Evolution of Galaxies from ELT Photometry of Stellar Clusters
Arne Ardeberg1 and Peter Linde1,2
1
Lund Observatory
2
Malmö University
arne@astro.lu.se, peter@astro.lu.se
Abstract
The reasons for our ignorance concerning the nature, formation and evolution of galaxies,
normal as well as exotic, have several explanations. Dominating are the distribution of
galaxies, the difficulty of separating superposed populations and effects of interstellar
extinction as well as the lacking light-collecting power of and, especially, the spatial
resolution feasible with our largest telescopes for visual and adjacent wavelength regions.
Realistic studies of stellar populations could, until recently, be made for the Galaxy and
the Magellanic Clouds only. With Very Large Telescopes (VLTs) using adaptive-optics
(AO) systems, all of the Local Cluster of Galaxies (LCG) can be included. Still, no
galaxy outside the LCG can be reached for population studies even with the best VLTs.
The LCG has a poor sample of galaxies, covering none of the normal types, emphasised
by recent results showing considerable differences even between M 31 and the Galaxy.
Stellar clusters are excellent for studies of evolution. Also in external galaxies, they are
easy to identify and separate from surrounding star fields. They can be studied with small
effects of galaxy orientation. The sole reason limiting the use of stellar clusters for
evolutionary studies has been their volume density of stars. For this reason, also with AO
VLTs, adequate use of stellar clusters is limited to the LCG. The closest full samples of
galaxies of all types are in the Virgo and Fornax clusters of galaxies. Reaching one of
these clusters with adequate light collection and spatial resolution for studies of clusters
would imply a definite road to knowledge of and insight into the evolution of galaxies.
We used observed data of an open cluster, NGC 6192, and modelled and simulated a
young stellar cluster. We placed the cluster in a surrounding stellar field composed of two
populations and at distances between 1 and 30 Mpc. Sky background and Poisson noise
was added. We used the Strömgren uvby system for photometry with a 50 m ELT, the
Euro50. We presumed a close to diffraction-limited PSF, an atmospheric turbulence
corresponding to an FWHM of 0.3 arcsec, a Strehl ratio of 0.7 and an actuator density of
7 mm for the deformable secondary mirror.
At dense intervals between 1 and 30 Mpc, we made (u)vby photometry of individual
cluster stars and determined cluster ages from turn-off point (TOP) photometry and
[Me/H] from m1 versus (b-y). In addition, we placed the stellar cluster at distances from
10 Mpc to 1 Gpc and measured integrated (b-y) data for age determination.
ELT TOP age determination gives accurate and constant results out to and including the
distances of the Virgo and Fornax clusters of galaxies. Even beyond 20 Mpc, TOP ages
are useful. [Me/H] data are of excellent quality out to approximately 10 Mpc and of
adequate quality out to the distance of the Virgo cluster. Open stellar clusters can be
identified and their ages estimated from integrated (b-y) data out to around 1Gpc. These
results open entirely new possibilities for ELT studies of the evolution of galaxies.
Further work and improvement of our methods are in progress.
Key words: Galaxies, evolution, star clusters, extremely large telescopes, adaptive
optics, spatial resolution, light collection, colour-magnitude diagram, metallicity diagram.
Nature, formation and evolution of galaxies
For the great majority of objects, the latest decades of development of tools and analysis
for astrophysics have caused an avalanche of new knowledge and insight. In this respect,
galaxies are an unfortunate exception. Over an extended period of time, they have, to a
very large extent, resisted the flooding of new understanding that has swept astronomy in
general. This resistance has endured and still does.
Progress regarding the evolution of stars, individually and in clusters, associations and
larger entities, has been fast. In contrast, our understanding of processes forming galaxies
is regrettably close to static, as it is also with respect to the subsequent evolution of
galaxies. In addition, while interaction between galaxies has been studied in a large
number of cases, we still know little about the consequences of such events. In total, our
grasp of processes determining the development of large-scale star formation as well as
the evolution of structure, dynamics and chemistry of galaxies is frustratingly poor.
The dilemma of galaxies
There are some obvious reasons for our lack of insight into the formation and evolution
of galaxies. They are due to a combination of factors regarding the distribution of
galaxies and our observing tools. These limitations are strong enough to survive even the
great progress of observing power due to the Very Large Telescopes (VLTs) with
apertures in the 8-10 m range, including the support provided by modern space
telescopes. While these telescopes, compared to their predecessors, offer very much more
favourable conditions in terms of light collection and spatial resolution, they still cannot
deliver the observational material necessary for a break-through concerning the evolution
of galaxies.
