Detection of ULIGs in the WIRE Primary Survey Data

I'm under construction!

1. Overview
2. Construction of Empirical ULIG SEDs
3. Observed ULIG Colors
4. Maximum Redshift of Detectable ULIGs

1. Overview

While a great deal of work has been done examining the role of starburst galaxies and AGN in the WIRE primary survey, not as much has been done to specifically address Ultraluminous Infrared Galaxies (ULIGs). ULIGs are the most luminous systems found by IRAS; they have bolometric luminosities exceeding 10^12 solar luminosities, making them as luminous as optical QSOs. Almost all are known to result from the merger of two gas-rich spiral galaxies. It has been proposed that they are the early dust-enshrouded stages in the genesis of quasars, other have concluded that they are powered by super-starbursts. In gross properties they have characteristics similar to both starbursts and AGN, and so most modeling has approximated the most luminous systems as one or the other. However, this is not appropriate for these types of objects - the ULIGs are not scaled-up versions of less luminous local starbursts, nor can they be easily approximated as dusty AGN. Instead, they have complex spectral energy distributions (SEDs) that result from the composite nature of the merger remnant. The near-UV/optical part of the SED is dominated by emission from hot young stars with very little obscuration that are physically located in the outskirts of the merger. The near-IR and mid-IR is dominated either by hot dust heated by an AGN or by dust continuum and spectral features associated with more deeply embedded star formation. The far-IR is all reprocessed thermal emission from a very deeply (hundreds of A_V) ultraluminous power source. Cracks in the dust shroud may allow some of the shorter wavelength light from this power source to leak out. The biggest difference in comparison to local starbursts and AGN is the presence of the deeply embedded central engine - as a result, the SEDs are much more heavily weighted towards the mid and far infrared. Additionally, the presence of lightly embedded young star clusters causes the SED to become more shallow at short wavelengths than one might expect based on the mid/far-IR. Note that while these are the only local population of ultraluminous infrared galaxies, this may not be the case at high redshift. Specifically, the known ULIGs are all major merger remnants - the high redshift ULIG population believed to exist on the basis of the apparent high rate of luminosity evolution may well represent another kind of luminous galaxy population. That is the point of making the observations, after all!

There are several local well-studied ULIGs, from which empirical SEDs can be determined. I describe here the construction of ULIG SEDs, as well as composite templates, in an attempt to predict how will they appear in the WIRE data.

2. Empirical Spectral Energy Distributions

There are almost two dozen ULIGs in the local (z<0.15) universe for which we can construct detailed SEDs. I have chosen 13 local ULIGs which are well observed over a wide range of wavelengths from the near-UV (3100 angstroms) to the far-infrared (100 microns) and which span a range of properties. These systems are listed explicitly in the table in Section 4 below. The data were derived primarily from the following sources:

Near-UV (0.3 um)
Surace 1998
Optical (0.4-0.9 um)
Surace 1998, Surace et al. 1998, Sanders et al. 1988a
Near-IR (1-2.2 um)
Surace 1998, Surace et al. 1999, Sanders et al. 1988a,b
Mid-IR (3-15um)
Genzel et al. 1998, Zhou et al. 1993, Dudley et al. 1998, Sanders et al. 1988a, Sanders (unpublished)
Far-IR (15-100um)
Genzel et al. 1998, IRAS FSC

Several additional single data points were culled from NED. Flux densities were derived using the conversions given in Bessel 1979, Neugebauer et al. 1987 and Tokunaga (unpublished).

The empirical SEDs for the six "warm" ULIGs.

As was expected the SEDs fall into two broad categories. It has long been known that the 25/60 micron flux ratio ("warmness") separates starbursts and AGN in the infrared. "Warm" galaxies (Log(f25/f60) > 0.2) typically have Seyfert spectra, while the "cool" galaxies are typically more similar to star-forming regions. "Warm" and "Cool" ULIG templates were constructed by averaging the galaxy SEDs after correcting them to a common redshift and normalizing them by their total energy flux from 0.4 to 90 microns.

The SED templates for the "warm" and "cool" ULIGs.

The most obvious difference between the two are the depression of the mid-IR of the cool ULIGs relative to the warm ULIGs. Additionally, the cool ULIGs all show evidence for strong PAH emission in the mid-IR band. This is the same result found by Genzel et al. 1998 and earlier by Sanders et al. 1988.

2. Predicted Optical/Mid-IR Colors

We will have three basic datasets: imaging at 0.64, 12, and 25 microns. Together, the flux ratios (f25/f12) and (f12/f0.6) define an optical/mid-IR "spectral curvature", which can hopefully be used to pre-select ULIG candidates from the imaging for subsequent follow- up spectroscopy. In the future, this same technique can be applied by matching the WIRE data with the Sloan Digital Sky Survey, which will have a variety of (non-standard Thuan- Gunn??!) filters available.

Optical/mid-IR colors of cool ULIGs, illustrating the wide
range of colors. They are roughly defined by
Log(f25/f12)+Log(f12/f0.6) > 2.4. The dots are in units
of z=0.1.

Optical/mid-IR colors of warm ULIGs,
illustrating the wide range of colors.

Optical/mid-IR colors of warm and cool ULIG galaxy templates.

Some additional modeling data in the near future for starburst galaxies at various redshifts will allow us to examine in detail the efficacy of these diagnostics.

4. Maximum Detectable ULIG Redshift

It is possible to determine the maximum redshift at which the local ULIGs could be detected by WIRE. Starting with the observed SEDs, each galaxy was placed at progressively higher redshifts and new K-corrections were computed using the WIRE 25 micron filter transmission. These were then applied to the observed rest frame flux, as was a modified inverse square law for a q_0=0.5 universe (Kolb & Turner 1990). Given in the following table are the maximum redshifts at which each ULIG could be detected by WIRE at 25 microns with a limiting flux of 1.5 mJy, corresponding to the limit associated with the moderate depth, moderate evolution survey.

"Cool" ULIGsmax z
UGC 5101 1.2
IR 12112 1.4
Mrk 273 1.2
IR 14348 1.4
IR 15250 1.4
Arp 220 1.2
IR 22491 1.4
"Warm" ULIGs
IR 05189 2.4
IR 08572 2.1
IR 12071 2.0
Mrk 231 2.9
Mrk 463 2.5
IR 15206 1.7

It is apparent that the "warm" ULIGs will be detectable at much higher redshift than their "cool" counterparts. This is primarily because they have much stronger mid-IR emission relative to their bolometric luminosity, as can be seen in the template SEDs above. As the WIRE bands are shifted to bluer rest wavelengths, the much higher slope of the cool ULIG SEDs causes the observed flux to drop more rapidly than that of the warms, whose bolometric luminosity peaks closer to the mid-infrared. Obviously, this assumes no luminosity evolution. A priori I don't expect there to be any with this kind of ULIG. I don't think there is any reason to expect galaxy-galaxy collisions of this kind to have been more luminous in the past. I do expect them to be more frequent, and hence the ULIG space density may increase. These may, therefore, be different from the other high luminosity dusty galaxies that WIRE may observe, which will possibly be the initial collapse of protogalaxies. It will be important to separate the different populations of high- luminosity objects in order to isolate their different evolution rates. In the case of major merger ULIGs, the tell-tale signs of merger morphology (primarily, 10-50 kpc tidal tails and multiple nuclei) will identify them, although these tidal features may be hard to detect at high redshift.