Emission is studied at luminosities lower than those for which individual discrete sources can be studied. It is shown that normal galaxies do not appear to provide the numerous low luminosity X-ray sources which could make up the 2-60 keV diffuse background. Indeed, upper limits suggest luminosities comparable with, or a little less than, that of the galaxy. This is consistent with the fact that the average optical luminosity of the sample galaxies within approximately 20 Mpc is slightly lower than that of the galaxy. An upper limit of approximately 1% of the diffuse background from such sources is derived

Boldt, E. A.

Marshall, F. E.

Worrall, D. M.

English

The hypothesis that normal galaxies are on the average more luminous in the X-ray region than the Milky Way galaxy or M31 and therefore are possible candidates for the low-luminosity sources of the 2 to 60 keV extragalactic diffuse background is tested. Data from the A-2 detectors on the HEAO-1 spacecraft were examined for emission from positions coincident with 76 selected normal galaxies, and upper limits to the average galactic luminosity for various luminosity distributions resulting in the observed count rate distribution were determined. For uniform and exponential galactic luminosity distributions, limits of 2.7 x 10 to the 38th erg/sec and 3.4 x 10 to the 38th erg/sec, respectively, at the 90% confidence level were obtained. It is shown that the Hubble-constant-independent upper limit to galactic emissivity is less than 1% of the diffuse background emissivity, indicating that normal galaxies are not responsible for the diffuse X-ray background and have luminosities comparable to that of the Galaxy

Explore Bristol Research

A limit to the X-ray luminosity of nearby normal galaxies

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19790022943 2020-03-11T17:55:22+00:00Z
NASA
Technical Memorandum 80321
A Limit to the X-Ray
Luminosity of Nearby
Normal Galaxies
D. M. Worrall, F. E. Marshall,
and E. ,A. Boldt
iNASA-TM-80321)	 A L:`.IIT TO THE X-RAY
	