The Local Cluster of Galaxies
Even the great majority of the members of the Local Cluster of Galaxies have remained
beyond our insight into evolutionary patterns and mechanisms. Also regarding the
Magellanic Clouds, the evolution is far from understood. New data continue to change
the situation (Romaniello et al., 2004; Ferraro et al., 2004; Dall’Ora et al., 2004;
González et al., 2004). The situation concerning the evolution of M31 and M33 is
comparatively more open to new results (Salow and Statler, 2004; Burstein et al., 2004;
Williams and Shafter, 2004; Ciardullo et al., 2004).
2
The similarity between our Galaxy and M 31, similarly classified as galaxies, has often
been emphasized and recently supported (Stephens et al., 2003). However, new data
show increasing amounts of differences, some of them far from minor (Durrell et al.,
2001; Ferguson et al., 2002; Brown et al., 2003; Burstein et al., 2004). As earlier shown
by van den Bergh (2000), the globular clusters of the Galaxy and M 31 have many
characteristics that are rather similar, while other parameters demonstrate striking
differences. In total, the globular clusters indicate evolutionary scenarios of quite
different type for the two galaxies.
While recent data indicate significantly new characteristics of the Magellanic Clouds and
M 31, the remaining galaxies of the Local Group remain even less understood. There are
many facts underlining our lack of basic knowledge of the majority of the Local Group
galaxies (van den Bergh, 2000). Thus, even for these very nearby galaxies, we lack a
consistent picture of evolution.
The main reason for our deficient understanding of the evolution of the galaxies in the
Local Cluster is our lack of adequate observational data. While light collection is a nonnegligible problem, the most prominent need is for spatial resolution. With VLTs
equipped with full adaptive-optics systems, we can expect a highly important
improvement. With VLT data, we should be able to get fundamental data for
evolutionary studies also of the most distant of the galaxies in the Local Cluster.
Beyond the Local Cluster of Galaxies
For an adequate understanding of the evolution of galaxies, an obvious requirement is an
observational sample that represents all major types of galaxies. Unfortunately, the Local
Cluster of Galaxies does not provide such a sample, not even marginally. In addition,
while VLT data can, with some effort, adequately cover all galaxies in the Local Cluster,
such data are quite inadequate beyond the Local Cluster.
To obtain a sample of galaxies that is representative, we have to reach, with adequate
data, the most nearby clusters of galaxies outside the Local Cluster. Thus, we have to
reach the Virgo and Fornax Clusters, at distances between 16 and 20 Mpc. In both these
clusters of galaxies, excellent samples can be obtained. With respect to studies of
evolution in the Virgo and Fornax galaxies, VLT data can deliver neither the light
collection nor, even less, the spatial resolution necessary.
Observational studies of the evolution of galaxies
At distances beyond 10 Mpc, photometry is in practice the only observational tool
realistically adequate for meaningful over-all studies of the evolution of galaxies. In
addition to photometric precision, it requires efficient light collection and, even more
challenging, high spatial resolution. While studies of colour-magnitude diagrams
(CMDs), yielding ages of populations, can be adequately performed with a variety of
pass bands, corresponding high-quality studies of abundance or metallicity diagrams
3
(MDs) require pass bands specially designed for the purpose. Further, for meaningful
results, MD studies need significantly higher photometric accuracy than corresponding
CMD studies.
General photometric studies of stellar populations in galaxies are met with a number of
practical difficulties. Often, the discs of the galaxies are unfavourably inclined relative to
the lines of sight. With different parts of the discs, characterised by different evolutionary
parameters, super-position leads to observational data hard to separate and interpret. The
amount of interstellar dust in galaxies varies laterally as well as in depth, causing
spurious values of magnitudes and colours. Thus, it is very hard to isolate homogenous
stellar populations and even harder to study their specific evolutionary signatures.
Stellar clusters
For studies of evolution, stellar clusters have many important advantages. For most
practical purposes, all stars in a cluster can be regarded as coeval. In the same way, it can
be presumed that all member stars have the same initial abundance of heavy elements,
later diversified through the action of evolution only. Further, at distances significantly
larger than the cluster dimension, all member stars can be regarded as co-distant. In
addition, the normally rather limited dimensions and/or evolutionary status of stellar
clusters mean that the amount of internal interstellar extinction can, normally, be
regarded as insignificant. Finally, the limited dimensions alone imply that also the
amount of foreground interstellar extinction often can be taken as constant for all member
stars.
