Rotation curve, mass distribution, and mass density of SA galaxy, NGC 68

Burbridge, E. M.

Burbridge, G. R.

Prendergast, K. H.

English

NASA Technical Reports Server

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. KITATION AND MASS OF THE Sa GALAXY, N& 681 
b 
E. MARGAREZF BUIiBIDGE and G. R. BURBIDGE 
University of California a t  San Diego 
and 
K. €I. PREXUERGAST 
Department of Astmnolqy, Columbia University 
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c, 
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ICODPI I 
3 0  
(cATcooRYl 
Received February , 1965 
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ABSTRACT 
/96& 
The Sa galaxy NGC 681 has been found t o  have emitting gas throughout i ts  
disk component so that a rotat ion curye, mas6 dist r ibut ion,  and m a s s  within 
a radius of 5256 pc have been derived. 
_ -  I- 
These prove t o  be very similar t o  those 
found for Sc galaxies, desp i t e  the S a  c lass i f ica t ion  'and the s t ~ c t u r a l  s imi l a r i t y .  
of this  gal- t o  the  well-known NGC- 
the  mean density 1.0 x gm/cm3, 
8 
and the mass-to-light r a t i o  3.6. 
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It is w e l l  known tha t  there i s  much l e s s  ionized gas present i n  e l l i p t i c a l ,  
SO and Sa galaxies than i n  Sb, .Sc, and irregular systems. 
study the conditions of excitation i n  the i n t e r s t e l l a r  gas through the whole 
In an attempt t o  
range of Hubble typs  we have made observations of a considerable number of 
s p i r a l  and irregular galaxies (Burbidge and Burbidge 1962). To extend th i s  
study t o  other types we took spectra in the  red wavelength region of a con:,. 
siderable number of e l l i p t i c a l ,  SO, and Sa galaxies l i s t e d  by Humason, Mayall, 
. c 
. 
./Ic 
' 
. and Sandage (1956) i n  whicki Humason had noted that the emission feature 
[0 1 1 1  A3727 is present. 
another paper. 
i s  c lass i f ied  Sa and which bears a s t r ik ing  resemblance t o  NGC 4594, the fanous 
Sombrero Nebula. I 
The -=sults of t h i s  survey w i l l  be reported i n  
Among the galaxies included in t h i s  study was  nTGC 681 which * 
/ 
A plate  of NGC 681 taken a t  the prime focus of the McDonald 82-inch t e l e -  . I 
scope i s  shown i n  Figure 1. It was found tha t ,  as well as showing emission 
lines of Ha and [N 1 1 1  h6583 in  the central  bulge, the emitting gas could I 
detected far out i n  the  disk. 
be 
. _- 
The Lick spectrogram described below showed tha t  
these two emission l i nes  are of approximately equal intensi ty  i n  the center of 
the galaxy, but have the  usual ra t io  of Ha/[N 11]  FW 3 i n  the disk, indicating 
t h a t  they are excited by the usual radiative mechanism operating i n  s p i r a l  arms. 
Thus it is c l ea r  t h a t  the furthest  out knots are in fact  H I1 regions' containing 
highly luminous 0 and B stars. This s i tua t ion  is  t o  be contrasted with that  in 
NGC 4594 where Mkch (1962) has obtained spectra of  apparently s i m i l a r  s t ructures  
i n  the am, '  but has found no emission features,  Thus he has concluded that  
these cannot be H I1 regions but must be star c lus te rs  which do not supply 
enough ul t ra-violet  radiation t o  ionize the gas, o r  in which very l i t t l e  gas is 
present. 
' ' 
The importance of our discovery of extended emission features in  NGC 681 
. -- 
is that  we can obtain a rotation curve and hence a mass and mass d i s t r ibu t ion ,  
f o r  a galaxy of t h i s  type. 
at  the prime focus of the 82-inch telescope of the McDonald Observatory. 
