Issue |
A&A
Volume 515, June 2010
|
|
---|---|---|
Article Number | A49 | |
Number of page(s) | 8 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200913527 | |
Published online | 08 June 2010 |
Interstellar extinction and the distribution of stellar populations in the direction of the ultra-deep Chandra Galactic field
M. Revnivtsev1,2,3 - M. van den Berg4 - R. Burenin2 - J. E. Grindlay4 - D. Karasev2 - W. Forman4
1 - Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching, Germany
2 -
Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
3 -
Visiting Fellow, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4 -
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
Received 22 October 2009 / Accepted 3 March 2010
Abstract
We studied the stellar population in the central 6.6
6.6
region of the ultra-deep (1Msec) Chandra
Galactic field - the ``Chandra bulge field'' (CBF) approximately 1.5
degrees away from the Galactic Center - using the Hubble Space
Telescope ACS/WFC blue (F435W) and red (F625W) images. We mainly
focus on the behavior of red clump giants - a distinct stellar
population, which is known to have an essentially constant intrinsic
luminosity and color. By studying the
variation in the position of the red clump giants on a
spatially resolved color-magnitude diagram, we confirm the anomalous
total-to-selective extinction ratio, as reported
in previous work for other Galactic bulge fields. We show that the
interstellar extinction in this area is
on average, but varies significantly between
3-5 on angular
scales as small as 1 arcminute. Using the distribution of red clump
giants in an extinction-corrected color-magnitude diagram, we
constrain the shape of a stellar-mass distribution model in
the direction of this ultra-deep Chandra field, which will be
used in a future analysis of the population of X-ray sources. We also
show that the adopted model for the stellar density distribution
predicts an infrared surface brightness in the direction of
the ``Chandra bulge field'' in good agreement (i.e. within
15%) with
the actual measurements derived from the Spitzer/IRAC observations.
Key words: stars: general - Hertzsrung-Russell (HR) and C-M
diagrams - stars: luminosity fonction, mass function - Galaxy:
bulge - galaxies: stellar content - X-rays: galaxies
1 Introduction
Our Galaxy hosts discrete X-ray sources of various types. The X-ray
luminosities of Galactic sources range from
erg s-1 for very luminous accreting neutron star and black hole
binaries down to
1026-1030 erg s-1 for coronally active
stars. Statistical properties of these different populations of
sources are poorly known with large allowed ranges or uncertainties, which in some cases might lead
to an underestimation of their contribution to the global X-ray
emission of our Galaxy and other galaxies.
Constraining the luminosity functions of the X-ray emitting populations is particularly relevant for understanding the faint, unresolved X-ray emission of the Galaxy that is distributed quite smoothly along the Galactic plane (the so called Galactic ridge X-ray emission, GRXE), which was discovered in early X-ray experiments (e.g. Worrall et al. 1982). Until recently, the nature of the GRXE remained unexplained, to a large extent due to our scarce knowledge of the statistical properties of populations of Galactic X-ray sources. Only recently it was shown that at least the majority of this unresolved X-ray emission arises from the cumulative emission of a large number of faint discrete sources, predominantly accreting white dwarfs and coronally active stars (Revnivtsev et al. 2006,2009).
Understanding the origin of the GRXE would have been impossible without the progress in understanding the statistical properties of faint Galactic X-ray sources, which was achieved through X-ray all-sky surveys (Sazonov et al. 2006). However, due to the limiting sensitivity of currently available all-sky surveys, these studies can probe only sources near the Sun, and the number of sources that are used to construct luminosity functions is small. This leads to significant uncertainties in the inferred properties and leaves the question about possible variations of populations throughout the Galaxy open. The resulting uncertainties have direct consequences for our studies of distant galaxies. As has been shown, the GRXE-type emission is a dominant component (after subtracting the contribution from low mass and high mass X-ray binaries with luminosities L >1035 erg s-1) for a significant fraction of non-starburst galaxies (see e.g. Revnivtsev et al. 2008,2007). Therefore it is important to understand the level of universality of the X-ray luminosity per unit solar mass inferred from studies of nearby objects. Another non-local sample of faint Galactic X-ray sources is required to address these questions.
