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THE ASTRONOMICAL JOURNAL, 119 : 126õ135, 2000 January
2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
(
YOUNG STELLAR NUCLEI IN THE LENTICULAR GALAXIES. II. NGC 72801
V. L. AFANASIEV
Special Astrophysical Observatory, Nizhnij Arkhyz, 357147 Russia ; vafan=sao.ru
AND
O. K. SILîCHENKO2
Sternberg Astronomical Institute, Moscow, 119899 Russia ; and Isaac Newton Institute, Chile, Moscow Branch ; olga=sai.msu.su
Received 1999 June 3 ; accepted 1999 October 1
ABSTRACT
We have undertaken bidimensional spectroscopy of the central part of the nearby lenticular galaxy
NGC 7280 with the Multi­Pupil Fiber Spectrograph of the 6 m telescope of the Special Astrophysical
Observatory. We ïnd a rather young stellar nucleus, with a mean population age of 1.5 ^ 0.5 Gyr,
which is more metal­rich than the bulge at RB 1 kpc by an order of magnitude. The chemically and
age­decoupled nucleus seems to be spatially resolved : the circumnuclear absorption index isolines rep­
resent ellipses elongated in P.A. B 100¡õ110¡. The same orientation, P.A. \ 103¡, is found for the elon­
gated circumnuclear stellar structure, revealed from the morphological analysis of the Hubble Space
Telescope (HST ) WFPC2 and NICMOS images of NGC 7280 and seen best of all at R\ 1@@. Taking
into account the stellar kinematics inside RB 2@@, we conclude that this structure is a circumnuclear
stellar disk inclined with respect to the global plane of the galaxy. Meanwhile, both photometric and
kinematical data in the radius range 2Aõ8A imply the existence of an intermediate­scale bar elongated in
P.A. B 60¡. The circumnuclear ionized gas is distributed and rotates in the plane orthogonal to the plane
of the circumnuclear stellar disk.
Key words : galaxies : nuclei õ galaxies : individual (NGC 7280) õ galaxies : evolution õ
galaxies : structure
1. INTRODUCTION
Integrated photometric and spectral properties of early­
type galaxies imply that their major stellar population is
very old, almost as old as the universe. But the center of a
galaxy is a special place ; the stellar nucleus may have a
particular evolution that di+ers from the evolution of the
whole galaxy. By analyzing statistics of photoelectric multi­
aperture photometry of nearby galaxies (Silîchenko 1994),
we have found that in at least 25%õ30% of all elliptical and
lenticular galaxies, the nuclei are distinguished from the
surrounding spheroids by a prominently red color. We have
interpreted this phenomenon as being due to a signiïcant
fraction of chemically distinct stellar nuclei in early­type
galaxies.
Chemically distinct galactic nuclei are still poorly
studied ; their relation to a global galactic structure is quite
unclear, and so any speculations on their origin are some­
what premature. However, there were some hints about the
relation between the chemically distinct nuclei in elliptical
galaxies and the presence of fast­rotating circumnuclear
stellar disks (Bender & Surma 1992 ; Surma & Bender 1995 ;
Scorza & Bender 1995 ; Forbes, Franx, & Illingworth 1995).
On the other hand, a correlation between the absorption­
line index Hb and the fourth Fourier coefficient has been
a 4
noted in elliptical galaxies, so that elliptical galaxies with
inner disks have younger (on average) stellar populations
(de Jong & Davies 1997). The resulting picture is consistent
with the occurrence of secondary star formation bursts in
the centers of some elliptical galaxies that produced
compact circumnuclear stellar disks whose mean age is
õõõõõõõõõõõõõõõ
1 Partly based on observations collected with the 6 m telescope at the
Special Astrophysical Observatory (SAO) of the Russian Academy of Sci­
ences (RAS).
2 Guest Investigator of the RGO Astronomy Data Centre
younger and mean metallicity is higher than those of the
main galactic spheroids. The full set of these propertiesõ
higher nuclear metallicity, younger nuclear age, and photo­
metric signatures of circumnuclear disksõare directly
observed in the elliptical galaxies NGC 4365 (Bender &
Surma 1992 ; Surma & Bender 1995 ; Carollo et al. 1997a)
and NGC 4621 (Silîchenko 1997). Similar coincidence of the
chemically, morphologically, and dynamically decoupled
entities has also been found in the center of the lenticular
galaxy NGC 4816 (Mehlert et al. 1998).
We think that lenticular galaxies are more promising for
studying the connection between chemical distinctness,
young age, and diskiness of decoupled galactic cores. First,
their stellar nuclei are on average younger than the nuclei of
elliptical galaxies ; among nearby lenticulars, 50% show
prominent Balmer absorption lines Hc and/or Hd in their
nuclear spectra, implying a mean stellar age of less than 5
billion years (Silîchenko 1993a). Second, the same fraction,
50%, of nearby lenticulars appear to have separate circum­
nuclear stellar disks embedded in extended bulges (Seifert &
Scorza 1996). In the ïrst paper of this series (Silîchenko
1999a, hereafter Paper I) we studied two lenticular galaxies
with chemically decoupled nuclei, NGC 1023 and NGC
7332. We have detected intermediate­age stellar popu­
lations in the centers of these galaxies. The mean stellar age
in the nucleus of NGC 1023 is 7 billion years, and in the
nucleus of NGC 7332 it is 2.5 ^ 0.5 billion years. Moreover,
the intermediate­age stellar structures in the centers of these
lenticulars are not pointlike. An enhanced Hb absorption
line is observed up to 2A from the center in NGC 1023 and
up to 5A from the center in NGC 7332. The sizes of the
intermediate­age stellar structures coincide with the sizes of
embedded circumnuclear stellar disks, so for these galaxies
the connection between chemically and evolutionarily
decoupled cores and separate circumnuclear disks seems to
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YOUNG NUCLEI IN LENTICULAR GALAXIES. II. 127
TABLE 1
GLOBAL PARAMETERS OF NGC 7280
NGC Parameter (Source) Value
Type (NED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SAB(r)0]
R 25 (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 kpc
B T 0 (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.64
M B (LEDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [19.45