An analysis of absolute spectral energy distributions of interstellar gas for a galaxy (NGC 1275) is presented. Infrared spectra data shows heavy reddening. It is proposed that the interstellar gas may be ionized by shock waves or by nonthermal or stellar radiation. It is suggested, that high velocity, emission-line knots are H2 regions in a Perseus cluster galaxy or intergalactic gas cloud seen in projection against NGC 1275

Oke, J. B.

Shields, G. A.

English

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1l
ThE SPECTr`J-.L ENERGY DISTRIBUTION OF NGC 1275
by
G. A. Shields
Decartment of Astroncmy, University of Texas at Austin
and
J. B. Ok:
F.ale Observatories California Institute of Technology
Carnegie Institution if Washington
Received:
( NAS A-CR- 1430391 THE SPECTRAL ENERGY
	 N75-26939DISTRIBUTION OF NGC 1?75 (Hale
Ob servatories, Pasadena, Calit.)
	 19 p HC$3.25
	 CSCL 03A
	 Uriclas
G3/89 26623
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ABS'ri ACT
An analysis of absolute spectral energy distributions
for NGC 1275 (Per A) covering the wavelength interval
XX3300-10800 is presented. The data are consistent: with
the heavy reddening discovered by Wampler. The Ha intensity
varied by less than 10 percent between the times of Wampler's
earlier measurements and the two occasions of our observations.
The line emitting region has a characteristic density
ti10 4 ' S cm3 , a mass ti10 5 ' SM., and a volume filling factor
ti10 -5 . The gas may be ionized by shock waves or by
nonthermal or stellar radiation.
It is again suggested, in the vein of Minkowski's original
proposal, that the high velocity, emission-line knots
described by Minkowski are H II regions in a Perseus
cluster galaxy or intergalactic gas cloud seen in
projection against NGC 1275.
Y' 1
ti
V
3
I OBSERVATIONS
The nuclear region of NGC 1275 was observed with the
200-inch (5-meter) multichannel spectrometer on the nights
of 1970 November 19-20 and 1974 October 9-10. The
observations gave complete wavelength coverage in the
interval XX3300-10800 with bandpasses of 40 A for ,1 < 5900
and 80 A for a > 5900 A. An aperture diameter of 7" was
used for both observations. The overall energy distributions
are similar on the two dates but the continuum is 0.28 mag
brighter in 1974. The difference is almost certainly real.
Figure 1 shows the absolute spectral energy distribution for
the 1974 observation. The fluxes are based on the absolute
calibration of a Lyrae given by Oke and Schild (1970). A
comparison of the absolute intensities of the stronger
emission lines on the two nights shows no evidence that
they have changed in the interval. A comparison with
Wampler's (1971) data also shows no evidence for changes
in the emission-line strengths. Differences in weaker
line intensities can be attributed to low resolution and
uncertainty in the continuum level. The average of our
emission-line intensities, and those measured by Wampler
(1971) are listed in table 1.
r
i
4II REMARKS
a) Reddening
Wampler (1971) determined the reddening in NGC 1275 by
comparing the [S II] X10320 and X4072 line intensities.
Since our data in the infrared ars not of high quality we
cannot verify the measurements. We have adopted E B_V = 0.63,
which is very close to Wampler's result. In table 1
reddening corrected line intensities relative to H(;, are
listed; they are based on Whitford's (1958) reddening law.
The adopted reddening yields a ratio for Ha/H( of 4.6, if
we adopt the N II/Ha ratio found by Anderson (1970) and
Wampler (1971). This Ha/HR ratio is somewhat larger than
the radiative recovJ ination value of 2.9.
Further hints about the reddening can be obtained by
looking at the infrared measurements. In figure 2 absolute
fluxes (log f v ergs s-1 cm-2 Hz-1) are plotted both without
reddening corrections and then with the adopted (Wampler)
reddening. The infrared measurements are taken from Rieke
and Low (1972) and Penston et al. (1974). In the
unreddened case the overall energy distribution is
relatively smooth and steep although there is a possible
suggestion of an irregularity near log v = 14.0. When
reddening is allowed for this irregularity becomes the
approximate point where the continuum slope shows a,marked
change. The adopted reddening appears reasonable although
by no means proved by this argument.
I
li
F
I -
	
