

2 2 ABSTRACT A recent Chandra observation of the young elliptical NGC 1700 revealed a rotationally flattened X-ray cooling disk. This casts doubt on the common assumption that the gas in ellipticals is in hydrostatic equilibrium. If rotational support is common in ellipticals then this calls various dark matter halo masses and shapes into question as they are usually based on the assumption of hydrostatic equilibrium. This thesis adresses the question whether NGC 1700 is a unique object in two separate ways. In the first part, Chandra archival X-ray data of about 80 early-type galaxies will be analyzed. Resolved point sources and a model for the unresolved point source emission will be subtracted from the X-ray image to uncover the diffuse emission of the hot gas alone. Hydrostatic and hydrodynamical models will be required to reproduce the morphology of the diffuse X-ray emission while the influence of their dark matter potentials have to stay consistent with the stellar kinematics. A statistical analysis of the optical and X-ray isophotes will be conducted to estimate the general importance of rotational support for the gas in ellipticals. Possible correlations with environmental or kinematical properties of each galaxy will be investigated as well. The second part of the thesis will consist of detailed hydrodynamical simulations of the hot gas. We will use a technique called Smooth Particle Hydrodynamics to shed light on the evolution and fate of the hot gas. Committee: Thomas S. Statler Joseph C. Shields Brian R. McNamara Todd Young (Department of Physics & Astronomy) (Department of Physics & Astronomy) (Department of Physics & Astronomy) (Department of Mathematics)

3 3 1. Introduction 1.1. X-ray History of Elliptical Galaxies Before the launch of the Einstein Observatory in 1978, little was known about X-ray emission from early type galaxies. Ellipticals were thought to be simple gas-free stellar systems. But, with the help of this first X-ray imaging satellite, astronomers soon started to discover substantial extended emission from them. Unfortunately the low sensitivity of the Einstein High Resolution Imager (HRI), leading usually to less than 200 total counts per observation, made it hard to clearly identify the origin. Twelve years later its follow up, the Röntgensatellite (ROSAT ) made this possible for the first time. The collaboration of Germany, the US and the UK represented a significant improvement in imaging and spectral resolution, as well as sensitivity. It allowed astronomers to model the spectra of elliptical galaxies in a more sophisticated way, and to separate out two major spectral components: a harder component thought to originate from stellar sources and a softer component from the hot interstellar medium. In 1999 a new era in X-ray astronomy began with the launch of the new Chandra X- ray Observatory. With advances in X-ray mirror and CCD technology it was possible to create the first X-ray imaging satellite with subarcsecond resolution. Combined with a high spectral resolution and large collecting area, Chandra is ideally suited to explore faint X-ray sources even in crowded fields. The narrow point spread function (PSF) and high sensitivity represent a major breakthrough in the field and make it possible to study X-ray sources in much more detail than before Rotational Support of the Hot Gas in Ellipticals: Previous studies The diffuse X-ray emission of elliptical galaxies originates from the hot interstellar medium (ISM). The ISM is thought to be a product of stellar mass loss, heated through collisions and shocks to temperatures of K. This produces thermal emission that can be seen in the Chandra X-ray band at keV. Emitting radiation causes a net energy loss in the gas which leads to cooling. In the absence of reheating by supernovae or active galactic nuclei, the gas starts radiating more strongly as it moves deeper into the gravitational well. Thus it cools more efficiently resulting in a loss of pressure support in the inner regions. Thus gas from the outer regions starts to slowly fall inwards with velocities of 5 50km s 1. A so-called cooling flow has developed (e.g. Loewenstein and Mathews 1987).