N79-31114
LUMINOSITY OF NEARBY NOFMAL GALAXIES (NASA)
11 p HC A02/MF A01	 CSCL 03B
Unclas
G3/90 36071
JULY 1979
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 207T1
.rc
Ai i
A LIMIT TO THE X-RAY LUMINOSITY OF NEARBY NORMAL GALAXIES
D.M. WORRALL, F.E. MARSHALL, and E.A. BOLDT
Laboratory for High Energy Astrophysics
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
The origin of the extragalactic diffuse X-ray background is not yet
well understood. It is isotropic to the extent that a Galactic origin can be
excluded. Indeed, even a halo of radius as large as 12 kpc could provide no
more than 37o of the flux (ref. 1). The spectrum between 3 and 50 keV fits a 40
keV thermal bremsstrahlung form which is suggestive of emission from a hot
intergalactic medium (ref. 2). This ov some other truly diffuse mechanism may
be responsible. If, however, discrete sources play a major role, we require
either strong evolution of an already recognized X-ray emitting class, such as
quasars, or a hitherto elusive population of low luminosity emitter. In the
following we address the latter.
Without invoking evolution, the largest background contributions are
thought to derive from Seyfert galaxies and clusters of galaxies. Based on
estimated local densities, the total background can not be accounted for (refs.
2 and 3). Consistency with fluctuation measurements of the diffuse background
is only achieved if members of the additional non-evolved class of source,
which would be required to com plete the background, are sufficiently numerous
and hence of low luminosity. Compatibility requires a volume density ti (8 x
10 3 h3 ) Mpc
-3
 (ref. 3), where h is the present Hubble constant in units of
100 km s -1 Mpc -1 . The diffuse background between 2 and 10 keV has an emissivity
ti (4.7 x 1039h) erg s -1 Mpc. -3 , which places an upper limit of . (6 x 1041/h2)
erg s -1 on the source luminosity. (We quote luminosities between 2 and 10 keV
to ease comparison with other work. However, the source and diffuse background
data used here are both from the HEAO-A2 experiment and comparison will be made
0
I	 '
- 2 -
over the entire energy range for which we have response, i.e. 2-60 keV; with
highest efficiency 3-20 keV).
Such weak extragalactic sources do not present a high detection prob-
ability for non-imaging proportional counter experiments. The requirements
for source detection with the A2 detectors on the HEAO 1 spacecraft are dis-
cussed in ref. A. There is dependence on ecliptic latitude and consequent
exposure but a 5a detection threshold is ti 2 x 10-11 erg cm-2
 s-1 (2 - 10
xeV) requiring, for example, a 1040 erg s -1 emitter to be within ti 2 Mpc.
To date the only catalogued galaxies < 10 42 erg s-1 are SMC, LMC, M31,
Cen A and M82. Of these the latter two are radio galaxies but are, probably,
U	 atypical of their class in general. The others are classed as normal galaxies
and their X-ray luminosity is compatible with that of our Galaxy, estimated,
from the summed emissivity of known sources, to be u 2 x 1039
 erg s -1 . Taking
M31 and the Galaxy as standards, normal galaxies are estimated to contribute
only a few percent of the diffuse background (see refs. 3,5-6). However, as the
most numerous extragalactic objectsthey merit further examination in the search
for a class of low luminosity X-ray source.
In this work we report results of a test of the hypothesis that normal
galaxies are On average more luminous than our Galaxy and M31 and therefore
form a candidate class of numerous low luminosity emitters which may fulfill
requirements for contributing substantially to the diffuse background. The
imaging experiments on the Einstein Observatory will measure at energies
below ,,  4 keV where low energy absorption and soft sources will limit com-
parison with the 2-60 keV background. As noted above, the comparison between
source and background fluxes are made over the HEAO-A2 experiment energy
range. But, where 2-10 keV source luminosities are given, a constant
m.
- 3 -
multiplicative factor suitable for a 9 keV thermal bremsstrahlung spectrum
has been used.
Data from the A2 detectors on the HEAD 1 spacecraft (ref. 7) were
investigated for emission from positions coincident with normal galaxies
selected from the Second Reference Catalogue of Bright Galaxies (ref. 8).
Selection required a velocity less than 1100 km s -1 (i.e. D < 20 Mpc for
Ho = 55 km s -1 Mpc -1 ). Positions contaminated by catalogued X-ray sources
were excluded. Since only data from the 1';0
 x 30 field of view detectors
were used, a source free region 3 0 each side of the galaxy along the scan
direction and 50 in the perpendicular direction was demanded. Where galaxies
were clustered the closest member was selected. 76 galaxies satisfied require-
ments. A 250 field of scan angle about the galaxy position, accumulateL
over ti 4 days about the one of maximum exposure, was fit with a fixed source
at the galaxy position and as many other sources as necessary to fit the
remainder of the field. Negative values for the source count rate were
allowed. After correcting for effective exposure for each fixed source a
count rate distribution was found. This was consistent, to within la errors,
with zero. This result implies that we can only determine upper limits to
the average luminosity for various model luminosity distributions.
The simplest model which can be examined is one in which al l galaxies are
assumed to have a standard luminosity, Lh 2 . Then the probability cf observing
the ith member of the 76 galaxies with its measured luminosity, L i h 2 , and
associated standard deviation, a., is P(Lh2 ) _ (? i (2,r)^2 )
-1 
exp(-z i?si /2), where
z i = (L i - L)h21a i . Hence the liklihood function for the measured luminosities
of the complete sample is P s (Lh2 ) oc exp(- (Ei(z?/2))). The values for a i
 were
calculated from the standard deviation of the count rate distribution for
the selected galaxies after this value had been confirmed to be consistent
'Y
Y'
7!
^d
^.ir
- 4 -
with values derived using random sample positions on the sky. The function
Py02 ) is of a Gaussian form. The values of Lh 2 bounding 70% and 90% of
the area are 1.5 x 1038 erg s-1 and 2.7 x 1038 erg s -1 respectively, which
are the upper limits to the standard normal galaxy luminosity at these re-
spective confidence levels. Even for a value of H o of 50 km s -1 Mpc-1 the
results show that the average X-ray luminosity of normal galaxies is ti 1039
erg s-1 0 This is a little lower than the X-ray luminosity of our Galaxy.
However, we note that the average optical luminosity of the chosen galaxies
in the B system is (2 x 109/h2 ) Lo
 and the appropriate value for our Galaxy
with which this should be compared is 11 x 10 9
 L o
 (the mean orientation value
estimated from the Galactic pole value given in ref. 9).
More realistically, the luminosity function of the galaxies can be assumed
to be an exponential, i.e. F(L,Lo ) = (Lo h 2 ) -1 exp(-L/L oh 2 ). The probability of
observing the ith member of the 76 galaxies with its measured luminosity is now
P i (Lo h2 ) = J F(L,Lo ) P si (Lh2 )dL. The product over all values of i gives the
liklihood function for the complete sample. As shown in Fig. 1, it is approximately
exponential. The 70% and 90% upper limits to L o h 2 are now 1.8 x 1038 erg s -1 and
3.4 x 1038 erg s -1 , in reasonable agreement with the simple model.
The method generates a strong distance-dependent weighting factor.
Because the answer is dominated by the closest 10 galaxies, selection effects
may be important. To investigate this we divide the galaxies by distance into
i
3 bands. The number of galaxies per band increases with distance so that the
three calculated luminosity upper limits are roughly equal. We find that they
are ti 3 times those quoted above. Thus we conclude that, to within a factor of
3, the 76 galaxies form a homogeneous class of which the closest 10 are not
particularly anomalous. This is further confirmed in that the mean count
vmw-mew	 .:o .r.^.. ^..	 ^:k' 4* '71 A—_-
- 5 -
r	 rate for these 10 is within la of the mean of the total sample.
Based on these results, we can make an h-independent comparison of
the emissivity of galaxies with that of the diffuse background. The density
of normal galaxies is calculated to be =(0.13 h 3 ) Mpc-3 . This uses a likely
maximum value for the optical luminosity density of galaxies of
(2.6 x 108h) LO Mpc-3 (see critical review by Felten; Ref.10) and the
average luminosity for our sample as given above.
For the detector fields of view we are using (defined as R15 in
ref. 4) and applying the 90% upper limit criterion for the exponential
luminosity distribution model, we find an emissivity of < (2.7 x 1045h)
ct s -1 Mpc -3 . This is to be compared with the diffuse background emissivity
of - ( 3 x 1047h) ct s -1 Mpc-3 . Thus normal galaxies contribute <, 1%
Van Paradijs (ref. 6) has calculated the normal galaxy contribution to
the X-ray background by modelling the X-ray luminosities of galaxy con-
stituents. He estimates 0.5 - 2.8% (2-10 keV) for no evolution, but investi-
gates several ways in which this value may be increased. While we can not
address those involving evolution, our calculations indicate that further
non-evolutionary effects are not important.
In summary, by using a sample of objects we have investigated emission
at luminosities lower than those for which we can study individual discrete
1
	 sources and have shown that normal galaxies do not appear to provide the
_	 numerous low luminosity X-ray sources which could make up the 2-60 keV
diffuse background. Indeed, upper limits suggest luminosities comparable
with, or a little less than, that of the Galaxy. This is consistent with
the fact that the average optical luminosity of the sample galaxies within
ti 20 Mpc is slightly lower than that of the Galaxy. An upper limit of
ti 1%	 of the diffuse background from such sources is derived.
I
- 6 -
We thank J. Swank for discussions. F.E.M is a NAS/ARC Research Associate
and D.M.W. is also of the Department of Physics and Astronomy at the University
of Maryland. The A2 experiment on HEAO 1 is a collaborative effort led by
E. Boldt of GSFC and G. Garmire of CIT with collaborators at GSFC, CIT, JPL
and UCB.
I.`
^y .^^. _ -_
	 __._	
sy	
^^. r. r`i' ^"3 fi t. .^-_.........,,..
REFERENCES
Schwartz, D.A., 1970, Astropnys. J. 162, 439.
2. Marshall,
	
