Optical Mysteries of the Andromeda Galaxy’s Bulge
By: Brandon Patel
Astronomy 193- Galactic Ecosystems
Tufts University
November 2007
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Abstract
The bulge region of the Andromeda galaxy (M31) has been a mystery to astronomers for
decades. The first stars in the M31 bulge were resolved by Baade in the 1940’s, yet we still know
very little about this important region to this day. The extremely bright nucleus of M31 makes
stars in the bulge region very difficult to resolve. This paper focuses on the observations of the
M31 bulge in optical wavelengths. There have been few optical observations, but they have been
useful in understanding this region. I argue that further analysis is clearly necessary in order to
determine the color and brightness of the stars in this region. Moreover, current estimates for the
metallicity and age of the M31 bulge region are presented. I argue that both of these values are
poorly understood in the region. Finally, the M31 bulge star results are compared to Galactic
bulge stars. The Galactic bulge is well understood and astronomers are confident about the
metallicity, age, and luminosity of the stars in the region. The Milky Way serves as a good
comparison to M31 because they are relatively close to each other (M31 is the closest big galaxy
to the Milky Way), and they are both spiral galaxies.
Introduction
The Andromeda galaxy, M31, is the closest large galaxy to us, and the only other giant
spiral galaxy in the Local Group. Its coordinates are RA: 00h42m44.3s and DEC: +41d16m09s
(NED). The distance modulus to M31 is 24.47, which corresponds to a distance of 780,000
parsecs (Holland 1998). Like all spiral galaxies, M31 consists of a bulge, disk, halo, and dark
matter halo (see Figure 1). Moreover, the majority of the stars in M31 are in the bulge. At the
center of M31, there is thought to be a supermassive black hole, a property that is common to
many other large spiral galaxies (Bender et al. 2005).
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Figure 1: Schematic view of a spiral galaxy on its side (Arny 450). The bulge, disk, halo, and
dark matter halo are all clearly marked.
Due to its proximity and similarity to our Galaxy (the Milky Way), M31 has been a
popular galaxy to study. The galaxy has been observed for centuries by many astronomers
throughout the northern hemisphere. However, the true extra-galactic nature of M31 was not
revealed until 1929. That year, Edwin Hubble published a paper on Cepheid variable stars and
showed that Andromeda was too far away to be in the Milky Way (Hubble 1929). In the 1940’s,
during the World War II black outs, Walter Baade studied M31. Baade was the first person to
resolve individual stars in Andromeda, and he discovered that there were two types. He called
the younger stars in the disk of M31 Population I and the older stars in the bulge and halo
Population II, a classification system we still use today (Baade 1944). Since then, we have
learned more and more about this nearby galaxy.
Although we know a lot about M31, the bulge region of the galaxy is still somewhat of a
mystery to astronomers. M31’s bulge is very bright and very crowded with stars. That is, the
density of stars in the bulge is much greater than in the disk or the halo. This makes it very
difficult to resolve individual stars in the bulge region. The stars in the region blend together
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when observed, resulting in an overestimation for the luminosity of the stars. Furthermore, the
blending of stars results in fewer detected stars than are actually present.
In this paper, I will discuss modern optical observations of the bulge of M31. There have
been many observations of M31’s bulge in other wavelengths (cf Davidge et al. 2005) , but I will
limit the discussion to optical studies. The most revealing optical observations of the bulge are
those conducted by Hubble Space Telescope (HST). The reason for this is that HST, a spacebased observatory, has very good resolution of order 0.1 arc seconds. Generally, HST has ~10
times better resolution than most ground based telescopes (STSCI). In a region that is very
crowded, this is a necessity.
The HST observations reveal many different properties about M31’s bulge. However, I
will focus on what the observations tell us about the age and metallicity of bulge stars. The
reason is that age and metallicity essentially define a population of stars. Furthermore, I will
compare the age and metallicity of bulge stars in M31 and the Galactic bulge. The Milky Way
serves as a good comparison to M31 in two ways. First, the Galactic bulge is well understood
and we are confident about the age and metallicity of the stars in the region. Second, both M31
and the Milky Way are spiral galaxies and relatively close to each other; it would be odd if the
two galaxies had different populations of stars in their bulges.
HST Wide-Field Camera Observations of M31’s Bulge
Unfortunately, there are very few observations of the bulge of M31 by HST. One of the
first HST observations of the bulge of M31 was presented by Rich et al. (1995). They analyzed
two observations of the bulge using the original Wide Field and Planetary Camera (WF/PC). The
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first observation was centered on the nucleus (center) of M31 and the second observation was 2
arc minutes south of the nucleus (corresponding to a distance of 500 pc). Each image has a field
of view of 2.57 x 2.57 arc minutes. Figure 2 shows the location of the fields observed in M31.
