May 9, 1995 - survey results of Stark et al. (1992)). As well as this NH ..... layer in the plane (Koo & McKee 1992), and the lack of any observational evidence ...
0273—1177/93 $6.00 + 0.00
Adv. Space Res. Vol. 13, No. 12, pp. (12)361—(12)364, 1993 Printed in Great Britain. All rights reserved.
Copyright @ 1993 COSPAR
EUV EMISSION FROM NORMAL GALAXIES T. J. Ponman
and A. M. Read
School of Physics and Space Research, University of Birmingham, UK
ABSTRACT Using data from the Wide Field Camera EUV all-sky survey, we have established upper limits to the EUV flux from a sample of 30 bright, nearby, non-active spiral galaxies. These galaxies were chosen to be those most likely to be detected in the EUV on the basis of (i) low interstellar absorption within our own galaxy, (ii) brightness in other wavebands, (iii) high star formation activity, and (iv) proximity. The derived EUV upper limits are restrictive, and establish for the first time that the EUV flux escaping from galaxies does not constitute a major component of their bolometric luminosity, and in particular that it cannot be the sink for the energy injected into the interstellar medium by supernova explosions, as had been suggested following the failure to detect this power in the X-ray band. THE GALAXY SAMPLE The ROSAT Wide Field Camera performed an all-sky survey in the ETJV (~ 70 200 eV) band during the period July 1990 August 1991. A number of extraga.lactic sources were detected in the routine survey processing, but all convincing identifications were with well known AGN, such as Mk421 and PKS2155-304. We have reanalysed the survey data at the positions of a selected sample of non-active spiral galaxies using improved source searching software. The sample studied induded all those normal galaxies most likely to be detected in the ETJV, based on the criteria of: —
—
(a) Low nH The sample is restricted to galaxies with nH< 3 x 1020cm2. (b) Infrared brightness All non-active galaxies with a 100pm flux of> 64 Jy (from the IRAS bright galaxy sample /1/) and nH< 3 x 1020cm2 were included. Many of these are starbur.st galaxies. —
—
(c) X-ray brightness All galaxies with X-ray flux (0.2 Einstein Observatory observations /2/ are included. —
—
4.0 keV)
>
4.5 x 10’~ergcm2s1 from
(d) High supernova fluxes We added to the sample a number of galaxies which did not qualify as JR or X-ray bright, but had high values of ‘supernova flux’; the latter being calculated from the predicted supernova rate (see below) and the galaxy’s distance. These additions are nearby ordinary galaxies. —
EUV UPPER LIMITS The standard WFC survey analysis implicity assumed that sources were unresolved. Since the 50% enclosed energy radius of the survey PSF is only 2’, it can be seen from the galaxy sizes in Table 1 that this is not a good assumption. Hence the sensitivity of the search was tuned to the expected galaxy diameters by blurring the nominal instrument PSF with a Gaussian similar in size to the optical image for each. No significant emission was detected from any of the galaxies, so 90% confidence upper limits to the count rate of each galaxy were derived at the position given in the Second Reference Catalogue of Bright Galaxies /3/. Conversion of the count upper limits into flunes and hence to luminosities, requires the adoption of (12)361
(12)362
1. J. Ponman and A. M. Read
(b)
(a) ~
I
I
I
~-I—
T=2x1O5K.
0
0
Log Leuv/Lsn
~
~
~
~ I
T=5x1O5K
-
Log Leuv/Lsn
Fig.1 Histogram of the ratio between the EUV upper limit and predicted supernova luminosity for the galaxy sample, for emission from a hot (T = 2x105 K and 5x105 K) corona. The distribution for the starbursts within the sample is shown dashed. specific spectral models. A hot optically thin plasma is the most likely source of any diffuse EUV emission from an external galaxy, so two Raymond and Smith models (with solar metallicity) at temperatures of 2 x 105K and 5 x 105K were assumed. The flux from these models is subject to absorption in our own galaxy before reaching us, and also possibly in the host galaxy, depending on whether the emission originates from the galactic plane or from a hot galactic corona. The column in the host galaxy was estimated from its far-IR luminosity using the relations of Martin et al. /4/ corrected for galactic inclination. MULTIWAVELENGTH LUMINOSITIES In addition to the EUV upper limits (which apply to the band 80-200 eV), optical (B) luminosities were taken from Tully /5/, and FIR luminosities calculated from IRAS 60 and 100 pm fluxes /1/ using the expression (/6/) LFIR
=
3.65 X 10~[2.58S~j~jm(Jy) +
(Jy)] D2 (Mpc) L®
~
X-ray luminosities were taken from /2/, and scaled to H 0
=
75 km s~ Mpc~.
Supernova luminosities were calculated using the supernova rates (types 1 and 2 combined) for each galaxy type given in /7/. The suggestion of these authors that the rates should be simply scaled with blue luminosity is unsatisfactory for starburst galaxies, in which optical emission from the active star-forming regions is subject to substantial obscuration by dust. As a result, LB underestimates the star formation rate, and hence the supernova rate. We therefore distinguish between normal and starburst galaxies (here defined as those with LFIR/LB > 0.38). Supernova rates for the former were obtained by scaling with LB, whilst for the starbursts we scaled using an estimate of that component of LFIR associated with star formation (as distinct from cirrus emission). This estimate was derived using the method of Devereux & Eales /6/. Finally the supernova rate for each galaxy was converted into a luminosity, assuming an energy release of 1051 ergs per supernova. Typically LSN 1042 erg s~for galaxies in our sample.
