IRAS additional observations were used to obtain a sample of point sources at much fainter flux levels than hitherto available through the IRAS Point Source Catalogue. This sample is being used to compile an incomplete but representative catalogue of faint IRAS candidate Active Galactic Nuclei (AGNs) and to study the evolution of the infrared bright galaxies. Ground based follow up observations (optical spectroscopy) are mainly hampered by identification confusion

Degrijp, Marinus H. K.

Keel, William C.

Miley, George K.

English

NASA Technical Reports Server

IRAS OBSERVATIONS OF AGN CANDIDATES AT LOW FLUX LEVELS
N87- 24366
Marinus H.K. de Grijp
William C. Keel
Sterrewacht Leiden, Postbus 9513 RA LEIDEN, The Netherlands
George K. Miley
Space Telescope Science Institute, Baltimore MD 21218, U.S.A.
Affiliated to the Astrophysics Divisions, Space Science Department of
ESA; on leave from Sterrewacht Leiden.
ABSTRACT. IRAS Additional Observations have been used to obtain a sample of
point sources at much fainter flux levels than hitherto available through the
IRAS Point Source Catalog (hereafter PSC). This sample is being used to compile
an incomplete but representative catalog of faint IRAS candidate AGN's and to
study the evolution of the infrared bright galaxies. Ground based follow up
observations (optical spectroscopy) are mainly hampered by identification
confusion.
I. INTRODUCTION
We have previously shown that IRAS colours are a remarkably efficient
predictor of nuclear Seyfert activity (de Grijp et al. 1985). By investigating
the properties of sources in the IRAS Point Source Catalog (PSC) which have
similar "warm" IR spectra to those of known AGN's, we compiled a catalog of 563
IRAS AGN candidates. Optical spectroscopy has now been carried out on several
hundred of these objects, of which 69% have Seyfert spectra, most of them
previously unknown.
It is clearly of great cosmological interest to extend this study to lower
flux levels. We have begun to do this by examining serendipitous sources in the
IRAS Additional Observations (AOs). These AOs reach sensitivities of between -5
and -15 deeper than the PSC. The median redshift of 0.03, and highest of 0.9
obtained for the "strong" IR Seyferts from the PSC, extrapolate to redshifts of
0.12 and 2.0 respectively for intrinsically similar objects observed to be a
factor 15 fainter. It is hoped that this will bring us into realms where
cosmological evolution is important.
2. AO-GRIDS AND DATA REDUCTION
To extend our survey to weaker sources we considered all AO observations
carried out in the "DPS" observational mode. Each of these observations was made
using a raster scan resulting in a map of a region typically 0.5 by 1.5 degrees
around an object for which deep IRAS observations were deemed desirable. In
addition to the source under study, each of these fields usually contained as
"bonus" a fe_serend_itous sources. It is these sources in which we are
interested here.
Many fields were observed more than once; overlapping grids were added to
Carol J. Lonsdale Persson (Editor)
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M. H. K. DE GRIJP AND G. K. MILEY
obtain maximum signal/noise. Up to 11 grids of the same field were added; at
this level the maps started to become confusion limited. The standard IRAS
source extraction program (which was originally derived from the Leiden-
Westerbork image processing system) was used on the IBM 3700 computer at Leiden
to extract point-source data from the 4 bands and a band merging algorithm
developed by D. Gregorich was used to compile a list of sources observed in at
least 2 of the 3 longest wavelength IRAS bands (25, 60 and 100 microns).These
longer wavelengths are most sensitive to galaxies.
Data covering 1040 fields were considered in this study. These fields were
distributed throughout the sky and comprised all available data except for
fields at low galactic latitude (lbl<20 °) where contamination by galactic
sources is significant. The outer 2 arcminutes of each field was omitted from
consideration because of edge effects. To minimize problems with cirrus, grids
containing more than 20 sources at 100 microns were ignored.
All sources that passed the above selection criteria were flux-calibrated
as described by Young et al. (1986) and manually checked for confusion and
reliability; cases where the source extraction algorithm was confused by nearby
sources or cirrus were omitted. Figure 2c shows a colour-colour distribution for
the 900 resultant sources detected in all 3 bands.
3. COMPARISON OF PSC AND AO SOURCES
Whereas the PSC encompasses -95% of the sky above 20 ° galactic
latitude (-27000 square degrees), the AO grids have a total extent of -800
square degrees. However, because the AO grids are on average -10 times deeper
than the PSC (fig. I), both datasets include a comparable volume of space.
800