Not in Rich et. al 1995.
Figure 2 – (Image taken from Jablonka et al. 1999) Location of the M31 bulge fields observed
with the HST. Rich et al. (1995) observations are the squares, while Jablonka et al. (1999)
observations are the circles. The third square (where the arrow is pointing to) is erroneous; Rich
et al. (1995) did not analyze an observation at this location. That square should not be on this
image. See text for more details on Jablonka et al. (1999) observations.
The choice of filters for this observation were F702W (an R band filter) and F785LP (I
band filter). The R band filter is centered on 702 nanometers, whereas the I band filter is
centered on 880 nanometers (the name for the filter is 785). The authors expressed some concern
over the spherical aberration of the camera, but they were confident that the observations would
still be able to resolve individual stars. After performing photometry on the images to find the
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instrumental magnitudes of the stars in the fields, they converted the magnitudes to Cousins R
and I (thus, all values in the paper are reported in the Cousins system). Figure 3 shows a Color
Magnitude Diagram (CMD) of the stars in the field of view centered on the nucleus. The authors
found only 648 stars in the entire field of view; we are very sure that there are more stars than
that! Most of the stars are red giants, but there is a small population of young blue stars. This is
unexpected; you would not expect to see such young stars surrounded by red giants. The authors
are certain that they are not foreground stars, and take a guess that they are blue stragglers. Blue
stragglers are bright and young blue stars in a region of mostly old and red stars. We do not
know how they form, but the leading theory is that they are merging or merged binary stars. The
authors concede that further study of the blue stars would be useful (Rich et al. 1995).
The authors take a conservative claim that the tip of the Red Giant Branch (RGB) is at I =
19.5 magnitude in Figure 3 (using the distance modulus of 24.47, this corresponds to MI=-4.96).
The line on the CMD in Figure 3 is complicated; it indicates that the magnitudes of stars brighter
than it cannot be trusted to be completely accurate due to photometric errors. Stars above the line
are up to 0.5 magnitudes dimmer than reported. They also point out that the stars that appear as
circles and diamonds cannot be trusted because they are too blue or too red (Rich et al. 1995).
This paper clearly shows that even HST has a hard time resolving stars in M31’s bulge.
Surprisingly, there are constraints for both the brightest stars and the dimmest stars in the bulge.
The authors explain that the reddest M giants reported in Frogel et al. (1987) would have I = 22
(MI = -2) magnitudes in M31. Clearly, they would not detect a star that dim. Moreover, due to
the bright background of the bulge, there is a detection limit on the bright stars detectable (Rich
et al. 1995).
However, there was one conclusion that could be drawn from the WF/PC observations of
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Rich et al. (1995). The tip of the RGB, at MI = -4.96, is 0.9 magnitudes brighter than most metalpoor Galactic globular cluster RGBs described in Da Costa et al. (1990). Also, based on previous
work by Mould et al. (1986) (they found the RGB tip at 7000 pc from the center of M31 to be MI
~ -4), these results suggest that the RGB tip gets brighter as you move towards the center of M31
(Rich et al. 1995).
MI
Figure 3: CMD for the field centered on the nucleus (not corrected for extinction). The
graph is discussed in the text above.
WFPC2 Observations of M31’s Bulge
The next observation and analysis of M31’s bulge using HST came a few years later. The
observations were done using a different camera aboard HST: the Wide Field and Planetary
Camera 2 (WFPC2). The WF/PC cameras were replaced with WFPC2 in 1993. WFPC2 consists
of four cameras; three wide field cameras and one planetary camera. The planetary camera has a
smaller field of view than the wide field cameras, but better resolution. The resolution of the
cameras did not change (still 0.1"/pixel for the wide field cameras and 0.043"/pixel for the
planetary camera), but there is no spherical aberration in the WFPC2 camera (Baggett et al.
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2002).
Jablonka et al. (1999) looked at three globular clusters in the bulge of M31. The purpose
of this paper was to show that the conclusions drawn from Rich et al. (1995) were incorrect.
Although Jablonka et al. (1999) was not the first paper to question the results from Rich et al.
(1995), it was able to show the actual magnitude of the RGB tip in the bulge of M31. Their
location in M31 is shown in Figure 2 (on page 5), along with the location of the Rich et al.
(1995) observations. The Jablonka et al. (1999) observations were 1.55 kpc (thousands of
parsecs), 0.92 kpc, and 0.80 kpc from the center of M31. The observation to the top left of the
bulge is 0.92 kpc from the center (observation of globular cluster G170), the one closest to the
bottom of the image is 1.55 kpc from the center (observation of globular cluster G198), and the
last one is 0.80 kpc from the center (observation of globular cluster G177). (Jablonka et al. 1999)
Jablonka et al. (1999) used filters F555W (V band) and F814W (I band). As before, the
number in the filter name is the wavelength (in nanometers) the filter is centered on. The I band
filter used in this paper is different than the one used in Rich et al. (1995), but this does not effect
the conclusions of the paper much. The filters are close enough to compare the results from the
two papers. As with Rich et al. (1995), after performing the photometry on the data, the authors
converted the instrumental magnitudes in the I band to the Cousins system (Jablonka et al. 1999).