With their highly similar age, initial abundance, distance and extinction effects, cluster
stars define ideal entities for studies of stellar evolution. They are, however, not only
excellent as tools for evolutionary studies, they are also, normally, easy to identify as
well as to isolate from surrounding and super-posed populations. This also implies that
stellar clusters are easy to find and locate, also in a variety of circumstances and at large
distances. Finally, the properties of stellar clusters are normally relatively little affected
by the orientation of their parent galaxies.
Calibrations of fundamental stellar evolution parameters are regularly made using stellar
clusters as standards. An important example is the initial mass function (IMF). The
special role and importance of stellar clusters for evolutionary studies, and especially
concerning the IMF, were recently commented and confirmed by Chabrier (2003). In our
context, it is especially noteworthy that young stellar clusters are found to represent the
same stellar population as the stars of the surrounding parent-galaxy disc.
Spatial resolution
The limited dimensions of stellar clusters are, as discussed, very favourable from many
points of view. At the same time, the volume density of clusters makes them prone to
effects of image crowding and especially so at larger distances. This has, so far, very
much restricted the use of clusters as evolutionary probes for galaxies, also those in the
4
Local Cluster of Galaxies. In case of observations with an image quality defined by the
atmospheric turbulence, stellar clusters are difficult objects even in more distant parts of
the Galaxy.
Image crowding and ELTs
ELTs equipped with full adaptive optics systems define a new era in terms of image
quality. A 50 m AO ELT, such as the Euro50 (Andersen et al., 2003, 2004), will, at a
wavelength of 1000 nm, give images that are close to diffraction limited, with a
resolution of around 0.005 arcsec. When prepared for operation at optical-visual
wavelengths, it will, at around 500 nm, produce images with a resolution better than
0.003 arcsec. The corresponding Strehl ratio is foreseen to fall in an interval from 0.7 to
0.8.
Clusters, atmosphere and adaptive optics
We investigate the possibility of evolutionary studies of galaxies at large distances. As
tools for these studies, we have adopted stellar open clusters. These clusters have typical
diameters around 5 pc. This implies that the size of a representative open cluster at
around 500 kpc equals the isoplanatic angle at a good site. This implies that a singleconjugated adaptive optics system can be used for cluster studies.
Over the wavelength interval used for observations, we assume an image quality close to
diffraction limited. Over the same wavelength interval, we adopt instrumental and
atmospheric parameters as given in Table 1. We presume, as for Euro50, a deformable
secondary mirror.
Reference objects for the AO system
With increasing distance of the clusters, the probability of finding a single, natural
member star adequate as a reference object will decrease. While member stars will get
too faint and too crowded, non-cluster-member stars will, statistically, be available within
gradually smaller angular distances. However, at the same time, if they are members of
the galaxy studied, they will also get gradually fainter. Foreground stars suitable as
reference objects will be available only exceptionally.
At the same time, already at, in our context, modest distances, the complete stellar
clusters will be small enough to be included within the isoplanatic angle. Thus, the
clusters can, if rich enough, in their entity, be adopted as reference objects for the AO
system. While this possibility may be attractive at intermediate distances, at larger
distances, the luminosity of the clusters will not suffice.
At larger distances and in the most common case, with no nearby bright object available,
artificial reference objects is the option remaining. However, then the cone effect has to
be taken into account. Thus, for elimination of this effect, a number of artificial reference
objects will have to be stacked. This as well as the implications of perspective elongation
5
and its remedies have been investigated by Owner-Petersen and Gontcharov (2004) and
Beckers et al. (2004).
Photometry with an AO telescope
Unavoidably, with photometry carried out with an AO system, the quality of the
photometry is a function of the performance of the adaptive optics system and its
corrections during the observations. Inevitably, the corresponding point-spread function
(PSF) will show significant variations both in time and as a function of position within
the field of view (Ardeberg, 2004). In our case, the total fields covered by our
observations are small. Thus, field-dependent variations should be of tolerable
importance. In contrast, the variations of the PSF with time need special consideration.
While a number of precautions can be taken, here, we will just assume that the precision
obtained is sufficient for our purpose.