4 4 A question that immediately arises is whether the gas retains its angular momentum. Though the stellar component of elliptical galaxies is generally pressure supported and flattened by velocity anisotropies rather than rotation, the stars still show significant net rotation. If the hot ISM really comes from stellar mass loss, it should share the stellar angular momentum. If it was acquired during a recent merger the amount of rotational support might be even larger. Angular momentum conservation would have a significant impact on the fate of the gas. The gas would finally settle into a cooling disk which prevents it from falling further inward. These rotating cooling flow models predict flattened X-ray isophotes, if seen edge-on, and a lowered X-ray luminosity (Kley and Mathews 1995; Brighenti and Mathews 1996, 1997). Inspired by this work, Hanlan and Bregman (2000) have performed an isophotal analysis of ROSAT and Einstein X-ray data for 6 elliptical galaxies, but fail to find the predicted flattening. The limited sensitivity of these older X-ray missions restricts them to this small sample which could consequently be biased toward bright ellipticals. Because rotational support is believed to lower the X-ray luminosity, this bias may have eliminated galaxies with rotational support from the sample. At any rate, this result agreed with previous observations, leading Hanlan and Bregman (2000) to conclude that the angular momentum supported cooling flow model is ruled out and that hydrostatic equilibrium is favored. This result has far reaching consequences. Hydrostatic equilibrium implies that the X-ray emission should exactly trace the projected gravitational potential. Assuming a constant mass-to-light ratio in the stellar component, one can then use the optical surface brightness profile to subtract the stellar potential. In this way on can obtain the shape and mass of the non-luminous dark matter halo. The latter procedure is often referred to as the Geometrical Test for Dark Matter (Buote and Canizares 1994, 1998; Buote et al. 2002). Over time the assumption of hydrostatic equilibrium has been widely accepted. Yet little follow-up work has been done to account for the lack of rotational suppport. Brighenti and Mathews (2000) investigate a two possibilities. Mass dropout is able to circularize the X-ray emission in their models. As the gas is removed before it can travel far from its origin it does not have time to settle into a disk. Turbulent viscosity e.g. induced by supernovae, stellar mass loss or mass dropout is found to be insufficient for explaining the round X-ray isophotes.

5 Rotational Support of the Hot Gas in Ellipticals: Recent studies With the advance in sensitivity and resolution with Chandra, astronomers are now able to gain new insights into the detailed structure of the hot gas in ellipticals. Chandra observations of M84, a radio lobe galaxy in the Virgo cluster revealed a complex H -shape for the diffuse X-ray emission (Finoguenov and Jones 2001). Overlaid radio contours of the jet fill the gaps in the X-rays and suggest an interaction between radio jet and the hot ISM. Another example is the discovery of the very flat X-ray emission from the young elliptical galaxy NGC 1700 by Statler and McNamara (2002). Its X-ray isophotes are not only flatter than the deprojected stellar potential, but even flatter than those of the starlight (see figure 1). Statler and McNamara try to model the dark matter potential under the assumption of hydrostatic equilibrium. No potential can reproduce the flatness in the X-ray image without violating constraints based on stellar kinematics. They conclude that the assumption of hydrostatic equilibrium is not valid. The preferred explanation is that angular momentum plays a significant role in this galaxy, forcing the gas to settle into a cooling disk. The size of NGC 1700 s 15kpc disk is much bigger than expected from models assuming a stellar origin of the gas (Brighenti and Mathews 1997). The angular momentum excess and the inferred cooling time of 3 Gyr suggests that the gas was acquired during the last merger. These examples cast serious doubts on the common assumption of hydrostatic equilibrium and put question marks on some dark matter halo masses and shapes. For instance, Buote et al. (2002) apply the geometric test for dark matter to the elliptical galaxy NGC 720. The infer an extremely flat, yet not completely impossible, dark matter halo. This constraint could be significantly relaxed if the gas were allowed to keep some angular momentum. Another important issue is the often underestimated portion of the diffuse emission that is due not to hot gas but rather to unresolved point sources. Recent studies of Fornax A (Kim and Fabbiano 2003) show that it is crucial to take the incompleteness limits of the point source detection algorithms into account. Their extensive Monte Carlo simulations suggest that earlier estimates of the fraction of undetected point sources are probably too low and that the contribution of unresolved point sources to the diffuse X-ray emission is considerable Open Questions Despite substantial work on the question of rotational support for the hot gas in elliptical galaxies, a convincing and conclusive answer has yet to emerge. Chandra is probably the first instrument able to adress this problem. Its discovery of the cooling disk in NGC 1700