The f i r s t  spectrum w a s  taken with the B spectrograph 
This 
exposure w a s  made when the moon was moxe than half  full and has 'therefore a 
considerable background spectrum of scattered moonlight j consequently the plate 
was not suitable f o r  accurate measurement'. 
I 
It did clear ly  show, however, an 
extended and incl-ined emissioh l i ne  that was presumably Ha! with a considerable 
veloci ty  gradient. 
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On December 4, 1964, a spectrum of 4 h u r s '  expsoure was  taken with'the 
prime focus spectrograph of the Lick 120-inch telescope, with the s l i t  set along 
the major axis in posit ion angle 6 4 O ,  and with the grating and camera combination 
--- 
0 
giving a dispersion of 190 A/mm at  Ha. 
gram alone are plot ted i n  Figure 2, and the rotati'on curve obtained by folding 
these measures about a cent ra l  velocity of +1705 km/sec E l a t i v e  t o  the loca.L__- 
The ve loc i t ies  measured from t h i s  spectro- 
standard of rest is shown in  Figure 3. Two smooth curves have been f i t t ed  t o  
these points using the usual polynomial solution (cf .  Burbidge, Burbidge, and 
Prencergast 19591, with four  and EL.-. parameters, respectively. 
e 
The %cession velocity of the center of mass of t h i s  galaxy corrected f o r  
galact ic  rotat ion is +1727 km/sec. This value can be compared w i t h  the  value of 
+1768 lun/sec obtained by Humason (Humason e t  al. 1956). 
a value of the Hubble constant of 75 km/sec per  Mpc, the  distance of NGC 681 is 
then 23 Mpc and a t  t h i s  distance 1" = 111.7 pc. 
For t h i s  redshift ,  w i t h  
Since the  observations extend 
t o  a distance of 47" frbm the center, t h i s  corresponds in  linear'dimensions t o  
t .  
a radius of 5250 pc. It should be added here t h a t  the t o t a l  diameter out t o  
which luminosity can be detected on the  d i r ec t  p l a t e  is approximately 55" o r  
614.0 pc. The total ma88 of the system and the cent ra l  density, calculated by 
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our usual method, are shown i n  Table 1 as a f b c t i o n  of the nmber of parameters 
i n  the solution, f o r  values of c/a of 1/4, l/5, and 1/8. 
The galaxy is  very nearly edge-on. We concluded t h a t  the correction fac tor  
t o  be applied t o  the ve loc i t ies  for inclination of' t he  galaxy w a s  therefore very 
nearly unity. 
axes of the  flat component, t h a t  the incl inat ion 5 = 81O so t h a t  the correction 
fac tor  f o r  the mass equals cosec' 5 = 1.02; t h i s  has been applied i n  deriving 
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-~ From the d i rec t  p l a t e  we found, by'estimating the major and minor * 
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the values shown in Table 1. It w i l l  be seen f r o m  Table 1 t h a t  t he  mass does 
not depend sensi t ively on the assumed value of c/a and w e  may conclude t h a t  the  
t o t a l  m a s s  contained within a radius of  5.25 kpc i s  1.9 lto.2 x 10 
e 
10 * Ma. Since 
' the  galaxy has a large cent ra l  bulge, a rather laI'ge value of c/a is  indicated. 
/ The densi t ies  normalized t o  unity at  the center are plot ted against distance 
from the center i n  Figure 5 ,  f o r  the  4- and 6-parameter solutions, both f o r  
c/a = 1/b. 
radius is  1.0 x 10 -23 gm/cm3. 
modulus of 31.8 corresponding t o  a distance of 23 Mpc, M 
For c/a = 1/4 the  mean density of the  galaxy within the observed 
-- 
For t h i s  galaxy m * = 12.8 and with a distance 
= -19.0. Thus the 
Pg 
Pg 
. photographic mass-to-light r a t i o  is  3.6. 