EUV Emission from NormalGalaxies
(12)363
(b)
(a) NGC 5457
(C) I
I
I
I
I
I
I
‘cl—
I
‘1
Opt
-
FIR
cn’cJ-
‘cl-
NGC 253
I
I
-
+
+
FIR
Opt
+
+
-
LSN
I
LSW N >~F
N >~
-
U)
-
(I)
o
EUVi
CO
+
-
-
X—ray -
-
cr0
0 ~ C
-
oro
Radio (0
I
I
I
10
I
I
I
I
15
I
I
I~_~jL~
20
Log Frequency (Hz)
-
+
EU
~0
X—ray
-
-
+
Radio I
_L__1_I
10
I
I
15
I
I
I
20
Log Frequency (Hz)
Fig.2 Broadband spectra for (a) NGC 5457 and (b) NGC 253. The EUV points shown are upper limits for the flux escaping from the galaxy both with and without internal absorption in the host. The estimated supernova luminosity is shown for comparison in each case. RESULTS AND IMPLICATIONS Coronal Emission The existence of ‘superbubbles’ within the planes of spiral galaxies, containing hot, low density gas is now well established observationally. However, it is not yet clear how frequently such bubbles are able to break out of the plane and inject hot gas into a galactic corona. Such a corona would act as a sink for the substantial supernova luminosity, and would also account for the existence of high ionisation ions (such as N V) observed at high z, for high velocity clouds above the plane, and for at least part of the soft X-ray background. X-ray observations of certain nearby galaxies with Einstein /8,9/, have already demonstrated that in these cases the X-ray luminosity is not sufficient to account for the supernova energy. Comparison of L~and LSN shows that this is true for all galaxies in the present sample with measured X-ray fluxes. However, the Einstein bandpass is not sensitive to gas with T < 6x i0~K, which leaves open the possibility of a rather cooler corona. Investigation of this possibility has awaited the launch of an EUV telescope. For the case of coronal emission, no absorption in the host galaxy need be included. The ratio of our EUV upper limits to the supernova fluxes is shown graphically for this case in Figure 1. The EUV limit is at least an order of magnitude below the supernova luminosity in the case of several galaxies, many of which are starbursts. The Broadband Spectra of Galaxies The WFC results establish that for nearby, non-active galaxies (both normal and starburst) the EUV emission is not a dominant component in the emitted radiation. In the cases with best WFC statistics we find that LEUV ~Lx. In Figure 2 we show the broadband spectra for two galaxies for which our upper limits are parJASR
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(12)364
T. I. Ponman and A. M. Read
ticulary tight. NGC 5457 is a fairly normal galaxy viewed face-on, whilst NGC 253 is a well known starburst galaxy. In both cases is is evident that the energy budget of escaping radiation is dominated by the optical and FIR emission. The Supernova Energy Sink Having discounted the idea that the supernova energy is dissipated in the EUV, it seems there are only two remaining possibilities. Either (c.f. Figure 2) it is radiated in the optical or FIR, or it is not radiated at all, but is carried away in the kinetic energy of a galactic wind. The latter seems most unlikely for non-starburst galaxies, given the large numbers of correlated supernovae believed to be required to break out of the gas layer in the plane /10/. For these galaxies, the most likely possibility appears to be that the bulk of the supernova energy is radiated in the EUV, but is absorbed locally and degraded to lower energies, where it is hidden in the large IR-optical-UV flux, and will also make some contribution in the FIR through irradiation of dust. Allowing for a uniform absorbing layer within the host galaxy, we find that if all the supernova flux were emitted in the EUV from close to the disc plane, it could easily be attenuated to a point where it is consistent with our non-detections. However this simple calculation ignores the inhomogeneity of the neutral gas. In the first place the use of a single disc-averaged n~will give a conservative estimate of the opacity to EUV emission. For example, most galaxies show a deficiency of HI in their inner regions, yet this is where star formation commonly peaks. Secondly, the HI distribution in the discs of nearby galaxies is punctuated by ‘HI holes’, which can cover 10-20% of the disc even in normal galaxies / 11/. If, as is commonly supposed, these represent regions from which HI has been cleared by supernova bubbles, then they should coincide with the sites of much of the supernova-related EUV and X-ray emission. It then becomes quite difficult to see how, in a face-on galaxy such as NGC 5457, both the X-ray and EUV luminosities can lie two orders of magnitude below LSN (Figure 2a). One possibility is that the late development of supernova remnants differs from what has been commonly supposed. For example, Slavin & Cox /12/, taking full account of the effects of magnetic pressure, find that the hot central bubble is surrounded at late times by a thick shell of partially ionised gas. Such a structure would certainly reduce the escape of EUV radiation. REFERENCES 1. Soifer, B.T., Boehmer, L. & Neugebauer, G. 1989, Astron.J., 98, 766 2. Fabbiano, G., Kim, D.W. & Trinchieri, G., 1992, Ap.J.Suppl, 80, 531 3. de Vaucouleurs, G., de Vaucouleurs, A. & Corwin, H.G., Jr., 1976, Second Reference Catalogue of Bright Galaxies, (University of Texas Press) 4. Martin, J.M., Bottinelli, L., Dennefeld, M. & Gouguenheim, L., 1991, Astron.Astrophys., 245, 393 5. Tully, R.B., 1988, Nearby Galaxies Catalog, (Cambridge) 6. Devereux, N.A. & Eales, S.A., 1989, Ap.J, 340, 708 7. van den Bergh, S. & Tammann, G.A. 1991, Ann.Rev.Astron.Astrophy8., 29, 363 8. Bregman, J.N. & Glassgold, A.E., 1982, Ap.J., 263, 564 9. McCa.mmon, D., & Sanders, W.T., 1984, Ap.J., 287, 167 10. Koo, B. & McKee, C.F., 1992, Ap.J., 388, 93 11. Heiles, C., 1990, Ap.J., 354, 483 12. Slavin, J.D. & Cox, D.P., 1992, Ap.J., 392, 131
