Dissertation committee:
Δέσποινα Χατζηδημητρίου, Καθηγήτρια, τμήμα Φυσικής, ΕΚΠΑ
Ανδρέας Ζέζας, Καθηγητής, τμήμα Φυσικής, Πανεπιστήμιο Κρήτης
Απόστολος Μαστιχιάδης, Καθηγητής, τμήμα Φυσικής, ΕΚΠΑ
Θεοχάρης Αποστολάτος, Καθηγητής, τμήμα Φυσικής, ΕΚΠΑ
Βασίλειος Χαρμανδάρης, Καθηγητής, τμήμα Φυσικής, Πανεπιστήμιο Κρήτης
Στυλιανός Καζαντζίδης, Επίκουρος Καθηγητής, ΕΚΠΑ
Καλλιόπη Δασύρα, Επίκουρη Καθηγήτρια, ΕΚΠΑ
Summary:
The Small Magellanic Cloud (SMC) is one of the closest star forming dwarf galaxies
and the second nearest dwarf irregular galaxy to our own. The SMC is gas rich and
about two orders of magnitude less massive than our own galaxy. It is a member of
an interacting galaxy pair (the other member being the Large Magellanic Cloud - LMC),
which is believed to be infalling to (and interacting with) the Milky Way Galaxy. The aim
of this study is to analyze the stellar populations in the inner denser regions of this galaxy
and determine its star formation history and its star cluster population. Recent star
formation is exemplified by the presence of a large population of high mass X-ray binaries
(of the Be type). One of the aims of this thesis is to identify the optical counterparts of
known X-ray sources in the SMC central regions and ultimately link them to star forming
events.
Observations of four fields in the SMC central regions were obtained with the 6.5m
Magellan Telescope at the Las Campanas Observatory in Chile on October 4th, 2004,
using the Inamori Magellan Areal Camera and Spectrograph (IMACS). Each field had a
0.44 o diameter (SMC3: RA 00 56 53.6, DEC -72 17 16.7, SMC4: RA 00 49 36.1, DEC
-73 16 18.0, SMC5: RA 00 53 22.4, DEC -72 26 36.2, SMC6: RA 00 53 15.0, DEC -72 42
06.8).
The images were reduced following standard procedures. After bias subtraction and
flat fielding (using IRAF), an astrometric solution was calculated and applied to the data
using the 2MASS catalog as reference. The absolute astrometric accuracy of the reference
catalog is approximately 0.1 arcsec. The final mosaic image in each filter and field was
constructed using SWarp. Point spread function (PSF) photometry was then performed,
using the DAOPHOT package in IRAF, separately for each constituent CCD. Absolute
photometric calibration was achieved by using a set of secondary standards selected from
isolated relatively bright stars (B≤17.5mag and I≤18mag) in the Zaritsky et al. [2002]
photometric catalog. This analysis yielded a photometric catalogue of 1.068.893 stars
with a limiting magnitude of B ∼ 24.5, significantly deeper by at least 2 magnitudes than
any other published optical survey in the inner regions of the SMC, to date. Extensive
artificial star experiments were performed in order to estimate the level of completeness
of the data.
The photometric data were used to construct colour magnitude diagrams (B versus B-
I) in the four fields studied. These diagrams show a well defined main sequence (MS), the
subgiant (SG) and red giant (RGB) branches as well as the red clump (RC) and its vertical
extension to brighter magnitudes caused by the presence of younger stellar populations in
the fields studied. It is noted that in the case of field SMC4,the RC appears to be strongly
elongated (and strongly inclined, essentially along the reddening vector) towards redder
colours. This is caused by differential interstellar reddening, which is particularly severe
in this field.
The CMDs were used to study the stellar populations present in the SMC central
regions and estimate the star formation history (SFH), i.e. the star formation rate as a
function of look-back time. The latter was achieved by applying the method of Dohm-
Palmer et al. [1997] to MS stars. The method is more sensitive to relatively recent star
formation, as the time resolution and accuracy degrade with increasing look-back time.
A general conclusion of this analysis was that in all four fields star formation has been
more intense recently (in the past 1Gyr). More specifically, the star formation activity
seems to have been intense over the past ∼100Myr, decreasing rapidly over a period of few
hundred Myr (look-backtime). In fields SMC4 and SMC6, there appears to have been a
small enhancement in SF around 700-800 Myr ago. Beyond 1Gyr, from about 1 to about
3Gyr ago, star formation activity seems to have been lower, while it peaked again between
4 and 8 Gyr ago. Our results also confirmed that most of the stars in the SMC seem to
have been formed over the past 8 Gyr. In the two fields with fainter completeness limits
(fields SMC3 and SMC5) the peak in star formation rate occurred close to an age of 6-7
Gyr. The SFH beyond the 50% completeness limit is less reliable, therefore for fields 4
and 6 the displacement to somewhat different ages of the SFR peaks beyond 4 Gyr is most
probably the result of increased uncertainties and biases.