Photometric system adopted
Studies based on colour-magnitude diagrams (CMDs) and luminosity functions (LFs) can
be made in a variety of photometric systems. Also systems with wide pass bands are
adequate, although with some loss in precision. Regarding studies of the abundance of
heavy elements, in metallicity diagrams (MDs), the situation is different. For this
purpose, only a few, well defined systems are suitable. Among them, the system of
widest application is the Strömgren uvby system of intermediate-band photometry. This
system, with its well-defined pass bands is excellent for all purposes discussed in the
present study. While the system was, originally, defined for use with mainly mainsequence stars of spectral types A and F, it is highly suitable for use over a considerably
wider interval in temperature and gravity. We refer to the discussion and references in
Ardeberg and Linde (2004).
Alternative photometric systems
As will be discussed below, in general, the effects of image crowding are more important
for the resulting quality of the photometry of cluster stars in distant galaxies than those
due to photon starvation. This has some implications on the choice of photometric
system. First of all, it implies that, again in general, systems based on intermediate pass
bands are relatively more interesting than those dependent on wider pass bands. Second,
it emphasizes that the selection of photometric system should be made from specific
considerations of information content rather than from arguments based on photometric
efficiency. Finally, still, for very distant and faint sources, there may be compelling
reasons to try wide-band photometry, not least in connection with LF observations.
Photometric parameters
The evolutionary status of an open stellar cluster can be studied from several photometric
parameters. As long as individual member stars down to and below the main-sequence
turn-off point (TOP) can be observed with high accuracy, CMD photometry is an
6
excellent alternative, especially concerning age determinations. A well resolved CMD
lends itself to age data of high accuracy and reliability.
With decreasing photometric quality, and especially when the TOP falls below the limit
of reliable photometry, alternative age parameters should be approached. Convenient
parameters rely, for evolved clusters, on the position of the horizontal branch (HB), the
position and upper part of the asymptotic giant branch (AGB) and the position,
inclination and upper part of the red giant branch (RGB). These parameters can be used
to larger distances than TOP photometry, albeit at the cost of lower information content.
The abundance of heavy elements can be obtained with high precision from photometry n
the Strömgren uvby system. An m1 versus (b-y)0 diagram gives excellent abundance data,
as long as the photometric accuracy is adequately high. When this accuracy falls, an
alternative is to determine the abundance of heavy elements from the position and colour
of the HB. The general strength of HB photometry, also at lower photometric accuracy,
can be seen from the work of Monaco et al. (2003). In the present work, we concentrate
on age data from TOP photometry and heavy element abundance deduced from m1 versus
(b-y)0 data.
Beyond the distance interval adequate for data on TOP, HB, AGB and RGB, results on
evolution can be supported by data from the luminosity function (LF). At the largest
distances, image crowding will prohibit adequate photometry of individual stars and
thereby the use of all of the parameters mentioned. Still though, information on the
evolutionary status of the cluster can be obtained from integral cluster photometry as
discussed and demonstrated by Lata et al. (2002). With an information content that is,
unavoidably, lower than that based on data from individual stars, integral photometry can
be used over very large intervals in distance. The usefulness of the method extends in
distance as long as the spatial resolution permits safe identification of the cluster with
respect to its surrounding field.
Open stellar cluster
For our study, we used modelling and simulation based on an open cluster. As the
template for our model, we chose the intermediate-age open cluster NGC 6192. We took
advantage of recent uvby data for that cluster published by Paunzen et al. (2003). For the
abundance of heavy elements, we applied a value of [Me/H] = - 0.1. Paying attention to
various age determinations for NGC 6192, representing a considerable spread, we applied
an age of 700 Myr.
To improve our conclusions from statistics, we raised the number of simulated stars to
three times that of the number of real stars in NGC 6192. The additional stars were given
photometric data locating them at random positions in the CMD but consistent with the
over-all observed distribution. For the simulated stellar cluster, the input data are shown
in CMD form in Fig. 1.
7
Table 1. Simulation parameters
Image size
2048 x 2048 pixels
Image scale
0."001 / pixel
Field of view
2"x 2"
Pass bands
Strömgren b and y
Strehl factor in all pass bands
0.7
FWHM of seeing disc
0."3
FWHM of PSF in all pass bands
0."003
Maximum PSF definition size
512 x 512 pixels
Exposure time per pass band
36 000 seconds
Fig. 1. Above: The CMD
used for defining input
magnitudes and colours for
the simulated cluster.