The authors limit the discussion to the stars on the planetary camera. They find tens of
thousands of stars in each of the three observations. The dimmest stars they report are I ~ 26.5
magnitudes. The CMD for G170 is shown in Figure 4. Also, they find the RGB tip to be at MI ~ 2.5 magnitudes for all the fields. So, the authors detect dimmer stars and a dimmer RGB tip than
Rich et al. (1995). Also, the conclusions in this paper show the RGB tip is dimmer closer to the
center of the galaxy (as stated earlier, at 7 kpc, MI (tip) ~ -4); the exact opposite conclusion of
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Rich et al. (1995). The authors explain, “The observations presented in this study strongly
support the idea that [the] very bright stars [in Rich et al. (1995)] were likely the result of
spurious detections of blended stars due to crowding in WFPC1 …” (Jablonka et al. 1999).
Figure 4: CMD from Jablonka et al. (1999) of all stars in the planetary camera field of view for
G170. There are 53,036 stars detected in this field; the data are not corrected for extinction.
However, we cannot be sure which paper is correct. There should be a physical reason
why the brightness decreases or increases from the disk to the center of the galaxy. Bica et al.
(1991) indicates that higher metallicity populations usually have fainter I band RGB tips because
of TiO blanketing. So, a higher metallicity in the bulge region as compared to the disk would
account for the Jablonka et al. (1999) results. I will come back to this point later in the paper.
There are two major problems in comparing Jablonka et al. (1999) with Rich et. al
(1995). First, Jablonka et al. (1999) does not observe the center of M31. It is clear from Figure 4
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that the observations are of different fields. It is possible that the two papers were looking at a
different set of stars. Jablonka et al. (1999) admits to this shortcoming.
The second problem with the comparison is that Jablonka et al. (1999) observed globular
clusters near in the bulge of M31. Rich et al. (1995) observed open cluster stars in the bulge;
there is no particular reason that the two populations should be the same. For this reason, another
paper was written on these observations. Jablonka et al. (2000) looked at G170, G177, and G198
globular cluster fields studied in the previous paper in addition to three control fields. The three
control fields are adjacent to the three previous fields studied. Jablonka et al. (2000) find no
difference in age or metallicity between the globular cluster populations and the stars in the
control fields. This demonstrates that the results of Jablonka et al. (1999) can be extended to the
bulge stars outside of the globular clusters studied.
Metallicity and Age
In 2005, after decades of observations of M31, two groups independently and nearly
simultaneously developed a metallicity distribution function (MDF) for the bulge region of M31.
One of those papers was Sarajedini et al. (2005) who used the same observations as Jablonka et
al. (1999) to construct the MDF. In fact, they present the same CMD of the field around G170
(converting the apparent magnitudes to absolute magnitudes and correcting for extinction) with
metallicity-dependent isochrones. First, the authors assumed that the age of the stars in the field
was 12.6 Gyr. They explain that the precise age does not effect the results much; that is, if
10<age<15 Gyr, the peak metallicity will change by ±0.05 dex. Next, the authors use
metalliicty-dependent isochrones from Girardi et al. (2002) and Salasnich et al. (2000) to
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determine the metallicity of the stars in the field. These metallicity-dependent isochrones cover a
metallicity range from [M/H] = -2.28 to [M/H] = 0.566 (the values are given in terms of Z, with
the conversion: [M/H] = log10(Z/Zsun), where Zsun = 0.019). See Figure 5 (right graph). From
these metallicity-dependent isochrones, they construct a MDF for the bulge of M31. The stars
used for the MDF had MI < -2.1; there were a total of 7771 such stars. The peak metallicity is at
[M/H] ~ 0 with a quick decline for higher metallicities, and a gradual decline for low
metallicities; see Figure 5, left graph (Sarajedini et al. 2005).
Figure 5 – Right: “CMD for the field surrounding G170 adjusted for distance and reddening …
The solid lines represent the theoretical RGBs of Girardi et al. (2002) for an age of 12.6 Gyr and
metallicities of Z = 0.0001, 0.0004,0.001, 0.004, 0.008, and 0.019. The two reddest RGBs are
taken from Salasnich et al. (2000) for metallicities of Z = 0.04 and 0.07.” (Sarajedini et al. 2005)
Left: Metallicity Distribution Function for the G170 field. 7771 stars were used to construct it.