We have obtained rotat ion curves f o r  a considerabie number of Sb snd Sc 
galaxies. Three quant i t ies  which have been m & m d  i n  a l l  of these are the 
forms of the rotat ion curves, the t o t a l  masses, and the  m e a n  densi t ies .  Ih all 
of these respects th i s  Sa galaxy is  indistinguishable from bther galaxies of very 
L 
$,',t.;,. different  forms. To show t h i s  we give i n  Table 2 data concerning t h e - t o t a l  m a s s ,  
#C' ' ' 
n ~ & b O w ? t ' '  ;he radius out t o  which the  m a s s  was measured, and the mean densi t ies  f o r  a 
.. 1 VI i' . ;<, .*.E -... -\ 
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number of Sc and Sb galaxies. 
NGC 1084, an 
form of the rotat ion curve and its similar i ty  t o  that f o r  NGC 681, 
In\..Figure 4,tre show the rotation'  curve f o r  
1 F,Q$)c" -- .-. ~ 
\ G-?. 
cd 
'F * z 
Sc system (Burbidge, Burbidge, and Prendergkt  l963), t o  show the 
4 LW 
:!,a*$ 
* 
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While NGC 681 has a l l  of  these physical parameters which are indistinguish- 
I 
able from la ter  type sp i r a l  galaxies, it appears t o  be very similar i n  s t ruc tura l  
de t a i l s  t o  NGC 4594 though it is smaller, less luminous, and presumably less 
massive. F'rom the data given by Sandage (1961) fo r  NGC 4594 we find tha t  t h i s  
galaxy has an absolute magnitude of -21.5, cornpaEd with -19 f o r  NGC 681. The 
apparent l i n e a r  dimensions out t o  the points of lowest luminosity which can be 
easily distinguished on d i m c t  plates are 11.7 kpc (NGC 4594) and 5.3 kpc (NGC 681). 
Pease (1916) noted tha t  the absorption l i nes  i n  the spectrum of NGC 4594 could be 
measured out t o  2' fmm the cepter, and had an inclination with a slope of 
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2.78 lun/sec per  second of arc.  This gives a rough Keplerian estimate fo r  the 
m a s s  of NGC 4594 of 1.7 x 10l1 Mo within 7500 pc of the center (i.e., within 
about 2/3 of the  luminous extent clearly v i s ib l e  on d i rec t  p la tes ) .  If the  m a s s -  
to- l ight  ra t ios  f u r  these two galaxies were  comparable the t o t a l  m a s s  of NGC 4594 
would have t o  be about 2 x 10l1 Ma while i f  the mass-to-light r a t i o  fo r  NGC 4594 
were N LO as is  often thought reasonable fo r  Sa systems it would have t o  be about 
6 x 10 Mo. 
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-- We believe t h a t  these results which we have obtained f o r  NGC 681 are of. 
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considerable importance fo r  the ideas of t he  evolution of galaxies. We had . 
e a r l i e r  (Burbidge e t  al. 1963) remarked on the s imi la r i ty  i n  form between the  
, 
1 
two fat-anned Sc galaxies NGC 157 and NGC 1084 and t h e i r  very similarly-shaped 
rotat ion curves, and the contrast  between these and NGC 5055, an Sc galaxy w i t h  
th in ,  much-wound s p i r a l  arm6 and a ra ther  d i f fe ren t  rotat ion curve resul t ing 
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from a much greater central  concentration of mass. It had seemed t h a t  the v isua l  
form of a galaxy m i g h t  be w e l l  correlated w i t h  the shape of i ts  rotat ion curve. 
The results f o r  NGC 681 show t h a t  t h i s  is not t he  case. 
Further, one argument that has been advanced by ourselves, among others, . 
against the idea t h a t  galaxies can evolve through a sequence of ty-ges from 
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i - . .  ' I  
. i r regulars  t o  e l l i p t i c a l s  hiL:; been that  .Sa and e l l i p t i c a i  galaxies which a& 
t'nought t o  contain older an6 o r e  evolved s t e l l a r  populatiorir: h v e  l a rge r  masses 
than the l e s s  evolved galax; 
sequence without the a c c r e t '  3 of  nass. -mile t h i s  may F Li be generally the 
and so could not have procee'. A through t h i s  
case, all: the parameters asao -iated with fiGC 681 and spec-2ically i t s  m a s s ,  
rotat ion curve, snU re la t ive  density curve suggest t h a t  it may well have evolved 
through t h i s  sequence. 
