![]() | Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved |
4.1.2. Nature of ISOCAM Sources: Luminous Star-Forming Galaxies at z ~ 0.7
At the time of writing, the properties of the ISOCAM
galaxies at other wavelength bands have been studied in detail in two
fields: HDF (N)
(Mann et al 1997,
Rowan-Robinson et al
1997,
Aussel et al 1999a)
and the 1415+52 field of the Canada-France Redshift Survey (CFRS;
Flores et al 1999a,
b). In HDF(N),
Aussel et al (1999a,
b)
have extracted a list of 38 galaxies
( 100 µJy, 99%
confidence) with optical identifications in the catalog of
Barger et al (1999).
Of these, 26 galaxies have known redshifts. In the (10')2
CFRS 1415+52 field,
Flores et al (1999b)
detected 78 significant
(
3
) 15 µm
sources
(
250 µJy),
22 of which have spectra and redshifts. The median redshift in both fields
is near 0.7, and most redshifts range between 0.4 and 1.3. The ISOCAM
galaxies
have the optical colors of Sbc-Scd galaxies. The majority of the galaxies
are disk or interacting systems, and the remainder are irregulars and E/S0s.
The one elliptical in the HDF(N) sample is associated with an AGN (z =
0.96).
Typical absolute K-band magnitudes HDF(N) range between -22 and -25,
similar to, but on average somewhat fainter than, an
L* galaxy [M(K) = -24].
Thus, the ISOCAM galaxies are mainly luminous disk/interacting galaxies and
are definitely not part of the faint blue galaxy population responsible for
the excess in faint B-band counts
(Ellis 1997).
The local luminosity function at the mid-IR luminosities characteristic
of the ISOCAM galaxies (~ 5 × 1010
L)
is dominated by AGNs
(Fang et al 1998).
However, the rest-wavelength B-band spectroscopy in both the HDF(N) and
CFRS fields
(Flores et al 1999b,
Aussel et al 1999b)
and the rest-wavelength R-band observations of the HDF(S) field
(Rigopoulou et al
1999b)
suggest that most of the ISOCAM galaxies are dominated by star
formation, with no more than ~ 1/3 of them AGNs. The majority of the
galaxies in the CFRS field (15% or 70%) have e(a) optical spectra
10
characteristic of post-starburst systems (age
several hundred Myrs), or active starburst galaxies with large differential
intrinsic dust extinction hiding the active burst, with some starburst
activity
prior to a few hundred million years ago and at the same time, the latter
explanation is almost certainly the correct one
(Aussel et al 1999b,
Rigopoulou et al
1999b).
For 19 galaxies with complete radio to UV SEDs,
Flores et al (1999b)
identified more than half as (highly reddened) starbursts. Dusty
starburst galaxies such as M82
(Kennicutt 1992),
many bright ULIRGs
(Liu and Kennicutt
1995),
and 50% of the (U)LIRG sample studied by
Wu et al (1998)
have e(a) spectra in the B-band. Local e(a) galaxies have large
H
equivalent widths, at
the same time demonstrating active current star formation and
differential dust extinction.
Rigopoulou et al
(1999b)
have recently carried out near-IR, VLT-ISAAC spectroscopy of a sample of
0.6
z
1.3 ISOCAM galaxies in the
HDF(S) field. The ISOCAM HDF(S) galaxies have large (50-100 Å)
H
equivalent widths, which provides compelling evidence that most of them are
active starbursts. The simultaneous presence of heavily dust- enshrouded
present star formation and less extinct older star forming activity probably
indicates several starburst episodes
(Sections 3.2.1,
3.4.4).
