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Journal ArticleDOI

Absolute spectrophotometry in M31 and M32

15 May 1975-The Astrophysical Journal-Vol. 198, pp 63-70

AbstractFor a number of places in the bulge of M31, and for two places in M32, photometric scans from lambda = 3300 A to lambda = 10,600 A have been obtained with the multichannel spectrometer on the 5-m Hale telescope. The scans show that in both objects the color temperature (particularly shortward of 5000 A) decreases toward the center, and that the strength of the CN bands increases toward the center in both objects, in agreement with other earlier observations. The new data can all be interpreted in terms of an increase of heavy-element abundance toward the center in both objects by a factor probably less than 2, and by an excess of heavy elements in M31 compared with M32 by a factor probably greater than 2, in qualitative agreement with earlier conclusions of other observers.

Summary (1 min read)

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  • Alt}iough the instrument has both a star and sky aperture separated by 40 `, this is not useful for sky subtraction in extended hence objects;/the instrument was used either without the starsky chopper running, or by reducing data i n the two apertures completely separately.
  • There is some suggestion that changes in the CN band strengths are smaller than in the previous case.
  • Indeed the color difference turns out to be 0.04 mag, hard at the margin of measurability with the present data.
  • Altogether then, these observations give for the entire bulge of M31 a progressive reddening of the spectral gradient as one approaches the center, the spectral changes being minor in the outer portions of the bulge but increasing rapidly towards the center -quite parallel to the strength of the CN features (Spinrad et al. 1971 ).

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J. B. Oke
Hale Observatories, California Institute of Ter7hnology
Carnegie Institution of Washington
M. Schwarzschild*
Princ^zton University Observatory
(21?,SA-CR-140683) ABSOLUTE
S E^
Observatories, Pasadena, Cal.if.) 25 p HC
CsCL 03A
G3/89 53609
*Visiting Sherman Fairchild Distinguished Scholar,
California Institute of Te.

For a number of places in the bulge of M31 and for
two places in M32 photometric scans from
= 3300
have been obtained with the multichannel
slpectrometer"'on"the 5-meter Hale telescope.
The scans
show that,in both objects the color temperature
(particularly shortwards of 5000
decreases towards
the center and that the strength of the CN bands increases
towards the center in both objects in agreement with
arlier observations.
The new data can all be interpreted
in terms of an increase of heavy element abundance
towards the
center in both objects by a factor- probably-
less than 2 and by an excess of heavy elements in M31
compared to M32 by a factor probably greater than 2, in
qualitative agreement with earlier conclusions.
ubject Headings:

It has been known for many years that some galaxies
show a charge of color with distance from the center (Tifft
1961, 1963, de Vaucouleurs 1961, Hodge 1963). Sandage,
P.ecklin, and Neugebauer (1969) have shown that ultraviolet
color changes occur in M31 within 20
of the center. They
also report a color change in M32 which depends on distance
from the
The 6-color photometry of Stebbins and
Whitford (1948) demonstrated that over a wide spectral
range M31 and M32 had different energy distributions.
This was also shown by Oke (1962) using detailed scars.
In recent years-most of the observational work on
M31 and M32 has concentrated on measuring various line
— —
strengths or indices to study the metallicity in the
objects. We mention as examples the work of McClure
and Van den Bergh (1968), McClure (1969), Spinrad and
Taylor (1971), Spinra3 et al. (1971), Spinrad et al.(1972),
Joly dnd A,ndrillat {1973) .
In order to investigate in more detail the known color
changes with distance from the center of M31 and M32,
detailed energy distributions covering the wavelength
range for
3300 to X10,600 have been obtained in the
bulge of M31 and at two points in M32. Enough detail
is present in some cases to make comments not only on the
overall energy distributions but also on the strengths
of features such as the CN bands.

All the observations were made with the multichannel
spectrometer attached to the 5-meter Hale telescope. Alt}iough
the instrument has both a star and sky aperture separated
by 40
, this is not useful for sky subtraction in extended
objects;/the instrument was used either without the star-
sky chopper running, or by reducing data
n the two apertures
completely separately. Sky measurements were made well
outside of M31 and M32.
It was the original intention to measure separately
the tiny 3
5 diameter nucleus of M31; however, atmospheric
refraction effects were too large. All results given in
this paper refer to a round aperture of radius 5^. Energy
distributions are based on the absolute calibration of
a Lyrae as given by Oke and Schild
1970) and results are given
in terms of AB = -2.5 log f
48.60 when f is the flux in
ergs s
The highest resolution measurements, using a bandpass
of 40 A in the blue (1/k > 1.74) and 80 A in the red,
are given in table 1. Along with 1/a, where a is the
wavelength measured
r'microns, are the fluxes in the
center of M31,'oitoin a radius of 5% and the fluxes
in a 5
radio:.' .
erture 40
East of the center. The
resulting absolute fluxes
in the form
AB, for the

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