The average spectral energy distribution for a sample of bright elliptical galaxies is presented in the range lambda lambda 2000 to 3200 A. Spectral synthesis indicates that elliptical galaxies are most likely older than 9 Gyrs. The ultraviolet flux is consistent with a population of red horizontal branch stars, as those present in metal rich globular clusters. Data for distant first ranked cluster galaxies show indications of spectral evolution

Bruzual, A. G.

Spinrad, H.

English

NASA Technical Reports Server

THE ULTRAVIOLET SPECTRA OF EARLY TYPE GALAXIES
G. Bruzual A. and H. Splnrad
Astronomy Department
University of Californla, Berkeley, Callfornia 94720
ABSTRACT
The average spectral energy distribution for a sample of bright ellipti-
cal galaxies is presented in the range AA 2000 to 3200 A. Spectral synthesis
indicates that elllptica1 galaxies are most likely older than 9 Gyrs. The
ultraviolet flux is consistent with a population of red horizontal branch stars,
as those present in metal-rich globular clusters. Data for distant (z _ 1)
flrst-ranked cluster galaxies show indications of spectral evolution.
INTRODUCTION
Detailed knowledge of the ultraviolet spectra (AA 2000-3200A) of early
type galaxies is important for at leastthree reasons. First, it may provide
clues about the presence of a hot stellar population which does not contribute
appreciably at optical wavelengths. Second, spectral features identified in
the spectra of nearby galaxies can be used as redshift indicators for distant
galaxies (up to z _ i). And, third, combining this information with our
notions of stellar evolution, some conclusions about the spectralevolution of
Ealaxles can be derived. In partlcular, the stellar population present in
first-ranked galaxies in distant clusters can be compared withthat expected
for galaxies of different morphological types at the corresponding age, and
hence the correctnessof the assumption of similarity of all flrst-ranked
cluster galaxies can be tested.
OBSEEVATIONS
Low resolution IUE spectra with moderateslgnal-to-nolseratlo in the
range from 2000 to 3200 _ are now available for several early type stellar
systems. Table I gives galaxy Identlflcatlonand morphologlcal type, exposure
time and IUE observer for the sample used in this paper. Figure 1 shows the
average spectral energy dlstrlbutlon, where each galaxy spectrum has been
normaiized at 2900 A and weighted according to the exposure time.
The most conspicuous spectral features that can be seen in this spectrum
are the Mg I (2852) and MEII (279_) absorption lines and the spectral discon-
tinuities at 2420, 2640, and 2900 A, which result from the blends of many me-
tallic lines. This spectrum lscharacterlstlc of a metal-rlch stellar popu-
latlon, as can be seen from the IUE spectra of globular clusters (ref. 3).
These features are prominent in spectral types A7 and F8 (ref. 4). No
emission lines have been detected in this s_ectral range. The high fre-
quency structure for A < 2300 and A > 3200 A is due to the low slgnal-to-
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noise ratio of the individual spectra at their edges, and does not represent
reproducible spectral features. This has been checked by comparison with a
ground based spectrum of M32 of high signal-to-noise ratio, down to I 3200 _.
The apparent sharp absorption features at l% 2400, 2580, and 3100 _ are due
to reseau marks in the IUE camera.
INTERPRETATION
The quality of the present data is high enough to allow an interpretation
of the spectrum in terms of the stellar population present in elliptical ga-
laxies. As part of a separate investigation (refs. 5 and 6), evolutionary
models for galaxy populations and spectra have been constructed. Elliptical
galaxies are well represented by a model in which initial star formation
takes place at a constant rate for a time interval of 1 Gyr, with an initial
mass function similar to that observed in the solar neighborhood (x=1.35),
and is zero afterwards. Evolutionary tracks for solar composition from
Ciardullo and Demarque (ref. 7) are used to follow the subsequent evolution
of the stellar population. The observed luminosity function for giants in
the solar vicinity, as derived by Tinsley and Gunn (ref. 8), is used to
complete the evolutionary tracks onto the giant branch. Standard stellar
spectra for solar metallicity are used to construct the resulting galaxy
spectrum with a resolution of 50 _ in the optical region. In the ultraviolet,
0A0-2 (ref. 4) spectra for stellar types earlier than GS, and IUE spectra ob-
tained by the a_thors for late type giants, complete the spectra with a re-
solution of i0 A to 2000 _.
The following results have been derived from these models.
(a) In the region from 2000 to 4000 _ the observed spectra are well re-
produced by the model at an age of 5 Gyr. However, this model has B-V = 0.83,
which is about 0.15 magnitude bluer than a typical giant elliptical. In addi-
tion, this young age implies formation redshifts for qo = 0 of 0.34 (Ho = 50)
or 1.04 (Ho = i00) which seem much too low for current views of massive ga-
laxy formation. The predicted B-V at z = 0.46 is 1.26 (Ho = i00). This is
too blue for the observed color of 1.4 (refs. 9 and i0). For a different
interpretation, see ref. ii.
(b) In the region from 5000 to 8000 X, the observed spectrum is not