6 6 has called the hydrostatic view of ellipticals into doubt. My analysis is designed to determine how common rotational support is in ellipticals. NGC 1700 is a special object in the sense that its disk is seen almost edge-on. Only a statistical analysis of a large sample of elliptical galaxies will uncover how important angular momentum really is. The first part of my dissertation will use Chandra archival data on 81 early-type galaxies to shed light on this puzzle. Section 2.1 will show details about the composition and properties of the sample. The data reduction pipeline is briefly discussed and the point source analysis is described as well. Part 2.4 will deal with the isophotal analysis. The second half of my thesis will comprise hydrodynamical simulations of the hot gas ( 3). It will make use of a technique called smooth particle hydrodynamics which is briefly summarized in section 3.2. Further details on the simulations are then given in section 3.3. A short summary concludes the prospectus ( 4). 2. Analysis of Chandra archival X-ray Data of Ellipticals 2.1. The Sample My survey will comprise a sample of 81 early-type galaxies with Hubble types ranging from S0 to E6 (see table 1). All were targets of Chandra in Cycles 1 4 and were observed with either the ACIS-S or I imaging array. Most (88%) of the galaxies have been observed with ACIS-S due to the superior response at lower energies of its back illuminated CCD S3. Ten galaxies were observed with ACIS-I which has a larger field of view, and 5 on both arrays. Since we are primarily interested in imaging rather than spectral analysis, only nongrating data has been considered. Observations with exposure times < 5ks were discarded because a minimum number of total counts is required to complete the full analysis. In order to adress the question of environmental effects on the X-ray emission, the sample covers representatives of cluster, group and isolated field galaxies, characterized by Tully s ρ parameter for galaxy densities (Tully and Fisher 1988). It was also attempted to comprise the wide range of Active Galactic Nuclei (AGN) types, including radio galaxies, Seyfert I and II, LINERs, etc. to reveal their influence on the state of the hot gas. AGN classifications are taken from the AGN catalogue (10th edition) by Véron-Cetty and Véron (2001). Morphological information was extracted from the Lyon-Meudon Extragalactic Database (LEDA, see e.g. Paturel et al. 1995) and kinematical properties from HyperLEDA (e.g. Prugniel and Simien 1996). The cited distances, absolute luminosities in the B band and approximate Einstein and ROSAT X-ray luminosities were recently published by O Sullivan et al. (2003).

7 Data Reduction In order to compare different Chandra observations with each other, one has to make sure that they are reduced in a uniform and consistent way. Unfortunately this is not the case with the automatically reduced archive data. The automated data processing pipeline has changed significantly over time. The reasons for the ongoing changes in software and calibration include new insights into the deterioration of CCD chips caused by charge transfer inefficiency (CTI) or quantum efficiency (QE) degradation, time variability of the spectral and imaging responses, and updates to the Chandra Interactive Analysis of Observation (CIAO) Software. It is therefore necessary to go back to the first order event files and reprocess them with the most up-to-date software and calibration files. For this reason I developed a series of SHELL scripts to automatically call several data processing routines in CIAO and IDL (Interactive Data Language) to reprocess the data sets. The lightcurve is analyzed to remove periods of very high background flares. The energy range is restricted to 0.3 5keV in order to reduce the contribution of the particle background which starts to dominate at energies over 5keV. The lower limit is due to calibration issues at lower energies, though the effects of ACIS QE degradation might make a somewhat higher cutoff at 0.5keV necessary. After this basic reduction, another script produces instrument maps and exposure maps tailored to each observation. This is necessary to correct for instrumental features such as chip gaps and edges, which result in a spatially varying exposure time. Binned counts images are then extracted for five different energy bands (supersoft: keV, soft: keV, medium: keV, hard: keV and complete: 0.3 5keV) and finally divided by the corresponding exposure maps to compute flux calibrated images. The CIAO program wavdetect, a wavelet based point source detection tool, is run on all counts images to produce source lists. Exposure map variations are taken into account to reduce the number of false detections due to chip edges and node boundaries Point Sources Motivation Spectral analysis of early ROSAT observations of ellipticals already suggested that their X-ray emission consists of two main parts. The first part is due to the hot interstellar gas in which we are interested. The second originates from a population of discrete point sources. Thanks to the excellent spatial resolution of the Chandra telescope it is now possible to