To investigate probable differences in the mixture of stellar populations across the
SMC central regions, we followed a different approach, which takes full account of the dif-
ferences in completeness in the regions studied. The Luminosity Function (LF) of the MS
3stars encodes information on the ages of the stellar populations present. Combining the
LF of MS stars with the completeness of our data, we derived the completeness corrected
LF (CCLF). The MS CCLF can be used for a rough comparison of the mixture of stellar
populations of different ages present in the different fields. The CCLFs have been normal-
ized to older stars, assuming that older populations are more evenly distributed spatially
than younger ones. Based on this analysis, it was found that fields SMC3 and SMC5,
which are located in the North-Northeast, have a significantly higher contribution from
younger populations compared to the Southern-Southwestern fields SMC4 and SMC6.
Younger populations therefore, seem to be more abundant in the North-NorthEasern re-
gions, which was also noted in near-infrared studies (e.g. Rubele et al. [2018])). Our
analysis confirmed that the history of star formation is not uniform along the so called
SMC Bar. Therefore, the ”Bar” may not be considered as a unique entity, at least in
terms of stellar populations.
CMDs constructed over large regions may suffer from small but non-negligible resid-
ual systematic photometric errors due to PSF variability, different completeness levels,
differential interstellar absorption and possibly line-of-sight distance variations. All these
factors may result in ”blurring” specific features, such as distinct main sequence turnoffs,
in CMDs constructed over extended areas. In order to overcome these limitations, we
focused our study on smaller regions with optimal sizes of about 1.1 arcmin in radius.
Regions of this size contain enough stars to allow for detection of stellar evolutionary
features in the CMD, and at the same time they are small enough to be less influenced by
the previously mentioned systematic effects. We found that in a few regions (favoured by
low dust absorption) there are clear indications of distinct MS turnoffs (MSTOs), at 2.7
and 4 Gyr ago. This confirms beyond doubt that SF has not been continuous over this
period and that intense SF activity has been followed by periods of low SF. This may be
secular, or connected to the SMC-LMC interaction.
Apart from distinct turnoffs in intermediate ages, we also noted the existence of a
distinct turnoff around 56-120Myr old. Enhanced SF for this period was also reported
by Harris and Zaritsky [2004], Rubele et al. [2015] and Auchettl et al. [2019] who showed
that a large fraction of the supernova remnants in the SMC indicated a burst of star
formation between 50 and 200 Myr ago. We have investigated the kinematic behaviour
of this population using proper motion measurements from Gaia DR2. It was found that
the 56-120Myr stars are kinematically distinct from very young stars that still lie on the
zero-age main sequence (at least up to the saturation limit of the IMACS data). This is
a tentative result that needs to be further investigated using the new Gaia EDR3 data
and it alludes to the presence of distinct substructures proposed by Murray et al. [2019]
who also found that non-rotational motions are prevalent throughout the SMC. A first
analysis of the EDR3 data have shown that significant systematic errors were present in
the DR2 data, that probably created false kinematic trends.
Star clusters are often considered as the building blocks of galactic disks, as star
formation is generally clustered. The identification of star clusters (mostly small clusters)
in the SMC is an ongoing endeavour. It is only relatively recently that machine learning
and data mining methods and techniques have been employed to detect star clusters in the Magellanic Clouds. Our data provide a unique opportunity to investigate the validity of
these identifications and classifications and have superior spatial resolution to most data
used to date to search for star clusters. Applying criteria based on CMDs corrected for the
field contribution, number density profiles and image inspection, we examined all known
clusters and candidates in our fileds. We could only confirm about 70% of the objects
classified as clusters in the most recent compilation of Bica et al. [2020]. Most of the non-
confirmed objects are artifacts, caused by the inferior spatial resolution of some of the
older surveys. We also applied the well-known data clustering algorithm, ”Density-based
Spatial Clustering of Applications with Noise” (DBSCAN), to both the IMACS data and
to Gaia DR2 data. DBSCAN is a non-parametric algorithm based on density.The results
from the application of DBSCAN confirmed the classification resulting from the use of
the three criteria described above. The combined use of the two methods is a very useful
tool for the discovery and verification of star clusters in resolved galaxies.
The last part of the thesis concerns the optical identification of X-ray sources that
have been discovered in the direction of the SMC central regions. To identify candidate
high mass X-ray binaries, we cross-correlated the most recent X-ray source catalogue in
4the SMC central regions Antoniou et al. [2019] with the IMACS photometric catalogue.
In order to correctly interpret the results of the cross-correlation, we calculated statistical
tables of chance coincidence in different regions of the CMD: the CMD was divided in
colour-magnitude cells. For each cell, we calculated the ”chance coincidence”, i.e. the
probability to detect one or more optical matches (inside the specified cell) to a X-ray
source, within a search radius determined by the positional uncertainty. The resulting
chance coincidence probabilities in the various CMD cells for different search radii were
extracted. As expected, the chance coincidence probability is higher for regions of the
CMD with high stellar density, and increases with increasing search radius. We found a
total of 9 new candidate high mass X-ray binaries of the Be type, within a 2 sigma search
radius. Optical spectroscopy is needed to confirm these identifications.