­'M1	 c-•
i
S-,
4
	 5
There appears to be a break in the continuum slope near
log v = 14.9. if the continuum is not reddened (lower curve)
this break would correspond to that seen in normal elliptical
galaxies; it is produced by the H and K lines and other strong
absorption features farther in the violet. If the reddening
found for the emission lines applies (upper curve) the break
is much more likely to be the Balmer jump produced by hot
stars in the field of view.
b) The Helium Abundance
Osterbrock (1971) has suggested a low helium abundance,
N(He)/N(H) = 0.03, for NGC 1275. The absence of [NeV] emission
indicates that the weakness of X4686 may be an ionization effect,
and therefore He I lines such as X5876 are the main helium
abundance indicators. Our data indicate I(X5876)/I(H8) < 0.2.
The strength of [0 I] X6300 betokens a large region of partial
ionization where helium may be neutral. A hypothetical 00,
H* , He  region at 6500° K could contribute the observed [0 I]
emission and two-thirds of the HR but no He I; 	 in this
case the observed upper limit on X5876 would be consistent
with a normal helium abundance. If the line is weaker than the
upper limit quoted, then the helium abundance may indeed be low.
c) Physical Conditions
NGC 1275 has I(X7325)/I(X3727) = 0.12 and I(X4072)/I(X6720)
w 0.8. Each of these ratios is an order of magnitude larger
than expected in the low density limit at ti10 4. K, and this
4 ' 5indicates a characteristic electron density N e 10	 cm 3.
it
rr
6
For this density, the extinction corrected HR luminosity,
L(HO) = 1042.5 ergs s -1 , implies a mass of 10 5 ' 5 MO . If
the emission-line region has a typical radius of ti100 pc,
the emitting filaments have a filling factor ' L10-6 . A
larger mass of lower density gas may exist between the
filaments.
d)	 Ionization
NGC 1275 has, among Seyfert galaxies, a spectrum of
unusually low excitation; He II and [Ne V] are weak, whereas
[01] is strong. For I(a4686)/I(HR) < 0.3, as observed,
a power-law ionizing continuum must have a IS-1.5 (Williams
1972). This is steeper than usually observed in QSO
spectra (Oke et al., 1970) or inferred from the stronger
He II emission of other Seyfert galaxies (Shields 1974; Shields
and Oke 1975), but it is consistent with the observed v-2
continuum uncorrected for reddening. However, the UV extrapolation
of this observed continuum gives les -s than -J_i_:minimum ionizing
photon luminosity, Q(H 0 = 6 x 10 54 s-1 , implied by the
extinction-corrected Hg ltiminosity (Vr
 = 5300 km s-1,
H = 50 km s -1 Mpc-l ; cf. § 11I). On the other hand, a
continuum Iv = 5 x 10 27 (v /1 Ry) -1.5 erg s -1 Hz-1 gives the
required Q(I-I O ); and if it undergoes the same extinction as
the emission lines, this continuum would contribute roughly
half the observed continuum, the rest presumably being
i
_ j
M"
a
7
starlight. This is consistent with Walker's (1968)
detection of optical polarization in 14GC 1275. Alternatively,
an exponential continuum, L  a e-v/vc'^ with v c ^ 4 Ry,
or radiation from hot stars with T rb 1.1 r. 10 5`' K (Hummer
and Mihalas 1970) also would be consistent with
our upper limit for I(X4686).
Another possibility is ionization by shock waves with
v ti 150 km s -1 (Cox 1972) which produce little He +2 , relative
to H* . Shock models agree with the comparatively weak X5007
and strong X6300 emission from NGC 1275 (Osterbrock 1971).
Although shock models give I(X3727)/I(H6) P» 10, it can be
argued that X3727 is collisionally suppressed by about the
right factor to reduce the shock prediction to the observed
value.
Shields and Oke (1975) have argued that time-dependence
in photoionized nebulae can lead to spectra resembling those
of shocks, and this may be relsdant to NGC 1275. In particular,
Jura (1973) suggested that the weakness of X4686 in 3C273
can be understood if He+2 has already recombined since the
latest hypothetical flash of ionizing radiation. This may
be an explanation of the weakness of the He II and [Ne V]
lines of NGC 1275.
Z
8Although the low excitation of the emission-line
spectrum is consistent with ionization by stars, the presence