The brightest (
erg s-1) end of the luminosity
functions of various classes of faint Galactic sources can be probed
over distances of several kilo-parsecs by surveys of the plane with a
limiting sensitivity
erg s-1 cm-2 and this is now extensively explored
(e.g. Hands et al. 2004; Grindlay et al. 2005). But for fainter sources one needs
to perform much deeper observations (with limiting sensitivity
10-16-10-17 erg s-1 cm-2) and choose a region with
minimal obscuring dust and gas to avoid the effects of interstellar
absorption. These observations with a total exposure time of 1 Msec
were recently performed with the Chandra X-ray Observatory in a
direction approximately 1.5 degree away from the Galactic center.
This field, located at the galactic coordinates
and chosen for its low
extinction (
;
see e.g. Drimmel et al. 2003) and proximity
to the Galactic Center, was first observed with Chandra in 2005
for 100 ks (``LimitingWindow'' target) to study the nature and radial
distribution of hard (2-10 keV) X-ray point sources in the inner Galactic bulge (Hong et al. 2009).
Simultaneously, images with the Advanced Camera for Surveys Wide Field
Camera (ACS/WFC) on the Hubble Space Telescope (HST) were
obtained of the central 6.6
6.6
region (centered at
RA = 267.86968 deg, Dec =
-29.581033 deg , J2000) of the field
observed by Chandra to search for optical counterparts
(van den Berg et al. 2009). Analogous to other inner-bulge fields included
in this survey (e.g. Baade's Window and Stanek's Window, at
3.8-degree and
2.2-degree offsets from the Galactic
center, respectively) this field was called the Limiting Window,
which refers to the rapidly declining opportunities for optical
identification when moving closer to the heavily-obscured Galactic
center region. In 2008, Revnivtsev et al., who referred to this region
as the ``1.5 degree field'', indicating its approximate angular distance
from the Galactic Center - revisited this field with Chandra for 900 ks
to derive constraints on the resolved fraction of the GRXE
(Revnivtsev et al. 2009). Below we refer to this field as the CBF - ``Chandra Bulge field''.
![]() |
Figure 1: Three-color optical images of the field containing the ultra-deep Chandra observations of the CBF. The upper panel shows the mosaic image obtained by the 1.5 m Russian-Turkish telescope (RTT150 telescope) (red shows the i' spectral band, green - r', blue - g'), while the lower panel shows a smaller area observed with the HST ACS/WFC (red - F625W, green - F658N, blue - F435W). From the top image it is evident that the interstellar extinction is strongly variable across the field (the regions close to boundaries of the figure are darker than the central regions). It is seen that the majority of stars are relatively faint and yellow - these are the old bulge population of the Galaxy. |
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In comparison with the Solar neighborhood, the analysis of the origin of the X-ray emission from the CBF has its own complications:
- First of all, we need to know the stellar density enclosed by the volume studied by Chandra.
- We need to understand the importance of the absorption that results from cold interstellar matter along the line of sight.


2 Extinction in the CBF
2.1 Red clump giants
The red giant branch in the Hertzprung-Russell diagram shows a distinct
feature that corresponds to a special subpopulation known as the red
clump giants (RCGs). The
intrinsic colors and absolute magnitudes of these core helium-burning
stars are well-defined with little dispersion
(e.g. Paczynski & Stanek 1998) and only a weak dependence on age and
metallicity. Therefore RCGs can play the role of
standard candles and are very useful for various studies of either
stellar populations or properties of the interstellar medium. In
particular, these stars have been used to map the Galactic bar
(Stanek et al. 1997,1994), to measure distances to the Galactic
Center (Paczynski & Stanek 1998) and to the galaxy M 31 (Stanek & Garnavich 1998), and to
measure the properties (e.g. Wozniak & Stanek 1996) and spatial variations
(Sumi 2004) of interstellar extinction in the Galactic bulge.