Below: The CMDs used for
defining the cluster
background.
8
Point-spread function
Using the parameters given in Table 1, we generated stellar images in the (u)vby system.
We assumed observations with the Euro50 in diffraction-limited AO mode. From this
assumption, we constructed a somewhat simplified PSF. The image profile was defined
from a symmetric Airy function with a residual seeing-limited disc modelled with an
adequate Moffat function.
We neglected the effects of the segmentation of the primary mirror. In addition, we did
not take into account the fact that the AO system will correct the atmospheric PSF to a
certain radius only, depending on the configuration of the corresponding actuators.
Further, for numerical reasons, we had to truncate the PSF when we performed the
simulations. As far as we have been able to verify, none of these simplifications should
imply any significant influence on our results. For a more detailed discussion of the PSF,
see Ardeberg et al. (1999).
Sky background
We added an increase in the local sky level as due to residual imperfections in the Euro50
AO system. This effect can be distinguished as a weak haze surrounding the brighter stars
in the simulated images. Extrapolation on the basis of the ELT exposure estimator
provided by the European Southern Observatory (ESO) was used for estimation of the
background provided by the natural sky level. We added adequate Poisson noise
The simulated stellar cluster was embedded in an environment constructed from two
stellar populations. One population was given an age of 3 Gyr and an abundance of heavy
elements corresponding to [Me/H] = - 0.5. The other population were given
corresponding data of 9 Gyr and [Me/H] = - 1.3. The cluster environment was defined to
contain 65 % of the first population and 35 % of the other. Also, a binarity fraction of 0.6
was assumed. The stellar density of the environment was adjusted to correspond to that of
the LMC halo. In Fig. 1, the CMD of the cluster environment is shown. When
constructing the environment, we adopted a code written by Meynet et al. (1994).
Member star photometry
We located the stellar cluster at a number of distances out to and beyond the distance of
the Virgo and Fornax clusters of galaxies. Simulated stellar images in v, b and y were
constructed employing software of Spännare (2003). The analysis of the images was
made with standard PSF fitting techniques based on DAOPHOT (Stetson, 1987).
Allowing for some adaptation required due to the increasing image crowding and
decreasing signal-to-noise ratio, we used an automated analysis mode. Subsequently, we
used the photometric data obtained for individual stars to construct CMDs and MDs.
From these diagrams, we derived age and abundance data.
9
10 Mpc
30 Mpc
50 Mpc
80 Mpc
140 Mpc
240 Mpc
500 Mpc
750Mpc
1000 Mpc
Fig. 2. The simulated cluster as seen from distances corresponding to 10 to 1000 Mpc. For
each frame the linear size of the cluster has been kept, emphasizing the decreasing image
resolution.
10
Accuracy of photometry
Various error sources influence the precision of the stellar photometry performed in the
cluster. The procedure used for image analysis is, for instance, not quite optimal for our
purpose. As we employed DAOPHOT in a rather automatic manner, there may still be
room for improvement. Such improvement should be especially important when the
image crowding is most pronounced. Thus, applying even more specialized algorithms in
the image reduction and analysis seems a possible way to stretch the usefulness of cluster
star photometry with respect to distance.
Major effects, in addition to the image analysis procedure, influencing the precision of
stellar photometry are image crowding and image disentangling, lack of photons and
effects of background interference. The effects of image crowding and photon starvation
have recently been analysed, especially concerning their relative contribution to the
photometric errors (Ardeberg and Linde, 2004). We identified image crowding as the
factor dominating photometric imprecision, being significantly more serious than low
photon flux, except for the case of very short exposures, with exposure times
uninteresting for our purpose. In all practical cases, we found the effects of the
interference of the sky background to be insignificant compared to those caused by image
crowding and weak flux of photons.
While important in a number of respects, the image crowding has a special effect as a
function of distance. In any representative cluster, from a certain distance, the stars in a
gradually growing core region are excluded from meaningful photometry. Thus, at larger
distances, our photometry and age and abundance analysis have to rely increasingly on
the stars in the outer parts of the cluster. This, in turn, tends to decrease the contrast of the
cluster stars versus the background stars. In our discussion below, this has been
commented.
The significant dominance of the effects due to image crowding over those depending on
photon starvation has its implications concerning the selection of exposure times. Given
the strong contribution of image crowding to photometric imprecision, at larger distances,
an increase of the exposure time may well have only small or negligible effects on the
resulting quality of the photometry.
