MORPHOLOGY AND EVOLUTION OF ULTRALUMINOUSIR GALAXIES

Testing the Merger-QSO Scenario

E. STURM∗, R. GENZEL, D. LUTZ, D. RIGOPOULOU, D. TRAN and H.DANNERBAUER

Max-Planck-Institut für extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany;∗E-mail: sturm@mpe.mpg.de

A.F.M. MOORWOOD and H.W.W. SPOONEuropean Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching, Germany

Abstract. Ultraluminous infrared galaxies (ULIRGs) are probably the local analogues of a majormode of galaxy formation in the early universe. Here we give a brief progress report on our on-going programme to study the nature and evolution of ULIRGs. Our near-IR imaging of a largesample of these highly disturbed merger systems provides a data base of morphological parameterslike tidal features or projected separation of the nuclei. Together with ISO Mid-IR spectroscopy thesemorphological parameters allow us to explore the merger dynamics and evolution of ULIRGs as theymay progress from starburst to buried AGN to exposed QSO. We find that the fraction of ULIRGspredominantly powered by AGNs increases with luminosity above∼ 3× 1012 L�, but that there isno obvious trend for ULIRGs to be more AGN-like with more advanced merger phase.

1. Introduction

Ultraluminous infrared galaxies (ULIRGs) are the most violent mergers in thelocal universe. It is becoming clear that galaxy mergers play a critical role in theevolution of galaxies (e.g. Genzel, Lutz and Tacconi, 1998). Recent deep submmsurveys with SCUBA (Submillimeter Common User Bolometer Array; Hollandet al., 1999) have uncovered numerous submm-luminous galaxies with propertiessimilar to the ULIRGs observed locally, were those ULIRGs to be seen at highz (Bargeret al., 1998; Smailet al., 1998; Trentham, Blain and Goldader, 1999).Hence, by studying nearby ULIRGs we hope to gain a better understanding ofgalaxy formation and evolution also at higher redshifts.

The nature of ULIRGS and their possible evolutionary connection to quasarshas been the subject of intense debate since their discovery by IRAS more thana decade ago. Although evidence for both starburst and AGN activity in ULIRGshas been found during this period, the question as to which generally dominates theluminosity has remained largely unsolved, mainly due to observational difficultiesassociated with their large dust obscuration. With the advent of ISO, sensitive mid-infrared spectroscopy became available as a new tool capable of penetrating the

Astrophysics and Space Scienceis the original source of publication of this article. It is recom-mended that this article is cited as:Astrophysics and Space Science269–270:399–402, 1999.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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obscuring dust. In the following we give a brief progress report on our on-goingprogramme to study the nature and evolution of ULIRGs with ISO mid-infraredspectroscopy and with ground-based near-infrared imaging and spectroscopy.

2. Previous Results

Our initial observations of mid-infrared fine structure emission lines with ISO-SWS as well as of PAH dust features with ISOPHOT-S for a sample of 15 ULIRGshave shown that these spectroscopic diagnostics are powerful tools for probing theluminosity source in ULIRGs (Genzelet al., 1998). We expanded the observationsto a larger sample of 62 ULIRGs observed with ISOPHOT-S, focusing on the spec-tral regime from 5 to 11µm, where most of the PAH features arise. First resultsfrom this survey have been summarized in Lutzet al. (1998), demonstrating howthe line-to-continuum ratio of the 7.7µm PAH features can be used to discriminatebetween starburst and AGN activity as major energy source (see also Figure 2). Wefound that 80% of ULIRGs display starburst characteristics with only 20% beingAGN dominated. We confirmed that the ULIRGs showing AGN activity on averageare ‘warm’ ULIRGs (measured in terms of S(25)/S(60) or S(5.9)/S(60)).

Lutz, Veilleux and Genzel (1999) have compared ISO classifications (starburstor AGN) with classifications from optical spectroscopy. The agreement betweenmid-infrared and optical classification is excellent, if optical LINER spectra areassigned to the starburst group. LINER spectra in infrared-selected galaxies, ratherthan being an expression of the AGN phenomenon, are probably due to shocks thatare related to galactic superwinds. The comparison also suggests that AGN activity,once triggered, quickly breaks the obscuring screen at least in certain directions,thus becoming detectable over a wide wavelength range.

3. New Results

Additional observations with the ISOCAM-CVF spectrometer have been definedfor the ‘ZZULIRG’ consortium ISO proposal, with the specific target of betterstudying the most luminous ULIRGs, covering the gap to the AGN-like so-calledhyperluminous sources (Taniguchiet al., 1997; Ausselet al., 1998). From the com-bination of all observations we find that the fraction of AGN powered ULIRGs in-creases with increasing luminosities. However, some starburst-dominated systemsare found up to luminosities of at least 5× 1012 L� (Lutz et al., 1999).

To probe the evolution of ULIRGs in more detail, we have obtained high res-olution near-infrared imaging for (so far) 27 ISOPHOT-S ULIRGs (Rigopoulouet al., 1999), using SOFI on the ESO-NTT (see Figure 1). The majority of theseULIRGs are double systems, and all show distorted morphologies. We used the(projected) separation of the double nuclei and other morphological diagnostics

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Figure 1.Three examples of our NIR imaging, representing three different stages of merger evolution.

Figure 2.7.7 PAH L/C as a function of nuclear separation of the interacting components of a ULIRG(from Rigopoulouet al., 1999). The dashed line indicates the adopted separation between starburstsand AGN at L/C = 1.

(tidal tails, bridges), measured from our new imaging as well as from the literature,to determine the evolutionary state or age of the merger. We then examined whetherULIRGs that are more advanced mergers are more AGN-like. Such a trend wouldbe expected in the plausible scenario in which ULIRGs are the precursors of QSOs(Sanderset al., 1988). However, we do not find evidence for this (Figure 2). AGN orstarburst dominated ULIRGs can be found at all nuclear separations. This suggeststhat the dominance of AGN or starburst may depend on local and shorter termconditions (compression of the circum-nuclear interstellar medium as a function ofgas content and galaxy structure, local accretion rate onto the central black hole,etc.) in addition to the global state of the merger.

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The complete spectral atlas of 62 ISOPHOT-S ULIRG spectra will be publishedin Rigopoulouet al. (1999), along with the detailed results from the investigationof their evolutionary stage based on the near-infrared imaging. The additional 16ZZULIRG ISOCAM-CVF atlas, together with an improved method to quantitat-ively characterize the relative contribution of star formation and AGN activity tothe mid-infrared emission of ULIRGs, shall be published in Tranet al. (in prep.).

Acknowledgements

This work is based on observations with ISO, an ESA project with instrumentsfunded by ESA Member states and with the participation of ISAS and NASA.SWS and the ISOSDC at MPE are supported by DLR. Near-Infrared imaging wasperformed at the European Southern Observatory, Chile, ESO No. 62.P-0315.

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