8 8 resolve a large percentage of these sources for the first time. They have to be removed from the images, since we are interested in the emission from the hot gas only. The real difficulty lies in the fact that not all of the discrete sources are resolved. Those under Chandra s detection limit are blurred into the diffuse background, especially in the central region of the galaxy, where the point source density increases sharply. As this changes the shape of the observed diffuse emission, their contribution has to be correctly estimated and then subtracted Nature of the Point Sources Detailed analysis of the spectra of point sources shows that most of them are of stellar origin (Sarazin et al. 2001). They are thought to be mostly so-called low-mass X-ray binaries (LMXBs), in which a main-sequence star overflows its Roche lobe and starts accreting onto its compact companion a white dwarf, neutron star or small black hole. Eventually a very hot, thin accretion disc develops and emits X-rays via thermal bremsstrahlung. The rest of the point sources consist of supernova remnants (SNR) and a background population of AGN. A nice summary of point source properties is given by Fabbiano and White (2003). Hardness ratios of single sources provide a suitable means to disentangle the different classes. Supersoft sources (SSS) are in general believed to originate from old SNRs or white dwarf accretors and emit essentially all their photons below 1keV. They usually have relatively low luminosities down to well below ergs s 1 and spectra that can be fitted with blackbody models of temperatures below 0.3keV. Sources with luminosities over ergs s 1 are generally classified as Ultra Luminous X-ray sources (ULX). Since this is well above the Eddington limit of a istropically radiating neutron star, they are thought to be harboring intermediate mass black holes The Luminosity Function The point source luminosity function has been determined for a few early type galaxies and bulges, and seems to be well represented by a steep single or broken power law. Sarazin et al. (2001), for example fitted the luminosity function of NGC 4697 with a broken power law. They explain the break by the transition between LMXBs hosting neutron stars and small black holes. Kim and Fabbiano (2003) present a similar but more thorough analysis of Fornax A. They correct their distribution function for incompleteness, which is significant

9 9 for low luminosities. This correction removes the need for a break in the luminosity function. This casts doubt on the Sarazin et al. s physical interpretation of the break and highlights the need to take incompleteness into account. We use an iterative Bayesian algorithm (see Bak and Statler 2000, for further details) to obtain a non-parametric estimate of the luminosity distribution function of all point sources in each galaxy in a way that does not require binning of the data. The algorithm adds the individual likelihoods of the source luminosities and then applies a smoothing spline to the sum to produce a first estimate for the prior distribution function. The smoothing parameter is chosen by maximizing the cross-validation score. The procedure is then repeated by summing the new improved posterior probabilities and iterated until the algorithm converges onto the underlying parent distribution, the luminosity function of the point sources. One complication here is to include incompleteness of the source detection. Variations in the diffuse local background and a strong off-axis angle dependence of Chandra s point spread function (PSF) influence the efficiency and the lower detection limit of the identification software. This results in a luminosity function more and more affected by incompleteness at lower luminosities, see figure 3. We correct for this effect by weighting the likelihood with a correction factor so that each source is representative of non-detected sources with similar flux as well. These incompleteness factors are dependent on the local background count rate, total source counts, and off-axis position, and are inferred by analyzing extensive Monte Carlo simulations Objectives This thesis will for the first time carry out a detailed analysis of the point source luminosity function for all early-type galaxies in the Chandra archive. The results will either confirm or rule out the presence of a break in the luminosity function. The source catalogues will comprise information about count rates, PSF and exposure corrected fluxes in various bands, hardness ratios, accurate positions, and source luminosities. It will be possible to accurately measure the total luminosity L P SRC of all point sources and correct it for incompleteness. Due to the mostly stellar origin of the point sources it is reasonable to assume that a tight correlation exists between L P SRC and the absolute blue luminosity L B of the galaxy. This has already been proposed by O Sullivan et al. (2003). The amount of X-ray emission of hot gas could then explain the quite large scatter in the existing L X L B relation. But the true goal of the point source investigation is actually to remove their contribution

