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Spitzer/IRAC Observations of the Active Merger Remnant NGC 6240

Spitzer/IRAC Observations of the Active Merger Remnant NGC 6240. Stephanie Bush Advisor: Giovanni Fazio Collaborators: Zhong Wang & Margarita Karovska. Why are we interested in nearby Interacting Galaxies?. Galaxy interactions affect a wide range of astrophysical applications:

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Spitzer/IRAC Observations of the Active Merger Remnant NGC 6240

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  1. Spitzer/IRAC Observations of the Active Merger Remnant NGC 6240 Stephanie Bush Advisor: Giovanni Fazio Collaborators: Zhong Wang & Margarita Karovska

  2. Why are we interested in nearby Interacting Galaxies? Galaxy interactions affect a wide range of astrophysical applications: • Galaxies - galaxy evolution • Cosmology - epoch of galaxy collisions z~2-3 • Star formation - Different process than in quiescent galaxies: shocks, starbursts, etc…. • Outstanding Questions: • How do galaxies form? • Can spiral-spiral mergers create the detailed structure of elliptical? • How are active galaxy stages related to one another? • How does small scale star formation relate to the observed global star formation in galaxies?

  3. 1985: What is that thing? IRAS observed hundreds of objects that emit more in the infrared than in all other wavelengths combined. LIRGs: LIR > 1011 L ULIRGs: LIR > 1012 L (Sanders & Mirabel 1996)

  4. 2007: What (specifically) is that thing? • Possibilities: • Starburst • AGN • Combination • Most likely evolves through these stages • How do we determine what stage a remnant is at? Hopkins et al. 2006

  5. NGC 6240 Chandra X-Ray: NASA • z = .02, d = 98 Mpc • Multiple tidal features • LIRG (LIR = 7.1 x 1011 L), transition object! • Double Nuclei (optical, NIR, X-Ray) • Outflows! (H, CO, X-Ray) HST B: NASA HST - H: NASA

  6. Hopkins et al. Merger Driven Active Galaxy Evolution Hopkins et al. 2007

  7. Objectives Use IRAC photometry to determine: • the distribution of stars and dust • AGN/starburst contribution Use this to comment on stage of the merger

  8. Advantages of Spitzer/IRAC • Angular resolution (better than ISO by 2-3x) • Traces • Old stars (3.6 m) • Polycyclic Aromatic Hydrocarbons (type of dust, 8.0 m) Dust Peak PAH Features Stellar Peak http://sings.stsci.edu/proposal/ IRAC Bands

  9. Mid-IR Classification Techniques Local Analysis (e.g. Smith et al. 2005) Global SEDs (e.g. Lacy et al. 2004) Log (S8.0/S4.5) Log (S5.8/S3.6)

  10. Observations & Methodology • 10.8 mins exposure per pixel • PRF FWHM ≤ 2.0” (950 pc) • Reduced with IRACproc (Schuster 2007) • Bright source corrected (Carey 2007) • Photometry • Look at radial changes with elliptical isophotes • Global and Nuclear SEDs • Use smaller apertures to examine regional differences

  11. PRF Spikes North North Nucleus Nucleus NE NE SE SE South South ACS B-Band and IRAC 3-Color Bush, S. et al. 2007 Blue - 3.6 m Green - 4.5 m Red - 8.0 m

  12. Stars Dust IRAC 1 (3.6 m) & 2 (4.5 m) on B Bush, S. et al. 2007

  13. Stars Dust IRAC 3 (5.8 m) & 4 (8.0 m) on B Bush, S. et al. 2007

  14. Elliptical Photometry • At a given semi-major axis: e, pa, x0, y0 allowed to vary • Flux constant at large radii (enclosing total flux) • Longer wavelengths more concentrated flux! Enclosed Normalized Intensity Bush, S. et al 2007 Blue - 3.6 m Yellow - 5.8 m Green - 4.5 m Red - 8.0 m

  15. Color-Color Diagram • Red objects - upper rightBlue objects - lower left • Dashed lines are Lacy et. al 2004 empirical AGN region • Ratios change as a function of radius • Colors agree with obscured AGN, especially in the nucleus  Large dust concentration in nucleus Bush, S. et al. 2007