Ground based observations with their limited angular resolution,
encounter severe confusion problems in regions close to the Galactic
Center (GC), which complicates the stellar photometry. Because of this
problem and also due to the very large extinction, the majority of RCG
studies were previously limited to angular distances larger than 2
from the GC, while our field of interest is located at
1.5
from the GC. As a demonstration of the confusion
problems, we compare images of our field as observed by Magellan/IMACS
with a seeing of 0.8
and by HST ACS/WFC in
Fig. 2. As the figure demonstrates, the ground
based data are strongly
confusion limited. Photometric measurements of stars with magnitudes fainter than
17-18are strongly affected by the contribution of fainter unresolved stars.
![]() |
Figure 2: This comparison of a small part of the CBF as observed by Magellan/IMACS (R) ( left) and the HST ACS/WFC (F625W) ( right) demonstrates the problem of confusion in ground-based images. |
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2.2 Intrinsic properties of RCGs in the F435W and F625W filters
To understand the luminosity and intrinsic colors of an unabsorbed population of
RCGs in the selected HST/ACS filters, we used the catalog of
284 Hipparcos RCGs from
Alves (2000). As we are unable to make real observations of all
listed RCGs in these two filters, we calculated their apparent
brightnesses with the Kurucz model atmosphere spectra as included in the
SYNPHOT/STSDAS/IRAF
package (Bushouse & Simon 1994) as follows. For all stars in Table 1 of
Alves (2000) we selected the stellar models that best
match the combination of B-V and V-I colors as presented in
Alves (2000). The resulting spectra were then convolved with the
F435W and F625W transmission curves in SYNPHOT to calculate the
magnitudes in these filters. We assumed that all stars in
Alves (2000) have zero extinction, because they are all very
nearby. As a check of the overall approach, we calculated the
predicted magnitudes of the stars in the K band and compared them
with their observed K magnitudes as listed in
Alves (2000). The difference between the predicted and observed magnitudes has a normal
distribution with zero mean and rms scatter of 0.13 mag, demonstrating
that our approach is reasonable. We might expect that the accuracy of
the predicted brightnesses in the F435W and F625W filters is even
better because their transmissions are closer to those of the B, Vand I filters - which we used as a reference for the spectral
modeling - than to the K-band response.
![]() |
Figure 3:
Distribution of the absolute brightness of the 284 red clump giants in
Table 1 of Alves (2000), converted into the F625W HST/ACS
filter. The solid curve is the model fit to the distribution. It
consists of a linear function plus a Gaussian with centroid at
MF625W=0.75 and
|
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The resulting distribution of absolute magnitudes of RCGs in the
F625W filter is presented in Fig. 3. The distribution is
clearly peaked, with centroid at
MF625W=0.75 and an rms
scatter around this value of 0.19. The solid line model includes also the
linear component following the approach of Alves (2000), which reflects
some ``background'' population of
red giants in the considered color and brightness intervals.
The color of the peak of the
RCG distribution is
with an rms
scatter 0.18. There is no strong indication for the dependence of
the position of the peak on color.
As an additional check, we repeated the above analysis with the SYNPHOT spectra from the Pickles atlas (Pickles 1998) and found the same distribution for MF625W.
2.3 Ratio of the total-to-selective extinction
For our subsequent analysis, we used the HST/ACS images taken with the F435W and F625W filters, which are somewhat similar to the SDSS g' and r' filters, to identify the RCGs in the CBF and derive their magnitudes. Details of the photometric analysis are given in van den Berg et al. (2009). Below we will use the STMAG photometric system.
It has been shown by several authors that the extinction law towards
the Galactic bulge differs from the standard one and
in addition shows variations between different lines of sight
(see e.g. Popowski 2000; Udalski 2003; Sumi 2004). Therefore we must
determine the extinction law before we
are able to determine the total extinction in our field. Like Udalski (2003) and
Sumi (2004) we used the shift of the RCGs in the
(mF625W,
mF435W-mF625W) color-magnitude
diagram due to variable extinction in our field to estimate the ratio
of the total-to-selective extinction. This shift can be clearly seen in
the spatially-resolved (mF625W,
)
color-magnitude diagrams in Fig. 2 of
van den Berg et al. (2009). Then, using the mean intrinsic color of RCGs as
determined above, we calculated the total extinction towards the CBF.