Analysis
Using our simulated and measured stellar b and y data, we constructed CMDs with y
versus (b-y). A complete simulation of data plus full photometric measurements were
made for every 200 kpc in distance, starting at 1 Mpc and proceeding to 20 Mpc. From
20 to 30 Mpc, corresponding simulations and measurements were made for every 500
kpc in distance. In all cases, CMDs were defined. In all CMDs, we fitted zero-age mainsequence relations and estimated the corresponding cluster ages from the position of the
TOP. These estimates were performed to a nominal accuracy of 50 Myr.
In a corresponding manner, simulated and measured v, b and y data were used for
construction of MDs. Again, we made full simulations of data followed by complete
11
photometric measurements for every 200 kpc in distance from 1 Mpc to 20 Mpc and from
20 to 30 Mpc for every 500 kpc in distance. In the MDs constructed, we fitted a global m1
versus (b-y)0 relation to the data set defined by the brighter main-sequence stars. Our
estimates were made to a nominal precision of 0.05 dex in [Me/H].
Integral photometry was chosen to continue the evolutionary studies of the stellar cluster
beyond the limits for CMDs and MDs. The complete cluster was measured with aperture
photometry in b and y. The colour index (b-y) was adopted as our age parameter at the
largest distances. Integral photometry was performed from 10 Mpc to 1 Gpc. The
photometry was carried out for every 10 Mpc in distance between 10 and 100 Mpc, for
every 20 Mpc between 100 Mpc and 500 Mpc and for every 50 Mpc between 500 Mpc
and 1 Gpc. During this study, as well as in this work as a whole, all effects of redshift
were neglected.
Results
In Fig. 2, we show a series of images of our stellar cluster, referring to increasing
distances. In these images, the effects of distance on the resolution are emphasised.
Comparing the image of the cluster at the distances 500, 750 and 1000 Mpc, the
decreasing visibility of the cluster at large distances can be appreciated.
A sequence of CMDs for our cluster, placed at a series of distances, between 1 and 24
Mpc, is shown in Fig. 3. In all CMDs displayed, we have shown both the template data
(red dots) and the corresponding data observed at the actual distance (yellow dots). These
CMDs are part of the total of 115 CMDs employed for the construction of the age versus
distance diagram shown in Fig. 4. Here, age in Myr is given as a function of distance in
Mpc.
The integral measurements of (b-y) for the cluster, expressed in magnitudes, are shown
versus distance in Mpc in Fig. 5. The interval covered in distance is from 10 Mpc to 1
Gpc. The relation in Fig. 5 should be compared to the cluster images shown in Fig. 2.
Representing a distance interval from 1 to 30 Mpc, Fig. 7 shows a sequence of MDs for
our cluster, placed at increasing distances. All MDs give the template data (red dots) as
well as the data observed at the specific distances (yellow dots). Part of a total of 115
MDs, these MDs were used for construction of the [Me/H] data, expressed in dex versus
distance in Mpc in Fig. 6.
Discussion of age data based on TOP photometry
The high quality of CMD photometry feasible with an AO ELT is demonstrated in Fig. 3.
For an evaluation of the quality, the template data are convenient standards. These data
have a one-to-one correspondence to the data observed.
12
1 Mpc
2 Mpc
3 Mpc
5 Mpc
8 Mpc
12 Mpc
16 Mpc
20 Mpc
24 Mpc
Fig. 3. Resulting CMDs from measurements of the cluster at selected distances from 1 to 24
Mpc. The yellow dots are observed values while the red dots correspond to input data.
13
Fig. 4. Cluster ages, derived
from TOP estimation, as a
function of distance.
Fig. 5. Cluster colour,
derived from integral
photometry, as a function
of distance.
Fig. 6. Cluster metallicity,
derived from Strömgren m1
photometry, as a function
of distance.
14
Over the first few Mpc, the sharpness of the definition of the CMD is left rather intact.
Between, say, 5 and 10 Mpc, the noise added is still of a minor order only. Even out to 15
Mpc, the contribution of noise is low enough not to influence age analysis in any serious
way. This seems, largely, true also up to around 17 Mpc, corresponding to just beyond
the Virgo cluster of galaxies. At this distance, the CMD retains all its main features in
spite of a noise level significantly higher than at lower distances. The corresponding loss
of measured data corresponds mainly to effects of unavoidable image crowding in the
central area of the cluster. These effects get increasingly strong at distances of 20 Mpc
and more.