  16. Spectral Energy Distribution • Derived from outer elliptical isophote (sma 43”, 20.75 kpc) • Matches well with ISO, IRS (Armus et. al 2006) IRS Bush, S. et al. 2007 IRAC

  17. Comparing the SED Global Nucleus M 82: Starburst Arp 220: ULIRG and late merger NGC 1068: AGN M 31: Quiescent edge on spiral Intensity (Jy) Bush, S. et al. 2007 rest (m) rest (m)

  18. Estimating AGN and Starburst Contributions Global Nucleus • Nucleus: • 30% AGN • 70% starburst • Consistent with ISO spectra estimates, slightly high for IRS spectra estimates (20-25% AGN) • Global • 45% Starburst • 55% Quiescent Intensity (Jy) Bush, S. et al. 2007 rest (m) rest (m)

  19. Regional Photometry • More detailed way of determining the distribution of stars and dust • Need Spitzer/IRAC resolution to do this! • 4.3”, 2 kpc radius apertures placed on remnant features by eye • Look for trends Bush, S. et al. 2007

  20. Regional Photometry Color -Color • Yellow - foreground stars • Nucleus reddest point • Always gets bluer with radius Bush, S. et al. 2007

  21. R1/4 law doesn’t fit well  not relaxed! Structure: Elliptical or Disky? 3.6 micron allows us to trace structure of old stars Disk profile fits in outer regions (red - north blue - south) Bush, S. et. al 2007 Bush, S. et al. 2007 Both! - Not relaxed, disk remnants in outer regions

  22. Structure: Multi-wavelength X-Ray seems to form an “X” around the major axis of NGC 6240 Bush, S. et al. 2007

  23. NGC 6240 Merger History Clues: • Obscured AGN  Dust in nucleus, not in blowout phase! Supported by X-ray (Komossa et al. 2003) • Portions of a disk remain  not fully relaxed! Supported by NIR (Max et al. 2005) • Tidal tails, outflows Hopkins et al. 2007

  24. Conclusions: NGC 6240 • Obscured AGN • Not relaxed, disk remnants in outer regions • 30% AGN contribution to nuclear emission • 45% Starburst contribution to total emission • Young, pre-blowout, early in evolution • Agrees with other observations and theoretical merger evolution

  25. Conclusions: Detailed IRAC Studies of Mergers and the Future IRAC data dissects the structure of galaxies -- this is particularly important in merger remnants where structure is complex. Large samples of simulations of interacting galaxies are needed to match these characteristics and constrain NGC 6240’s history.

  26. References • Armus, L., et al. 2006, ApJ, 640, 204 • Bush, Wang, Karovska and Fazio in preparation • Carey, S. 2007 Private communication • Hopkins, P.F., Cox, T.J., Keres, D., & Hernquist, L. 2007, ArXiv e-prints, 706, arXiv:0706.1246 • Hopkins, P.F., Hernquist, L., Cox, T.J., Di Matteo, T., Robertson, B., & Springel, V. 2006, ApJS, 163, 1 • Komossa, S., Burwitz, V., Hasinger, G., Predehl, P., Kaastra, J.~S., & Ikebe, Y. 2003, ApJL 582, L15 • Lacy, M., et al. 2004, ApJS, 154, 166 • Max, C.E., Canalizo, G., Macintosh, B.A., Raschke, L., Whysong, D., Antonucci, R., & Schneider, G. 2005, ApJ, 621, 738 • Sanders, D.B. & Mirabel, I.F. 1996, ARAA, 34, 749 • Schuster, M.T., Marengo, M., & Patten, B.~M. 2006, procspie, 6270, • Smith, B.J., Struck, C., Appleton, P.N., Charmandaris, V., Reach, W., & Eitter, J.J. 2005, AJ, 130, 2117

  27. Image Credits NGC 6240 ACS: NASA MAST IRAS: http://www.ipac.caltech.edu/Outreach/Gallery/IRAS/iras_sat.html NGC 6240 Halpha: http://www.lcsd.gov.hk/CE/Museum/Space/AstroNews/e_astronews_04Jan.htm NGC 6240 X-Ray: NASA/CXC/MPE/S. Komossa et al. Spitzer Telescope: http://tv.gsfc.nasa.gov/G05-074_space.htm

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