The infrared derived star formation rates are substantially greater than
those determined from the [OII] (or
H) lines. On the basis
of the SEDs, Flores et al find that the median 8-1000 µm
luminosity of the CFRS sample is ~ 3 × 1011
L
(star
formation rate ~ 30 to 50
M
yr-1). The HDF(N) results are similar. For HDF(s)
Rigopoulou et al 2000
and Franceschini 2000 (private communication) deduced star formation
rates of a few tens of
M
yr-1 from the
H
emission but typically three times greater values from the mid-IR data. Most
of the faint ISOCAM galaxies thus appear to be LIRGs. A smaller fraction
(~ 25%) of the CFRS sources have ULIRG-like luminosities (1012
L
), in
agreement with the work of
Rowan-Robinson et al
(1997)
for HDF(N). Other confirmations of the identification of the faint ISOCAM
galaxies as luminous starbursts come from observations of several galaxies
near z ~ 1 that are lensed by foreground clusters
(Lemonon et al 1998,
Barvainis et al 1999).
All e(a) galaxies in the z = 0.2 cluster A1689 are 15 µm
ISOCAM sources
(Duc et al 2000).
Of the k+A and e(a) galaxies in Coma, only those with emission lines have
excess mid-IR emission indicative of active star formation
(Quillen et al 1999).
In the LW2 filter the situation is similar, but the relative number of AGNs and ellipticals is proportionally larger. Flores et al (1999a) identified 40% of the 15 CFRS/6.75 µm galaxies with spectra as AGNs, and 53% as active starbursts or S+A galaxies. Aussel et al (1999b) classified 4 of the 6 confidently detected 6.75 µm sources in HDF(N) as elliptical galaxies.
For the great majority of ISOCAM sources, the far-IR counterparts are not yet known. Still, template spectra and the overall background constraint can be used to draw first-order conclusions (Figure 15). If all the ISOCAM sources had ULIRG spectra as cool as Arp 220 [L(80 µm) / L(8 µm) ~ 70 11], they would significantly overproduce the far-IR/submm background and would give rise to counts at 850/175 µm, well in excess of the observations (Aussel et al 1999b, Elbaz et al 2000). Thus, on average, they must have SEDs at least as warm as LIRGs [like M82 or NGC3256: L(80 µm) / L(8 µm) ~ 5..12], in which case they still produce 30-60% of the far-IR background at 140 µm (Figure 15). A similar contribution of the ISOCAM sources to the background would be expected if they were predominantly dust-enshrouded AGNs (Figure 15). This interpretation is not supported by the HDF(N,S) and CFRS samples, however.
![]() |
Figure 15. Cosmic UV to mm, extragalactic
background. Open squares give the lower limits from ISOCAM 15
µm, ISOPHOT 175 µm, and SCUBA 850
µm
(Blain et al 1999)
sources. The optical-UV points are from
Pozzetti et al
(1998).
The COBE FIRAS (grey shaded) and DIRBE 140/240 µm
(filled circles) data are from Lagache et al (1999) (from
Elbaz et al 2000).
Different SEDs are shown and normalized to the 15 µm ISOCAM
limit: M82 (continuous line), Arp 220 (long dashes).
An Arp 220-like SED would significantly overproduce the COBE background.
With an M82-like background, the ISOCAM galaxies would contribute about 30%
to the COBE background. The SED of a typical Seyfert 2 galaxy (and matching
the COBE background) is shown as a dash-dotted line. The
hatched upper limits in the mid-IR are derived from the lack of
attenuation of high-energy
|
10 'e(a)'
(Poggianti and Wu 1999)
or 'S+A'
(Hammer et al. 1997)
galaxies have moderate EW ([OII])
line emission and at the same time strong Balmer
(H etc.) absorption or
(at low spectral reolution) a
large 3550-3850 Å Balmer continuum break, characteristic either of
A-stars or very large extinction. In
the A-star model the characteristic age of the (optically visible) star
formation is ~ 0.5 - 1 Gyrs.
Galaxies with Balmer absorption but no emission lines are called 'E+A',
or 'k+A'.
Back.
11 Arp 220 has an extremely cool SED. Average ULIRGs have L(80 µm) / L(8 µm) ~ 25 which would still exceed the far-IR/submm background but only marginally so, yet leaving no space for a higher z population. Back.