![]() |
Figure 4:
Dependence of the mean magnitude of RCGs on their
mean colors for different locations within the
HST/ACS field. The slope of the linear fit to these
positions (solid line) measures the ratio of the total to selective
extinction
|
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We divided the area observed by HST/ACS into rectangular bins of
the size
.
Of the stars detected in each
angular bin
we filtered out those that are clearly not associated with
the red clump, viz. those that are fainter in F625W than
mF625W>17+(mF435W-mF625W) and have a color
(mF435W-mF625W) less than 2.0. To measure the mean magnitude and
color of RCGs, we then determined the mean color
and magnitude of the stars within an ellipse of the height
mag and width
mag
which is centered on the peak of the number distribution of the
remaining stars in the color-magnitude diagram. The resulting
dependence of the mean magnitude of RCGs on their mean color is
presented in Fig. 4. The variation of the RCG
mean magnitude with color can be adequately
described by a linear function with a ratio of the total-to-selective
extinction
.
The scatter of the measured centroids of
the RCG positions around this fit is compatible with the statistical
uncertainties in the individual centroids. The total number of RCGs
used in these calculations is on the order of a few hundreds per
bin. Note that the standard extinction law predicts
-1.9
(Sirianni et al. 2005), thus larger than we measure in the CBF (a similar conclusion
regarding the smaller ratio of the total-to-selective extinction was found previously by e.g. Popowski 2000; Udalski 2003; Sumi 2004)
The position of the red clump in the color-magnitude diagrams of the two
different spatial bins is shown in Fig. 5. A color-magnitude diagram of
all photometered stars in the HST/ACS images with the extinction
reduced to the value of
is shown in Fig. 6.
![]() |
Figure 5:
Color-magnitude diagrams in two areas of the HST/ACS
field, with the number of stars per image bin color coded. The
number of faint stars (
mF625W>20.5) is artificially depressed
via multiplication by a factor
|
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![]() |
Figure 6:
Color-magnitude diagram of all photometered stars in the
HST/ACS field. The magnitudes and colors of stars in each
rectangular
|
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Now we computed the apparent magnitude of RCGs corrected for
extinction (
mF625W, 0) in each spatial bin with the
assumption that their intrinsic mean color is
:


The resulting average value is






3 Constraints on the stellar density distribution in the CBF
The shape of the clump formed by the RCGs in Fig. 6
suggests that their position is still affected by additional
extinction - the distribution of the RCGs appears a bit elongated in the direction of the
reddening. This shows that the extinction varies even on angular scales
smaller than
.
Note that this effect is almost absent
in regions with smaller total extinction - after correction for
extinction, the RCGs form a quite compact locus without an extension in the
reddening direction (see e.g. Sumi 2004).
However, apart from the possible elongation connected with any residual reddening effects, the RCGs define a relatively narrow clump, which can be used to constrain the distribution of stars along the line of sight toward the CBF.
3.1 Model of the stellar distribution
In any direction within the Galaxy stars are distributed over a range of distances along the line of sight. In the regions far away from the GC, the dominant stellar component is the stellar disk, while at small angular distances from the GC the majority of the stellar mass is provided by the Galactic bulge and/or bar, nuclear stellar disk or nuclear stellar cluster components. In our subsequent analysis we will consider only three components of the stellar distribution: the Galactic bulge, Galactic disk and nuclear stellar disk. We will adopt the distance to the Galactic Center of 8 kpc, which is some intermediate value between measurements of Eisenhauer et al. (2005) and Ghez et al. (2008). The nuclear stellar cluster is important only for the very central parts of the Galaxy, at distances less than 10-20 arcmin (see e.g. Launhardt et al. 2002), and, therefore we will not consider it here. We use the following simplified models of the three stellar components. Here r is the Galactocentric distance, z is the height above the Galactic plane.
