7.3.4 ULIRGs, HLIRGS, EROs and EMNOs

We find a surprisingly large population of Arp220-like ULIRGs ( $L_{ir} > 12.22$) in this survey, 14% of 15 $\mu$m galaxies with known $z$).

Morel (2001) reported the first HLIRG in the survey. In fact there are 9 candidate HLIRGs ( $L_{ir} > 13.22$), ane these are listed in Table 7.3 (note that for galaxies with $n_{ir,t}$ = 6, the luminosity in the starburst and AGN dust torus components must be added to get the total infrared luminosity). A further 45 galaxies would be hyperluminous if their 15 $\mu$m emission were interpreted as due to a starburst rather than the more conservative assumption that it is due to an AGN. All 9 objects in Table 7.3 appear to be quasars: selection at 15 $\mu$m does favour detection of AGN dust tori. For 4 galaxies this classification is based only on a photometric redshift, and this needs to be confirmed by spectroscopy (especially where $z_{ph} =$ 2-2.5, since there is a strong possibility of aliasing.

The large number of ULIRGs and HLIRGs probably reflects the very steep rise in the star formation rate between $z = 0$ and $z = 2$ and the associated strong evolution in the AGN population, though gravitational lensing could also play a part.

Table 7.3: Hyperluminous Infrared Galaxies (HLIRGs) in the ELAIS Band-Merged Catalogue.

ELAIS Name	$m_r$	$n_{p,t}$	$z_{ph}$	$z_{sp}$	$n_{s,t}$	$n_{z,r}$	$L_{opt}$	$n_{ir,t}$	$L_{ir}$	$L_{tor}$
C15_J002925-434917	18.63	-	-	3.09	5	3	13.59	2	14.80	13.54
C15_J003213-434553	17.09	-	-	1.70	5	3	13.57	2	13.97	13.29
C15_J050152-303519	17.86	4	0.02	1.81	5	4	13.62	4	13.25	13.07
C15_J160419+541524	18.00	8	2.54	-	-	-	13.42	2	14.51	13.39
R20_J160758+542353	22.70	8	2.31	-	-	-	11.48	2	13.80	12.57
C15_J161259+541505	19.06	8	2.54	-	-	-	12.98	6	13.76	13.03
C15_J163739+405643	-	-	-	1.43	5	1	-	2	13.42	12.67
C15_J164010+410521	16.95	8	1.39	1.09	5	6	13.17	6	13.39	12.92
C15_J164018+405812	18.06	8	2.31	-	-	-	13.33	2	14.33	13.30

We also find 9 EROs in the survey, defined as $r-K > 6$, all from the $K$-band photometry of Väisänen (2002) and Rigopoulou et al. (2004), and these are listed in Table 7.4.

3 of the objects have photometric redshifts and have the SEDs of elliptical galaxies at $z \sim 1$. Predicted $r-K$ colours as a function of $z$ show that elliptical galaxies have $r-K> 6$ for $1 < z < 4$. In the optical and near infrared the SEDs of dusty starbursts like M82 and Arp220 look very much like ellipticals due to the extinction of the young stellar component. Table 7.4 gives the values of $\log\,(S_{15}/S_r)$ and $\log\,(S_{rad}/S_r)$, which are consistent with all 9 objects being highly extinguished starbursts like Arp220, at $z \sim 1$. However only a small fraction of EROs will fall into this category of highly extinguished starbursts. None of the 17 EROs with $r-K > 6$ in Table 2 of Roche (2002), which are located within a small 81.5 $\square^\prime$ area of N2, are detected by ISO (one is an ELAIS radio source, 163657+410021).

Table 7.4: Extremely Red Objects (EROs) in the ELAIS Band-Merged Catalogue.

ELAIS Name	$r$	$K$	$n_{p,t}$	$z_{ph}$	$\log\,(S_{15}/S_r)$	$\log\,(S_{rad}/S_r)$
ELAISR20_J160721+544757	24.11	18.07	-	-	-	3.60
ELAISC15_J160913+542320	24.29	17.18	-	-	3.48	-
ELAISR20_J161030+540247	23.53	16.40	-	-	-	2.91
ELAISR20_J161046+542329	23.15	17.09	2	1.138	-	2.28
ELAISC15_J163536+404754	23.38	17.34	1	0.995	2.71	-
ELAISR20_J163555+412233	23.29	16.78	1	1.291	2.62	-
ELAISR20_J163723+410526	23.71	17.44	-	-	-	3.33
ELAISC15_J163748+412100	24.50	17.92	-	-	3.14	-
ELAISR20_J163758+411741	23.79	17.41	-	-	-	2.32

Counts of EROs in the N1 and N2 areas, and estimates of their space-density, are given by Väisänen et al. (2004a); Väisänen et al. (2004b). They conclude that the redshifts of their ERO sample lies in the range $z =$ 0.7-1.5 and estimate the fraction of strong starbursts ( $> 30 M_\odot / \mathrm{yr}$) to be $<$ 10%.

We also find a sample of 9 objects characterized by extremely high mid-to-near infrared flux ratio ( $f_\nu(15\,\mu\mathrm{m}) / f_\nu(2.2\,\mu\mathrm{m}) > 25$). Johansson (2004) argue that these Extreme Mid-to-Near Infrared Objects (EMNOs) may belong to a new extragalactic population of obscured AGN and very strong dusty starbursts at $z > 1$ which was not detected in deeper ISO surveys because of their small covered area. If this tentative discovery is confirmed, Spitzer surveys such as SWIRE (see Section 8.4) are bound to uncover large numbers of these objects, which will provide a useful window into the high-redshift obscured nuclear activity and star formation phenomena in a similar way the discovery of EROs has been used to define samples of high-redshift early-type galaxies.