Peak at [M/H] ~ 0.
The results from Sarajedini et al. (2005) are confirmed in Worthey et al. (2005) who
looked at several WFPC2 fields from the bulge to the disk; observations ranged from 4 kpc from
the center of M31 to 50 kpc from the center. The authors got very similar results for the bulge of
M31 as Sarajedini et al. (2005). Moreover, the authors found that the metallicity decreases from
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the bulge to the disk of M31. This fact pretty much confirms the results of Jablonka et al. (1999)
(see previous section). The higher metallicity would create a dimming effect in the I band due to
TiO blanketing, which would make the RGB tip appear dimmer in the bulge than in the disk
(Jablonka et al. 1999).
Comparison to the Milky Way
Our understanding of the stars in the Galactic bulge is much better than that of M31’s
bulge. As shown in Santiago et al. (2006), there is a large range of stars in the Galactic bulge.
We see stars brighter and dimmer stars in the Galactic bulge as compared to M31. This is to be
expected; we see dimmer stars because the Galactic bulge is closer to the Earth than M31. We
see brighter stars in the Galactic bulge because the bright, unresolved nucleus of M31 puts an
upper limit on the brightest stars we can detect.
A much more useful comparison is between the metallicity and age of the Galactic bulge
and M31’s bulge. Zoccali et al. (2003) provides the most comprehensive MDF available. In this
paper, the MDF is derived from the photometry of the Galactic bulge in the V and K bands.
Several Galactic globular clusters (with known MV, MK, and metallicities) are used to determine
the appropriate metallicity for the stars observed. Figure 6 shows the resulting MDF (containing
503 stars). There are three graphs; the purpose is to compare the work in this paper to the results
from previous papers. The authors derive the MDF in units of [M/H], but convert these values to
[Fe/H] for the comparison. As you can see from Figure 6, the MDFs are different from paper to
paper, but not by very much (Zoccali et al. 2003). The sole exception is the MDF of Sadler et al.
(1996), which has a similar peak but a greater range toward higher values.
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Zoccali et al. (2003) report a peak metallicity in the Galactic bulge of [M/H] = -0.1. This
is 0.1 dex lower than Sarajedini et al. (2005) report for the metallicity of M31’s bulge. The
results are in pretty good agreement, as can be seen in Figure 7. Sarajedini et al. (2005) overlaid
the Galactic bulge MDF onto their MDF for M31’s bulge. However, to be consistent, they rederived the Galactic bulge MDF using the Girardi et al. (2002) and Salasnich et al. (2000)
isochrones. That is, they took the photometry of the stars in the Galactic bulge (as presented in
Zoccali et al. 2003) and used the metallicity-dependent isochrones to determine metallicity. The
MDFs are essentially the same for both galaxies. Additionally, it is known that metallicity
decreases from the bulge to the disk in the Milky Way (Elmegreen 1997). Worthey et al. (2005)
showed that this occurred in M31 as well.
Figure 6 – Comparison of the Galactic bulge MDF derived in Zoccali et al. (2003) to
Ramirez et al. (2000), Sadler et al. (1996), and McWillam et al. (1994).
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The age of the Galactic bulge is also discussed in Zoccali et al. (2003). The authors have
determined that there is no age difference between the Galactic bulge stars and the Galactic
globular clusters (specifically, 10-13 Gyr). Sarajedini et al. (2005) estimated the age of M31’s
bulge to be between 10 and 15 Gyr. So, as expected, the stars in the Galactic bulge are very
similar to the stars in M31’s bulge.
Figure 7 – The MDF from Sarajedini et al. (2005) is represented by the dashed line and circles.
The re-derived Galactic bulge MDF is represented by the histogram.
Conclusions
Although HST has improved our understanding of M31’s bulge, the region needs to be
studied further. The regions that Rich et al. (1995) studied need to be observed with WFPC2 to
confirm the conclusions of Jablonka et al. (1999). Moreover, more regions should be examined
to confirm the age and metallicity results from Sarajedini et al. (2005) and Worthey et al. (2005).
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However, before we can be confident about our knowledge of M31’s bulge, we must
determine the correct extinction to it. Many astronomers believe we have an incorrect value for
the extinction to M31’s bulge (Liu, private communication). They feel that the current extinction
value underestimates the true value. If this is the case, in addition to the reported brightness and
color of the stars in the region being incorrect, the metallicity of the bulge stars would be
reduced. Moreover, if the true extinction value is much greater than the accepted value, then the
age of the bulge population will be less than 10 Gyr. This would make the MDF presented in
Sarajedini et al. (2005) inaccurate, since they created their MDF with the assumption that
10<age<15 Gyr. Thus, it is very important that we find out the true value of the extinction to the
bulge of M31 through further observations.
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