10 10 to the X-ray emission. Resolved sources are simply blanked out and replaced by a background plus Poisson noise according to the local background count rate and its gradient. The treatment of the unresolved point sources is trickier. The corrected luminosity function has to be extrapolated to lower luminosities to estimate the missing flux. Due to their stellar origin, their spatial distribution should follow the starlight. This makes it possible to use published optical surface brightness profiles to subtract the unresolved component from the diffuse X-ray image. We are then left with the uncontaminated emission from only the hot gas, which is needed for the analyis described in the following section. Figures 1 and 2 show adaptively smoothed diffuse X-ray emission of NGC 1700 and NGC 4365 respectively. These images have gone through part of the process. Only resolved sources have been removed. The X-ray isophotes are already flatter then the starlight, as can be seen from the DSS images on the right, on which X-ray isophotes are overlaid for comparison. Thus subtracting the rounder unresolved point source emission will flatten the X-ray isophotes even more, leaving us with even stronger cases for rotational support Isophotal Analysis In order to analyze the diffuse X-ray image of the hot gas quantitatively we have to fit the isophotes. The FORTRAN code shape that we are using was originally developed by Tom Statler to fit 3-dimensional density surfaces to particle distributions in N-body simulations. We modified the algorithm to fit 2-dimensional isophotes to a list of X-ray photon positions instead. Exposure map effects are taken into account since neglecting to do so could introduce an artificial flattening at node boundaries. To improve the accuracy we evenly spread the counts within a single pixel according to the local gradient determined from neighboring pixels. The procedure outputs axis ratios and major axis position angles of the best-fit elliptical isophotes. The X-ray ellipticities can then be compared to those of the optical isophotes to see whether there is a statistically significant difference consistent with rotation. By splitting the sample into several subsamples we will also investigate the possibility of correlations with environmantal properties or AGN activity. It will also be interesting to see if the flattening shows a dependence on stellar kinematical parameters such as V rot /σ since it is still unclear whether the origin of the gas is stellar mass loss, mergers, or other special events. The dependence of the flattening on the stellar rotational velocity combined with a comparison of rotation axes will show if a stellar origin of the gas is consistent with the data or if external acquisition of the gas is more likely.

11 11 3. Simulations of the Hot Gas 3.1. Introduction The second part of my dissertation will be completely different in nature. Since we want to be able to explain the results of the Chandra archival analysis, we will develop hydrodynamic models that simulate the evolution of the gas. It is our goal to find out under what conditions the gas settles into extended cooling disks. We will use a simulation technique called Smooth Particle Hydrodynamics (SPH) whose basics are briefly discussed in the next section. This work will be part of a larger collaboration. Tom Statler (Ohio University, Athens) and Mateusz Ruszkowski (University of Colorado, Boulder) will be responsible for calculating the equivalent models using the FLASH hydrodynamics code (e.g. Fryxell et al. 2000) which is an Eulerian code based on Adaptive Mesh Refinement (AMR). My responsibility will be the SPH counterpart under the partial supervision of Christopher Fryer in Los Alamos National Laboratories (LANL) who has extensive knowledge and experience in SPH simulations (for example Fryer and Heger 2000). Fryer has used SPH to model supernova explosions and mergers between black holes and neutron stars. I will use his SPH code and modify it to simulate the evolution of the hot gas in elliptical galaxies. The different nature of these two approaches, namely N-body and grid, make them well suited to check and confirm each other s results Smooth Particle Hydrodynamics This Lagrangian method was first developed almost simultaneously by Lucy (1977) and Gingold and Monaghan (1977) to simulate nonaxisymmetric phenomena in astrophysics. Since then, SPH has been applied to a wide variety of problems in different areas such as accretion disks, stellar collisions, galaxies, cosmology, relativity, and planetesimal collisions (see Monaghan 2001, and references therein). Simulations outside of astrophysics including problems in elasticity, fragmentation, water waves or impact problems has shown a very good agreement with experiment. The principle idea behind SPH is an interpolation method. Any function A(r) can be represented by its integral interpolant A I (r) (Monaghan 1992) such that A I (r) = A(r ) W (r r, h) dτ, (1) where W (r, h) is a kernel normalized over the volume elements dτ, and h is the smoothing