For the stellar disk density we use (from Revnivtsev et al. 2006)

The normalization of this component was chosen to yield an integrated stellar disk of


For the Galactic bulge component we used the simple analytic form from
Dehnen & Binney (1998) and Grimm et al. (2002)

The adopted total mass of the Galactic bulge is

We assumed that the nuclear stellar disk (NSD) has the following density distribution

where





The resulting assumed distribution of the stellar density along the
line of sight in the direction of the CBF is presented in
Fig. 7. For the adopted model, the total
surface mass density in this direction is
sq.arcmin-1, while the bulge component contributes
sq arcmin-1, the stellar disk
contributes
sq arcmin-1, and the
nuclear stellar disk contributes
sq arcmin-1.
![]() |
Figure 7:
Distribution of the density in the different adopted stellar
Galactic components along the line of sight in the direction of the
CBF, i.e. towards
|
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![]() |
Figure 8: Crosses - observed differential distribution of the number of stars in the HST/ACS field with color mF435W-mF625W>2.5 as a function of their apparent F625W magnitude after correcting all magnitudes to a common value of the extinction that corresponds to some arbitrary chosen position in the field of view (see color-magnitude diagram on Fig. 6). The size of the magnitude bins is 0.085 mag. Solid histogram - modeled differential distribution of RCGs as a function of their apparent F625W magnitude in the field of our study. The dashed line shows a part of the same distribution but with some linear function added to match the surrounding magnitude bins. |
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3.2 Comparing the shape of the stellar model distribution with HST observations
By choosing some fiducial number of the RCGs per unit mass of the stellar
population, we can use these stellar distribution models together with
the intrinsic magnitude distribution of RCGs (Fig. 3)
and the derived extinction in our field, to construct the differential
distribution of RCGs as a function of their apparent magnitude in the
F625W band and compare this prediction with the data. The model
distribution and the observations (distribution of photometered
stars from Fig. 6 with colors
mF435W-mF625W > 2.5) are shown in Fig. 8. Here we
assumed that all (or the majority of) stars are located behind the
extinction region. This assumption is supported by studies of the
distribution of the interstellar extinction along the line of sight by
e.g. Drimmel et al. (2003) or Marshall et al. (2006). See also Fig. 1 in
van den Berg et al. (2009), which shows that according to these models most
of the absorbing material is closer than 6 kpc, whereas
Fig. 7 in this paper shows that the bulk of
stars lies further away than
6 kpc.
Figure 8 shows the model
distribution of stars in the studied region (histogram and dashed line). It was obtained
via convolution of the model of the stellar mass distribution over the line of
sight with the distribution (see Fig. 7) of RCGs over
their absolute magnitudes (see Fig. 3), assuming the denisty
of the RCGs in their gaussian component
.
This normalization value was chosen to fit the observed points. This RCG density
number in general looks reasonable (see e.g. López-Corredoira et al. 2002), however, a more
quantitative comparison of this number with those obtained in different observational
and theoretical works requires more accurate treating of different selection effects.
The figure shows that the model distribution very closely follows the
measured points (crosses) around the corresponding peak of the
color-magnitude diagram. The linear function that is added to make the
model distribution match with the magnitude bins that surround the
observed peak could originate from red giants and asymptotic giant
branch stars that also lie in this part of the color-magnitude diagram,
but do not belong to the red clump (see also Alves 2000).
There are some other subtle differences to note. In particular, the model (which takes into
account the intrinsic dispersion of the RCGs brightnesses at the level
of 0.19 mag) has a Gaussian width of the main peak of 0.23 mag. However, the Gaussian width of the RCG peak is
in the CBF. This is somewhat higher than we
might expect from our model.
We should keep in mind though that there may be several complications
that could result in the observed additional broadening. For example,
it is likely that we have not
completely corrected for all the extinction in our field due to its
variations on small angular scales (see the discussion of the shape of
the
red clump in Fig. 6); with the adopted technique based
on RCGs, these variations are difficult to account for, because the
density of RCGs is too low to use smaller spatial bins. Residual
variations of the total extinction in our field with an amplitude
mag can result in the observed
broadening of the RCG distribution.