of a nonthermal radio source in the nucleus supports the idea
of a nonthermal ionization mechanism. NGC 1275 coincides
with the radio source 3C 84, which has a variable nuclear
component with complex structure at VLB resolution (Shaffer
and Schilizzi 1975), a larger component ti5' in diameter, and
a halo ti26' in diameter (Ryle and Windram 1968). Especially
interesting is the apparent interaction between.NGC 1275 and
the neighboring galaxies NGC 1265 and IC 310, !rhich Ryle and
Windram have interpreted in terms of particles ejected from
NGC 1275 radiating in the magnetic fields of the other
galaxies. This brings to mind the possibility that the
emission-line region of NGC 1275 is ionized by fast particles.
Souffrin (1969) showed that for the high ionization stages
observed in typical Seyferts, the required particle flux
would cause excessive Coulomb heating; but for the lower
degree of ionization in NGC 1275, a consistent model using
fast particles may be possible.
III A SECOND GALAXY
The emission-line spectrum of NGC 1275 shows two
distinct velocities. The broad emission lines of the nucleus
have a redshift of 5300 km s -1 (Burbidge and Burbidge 1965);
and presumably this is the measured velocity of the galaxy,
I
I
7
4
x
x	
—	 n
9
since it equals the mean velocity of the Perseus cluster, in
which NGC 1275 is centrally located (Humason, Mayall, and
Sandage 1962). Interfer nce-filter photographs in Iia at
this velocity show a web of luminous filaments around the
nucleus, resembling the Crab nebula (Lynds 1970). Minkowski
'(1957) discovered Ha emission knots NW of the nucleus with
a velocity of 8200 km s-1 , and he proposed that the system
consists of two galaxies in collision. however, Burbidge and
Burbidge (1965) argued for explosive ejection of the filaments
from the nucleus.
Recei,t radio observations of Per A by De Young, Roberts,
and Saslaw (1973) revealed a 21-cm absorption line at 8100
km s -1 , with a half-width of only ti6 km s -1 . De Young et al.
suggested two models to explain the coincidence in velocity
between the radio feature and the high-velocity Minkowski
knots: (1) Gas has been ejected from the nucleus and
closely regulated to a relative velocity of rv2900 km s-1 by
radiation-pressure.line-locking or by a "nozzle" effect.
This material is behind the optical continuum source but
still inside the nuclear radio source. (2) There is a second
galaxy on the line of sight to NGC 1275. (A relative velocity
of 2900 km s -1 is consistent with the extremes of the
velocity distribution of the Perseus cluster.)
We note two arguments against the first hypothesis.
(1) If gas were ejected at a single magic velocity of
r
10
ti2900 km s-1 , the velocity component projected on the line of
sigh:. to the earth would depend on a factor cos 0 leading to
different radial velocities for the radio feature and the
various high velocity knots. (These knots occupy a range of
position angles relative to the nucleus.) (2) if the high
velocity material were ejected from the nucleus, it should
now lie on the far side of NGC 1275. On the contrary,
photographs by N.inkowski (1957) and Burbidge and Burbidge
(1965) show absorption lanes in (and only in) the sector
containing the high velocity knots. This implies that the
high velocity material is in front of NGC 1275, as does the
straightforward interpretation of the 21-cm absorption.
We therefore propose the following picture for NGC 1275.
The Crab-nebula-like system of filaments with a redshift of
5300 km s -1 (Lvnds 1970) has bean ejected from the nucleus,
as argued by Burbidge and Burbidge from their study of the
velocity field. The radio absorption feature, the 8200 km s-1
emission knots, and the dust lanes in the MI sector, occur
U
r
in a second, intervening galaxy.
have a density of X100 cm 3 , then
by the Seyfert nucleus [Q(H 0 ti 1
105
 pc. Alternatively, the knots
If these emission knots
they can be photoionized
)5 4 s-1] if they are within
may be H II regions ionized
by hot stars in the second galaxy, in which case the two
galaxies may be far apart. Burbidge and Burbidge (1965)
have noted that the high velocity knots have a low value of
s
7	
^J
	