12 12 length which can be thought of as the resolution size of the simulation. Though there is a wide range of acceptable choices for the smoothing kernel, computational reasons favor cubic splines. A I (r) itself can be approximated by a summation interpolant: A S (r) = N b=1 A(r b ) m b ρ b W (r r b, h). (2) This step can be thought of as dividing the fluid into N particles of mass m b and densities ρ b. The field A is then evaluated at the position of these particles which can be distributed randomly in space. By replacing every field property of the gas (density, temperature, etc.) by its SPH equivalent, one is able to establish a complete N-body formalism in which the particles obey continuity equations and certain conservation laws. Angular momentum conservation can be strictly enforced. This property, in addition to the algorithm s relative simplicity in including complicated physics, make it a suitable choice to adress our problem Objectives We will use SPH simulations to model the evolution of the hot gas in elliptical galaxies. The goal is to reveal the importance or presence of rotation of the X-ray emitting gas. This will require simulations that include different shapes and masses for the dark matter halo combined with variations in the total angular momentum. These results will be compared to the hydrostatic scenario where the diffuse X-ray emission traces the total gravitational potential. In each case the models will be required to reproduce the shape of the emission and to stay consistent with stellar kinematics. An algorithm will be developed to evaluate the likelihood of each model given the data. A statistical analysis of these probabilities will provide a means to assess the question of rotational support. Early simulations will probably simply consist of dropping gas with a certain amount of angular momentum into the existing stellar potential. Then other effects will be gradually included. A dark matter potential and self gravitation of the gas will be added. Another difficulty will be to integrate sinks and sources such as star formation or stellar mass loss respectively. The effects of supernovae, winds, or turbulent viscosity (Brighenti and Mathews 2000) will need consideration as well.

13 13 4. Conclusions Since the launch of Chandra, many people have worked on various aspects of X-ray emission from ellipticals. However, changes to data reduction techniques and individual choices of energy bandpasses make it almost impossible to compare quantitatively the results of different authors. These problems will be overcome with this analysis, which will be the first time that such high quality Chandra data for a large sample of elliptical galaxies has been reduced homogeneously according to the newest software release. The point source catalogues derived from this sample can be used to put new constraints on the point source population of early-type galaxies. Higher number statistics might give a better clue to the origin of this emission. Dividing the point sources into several groups according to hardness ratio and analyzing the combined spectra separately could provide a means to distinguish between separate families of sources. The analysis of the diffuse X-ray emission will be the first to take emission of unresolved point sources into account and thus expose the true shape of the emission of the hot interstellar gas. Modelling this emission will show the importance of angular momentum for the gas. There are a 3 different scenarios imaginable: Maybe rotation is rare and NGC 1700 is a nearly unique object. Hydrostatic equilibrium holds generally and the shape of the X-ray emission traces the total projected gravitional potential, thus revealing total mass and shape of the dark matter halo. Rotational support is a common feature of ellipticals, and results based on hydrostatic equilibrium (such as dark matter halo shape and mass) have to be reexamined. Angular momentum is only apparent in a subset of galaxies. Objects like NGC 1700 are caused by special events. It is expected that a statistical analysis of the shapes will show which scenario holds true. With the uniformly reduced data sets at hand at the end of the analysis it might also be possible to identify cooling flows in non-cluster ellipticals by their temperature gradients. The present deterrent to using techniques common in cluster analysis (e.g. annular spectral fitting or temperature maps) on normal ellipticals is simply that no observation has enough counts to do so. We may be able to scale data sets of similar ellipticals to a common size scale and combine them to produce a simulated high counts observation. The combined increased count rate may then allow sophisticated cooling flow models to be fitted. So far the data reduction pipeline has been set up and successfully applied to 8 different data sets. Some minor changes have to be made to adapt to the changes in the recent CIAO

14 and CALDB 2.23 release. The point source analysis algorithm is nearly completed and working well. The procedure is able to remove resolved point sources automatically and derive their luminosity function. The subtraction of the unresolved point source emission has yet to be implemented. The isophotal analysis algorithm has already been modified and applied successfully. So far, at least one further candidate for significant flattening of the X-ray isophotes can already be identified: NGC 4365 (see figure 2). Its discovery favors the conjecture that NGC 1700 (see figure 1) is actually not unique.

