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

• 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)

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.

Local ULIRGs:Analyzed to Death, yet Surprisingly Controversial

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)

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).

IRAS 08572+3915

As we move towards emission arising more in the optical, the underlying galaxy body begins to appear.

IRAS 08572+3915

In the optical, dust lanes and other features start to appear against the underlying galaxy body.

IRAS 08572+3915

This deep image shows the entire galaxy body.

IRAS 08572+3915

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.

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.

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.

Spitzer: Sensitivity, not Resolution

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.

Keck Spitzer

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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.

Arp 220: What We Knew It Looked Like

Data from Trentham et al. 1999, Surace et al. 2000, Soifer et al. 1999

Arp 220: IRAC

3.6, 4.5, 8µm composite

Arp 220: IRAC


Compact core, at same scale.

Arp 220: IRAC


Old stellar population, from progenitor galaxies, which appears blue at IRAC wavelengths.

Arp 220: IRAC


Noticeably extended emission at 8µm!

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?

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!

Mrk 273

Optical IRAC Composite

3.6µm 4.5µm - stellar continuum

8µm - stellar continuum

Mrk 273




PAH-colored emission in tails.

Only 20% of 8µm emission originates outside central point source. Resolution effects?

UGC 5101




UGC 5101




Similarly, UGC 5101 has less than 20% of the total 8µm emission outside the nuclear source.

Mrk 23150

”

I-band WFPC2

IRAC Composite

A “difficult” object. Extended emission is overwhelmed by the complex Spitzer PSF.

MIPS ULIRG Observations

This didn’t work out so well. Local ULIRGs are:

24µm 70µm 160µm

Arp 220

• Too bright. Most are saturated or very near saturation.

• Too small. PSF too complex to easily identify low surface brightness features.

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.

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??

Some Ellipticals Without Dusty Extended Bodies

NGC 596

NGC 636

NGC 1700

NGC 3610 NGC 5557

NGC 5982

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.

Some Ellipticals with Dusty Extended Bodies

IRAC color composite


NGC 5018: extreme IR excess, dust emission distributed along optical dust absorption features and shells

Some Ellipticals with Dusty Extended Bodies

IRAC color composite


HST Optical

NGC 3156: dust emission along optical dust structures.

These Ellipticals Probably Reflect General Merger Population, not ULIRGs…

Two obvious ways to select for ULIRG-Elliptical bridge objects:

• Ellipticals that were demonstrably dusty mergers.

• Ellipticals that were far-IR luminous.

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.

What is happening out there??

Background: < 0.1% of SWIRE