F.E.,	 Boldt,	 E.A.,	 Holt,	 S.S.,	 Miller,	 R.,	 MuShotzky,	 R.,
Rose,	 L.A.,	 Rothschild,	 R,F., and Serlemitsos, P.J.,	 1979, Astrophys. 	 J.,
submitted.
3. Schwartz, D.A., 1978, Proc.	 IAU/Cu'WR Symp., Innsbruck, Austria
4. Marshall,
	
F,E.,	 Boldt,	 E.A.,	 Holt,	 S.S., Mushotzky,	 R.F.,	 Pravdo,
S.H.,	 Rothschild,	 R.E., and Serlemitsos, 	 P.J., 1979, Astrophys. J.
Suppl,	 40,
5. Rowan-Robinson, M., and Fabian, A.C., 	 1975, Mon.	 Not.	 R.	 Astr.	 Soc.,
170,	 199.
6. van Paradijs, J., 1978, Astrophys. 	 J.	 226, 586.
7. Rothschild,	 R.,	 Boldt,	 E.,	 Holt,	 S.,	 Serlemitsos,	 P.,	 Garmire, G.,
Agrawal,	 P.,	 Riegler, G.,	 Bowyer, S., and Lampton, M.,	 1979, Space
Sci.	 Instr.,	 4,	 269.
8. de Vaucouleurs, G., de Vaucouleurs, A., and Corwin, H.G., 1976,
"Second Reference Catalogue of Bright Galaxies", Univ. Texas Press.
9. de Vaucouleurs, G., and Pence,	 W.D.,	 1978, Astronomical	 J.	 83, 1163.
10. Felten, J.E.,	 1977, Astronomical	 J. 82, 861,
I'
I,
_.--	 —
FIGURE CAPTIONS
Figure 1 - The Loh2 probability function resulting from the application
of a luminosity function F(L,L0 ) = ( 1/Lo h 2 ) exp (-L/L oh2 ) to
the sample of 76 normal galaxies. The result fits the indicated
exponential function. The values of Loh2 bounding 70% and 90;0
of the area are shown, i.e. the upper limits to L oh 2 at these
respective confidence levels.
	 uz
)11
TI
J?
-co co ,.., 
N 0 
.s:: 
0 CD 
.-J V l"-I . 
- CO 
-
,.., 
a. lD '0 ~ Q) 
~ 
~ 
-LO -. 
-
, 
N <t 
'" ..c W 
0 0:: C' 
--.J <t '¢ ~ Q) 
- -a.. ~ 0 
<t 0 
C-I 
(]'I t<'> ..c:. W 0 
0:: --.J 
<t 
co N o 


A limit to the X-ray luminosity of nearby normal galaxies

Abstract

Similar works

Full text

Available Versions

Explore Bristol Research

NASA Technical Reports Server