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Philip J. Humphrey (UC Irvine)

The evolving LMXB populations of early-type galaxies. Philip J. Humphrey (UC Irvine). David A. Buote (UC Irvine), Andreas Zezas, Ewan O'Sullivan (SAO). LMXBs in early-type galaxies. Chandra has allowed LMXBs to be found & Characterized in early-type galaxies (eg Fabbiano 2006)

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Philip J. Humphrey (UC Irvine)

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  1. The evolving LMXB populations of early-type galaxies Philip J. Humphrey (UC Irvine) David A. Buote (UC Irvine), Andreas Zezas, Ewan O'Sullivan (SAO)

  2. LMXBs in early-type galaxies • Chandra has allowed LMXBs to be found & Characterized in early-type galaxies (eg Fabbiano 2006) • Provides a rich LMXB population to assess questions of formation and evolution. NGC 1399

  3. Long-term LMXB evolution 8 Gyr ASCA White, Sarazin & Kulkarni (2002)

  4. Our Galaxy Sample • Sample of 19 early-type galaxies from Chandra archive • Added 4 new datasets with recent mergers (Schweizer & Seitzer 1992). • Sample contains 6 of the 7 most disturbed in SS92. • Stellar “ages” from SS92 or Lick index fitting. • Central WFPC2 pointings for all galaxies to study GCs. • Detailed study of subset presented in Humphrey & Buote (astro-ph/0612058) • Numbers of LMXB, GC inferred by LF fits. • For shallow data, LMXB LX from spectroscopy.

  5. Evolution in the XLF? • Does the XLF shape change with age? • If sources spend most of their “active” lives at similar Lx, the XLF will evolve (Wu 2001). • The XLF can evolve a “knee” which changes with age. • Note: This cannot explain the XLF break unless lifetime ~6 Gyr

  6. An evolving field population?

  7. Our LMXB-GC relation Omits very young systems

  8. LMXB luminosity evolution with age

  9. A simple evolutionary model • In lieu of a detailed population synthesis model, simple approach based on White & Ghosh (1998) • Post-supernova binaries (PSNBs) evolve over timescale τp (~1Gyr) into LMXBs, which have lifetimes τL. • PSNBs form at a rate proportional to SFR. • Assume 2 bursts of star formation; a Hubble time ago and at the time of the last merger. • Use SS92's estimate of fraction of stars born in the merger.

  10. Normalizing the model • Problem: we actually measure LX, which depends on number of sources and duty cycle, beaming factor, etc... • How do we normalize the model to relate it to SFR? • Solution: Use the Milky Way! • Assume ~constant SFR history for MW disk; SFR~2 Msun yr-1 • From LX,excess(MW) we can normalize the model, for a given τp and τL. • We further assume τp=1 Gyr (following White & Ghosh 1998).

  11. Amount of field formation

  12. Results

  13. Conclusions • Sample of 22 early-type galaxies, 6 post-merger. • Shape of the XLF insensitive to population age. • Accounting for production in GCs, number of LMXBs per unit light does not evolve with age. • Consistent with a short (~Gyr) LMXB lifetime. • This implies negligible field LMXB production. • Supported by specific frequency of LMXBs being proportional to specific luminosity of GCs.

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