From the results displayed in Fig. 4, it is obvious that ELT CMD photometry of stellar
clusters gives a highly favourable access to the evolution of distant galaxies. Out to a
distance of approximately 17 Mpc from the Sun, the quality of the age determinations is
rather high, very much the same as for the smallest distances treated. The gradual
deterioration of the quality of the CMD at distances larger than 17 Mpc is clearly
reflected in the age data displayed in Fig. 4.
The age data based on CMDs obtained between around 18 and 30 Mpc show a gradual
decrease of quality. First, the spread in the age estimates increases with distance. Second,
in the same distance range, there is a clear average trend towards an estimation of higher
ages. This trend can be explained from the relation between the cluster population of stars
and the corresponding mixed population of the surrounding galactic field.
At more modest distances, the cluster stars are, in the field of observations, much more
numerous than those belonging to the background. Thus, at these distances, the CMD
defined for the cluster is virtually unaffected by the field stars. When the distance
increases, also the image crowding increases. In a more significant manner, the influence
of this crowding grows from the cluster centre and outwards. Consequently, at some
distance, in our case around 10 Mpc, the number relation between cluster stars and field
stars enters a phase of steady modification.
In Fig. 3, it can be seen that from a distance of approximately 10 Mpc, the density of data
in the CMD decreases significantly, reflecting the gradual loss of stars in the cluster
centre. Hence, the influence of the field stars is correspondingly increased. As these stars
belong to older populations than the stellar cluster, the result is a gradual increase of the
average age estimated from the TOP of the CMD. Still, as far out as at 30 Mpc, the age
data obtained, albeit scattered, are of a quality sufficiently good to show that the cluster
belongs to a young galactic population.
Our results may, to some extent, be compared with those obtained by Frayn (2003).
Frayn considers general stellar populations as compared to or choice of clusters. As has
been discussed above, clusters are much better evolutionary probes than general stellar
populations hard to define in a stringent manner. On the other hand, clusters are also
more vulnerable to effects of image crowding than are general stellar populations. Taking
the effects of image crowding into account, our results and those of Frayn are in good
agreement.
15
Discussion of age data based on integral photometry
With the morphological changes of the CMD accompanying the evolution of a stellar
cluster, the integrated colour of an open cluster is a natural measure of its evolutionary
phase. This has been discussed recently by Lata et al. (2002). With our integrated (b-y)
data, converting from the (B-V) colour frame adopted by Lata et al. to a corresponding
(b-y) frame, we can deduct both an average typical age of NGC 6192 and the
corresponding apparent age variation and age spread as a function of distance. We refer
to Figs. 2 and 5.
Fig. 2 demonstrates the strength of the method of integrated colour data for open clusters
in galaxies at large distances. The high image contrast of the cluster even at rather large
distances is obvious. Also, the resolution of the ELT preserves the difference between a
cluster and correspondingly intensive point sources rather well out to considerable
distances. Thus, identification of the cluster seems quite safe over large distances. Fig. 2
shows that, out to at least beyond 750 Mpc, probably somewhat more, the open cluster
can be well distinguished from a point source.
Our typical, at least for distances out to around 300 Mpc, (b-y) value, 0.15 mags, as
shown in Fig. 5, corresponds to an age of approximately 300 Myr. This is significantly
less than the age of NGC 6192 as adopted from the collected age data from several
authors. However, these data show a large spread. Moreover, from Lata et al. (2002), we
find that they, for NGC 6192, adopt an even lower age, around 100 Myr, than that
resulting from our integrated (b-y) data. In their diagram with (B-V)0 versus logarithmic
age, the data for NGC 6192 deviates significantly from the corresponding average age
relation. For distances larger than 300 Mpc and out to 1 Gpc, our integrated (b-y) value
decreases to an average of first around 0.14 mags, then 0.13 mags, corresponding to
approximate ages of 270 Myr and 210 Myr, respectively.
Out to around 300 Mpc, the spread in our integrated colour data is around 0.02 in (b-y).
This corresponds to an age spread of approximately (300 ± 40) Myr. For distances larger
than 300 Mpc, the corresponding spread in the (b-y) colour is approximately 0.03 mags.
This is equivalent to a spread in age of the order of (240 ± 60) Myr. Given the high
distance range and the simplicity of the method, the spread of the age data is seen as
rather low.