•_ 600
400
v
o
Fig I: Noise statistics of A0 grids
''''"l ' ' ''''"[
25 micron bv
f--
J
=1
I
I
I
r- I
i0 -2
' ' '"'I
ad
' ' '"'I
m
D
D
' ' '''"'I ........ | I
60 micron ban_t
j__--, I
I
I
I
-)
, I ,,,,,,I , ,,,,, , , ,,,,,,I , , I,,,,,I ,
10 -1 I 10-2 10 -I I
noise level (Jy) noise level (Jy)
''""1 ' ' ''""] ' ' '''"
100 micron band
E
I0 -_ I0 -I I
noise level (Jy)
I '
Figure I: Each AO grid has different noise levels and size. This figure gives an
overview of the total area covered at different noise levels. The
lowest noise figures are reached for grids that are averages of up to
11 originals grids.
The colour-colour distribution (fig. 2a and 2c) of PSC and AO sources is
virtually identical (see also next section); the major difference between them
is an excess at _(25,60)=-4, _ (60,100)= -3. These very cold sources are
probably due to cirrus.
' 694
IRASOBSERVATIONSATLOWFLUXLEVELS
2
go
-2
-4
Fig 2: Colour - colour distribution of sources
Point Source Catalog Vorcn-Cetty & Veron AGNs Paint Sou
, I , , , I , , , I , , , I , , I _ I I I ,/, , I , , , I _ _ Ii i i i i , , , i i , i i i
-4 -2 0 2 4 -4 -2 0 2 4 -4 -2 0 2 4
ALIA(25,60) AI,FA(25,60) ALFA(25,60)
Figure 2: Source density distributions in the 25 - 60 - 100 micron colour-colour
diagram. This figure shows that AGN's have flatter spectra than normal
galaxies, and that faint source counts are similar to counts for
bright sources. A contour representation of binned data was chosen
rather than a scatter diagram to assist in a quantitative analysis of
source counts. The source density units are sources per square degree
per unit colour bin; the spectral index a is defined as
S_ _ va. The counts in fig. 2c are weighted with a factor (flux limit
-15
P.S.C./Flux limit AO) " ; so that if the counts are from a non-
evolving population, they should be the same. Furthermore, the source
counts are corrected to a uniform sensitivity at 60 microns; the other
bands do not limit the sensitivity in this diagram.
a - Point Source Catalog sources above Ibl=20 degrees; stars are
concentrated at a(25,60)=2.0, a(60,I00)=2.0 (Ray!eigh-Jeans
limit); galaxies cluster around a(25,60)=-2.5, a(60,I00)=-1.5.
The contour levels are linear increments of 0.02.
b - Point Source Catalog sources that were identified with AGN's
(Veron-Cetty & Veron, 1985). Note that they extend to much flatter
spectra than normal galaxies. Contour levels are linear increments
of 0.001.
c - AO sources. The similarity with fig. 2a is obvious; differences
are a number of stars with IR excess (probably due to mass loss),
and an excess of very cold sources, which is probably due to
cirrus. Contour levels are as in fig. 2a.
The galaxy counts in fig. 2a and 2c are consistent with a non-evolving
population with N _ S-1"5
4. SOURCE COUNTS; EVOLUTION
A detailed examination of fig. 2 can be used to assess the amount of
evolution as revealed in the source counts. As shall be discussed in the next
section, redshifts for most of the sources under consideration are so low that
K-corrections are still unimportant. Therefore it is possible to directly
compare figures 2a and 2c: a spot in the colour-colour diagrams refers to sources
with the same intrinsic spectral shape, but at different flux levels. Overlaying
these figures reveals no significant differences between them other than an
695
M.H.K. DE GRIJP AND G. K. MILEY
excess of very cold AO sources discussed in section 3. This comparison is not
[]eaningful for stars, of course, since at the galactic latitudes concerned
distant stars are not as nu[]erous.
As a whole these source counts do not show evidence of evolution. In the
next section we will investigate the redshifts probed, and thus the level up to
which evolution does not see[] to play an important role.
5. OPTICAL FOLLOWUP
Our original AGN detection criterion imple[]ented on the PSC was based on
the spectral indices between 25 and 60 []icrons of -1.5<a(25,60)<0.0 (de Grijp
and _iley 1986). Fro[] the AOs we have co[]piled a source list of []ore than 100
sources which have the sa[]e colours and we are si[]ultaneously carrying out a
progra[] of follow up optical spectroscopy.
Spectra for a nu[]ber of the discussed sources have been obtained at the
European Southern Observatory (3.6 [] telescope + EFOSC) and at Roque de los
Muchachos Observatory (2.5-m Isaac Newton Telescope + faint-object
spectrograph).
Fig 3: Spectrum of Seyfert 1 galaxy IRAS0622-645, ESO 3.6M + EFOSC
2500 , , , ,
E._
Z
0
L_
2000
1500
1000
5OO
REDSHIFT = 0.129
i i
4000 5000
WAVELENGTH (ANGSTROM)
0 I , , , , I
6000 7000
Figure 3: Spectru[] of a previously unknown Seyfert I galaxy obtained at ESO.
This elliptical galaxy was identified by taking a slitless spectrogra[]
for a 2 x 3 arcmin field around the IRAS position. This e[]ission line
object was less than 20" fro[] the IRAS position.
696
IRASOBSERVATIONSATLOWFLUXLEVELS