s
11
I([NII))/I(Ha). The same holds for the giant HII regions
in the outermost spiral arms of 1433 and M101 (Searle 1971),
and for the "isolated extragalactic I3 II regions" observed
by Searle and Sargent (1972). If the high velocity knots
are metal-poor H II regions in a superimposed galaxy, their
sne:tra should show narrow lines, strong [OIII] emission,
and a flat stellar continuum leading to an H$ equivalent
cridth of ti50 to 200 A.	 The reddening corrected continuum
	 1
(see above) does suggest the presence of hot stars. 	 j
This work was .supported in part through grant NGL 05-002-134
from the National Aeronautics and Space Administral:ion.
ORIGD4AL PAGE IS
POOR QUALITY,
12
TABLE 1
NGC 1275 Emission-Line Fluxes
I(^)Emission line flux I01
-13 crQs(10 -1	 -2s	 cm (corrected for
Icn ), Wampler	 (1971)	 Thisr- ^ -paper rc:dd edd 	 in^^;n 1
15 7 1I1 9532,9069 3.3* 1.4 0.30
(O+Ij 7325 1.5 1.1 0.35
[SiI) 5720 4.7 3.5 1.33
?= 6563 14.3	 l
} 13.9 5.44
(:+=1; 6564,6548 2.7	 JJJ
10., 6363,6300 2.3 3.: 1. O
reI 5876 <0.3: <0.3 <0.2
[oil=] 5007,4)59 6.8 5.6 4.33
E6 4861 1.3 1.2 1.00
Ee?I  - <0.3 <0.3
[OTIL] 4363 -
0.3 0.33
y 4340 0.24:
FS 4101 0.2
0.9 1.11
(S1i 4072 0.6
['14'e =:7] 3869 - 1.0 1.43
[OiI} 3727,3729 2.7 1.8 2.90
3426 - <0.3 <0.5
'W :--=-7e_ corrected this to 8 x 10 -13 to allow for telluric if20
4
absc=zz.:on; zresumably, a similar correction factor should be
-^clied to our measurements.
r
13
REFERENCES
r.:'erson, K. S. 1970,	 Ap.	 J., 162, 743.
^.:rbidce, E. M. and Burbidge, G.	 11. 1965, Ap	 J.,	 142,	 1351.
Cox, J. P. 1972, E_J _, J .?8, 143.
-- e Young , D. S., Roberts, M. S., and Saslaw, W. C. 197 ,
Ap. J., 185, 809.
i3umson, M. L., Mayall, N. U., and Sandage, A. R. 1956,
J. , 63 , 97.
E •r-^.:er, D. G. and Mihalas, D. 1970. M. N. R. A. S., 147, 339.
Jura, t•S. 1973,  Ap . J., 181, 627.
1-1nds, R. 1970, An. J. (Letters), 159, L151.
Minkokski, R. 1957, in Radio Astronomy, IAU Symposium No. 4,
ed. 1 1. C. van de Hulst (Cambridge: Cambridge Univ. Pross),
o. 107.
C::e, J. B., Neugebauer, G., and Becklin, E. E. 1970, j^p. J.,
159, 341.
Oke, J. B. and Schild, R. L. 1970, ^. J., 161, 1015.
U=_te:brccri, D. E. 1971, "Nuclei of Galaxies;' Pont. Acad. Sci.
Scri;:)ta Varia, ed. D. O'Connell, 35, 151.
=ens:,on, M. V., Penston, M. J., Selmes, R. A., Becklin, E. E.
and Neugebauer, G. 1974, M. N. R. A. S., 169, 357.
G. H. Loca,	 F. J. 1972, A. J. (Letters), 3.76,	 L95.
R le,	 V., and Windram, M. D. 1968, M. N.	 R.	 A. S.,	 138,	 1.
Searle, ;--	 1971, Ap.	 j., X68, 327.
Earle, L.,	 and Sargent, W.	 L. W.	 1972,	 AA.	 J.,	 173,	 25.
_a_'-`er, D. and Schilizzi, R. 1975, preprint.
ORIGINAL PAGE- IS
OF POOR QUAI.I Y,
a^	 a
r
14
Shields, G. A. 1974, Ap. J., 191, 309.
Shields, G. A. and oke, J. B. 1975, Ap. J., 197, 5.
sous_`rin, S. 1969, Astr. and Ap, 1, 414.
Walker, M. F. 1968, A. J., 73, 854.
Rampler, E. J. 1971, Ap. J., 164, 1.
Whitford, A. E. 1958, A. J., 63, 201.
Yilliams, R. E. 1971, Ap. J. (Letters), 167, L27.
Figu;.e Captions
Fig. 1.- Absolute spectral. energy distribution for NGC 1275
on 9-10 October 1974 using an aperture of 7" diameter. The
flux fu
 is in units of ergs sec -1 cm 2 Hz -l . Emission
lines of interest are marked even if not present. The
statistical errors of the points are no larger than the
dots except for those at the extreme ends of the plot.
Fig. 2.- Infrared and visual fluxes for NGC 1275. The
lower solid curve is the continuum from fig. 1. while
the upper curve is the continuum corrected for reddening
EB-V = 0.63.
	
Solid dots are observations of Rieke and
Low (1972) probably using a G" diameter aperture. The
open circles are measures by Penston et al. (1974) using a
10" aperture made on 21 September 1971 and 27 September
1971. Points marked X indicate reddening corrected fluxes.
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The spectral energy distribution of NGC 1275

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