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: –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?

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

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

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

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

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

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 Stellar Peak PAH Features IRAC Bands

Mid-IR Classification Techniques Global SEDs (e.g. Lacy et al. 2004) Local Analysis (e.g. Smith et al. 2005) Log (S 5.8 /S 3.6 ) Log (S 8.0 /S 4.5 )

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

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

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

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

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! Bush, S. et al 2007 Blue  m Yellow  m Green  m Red  m Enclosed Normalized Intensity

Color-Color Diagram Red objects - upper right Blue 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

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

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

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

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

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

Structure: Elliptical or Disky? Disk profile fits in outer regions (red - north blue - south) Both! - Not relaxed, disk remnants in outer regions 3.6 micron allows us to trace structure of old stars R 1/4 law doesn’t fit well  not relaxed! Bush, S. et. al 2007 Bush, S. et al. 2007

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

NGC 6240 Merger History NGC 6240 Merger History Hopkins et al 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

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 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

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.

References Armus, L., et al. 2006, ApJ, 640, 204 Bush, Wang, Karovska and Fazio in preparation Carey, S Private communication Hopkins, P.F., Cox, T.J., Keres, D., & Hernquist, L. 2007, ArXiv e- prints, 706, arXiv: 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

Image Credits NGC 6240 ACS: NASA MAST IRAS: _sat.html NGC 6240 Halpha: e_astronews_04Jan.htm NGC 6240 X-Ray: NASA/CXC/MPE/S. Komossa et al. Spitzer Telescope: 074_space.htm