It is also possible that the intrinsic (i.e. approximately extinction-corrected) brightness of the RCGs in the Galactic bulge has a slightly higher scatter than that in the Solar vicinity, which we used to compute the model distribution.
It is also possible that the extra broadening is the result of our oversimplified picture that all the extinction is located in front of the majority of the Galactic bulge RCGs. If some residual extinction, distributed within the Galactic bulge, is present in our field, it will similarly result in a slight widening of the RCG peak in the differential number-counts plot.
Finally it could be that the adopted model for the stellar density distribution of the bulge oversimplifies the true distribution, resulting in a density peak that is too narrow. However, our tests to vary the parameters of the stellar model showed that the latter is unlikely.
Summarizing the above findings, we conclude that the shape of the adopted model of the stellar density distribution in the Galaxy adequately describes the observed distribution in the CBF.
3.3 Stellar surface brightness in the near-infrared
Another important check of the adopted stellar mass distribution in
the direction of the CBF is the total surface brightness provided by
stars. We took the value of the total luminosity density of the
Galactic bulge as measured by Dwek et al. (1995) at a wavelength 3.5 m -
erg s-1 Hz-1 - and assigned
it to our adopted total mass of stars in the Galactic bulge -
- to calculate a specific infrared luminosity
density per unit mass. Then we integrated the surface brightness
provided by all stars in our model (assuming the same
luminosity-to-mass ratio for all stellar components). We point out that
due to
differences in age and/or composition compared to the bulge, the true
specific luminosity density at 3.5
m could be slightly different
for the (nuclear) disk, although their contributions
to the total surface brightness are relatively small (see below). The
result of this integration is shown in Fig. 9. The
observed infrared surface brightness at 3.6
m in our field
is
MJy/sr, or
Jy sq arcmin-1, as measured by
Spitzer/IRAC
(see Revnivtsev et al. 2009). It is seen that the adopted model
correctly predicts
the observed surface brightness. The majority of the 3.5
m flux is provided by the stellar population in the Galactic
bulge (
82%) with small contributions from the stellar disk
(
9%) and the nuclear stellar disk (
9%). The dominance
of the Galactic bulge stars to the total stellar mass budget in the field
is also evident from Fig. 1. The vast majority of stars in
the high resolution HST/ACS image are faint and have a
yellow/red color, which shows that they are of a relatively low mass and
are located at a large distance from us. This is exactly what is expected
for a stellar population of the Galactic bulge with an age of
10 Gyr.
![]() |
Figure 9:
Cumulative stellar surface brightness at 3.5 |
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4 Conclusions
The study of populations of Galactic X-ray sources requires knowledge of the properties of the underlying population of normal stars, because only then one can estimate the statistical properties of X-ray sources and their systematic dependencies on the age, metallicity, density etc. of the stellar population. One of the largest projects performed recently by the Chandra X-ray Observatory is the ultra-deep observation of a region close to the Galactic Center - the CBF. This observation provides us with a wealth of data on the X-ray population in this area. To connect these properties to the properties of normal stars we must study this area in the optical and near-infrared bands.
We analyzed HST/ACS images of the central 6.6
6.6
field of the CBF in the F435W and F625W bands and found the following:
- Interstellar extinction in the direction of the CBF is
and variable on an angular scale of
1
. Variations of the extinction in this area on larger scales were previously noticed by Dutra et al. (2003) (
scales), Revnivtsev et al. (2009) (
scales), van den Berg et al. (2009) (
scales).
- The color-magnitude diagram corrected for extinction, as determined in
angular bins of
, suggests that there is still residual extinction on smaller scales at the level of
mag or less. This would be difficult to measure with RCGs due to their decreasing numbers.
- We determined the extinction law (in particular, the ratio of the
total-to-selective extinction
) in the direction of the CBF -
- and found that it is significantly different from the canonical one
1.9. The discrepancy shows a similar trend as that determined by various authors using OGLE data of the bulge (e.g. Popowski 2000; Udalski 2003; Sumi 2004). This suggests that we might expect deviations from the standard correspondence between the photoabsorption column
, measurable in the X-ray band, and extinction in the optical/NIR bands. This likely differing X-ray (vs. IR) absorption in the bulge region will affect derived intrinsic X-ray spectra and thus (in some cases) the inferred source types (e.g., with less X-ray absorption for a given color excess, soft spectral components will be decreased, etc.).
