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Spitzer Imaging of nearby ULIRGs and their Progeny: Merger-Formed Ellipticals

Spitzer Imaging of nearby ULIRGs and their Progeny: Merger-Formed Ellipticals. Jason Surace (Spitzer Science Center) ULIRGs: Z.Wang, S.Willner, H.Smith, J.Pipher, W.Forrest,G.Fazio Ellipticals: J.Hibbard, A.Evans, F.Marleau, L.Yan. Ultraluminous IR Galaxies: Some Background.

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Spitzer Imaging of nearby ULIRGs and their Progeny: Merger-Formed Ellipticals

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  1. Spitzer Imaging of nearby ULIRGs and their Progeny: Merger-Formed Ellipticals Jason Surace (Spitzer Science Center) ULIRGs: Z.Wang, S.Willner, H.Smith, J.Pipher, W.Forrest,G.Fazio Ellipticals: J.Hibbard, A.Evans, F.Marleau, L.Yan

  2. Ultraluminous IR Galaxies: Some Background • Discovered in large quantities by IRAS. • Most luminous local starbursts; bolometric luminosities similar to QSOs • 50-90% of luminosity emitted in the mid/far-IR. Optical luminosities non-descript. • Optical spectral properties similar to both QSOs and starburst galaxies • Local space densities relatively low (roughly similar to QSOs), but undergo rapid increase with (1+z) • Almost all are advanced major merger systems, and are postulated to be critical phases in merger-driven AGN and galaxy evolution scenarios. (which is why people care if they are starburst or AGN driven, or both)

  3. Local ULIRGs:Analyzed to Death, yet Surprisingly Controversial Local samples of about 20 systems (closest) subjected to a great barrage of observations, with almost every imaginable telescope and at all wavelengths. Results have been highly conflicted. Results at different wavelengths indicate different things. A general understanding has arisen that this results from an extremely complex emission and absorption geometry, arising from the galaxy merger process.

  4. IRAS 08572+3915 This is a canonical merger system, with two merging spiral galaxies, tidal arms, and clustered star formation. This system makes a good case study for understanding the merging process. Far-UV to near-IR composite (data from Trentham, Goldader, Surace, Scoville)

  5. IRAS 08572+3915 At 1600 angstroms, emission is dominated by lightly extinguished star-forming regions in the extended tidal structure. These regions undergo bursts in star-forming super-star clusters (SSCs).

  6. IRAS 08572+3915 As we move towards emission arising more in the optical, the underlying galaxy body begins to appear.

  7. IRAS 08572+3915 In the optical, dust lanes and other features start to appear against the underlying galaxy body.

  8. IRAS 08572+3915 This deep image shows the entire galaxy body.

  9. IRAS 08572+3915

  10. IRAS 08572+3915 Near the peak of emission of the old stellar population, the nuclei (and in deeper images the galaxy body) are seen as relatively featureless. But note the NW nucleus.

  11. IRAS 08572+3915 As thermally heated dust becomes more important, a point-like NW nucleus dominates the system. It is size-constrained to well under 200 pc.

  12. Spitzer Observations As part of the IRAC GTO program, we observed many of the nearby ULIRGs with IRAC and MIPS. Includes all of the canonical favorites, such as Arp 220, Mrk 231, and UGC 5101. Mapping of systems to several times the known optical extent of the known extended optical emission. Observations designed to be deep enough to reach the emission from stars in the extended galaxy.

  13. Spitzer: Sensitivity, not Resolution Blue: I-Band (Surace et al. 1998) Keck Spitzer Soifer et al. 2000 Despite being a space telescope, the diffraction limited spatial resolution of Spitzer is quite poor due to its small mirror size. However, the field of view and sensitivity are immensely greater.

  14. Arp 220: What Is It? Arp 220 is the closest (by more than a factor of 2) ULIRG at z=0.018. Commonly used as the “archetype”, although in reality it is not very typical of the local population in general (redder, colder, few optical SSCs). Tidal structure believed to arise from major merger. Probably the most commonly used template for extreme starbursts.