Uncertainties in the IRAS positions have so far proved to be the major
difficulty in identifying optical counterparts. Since experience with the PSC
showed that 90% of all objects with -1.5<a(25,60)<0.0 are emission-line objects,
this can in principle be used as a secondary identification criterion. Slitless
spectroscopy with EFOSC proved to be a boon in this respect.
Although the sample observed is by no means statistically complete, the
redshifts found are on average 3 times higher than for objects in the PSC, and
AGNs cluster in the same part of the colour-colour diagram. As 20% of them have
redshifts above z=0.2, careful selection of the sources in the deepest grids may
yield a luminosity function for Seyferts at redshifts of -0.3. Also a number of
sources at redshifts of I to 2 can be expected.
We conclude that although uncertainties in the IRAS positions is a major
limitation in identifying optical counterparts, high redshift objects obtained
from the AO data may prove to be important for investigating cosmological
evolution of AGNs.
6. ACKNOWLEDGEMENTS
The optical observations discussed here were carried out at the ESO
observatory in Chile and at Roque de los Muchachos observatory on La Palma. We
thank their staffs for support at the telescope. We thank H. van der Laan for
useful discussions.
7 • REFERENCES
de Grijp, M.H.K., Miley, G.K., Lub, J. and de Jong, T. 1985, Nature, 314, 240
IRAS Catalogs and Atlases, Explanatory Supplement, 1985 ed. Beichman, C.A.,
Neugebauer, G., Habing, H.J., Clegg, P.E. and Chester, T.J. (Washington DC:
US Government Printing Office)
IRAS Point Source Catalog, 1985, Joint IRAS Science Working Group. (Washington
DC: US Government Printing Office)
de Jong, T., Clegg, P.E., Soifer, B.T., Rowan Robinson, M., Habing, H.J., Houck,
J.R., Aumann, H.H. and Raimond, E. 1984, Ap. J. (Letters) 278, L67
Miley, G.K., Neugebauer, G. and Soifer, B.T. 1985, Ap. J. (Letters) 293, L11
Neugebauer, G. et al. 1984, Ap. J. (Letters), 278, LI
Veron-Cetty, M.P. and Veron, P. 1985, A Catalog of Quasars and Active Nuclei
(Munich: European Southern Observatory), Scientific Report No.4.
Young, E.T., Neugebauer, G., Kopan, E.L., Benson, R.D., Conrow, T.P., Rice, W.L.
and Gregorich, D.T. 1986, A User's Guide to IRAS Pointed Observation
Products (Washington DC: US Government Printing Office)
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M.H.K. DE GRI JP AND G. K. MILEY
8. DISCUSSION
MALKAN: To find a reasonable number of AGN's you adopt quite restrictive IRAS
colour criteria. Our work on an unbiased sample of Seyfert galaxies shows that
you exclude about 80% of all AGN's. How, then, is it possible for you to
construct a meaningful AGN luminosity function?
DE GRIJP:You are correct that we can only construct a luminosity function for
AGNs whose IR spectra are consistent with our colour-criteria. This is about 70%
of all Seyfert-like AGNs. Comparison with optical/UV-selected samples of AGN's
(Neugebauer et al. 19_4, Miley et al. 1985) shows that we miss 25-30%, mainly at
the steep spectrum side of the adopted colour range. Presumably these steep
spectrum AGN's are dominated by cold disk emission, in other words their nuclei
are relatively faint. So we expect that a luminosity function constructed on the
basis of our colour criterion might be underestimated at the low luminosity end,
but essentially complete at high luminosities.
WINDHORST: You mentioned in your talk that your AO source counts do not support
(cosmological) evolution. Hacking yesterday claimed from somewhat deeper 60
micron counts evidence for evolution. Do your and his counts agree in amplitude
and slope?
DE GRIJP: I have not yet been able to compare Hacking's figures with mine in
detail. From what I understood about his talk, it is not the source counts as
such that are high, but the prediction of the number of faint sources that is
low. This is because for his model Hacking used a luminosity function of Rowan
Robinson that is low by a factor of 2 (Weedman, private communication) as
compared to more recent results. (Editor's note: Hacking in fact used the
local luminosity function of Soifer et al. (these proceedings)).
WINDHORST: What fraction of your 60 micron galaxies is at cosmological distances
(z>>0.1)? Namely, if this fraction is considerable, even a Euclidean slope of
-1.5 would be indicative of a non-uniform source distribution in a relativistic
universe.
DE GRIJP: Some 20% have redshifts over 0.2, and a (small) number should have
redshifts over 1.0 (based on extrapolation from the Point Source Catalog). So
although there is a fraction of sources at high redshifts, I don't think that
source counts alone will be enough to make a hard claim of evolution because of
the low numbers involved.
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IRAS observations of AGN candidates at low flux levels

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