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Finally, we note t h a t  l i t t l e  can be sa id  about the population of the  large 
cent ra l  bulge of NGC 681. The Lick spectrogram showed the blend of  the X a  I D 
l ines  at  A5893 i n  absorption i n  the s t r i p  of continuum from the central  region 
of  the  galaxy, but t h i s  refers  only t o  a distance o f  2" each side of the center, 
i.e., a radius of 400 pc. 
veloci ty  gradient of about the same magnitude as t h a t  shown by the emission 
l i nes .  
This absorption feature i s  inclined, indicating a 
It was not, however, measured, as the  spectrogram is not i n  bes t  focus _- 
here, the focus having been set for the  red spec t ra l  region. This was the only 
0 
absorption feature seen i n  the raiige 5800 - 6800 A ( there  i s  8 strong airglow 
spectrun on our p l a t e ) .  
population i s  t h a t  usually found i n  the nuclei  of SO,  Sa, and E galaxies. 
Bumason's spec t ra l  type of G5 suggests Kiaz the  stellar 
'Tne 
D l i nes  are blended on our p l a t e  but do not show great additional broadening, 
so  we can i n f e r  t'nat the veloci ty  dispersion of the stars i n  the  bulge component, 
which has l i t t l e  f la t tening,  i s  not inconsistent with the  mass t h a t  we have 
de rive d . 
. 
We wish t o  thank Anneila I. Sargent and J. W. Shelton f o r  carrying out -- 
measurements and reductions. This work has been supportea i n  pa r t  by a grant 
from the  NationaL Science Foundation and i n  p a r t  by the National Aeronkutics 
and Space Administration under G r a n t  NsG-357. 
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B u r b i d g e ,  E. M. and Burbidge, G. R. 1962, Ap. J., 
Burbidge, E, M., Burbidge, G. R., and Prendergast, K. H. 1959, Ap. J., P& '739. 
694. 
1963, AP. J., zj 376- 
Humason, M. L., Mayall, N. U., and Sandage, A. R. 1956, A. J., 6&' 97. 
Mhch, G. 1962, Problems of Extragalactic R e s e a r c h  (I.A.U. Symposium No. 15, 
ed. G. C. McVittie, Macmillan Co., New York), p. 69. 
Pease, F. G. 1916, Proc. N a t .  A c a d .  Sci., 2 
Sandage, A. fi. 1961, Hubble A t i a s  of Galaxies (Carnegie Ins t i tu t ion  of Washington). 
517. n2 I 
4 
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TABLE 1 
- _  - . -  MASS AM> (X3I'TRA.L DENSITY IQOM VARIOUS SOLUTIONS 
No. of X/Ma x 10-l' Central Density ' 
r c/a 
Parameters po i p / c m  3 
1/4 3 2.01 1.09 
1/4 4 2.04 1 .oo 
1/4 5 2.09 1.12 
1/4 7 1.64 1.79 
1/ 4 6 3 1.74 2-39 
1/5 
1/5 
1/5 
1/5 
1/5 
1/8 
' 3  
4 
5 
6 .  
7 
3 
1.99 
2.02 
2.05 
1.86 
1.81 
1.95 
1.30 
1.19 
1.32 
2.84 
22.3 
1.92 
1/8 4' 1.97 1.76 
1/8 1 5 1.98 1.96 
6 1.92 4.19 
7 1.91 3.14 
. 
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MASSES, RADII, AM) MEAN DENSITIES OF NGC 681 
TOGETHER WITX SOME Sc AND Sb-GALAXIES 
Galaxy m e  Radius Mean Densit* 
NGC 157 
NGC 253 
NGC 1084 
XGC 1792 
NGC 2146 
NGC 2903 
NGC 3521 
3556 
L 
NGC 4258 h - 
/ '  NGC 5005 
NGC 5055 
nrnn Lr 
I Y U b  UJ03 
NGC 7331 
NGC 681 Sa 
s sc 
sc 
sc ' 
s c  
S b C  
s c  
Sb 
sc . 