15 15 REFERENCES J. Bak and T. S. Statler. The Intrinsic Shape Distribution of a Sample of Elliptical Galaxies. AJ, 120: , July F. Brighenti and W. G. Mathews. Structure and Evolution of Interstellar Gas in Flattened, Rotating Elliptical Galaxies. ApJ, 470:747 +, October F. Brighenti and W. G. Mathews. Evolution of Interstellar Gas in Rapidly Rotating Elliptical Galaxies: Formation of Disks. ApJ, 490:592 +, December F. Brighenti and W. G. Mathews. Why Are Rotating Elliptical Galaxies Less Elliptical at X-Ray Frequencies? ApJ, 539: , August D. A. Buote and C. R. Canizares. Geometrical evidence for dark matter: X-ray constraints on the mass of the elliptical galaxy NGC 720. ApJ, 427:86 111, May D. A. Buote and C. R. Canizares. X-ray isophote shapes and the mass of NGC MNRAS, 298: , August D. A. Buote, T. E. Jeltema, C. R. Canizares, and G. P. Garmire. Chandra Evidence of a Flattened, Triaxial Dark Matter Halo in the Elliptical Galaxy NGC 720. ApJ, 577: , September G. Fabbiano and N. E. White. Compact Stellar X-ray Sources in Normal Galaxies. ArXiv Astrophysics e-prints, July A. Finoguenov and C. Jones. Chandra Observation of M84, a Radio Lobe Elliptical Galaxy in the Virgo Cluster. ApJ, 547:L107 L110, February C. L. Fryer and A. Heger. Core-Collapse Simulations of Rotating Stars. ApJ, 541: , October B. Fryxell, K. Olson, P. Ricker, F. X. Timmes, M. Zingale, D. Q. Lamb, P. MacNeice, R. Rosner, J. W. Truran, and H. Tufo. FLASH: An Adaptive Mesh Hydrodynamics Code for Modeling Astrophysical Thermonuclear Flashes. ApJS, 131: , November R. A. Gingold and J. J. Monaghan. Smoothed particle hydrodynamics - Theory and application to non-spherical stars. MNRAS, 181: , November P. C. Hanlan and J. N. Bregman. X-Ray Emission from Rotating Elliptical Galaxies. ApJ, 530: , February 2000.

16 16 D. Kim and G. Fabbiano. Chandra X-Ray Observations of NGC 1316 (Fornax A). ApJ, 586: , April W. Kley and W. G. Mathews. Rotating cooling flows. ApJ, 438: , January M. Loewenstein and W. G. Mathews. Evolution of hot galactic flows. ApJ, 319: , August L. B. Lucy. A numerical approach to the testing of the fission hypothesis. AJ, 82: , December J. J. Monaghan. Smoothed particle hydrodynamics. ARA&A, 30: , J. J. Monaghan. Smoothed Particle Hydrodynamics Code Basics. Journal of Korean Astronomical Society, 34: , December E. O Sullivan, T. J. Ponman, and R. S. Collins. X-ray scaling properties of early-type galaxies. MNRAS, 340: , April G. Paturel, L. Bottinelli, and L. Gouguenheim. LEDA: The Lyon-Meudon Extragalactic Database. Astrophysical Letters Communications, 30:13 +, P. Prugniel and F. Simien. The fundamental plane of early-type galaxies: stellar populations and mass-to-light ratio. A&A, 309: , May C. L. Sarazin, J. A. Irwin, and J. N. Bregman. Chandra X-Ray Observations of the X-Ray Faint Elliptical Galaxy NGC ApJ, 556: , August T. S. Statler and B. R. McNamara. A 15 Kiloparsec X-Ray Disk in the Elliptical Galaxy NGC ApJ, 581: , December R. B. Tully and J. R. Fisher. Catalog of Nearby Galaxies. Annales de Geophysique, M. P. Véron-Cetty and P. Véron. Quasars and Active Galactic Nuclei (10th Ed.) (Veron+ 2001). VizieR Online Data Catalog, 7224:0 +, April This preprint was prepared with the AAS L A TEX macros v5.0.

17 17 Fig. 1. NGC 1700: adaptively smoothed X-ray image in the 0.3 5keV band with resolved point sources removed (left); DSS optical image with X-ray contours overlaid (right) Fig. 2. NGC 4365: adaptively smoothed X-ray image in the 0.3 5keV band with resolved point sources removed (left); DSS optical image with X-ray contours overlaid (right)

18 18 Fig. 3. Differential luminosity function of NGC 720: corrected for incompleteness (green) and uncorrected (red)