  15. Arp 220: What We Knew It Looked Like Data from Trentham et al. 1999, Surace et al. 2000, Soifer et al. 1999

  16. Arp 220: IRAC 3.6, 4.5, 8µm composite

  17. Arp 220: IRAC 3.6, 4.5, 8µm composite

  18. Arp 220: IRAC Compact core, at same scale. 3.6, 4.5, 8µm composite

  19. Arp 220: IRAC Old stellar population, from progenitor galaxies, which appears blue at IRAC wavelengths. 3.6, 4.5, 8µm composite

  20. Arp 220: IRAC Noticeably extended emission at 8µm! 3.6, 4.5, 8µm composite

  21. Arp 220: Red Underlying Galaxy The compact core is extraordinarily red, much redder than the galaxy, and contributes an increasing fraction of the total luminosity. But, the galaxy body of Arp 220 is clearly extended at 8µm. Nearly 2/3 of the total 8µm emission arises in a low surface-brightness component extended over 10”, similar to the extended CO distribution. Redder than starlight. Colors are similar to PAH emission in late-type spirals. Emission is almost certainly dust. Dust in progenitors expected to be coupled to gas. Where does this current dust come from? Where does it go?

  22. Challenging Analysis: Some Additional Systems Unfortunately, Arp 220 is the closest ULIRG. Additional targets in sample are more challenging to analyze, due to high degree of central concentration and complexity of Spitzer beam. Requires careful analysis of the beam shape!

  23. Mrk 273 Optical IRAC Composite 3.6µm 4.5µm - stellar continuum 8µm - stellar continuum

  24. Mrk 273 Optical IRAC Composite PAH-colored emission in tails. Only 20% of 8µm emission originates outside central point source. Resolution effects? 3.6µm 4.5µm - stellar continuum 8µm - stellar continuum

  25. UGC 5101 Optical IRAC Composite 3.6µm 4.5µm - stellar continuum 8µm - stellar continuum

  26. UGC 5101 Optical IRAC Composite Similarly, UGC 5101 has less than 20% of the total 8µm emission outside the nuclear source. 3.6µm 4.5µm - stellar continuum 8µm - stellar continuum

  27. Mrk 231 50” IRAC Composite I-band WFPC2 A “difficult” object. Extended emission is overwhelmed by the complex Spitzer PSF.

  28. MIPS ULIRG Observations Arp 220 24µm 70µm 160µm • Too bright. Most are saturated or very near saturation. • Too small. PSF too complex to easily identify low surface brightness features. This didn’t work out so well. Local ULIRGs are:

  29. Transformation of ULIRGs into Ellipticals N-body simulations of major merger events (like those that form ULIRGs) predict the merger remnant will strongly resemble an elliptical galaxy. Studies of the underlying galaxy bodies of ULIRGs show morphological and kinematic evidence of being similar to ellipticals (see Wright et al. 1990, Kormendy & Sanders 1992, etc.). At least some ellipticals show evidence of being merger remnants. These include bimodal stellar distributions (like the clustered star formation in major mergers) and merger-like morphology.

  30. Dusty Elliptical Sample In Spitzer GO-1, we selected a sample of 12 “young” (<2 Gyr) merger-produced ellipticals based on fine-structure and other discriminators (Schweizer & Seitzer 1992). So such systems have similarities linking them to the ULIRGs? Were they ULIRGs earlier in their evolutionary history??

  31. Some Ellipticals Without Dusty Extended Bodies NGC 636 NGC 3610 NGC 5557 NGC 5982 NGC 1700 NGC 596

  32. Some Ellipticals Without Dusty Extended Bodies Evidence of stellar shell structures and other merger features in the galaxy body isophotes. But, little or no widespread systemic extended dust emission or structure, as see in ULIRGs. Some evidence for excess emission in the galaxy cores, but uncertainties in the extended source calibration and IRAC PSF make this difficult to constrain accurately.

  33. Some Ellipticals with Dusty Extended Bodies IRAC color composite 8µm - stellar continuum NGC 5018: extreme IR excess, dust emission distributed along optical dust absorption features and shells

  34. Some Ellipticals with Dusty Extended Bodies 8µm - stellar continuum IRAC color composite NGC 3156: dust emission along optical dust structures. HST Optical

  35. These Ellipticals Probably Reflect General Merger Population, not ULIRGs… • Ellipticals that were demonstrably dusty mergers. • Ellipticals that were far-IR luminous. Two obvious ways to select for ULIRG-Elliptical bridge objects: While almost all ULIRGs are mergers, most mergers are not ULIRGs (a quick glance at the Arp Atlas confirms that). Probably we’re looking at merger-grown ellipticals that were a different merger geometry, or never underwent burst activity, or were ISM/dust-poor.

  36. What is happening out there?? Background: < 0.1% of SWIRE

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