* 
These m 
Sb 
Sb 
Sb c 
sc 
Sb 
b 
4 
. 
n densit ies refer on& 
1.9 
6.0 
20 
1.6 
1.8 
1.8 - 
3.7 
8 .o 
1.4 
8.2 
7.0 
7.6 
c,13 
8.0 
5 -2 
9.3, 
8 
5.1 
4.5 
2.3 
4; 4 
7.1 
7.5 
7.0 
6.0 
10 
1 .5  
10 
t o  the  observed masses in Col 
1.0 
1.2 
6.8 
2.0 
2.4 
4.5 
6.7 
3 :7 
0.4 
2.6 
5.4 
1.9 
6.8 
1.6 
___i_. - * _  
imn 3 and the 
r a d i i  within which the observations were made, given i n  Column 4; they should 
be systematically consistent as they have a l l  been derived by us using the 
sane method of analysis. 
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b FImm CAPTIONS 
' 0  Fig .  1 NGC 681, photographed at  prime focus of 82-inch telescope a t  the 
McDonald Qbservatory of the University of Texas, on baked Kodak 
I 
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IIa-0 plate through a,Schott Go13 filter.' 
fm s&pe plate t o  show center. 
8cale: '1 mm = 2.7". 
Observed velocities Elat ive t o  local standard of ~ s t  plotted 
against distance from center of NGC 681. 
Fig, 3 - Velocity. measures folded about central velocity of +1705 km/sec. 
Inset is a dark print , * 
* '  - -  
South is a t  top, east a t  left. 
I , ,  
I .  1 .  
l 
. 
. %  'Curves are polynamial solutions with 4 and 6 parameters, I.espectively: 
' f i t ted t o  the observed points; velocities have not been corrected 
i 
a p  
'. for inclina$ion of plane of ga luy  t o  line of sight. , A .  ' ' .  1, r , 
I ', ii . , * *  
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! ' * ' , i  1 
a ' .  ' 1  t, : Fig. $ i Rotation curve of the Sc galaxy NGC 1084, showing the gmat similarity 
with the rotation curve of NGC 681 given in Fig. 3. 
a m i t y  mlative t o  central value, colqptrted f o r  c/a = 1/4 with 
' !  9 .  
' .  < 
Fig. 5 - 
1900 
1800 
W 
Q) 
u, 
\ 
€ 
Y 
1700 
l60C 
. 
L 
sw 
Q., - 
- *  
. .  
, x  a
x x  e 
X 
. .  
0 
x 9 0  
X 
0 '  
X 
0 
0 
1 X 0 
NE . *  
I I I I I I I I 
distance from center (seconds of arc) 
I 
-40 -20 0 20 40 150( 
t .  
200 
150 
100 
x 
W 
0 
a 
c .- 
7 
>. 50 
X 
X 
0 
0 ,  
0 
0 
.- 
Q 
1 
I I I I I I I 1- 
10 . 20 30 ; 40 -50 1 0 
- 1  
ter (seconds' of -. arc') 
- ----_____L _ -  __ 
q .  . 
. 
0 
. \  
0 
* .  I 
. .  
rn . c3' . 
Q 
! 
*', 0 V' 0 
0 '  
0 
Q '  
I 
I 
! i t 
X 
! 
0 
X 
a 
n.. ' 0 ' _  
0 
\ 
\ 
0 
0 X 
. .  
4 
. .  m . .  , . . . .  
0 
0 '  \ -  
F 1.0 
. 
0.8 
t 
0.7 
2 0.6 
. S'o.4 
a, L 
0.3 
0 -2 
- 4 ,  
a '  
0 .1 
i 
0 
-0.1 
, .  
? 
i 


Rotation and mass of the sa galaxy, ngc 681

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