Infrared studies of the structure of the galaxy
Date
1986
Authors
Ruelas-Mayorga, R. A
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Abstract
A simple three-component model of the Galaxy is presented. The Galaxy is
represented by (1) a disk whose stellar density decreases exponentially with
distance from the galactic centre as well as perpendicularly as a function
of height above or below the galactic plane; plus a (2) ring with a radius of
R₀sin25 kpc considered to be an enhancement of the disk and finally by (3)
an oblate de Vaucouleurs spheroid meant to represent the galactic bulge.
The absorbing material is also assumed to be distributed as an exponential
layer.
The model is used to fit the 2.4 μm integrated flux observations of the
Galaxy, which it does remarkably well, and to determine some of the model
constants such as the radial length scale for the disk and the central densities
for the bulge and ring components. Based on this model and the derived
values of the constants, a series of stellar count predictions for the Km
agnitude (2.2 μm) were made. These predictions were com pared to those
made by the Bahcall and Soneira model of the Galaxy. The model predicts
an almost complete dominance of the bright K-magnitude (K ≤ 14-15)
realm by the disk population. At fainter magnitudes the bulge population
dominates. A transition region at which disk and bulge contributions are
similar occurs at different K -m agnitudes and depends on galactic longitude
(from K ~ +9.5 at l ~ 0 to K ~ +20 at l ~ 180). Stellar counts along the galactic plane in the range 220 ≤ l ≤ 60 have
been obtained, and have been fitted to the galactic model described above.
The model constants utilised in fitting the stellar counts are those derived
from the model fit to the 2.4 μm observed integrated flux. Considering
the general nature of the model (same constants for all regions), the agreement between the model predictions and the observations is remarkable.
The slight discrepancies which have arisen between theory and observations
have been interpreted, in most cases, as inhomogeneities in the absorbing
material.
In the longitude range 0 ≤ l ≤ 60, there appears to be a slight theoretical
excess over the observed counts at bright magnitudes. A carefully chosen
alteration of the absolute K-magnitude of all the stars in the Luminosity
Function (LF) may be necessary to produce a perfect fit between predicted
and observed Cumulative Counts Functions (CCF’s).
A low absorption window in the direction l — 0, 6 ~ -4, known as
Baade’s Window (BW) has been observed photometrically as well as spectroscopically.
A K-scan of this region provided an observational CCF which,
with the aid of the model discussed above, was decomposed into possible
disk plus bulge contributions. The following results were obtained:
• At bright K-magnitudes the slope of the disk CCF is different from
that of the bulge CCF. This suggests the presence of two radically different
stellar populations within a radius of ± 3 kpc from the galactic centre.
• Possible similarities between the real BW bulge population and globular
clusters stars is suggested from the similar slope of both stellar
populations’ CCF’s.
• A significant spread in the (J-H) vs. (H-K), as well as in the CO
vs. (J-K) diagram suggests an intrinsic spread in the colour of the
sources, which may indicate a possible spread in the metallicity of the
objects observed in BW.
• What may be identified as the brightest infrared (IR) magnitude locus for stars in BW, is composed of young (2 x 10⁹ years) Asymptotic
Giant Branch (AGB) stars with a luminosity of the order ~ 10⁴Lʘ.
• In BW ‘true’ bulge members appear to be CO-poor as opposed to
‘true’ disk members which seem to have higher CO-index values. This
suggests there is a difference in metallicity of the stars in the disk and
the bulge. ‘True’ bulge members appear to be more metal poor than
47-Tuc.
There is a hint of kinematical differences between CO-weak and CO-strong
stars which is consistent with our picture of ‘true’ bulge and ‘true’
disk populations respectively. Further photometric, spectroscopic and kinematical
studies of IR sources in low absorption regions will help to confirm
the points put forward in this thesis.
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