reproduced until an age of 8 to 9 Gyr. However, by this time, the model is
deficient in ultraviolet light. We have interpreted this deficiency as
due to the lack of horizontal branch stars in our model. To test this hypo-
thesis we have added some light to the models from stars in the range F0 to
F8, which resemble well the spectra of metal-rich globular clusters (ref. 3).
The fraction of this light added to the models is arbitrary and depends on
the age assumed for the galaxies. Table II shows the fraction of light in-
side the V-band for three different stellar groups at three possible galaxy
ages. The fit to the observed spectrum from 2000 to 8000 _ is equally good
for any of these models. Thus, it does not seem possible to derive the typi-
cal age of the population on spectral grounds solely. Assumptions about the
evolution of the horizontal branch population, together with observed spectra
of galaxies at z > 0.4 may provide clues about the most plausible age. This
732
is being explored in ref. i0.
c) Even though some light from giant stars hotter than those commonly
seen in the solar neighborhood is required to reproduce the galaxy data, these
stars are not hotter than FO (T_7200, B-V=0.30). In the HR diagram these stars
fall to the right of the RR Lyrae instability strip, and thus are character-
istic of a red horizontal branch (ref. 12). This is not surprising, given
the similarity of the galaxy spectra and those of metal-rlch globular clusters
(ref. 3). The number of horizontal branch stars required to reproduce the
spectra is equivalent to a few percent of the total number of stars in the
red giant branch. This ratio is given in the last llne of Table II. Most
likely these stars are more conspicuous in the elliptical galaxies than in the
solar neighborhood because of the intrinsic differences between both popula'
tlons and the larger volume sampled in the case of the galaxies.
SPECTRAL EVOLUTION
Figure 2 shows the observed spectral energy distribution for E galaxies
at z = 0, 0.2, 0.5, and i.i in the rest frame of the galaxies (refs. i0 and
3). The galaxies at z=0.5 and i.i are bluer in the range II 2700 - 3800
than the average nearby elliptical. Similarly, the amplitude of the 4000-_
discontinuity is lower in the distant E galaxies than in nearby ones. This
is expected from the normal evolution of the main sequence stars in these
galaxies (ref. 5). The extent to which star formation could be taking place
in these elliptlcals at z > 0.5, as well as the relative importance of hori-
zontal branch stars at the respective epoch is the subject of a separate inves-
tigation (ref. i0). Certainly, some luminous, red-color-selected E galaxies
have had no active star formation over the last 5-7 Gyrs.
CONCLUSIONS
Our spectral synthesis m_dels for the spectrum of elliptical galaxies
in the range l% 2000 to 8000 A imply that these systems are most likely older
than 8-9 Gyr. The flux in the region from 2000 to 3200 _ can be understood
as being produced by stars in the red horizontal branch, similar to those
observed in metal-rlch globular clusters. In this respect we disagree with
the recent results of O_Connell (ref. ii). Spectra of distant elliptical
galaxies show slight indications of evolution, consistent with our ideas about
stellar evolution.
REFERENCES
1. Johnson, H. M. 1979, Ap. J. (Letters), 230, L137.
2. Cappacioli, M., Bertola, F., and Oke J. B. 1980, (in preparation).
3. Dupree, A. K., Hartmann, L., Blacki J. H., Davis_ E. J., Matilsky, T.A.,
Raymond, J. C., and Gursky, H. 1979, Ap. J. (Letters), 230, L89.
4. Code, A. D., and Meade, M. R. 1979, Ap. J. Suppl., 39, 195.
5. Bruzual A., G. 1980, Ph.D. Thesis, University of California, Berkeley
(in preparation).
6. Bruzual A., G., and Kron, R. G. 1980, Ap. J. (in pres_).
7. Ciardullo, B., and Demarque, P. 1977, Trans. Astr. Obs. Yale Univ., 33.
733
I8. Tinsley, B. M., and Gunn, J. E. 1976, Ap. J., 203, 52.
9. Kristian, J., Sandage, A., and Westphal, J. A. 1978, Ap. J., 221, 383.
i0. Bruzual, A., G., and Spinrad, H. 1980 (in preparation).
ii. O'Connell, R. W. 1980, Ap. J., 236, 430.
12. Phillip, A. G. D., Cullen, M. F., and White, R. E. 1976,
Dudley Obs. Reports, ii.
13. Spinrad, H., Stauffer, J., and Butcher, H. 1980 (in preparation).
TABLES
Table I. Galaxies observed with IUE
Galaxy** Type LWR exposure Reference
M31 (nuc) Sb 8.5 hrs i,*
M32 dE2 5.5 *
NGC 3379 E1 7.0 2
NGC 4472 E2 10.5 *
• Data from authors.
•_ IUE aperture size was "large", an oval 10"x20"
in dimension.
Table II. Fraction of Flux in the V,band
Model age (Gyr) 9 13 16 •
Turnoff G5 G7 G9
[FO - F8] (HB) 0.05 0.09 0.13
Turnoff - M6V 0.44 0.37 0.34
Red Giants 0.51 0.54 0.53
N(HB)/N(RG)* 0.028 0.034 0.050
* Number ratio of horizontal branch to red giant stars.
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4 -
5.0
i_a.-'<
2.O
I.O
O0
-I0
2000 2500 3OOOO
WAVELENGTH ( A )
Figure i. Average spectral energy distribution for galaxies listed in Table
I. Each spectrum was normalizedat 2900 X and then weighted according to
the exposure time. (See text for details).
Figure 2. Observed spectralenergydistributionfor galaxiesat z=O, 0.2,
0.5, and 1.1 in the rest frame of the galaxles. (see refs. 10 and 13 for
details).
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The ultraviolet spectra of early type galaxies

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