19 19 Table 1. Early-Type Galaxy Catalogue of Chandra Targets Galaxy Type a ɛ a D b Instr. c Exp. c log(lx) b log(lb) b vrot d σ d ρ e AGN f IC1262 E 0.48 ACIS-S IC1459 E ACIS-S S3 IC4296 E 0.17 ACIS-S NGC0193 E-SO 0.20 ACIS-S 30 NGC0221 E ACIS-S NGC0315 E 0.22 ACIS-S S3h NGC0383 E-SO 0.18 ACIS-S NGC0404 E-SO ACIS-S S3 NGC0507 E-SO ACIS-S NGC0526 SO 0.47 ACIS-S 9 S1.9 NGC0533 E ACIS-S NGC0720 E ACIS-S NGC0741 E ACIS-S NGC0821 E ACIS-S NGC1132 E 0.51 ACIS-S 40 NGC1265 E ACIS-S NGC1316 SO 0.31 ACIS-S NGC1332 E-SO ACIS-S NGC1395 E ACIS-I NGC1399 E 0.08 ACIS-S NGC1404 E ACIS-S NGC1407 E ACIS-S NGC1549 E ACIS-S

20 20 Table 1 Continued Galaxy Type a ɛ a D b Instr. c Exp. c log(lx) b log(lb) b vrot d σ d ρ e AGN f NGC1553 SO ACIS-S NGC1600 E ACIS-S NGC1700 E 0.37 ACIS-S NGC1705 E-SO 0.23 ACIS-S NGC2110 E-SO 0.21 ACIS-S S1i NGC2434 E ACIS-S NGC2865 E ACIS-S NGC3115 E-SO ACIS-S NGC3245 SO 0.32 ACIS-S NGC3377 E ACIS-S NGC3379 E ACIS-S NGC3557 E ACIS-I NGC3585 E ACIS-S NGC3608 E ACIS-I NGC3923 E ACIS-S NGC4125 E ACIS-S NGC4261 E ACIS-I S3h NGC4365 E ACIS-S NGC4374 E ACIS-S S2 NGC4406 E ACIS-S NGC4472 E ACIS-S NGC4486 E ACIS-S S3 NGC4494 E ACIS-S

21 21 Table 1 Continued Galaxy Type a ɛ a D b Instr. c Exp. c log(lx) b log(lb) b vrot d σ d ρ e AGN f NGC4526 SO ACIS-S NGC4552 E ACIS-S S2 NGC4555 E ACIS-S NGC4564 E ACIS-S NGC4621 E ACIS-S NGC4636 E ACIS-S S3b NGC4649 E ACIS-S NGC4697 E ACIS-S NGC4782 E ACIS-S NGC4783 E 0.50 ACIS-S NGC5018 E ACIS-S NGC5044 E ACIS-S NGC5102 E-SO ACIS-S NGC5128 SO ACIS-I ? NGC5171 E-SO 0.26 ACIS-S 35 NGC5252 SO 0.40 ACIS-S S2 NGC5532 SO 0.10 ACIS-S NGC5845 E ACIS-S NGC5846 E ACIS-S NGC6251 E 0.14 ACIS-S S2 NGC6338 SO 0.37 ACIS-I NGC6482 E 0.12 ACIS-S NGC6868 E ACIS-I

22 22 Table 1 Continued Galaxy Type a ɛ a D b Instr. c Exp. c log(lx) b log(lb) b vrot d σ d ρ e AGN f NGC7052 E 0.47 ACIS-S NGC7618 E 0.16 ACIS-S 17 NGC7619 E ACIS-S NGC7626 E ACIS-I NGC4486B E-SO 0.05 ACIS-S PGC E 0.23 ACIS-S 30 PGC E-SO 0.28 ACIS-S S3 UGC03087 SO 0.33 ACIS-S S1.5 UGC03097 SO 0.46 ACIS-S 11 UGC11130 E 0.07 ACIS-S 10 HP ESO E ACIS-I ESO E ACIS-I a Morphological type and ellipticity from the LEDA database (Paturel et al. 1995) b Distances (in Mpc), blue and X-ray luminosities from O Sullivan et al. (2003) c Chandra instrument and approved exposure time (in ks) from CXC target lists ( lists/index.html) d Maximum rotational velocity and central velocity dispersion from the Hyperleda database (Prugniel and Simien 1996) e Galaxy density (in Mpc 3 ) from the Catalogue of Nearby Galaxies (Tully and Fisher 1988) f Classification from the AGN catalogue (Véron-Cetty and Véron 2001); S1 S2: Seyfert 1 2, S3: Seyfert3/Liner, HP: high degree of optical polarization, S1h:broad polarized Balmer lines detected, S1i:broad Paschen lines observed in infrared, S1n: narrow lines, S3b: broad Balmer lines, S3h: broad Balmer lines only in polarized part