Discussion of abundance data
While high-quality TOP data from CMDs require a photometric precision of the order of
0.04 to 0.05 mags, corresponding data for the abundance of heavy elements from MDs
demand a photometric accuracy in the colour indices m1 and (b-y) better than 0.02 mags.
Hence, MD abundance estimates are significantly more vulnerable to various sorts of
photometric inaccuracy than CMD TOP photometry. This is a fact clearly demonstrated
in our work, at the same time as the results of our abundance determinations are highly
positive.
16
1 Mpc
2 Mpc
3 Mpc
5 Mpc
8 Mpc
12 Mpc
16 Mpc
20 Mpc
24 Mpc
Fig. 7. Resulting metallicity diagrams from measurements of the cluster at selected distances
from 1 to 24 Mpc. Yellow dots are observed values while red dots correspond to input data.
17
From Fig.7, the quality of the MD data can be examined. As for the CMDs, we have
inserted the template data in all MDs. They are, as before, marked in red and have a oneto-one correspondence to the m1 versus (b-y) data observed, which are marked in yellow.
Out to and including 2 Mpc, the quality of the MDs is practically unaffected. Also for
somewhat larger distances, out to and including 5 Mpc, the quality is good enough not to
affect the over-all abundance determination very much. Still out to around 10 Mpc, the
decrease in quality is quite limited with respect to deduction of over-all abundance
parameters. Beyond 10 Mpc and out to around 18 Mpc, the decreasing MD quality is
obvious although still within quite tolerable limits concerning over-all abundance
determination. At distances larger than 18 Mpc, the quality of the MDs is doubtful.
The level of abundance of heavier elements given in Fig. 6 is approximately constant out
to a distance of around 7 Mpc. Between 7 and 10 Mpc, there is a slight increase in
abundance with distance. This is followed, between 10 and 18 Mpc, by an interval of
rather constant although slightly decreasing average abundance. At distances larger than
18 Mpc, the average abundance indicated is higher and of doubtful physical significance.
The noise present in the MD main-sequence relations begins to increase slightly already
for distances beyond around 2 Mpc. Subsequently, the noise level increases smoothly
with distance up to approximately 10 Mpc. At this level, the noise around the mainsequence relation is still relatively well behaved. Also, the influence of the decreased
sharpness of this relation on the deduced value of [Me/H] is quite limited.
Commencing at around 10 Mpc, the noise affecting the main-sequence relation increases
gradually. At this stage, a significant effect on the abundance data cannot be avoided.
Still, out to approximately 18 Mpc, the main-sequence relation can, in spite of its high
spread, be identified and a reasonable abundance be assigned. At distances larger than 18
Mpc, the corresponding analyses gets very problematic. Both systematic effects and
spread make the MDs highly questionable for abundance deduction at these distances.
At comparatively smaller distances, the main-sequence stars of the cluster dominate the
field of observations, as discussed for the CMDs. The field population is mainly
represented by its giant stars. Corresponding to the CMD case, the MD of the field is, for
smaller distances, nearly completely defined by the cluster stars. With increasing
distance, the crowding grows from the cluster centre and outwards. As a result, from
some distance, in our case around 10 Mpc, the number relation between cluster stars and
field stars is steadily modified.
Conclusions
For studies of the evolution of galaxies, the Virgo cluster is of fundamental importance,
with its rich display of all types of galaxies and well-determined distance. Our current
data show that we can, with ELT cluster photometry, reach the Virgo cluster and study
the evolutionary signatures of its galaxies. The TOP-related age data reach the distance of
the Virgo cluster with rather good accuracy. The MD-dependent abundance data show a
18
corresponding high-quality endurance out to around 10 Mpc, at the same time as they
reach the distance of the Virgo cluster with reasonably good definition. For studies of the
evolution of galaxies, these are important facts.
The age of distant open clusters can be determined from their integrated colour data. Our
results show that the method is applicable out to a distance of around 1 Gpc. While the
information content of the integrated colour data cannot successfully compete with that of
full CMD photometry, it defines an attractive method for galaxies at large distances.
It should be added that further improvements of our methods are, as discussed above,
possible and will be attempted. Also with only quite marginal improvements of the
results, even the Fornax cluster of galaxies will be within reach for ELT cluster
photometry and evolutionary studies. Such work as well as work on a wider application
of stellar clusters and their evolutionary tools are in progress for extended ELT studies of
the evolution of galaxies.
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