List of Past CTC Theory Lunches : 01-Jan-2003 to 01-Jun-2003

Date:   Thursday 30-Oct-2003
Speaker:   Chris O'Dea (STScI)
Title:  Hot News from Cool Cores

The central regions of clusters of galaxies exhibit a wealth of interesting phenomena. After decades of study these objects remain mysterious. We have obtained HST/STIS Far-UV images of Lyman-alpha and the nearby Far-UV continuum of the cooling core clusters A1795 and A2597. We have compared the FUV data with WFPC2 optical data and VLA high resolution radio imaging. We see a wealth of fine detail in the HST images which is not visible from the ground. The Lyman-alpha and FUV continuum exhibits a diffuse component aligned roughly with the radio jet axis, and bright knots which tend to lie along the edges of the radio source. There is a broad positive correlation between the FUV continuum and the Lyman-alpha flux.

The colors of the continuum knots are consistent with those of young stars with ages of about 10 million years. We suggest that the FUV knots are star clusters with ongoing star formation with star formation rates of several solar masses per year. It appears that star formation occurs through out the nebula, though it is strongly enhanced along the edges of the radio source. We suggest that hot young stars are the dominant source of ionization for the nebulae.

Although smaller and less luminous than the Lyman-alpha nebula in high-z radio galaxies, some key properties of the low-z cooling core nebula are similar. This suggests that similar processes occur in the low-z cluster cooling cores and the high-z radio galaxies.

Date:   Thursday 26-Feb-2004
Speaker:   Ed Colbert (C.U.A)
Title:  Old and Young X-ray Point Source Populations in Nearby Galaxies

I will present results from analysis of 1441 X-ray point sources in Chandra ACIS data of 32 nearby spiral and elliptical galaxies (Colbert et al 2004, ApJ). The total _point-source_ X-ray (0.3-8.0 keV) luminosity L_XP is well correlated with the B-band, K-band, and FIR+UV luminosities of spiral host galaxies, and is well correlated with the B-band and K-band luminosities for elliptical galaxies. This suggests an intimate connection between L_XP and both the old and young stellar populations, for which K and FIR+UV luminosities are reasonable proxies for the galaxy mass M and star-formation rate SFR.

We derive proportionality constants 1.3e29 erg/s/Msol and 0.7e39 erg/s/(Msol/yr), which can be used to estimate the old and young components from M and SFR, respectively. The cumulative X-ray luminosity functions for the point sources have significantly different slopes. For the spiral and starburst galaxies, gamma ~= 0.6-0.8, and for the elliptical galaxies, gamma ~= 1.4. This implies that the most luminous point sources -- those with L_X >~= 10^38 erg/s -- dominate L_XP for the spiral and starburst galaxies. Most of the point sources have X-ray colors that are consistent with soft-spectrum (photon index Gamma ~ 1-2) low-mass X-ray binaries, accretion-powered black-hole high-mass (BH HMXBs), or Ultra-Luminous X-ray sources (ULXs a.k.a. IXOs). We rule out hard-spectrum neutron-star HMXBs (e.g. accretion-powered X-ray pulsars) as contributing much to L_XP. Thus, for spirals, L_XP is dominated by ULXs and BH HMXBs. We find no discernible difference between the X-ray colors of ULXs (L_X >= 10^39 erg/s) in spiral galaxies and point sources with L_X ~= 10^38-10^39 erg/s. We estimate that >~= 20% of all ULXs found in spirals originate from the older (pop II) stellar populations, indicating that many of the ULXs that have been found in spiral galaxies are in fact pop II ULXs, like those in elliptical galaxies. We find that L_XP depends _linearly_ (within uncertainties) on both M and SFR, for our sample galaxies (M <~= 10^11$ Msol and SFR <~= 10 Msol/yr).

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