- We compared a model of the stellar distribution along the line of
sight towards the CBF with the observed distribution of red clump
giants in the color-magnitude diagram. We show that the
adopted model adequately describes the data. In the model, the
majority of the stellar mass in the direction of the CBF is provided
by Galactic bulge stars with only a small contribution from the stellar disk
and nuclear stellar disk components (
5% and
9%, respectively). Small discrepancies between the model and the data could be the result of variations in the extinction on small angular scales (
1
), differences between the local and bulge luminosity functions of red clump giants, uncertainties in the distribution of the extinction along the line of sight, or an oversimplified model of the stellar mass profile of the bulge.
-
We showed that the adopted stellar model predicts an integrated
3.5
m surface brightness of this part of the Galaxy that is only 13% higher than the surface brightness actually measured by Spitzer/IRAC in the 3.6
m band.
The authors thank Annamaria Donnarumma and Maxim Markevitch for their help in obtaining Magellan observations of the field. This research made use of data obtained from the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. This work was supported by grants of the Russian Foundation of Basic Research (07-02-01051, 07-02-00961-a, 08-08-13734, 07-02-01004, 08-02-00974, NSh-5579.2008.2) and programs of Presidium of RAS P04 and OFN-17. MvdB was supported in part by STScI/HST grant HST-GO-10353.01. Results in this paper are based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Institute which is operated by the Association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.
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Footnotes
All Figures
![]() |
Figure 1: Three-color optical images of the field containing the ultra-deep Chandra observations of the CBF. The upper panel shows the mosaic image obtained by the 1.5 m Russian-Turkish telescope (RTT150 telescope) (red shows the i' spectral band, green - r', blue - g'), while the lower panel shows a smaller area observed with the HST ACS/WFC (red - F625W, green - F658N, blue - F435W). From the top image it is evident that the interstellar extinction is strongly variable across the field (the regions close to boundaries of the figure are darker than the central regions). It is seen that the majority of stars are relatively faint and yellow - these are the old bulge population of the Galaxy. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: This comparison of a small part of the CBF as observed by Magellan/IMACS (R) ( left) and the HST ACS/WFC (F625W) ( right) demonstrates the problem of confusion in ground-based images. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Distribution of the absolute brightness of the 284 red clump giants in
Table 1 of Alves (2000), converted into the F625W HST/ACS
filter. The solid curve is the model fit to the distribution. It
consists of a linear function plus a Gaussian with centroid at
MF625W=0.75 and
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Dependence of the mean magnitude of RCGs on their
mean colors for different locations within the
HST/ACS field. The slope of the linear fit to these
positions (solid line) measures the ratio of the total to selective
extinction
|
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Color-magnitude diagrams in two areas of the HST/ACS
field, with the number of stars per image bin color coded. The
number of faint stars (
mF625W>20.5) is artificially depressed
via multiplication by a factor
|
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Color-magnitude diagram of all photometered stars in the
HST/ACS field. The magnitudes and colors of stars in each
rectangular
|
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Distribution of the density in the different adopted stellar
Galactic components along the line of sight in the direction of the
CBF, i.e. towards
|
Open with DEXTER | |
In the text |
![]() |
Figure 8: Crosses - observed differential distribution of the number of stars in the HST/ACS field with color mF435W-mF625W>2.5 as a function of their apparent F625W magnitude after correcting all magnitudes to a common value of the extinction that corresponds to some arbitrary chosen position in the field of view (see color-magnitude diagram on Fig. 6). The size of the magnitude bins is 0.085 mag. Solid histogram - modeled differential distribution of RCGs as a function of their apparent F625W magnitude in the field of our study. The dashed line shows a part of the same distribution but with some linear function added to match the surrounding magnitude bins. |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
Cumulative stellar surface brightness at 3.5 |
Open with DEXTER | |
In the text |
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