384 research outputs found
Machine learning techniques for galaxy imagery and photometry
Doctor of PhilosophyDepartment of Computer ScienceMajor Professor Not ListedIn the past two decades, autonomous digital sky surveys have enabled significant advances in astronomy by collecting massive databases of imagery and other information. The quantity of data, coupled with the variety of scientific questions that require its analysis, makes manual analysis of these data impractical. To address this challenge, machine learning algorithms have been widely adopted for data analysis and product generation in astronomy. In this dissertation I examine the efficacy of machine learning algorithms such as deep convolutional neural networks, support vector machines, and vision transformers for the purpose of astronomical data analysis, with emphasize on extra-galactic objects. These include algorithms that can annotate large datasets of galaxy images, and their application to premier digital sky surveys such as Pan-STARRS. Specifically, I address the following research question: How effective are machine learning algorithms for annotating astronomical data, and what are the downsides of using these algorithms for this purpose? Namely, biases that are typical to machine learning systems can influence the annotations, which may consequently lead to false conclusions when applying statistical analysis to data annotated using such systems. These biases are often difficult to identify. Overall, this research highlights the importance of careful consideration of machine learning algorithms and their potential biases when applying them to astronomical data analysis. Our findings have broad implications for the use of machine learning in astronomy and other scientific domains, as they demonstrate the importance of addressing potential biases in machine learning systems to avoid erroneous scientific conclusions
Gold-Catalyzed Intramolecular Aminoarylation of Alkenes: C-C Bond Formation through Bimolecular Reductive Elimination
Gold-ilocks and the 3 mol % catalyst: Bimetallic gold bromides allow the room temperature aminoarylation of unactivated terminal olefins with aryl boronic acids using Selectfluor as an oxidant. A catalytic cycle involving gold(I)/gold(III) and a bimolecular reductive elimination for the key CC bond-forming step is proposed. dppm= bis(diphenylphosphanyl)methane
A Comparison of Methods for Determining the Age Distribution of Star Clusters: Application to the Large Magellanic Cloud
The age distribution of star clusters in nearby galaxies plays a crucial role
in evaluating the lifetimes and disruption mechanisms of the clusters. Two very
different results have been found recently for the age distribution chi(t) of
clusters in the Large Magellanic Cloud (LMC). We found that chi(t) can be
described approximately by a power law chi(t) propto t^{gamma}, with gamma
-0.8, by counting clusters in the mass-age plane, i.e., by constructing chi(t)
directly from mass-limited samples. Gieles & Bastian inferred a value of
gamma~, based on the slope of the relation between the maximum mass of clusters
in equal intervals of log t, hereafter the M_max method, an indirect technique
that requires additional assumptions about the upper end of the mass function.
However, our own analysis shows that the M_max method gives a result consistent
with our direct counting method for clusters in the LMC, namely chi(t) propto
t^-0.8 for t<10^9 yr. The reason for the apparent discrepancy is that our
analysis includes many massive (M>1.5x10^3 M_sol), recently formed (t<10^7 yr)
clusters, which are known to exist in the LMC, whereas Gieles & Bastian are
missing such clusters. We compile recent results from the literature showing
that the age distribution of young star clusters in more than a dozen galaxies,
including dwarf and giant galaxies, isolated and interacting galaxies,
irregular and spiral galaxies, has a similar declining shape. We interpret this
approximately "universal" shape as due primarily to the progressive disruption
of star clusters over their first ~few x 10^8 yr, starting soon after
formation, and discuss some observational and physical implications of this
early disruption for stellar populations in galaxies.Comment: 21 pages, 5 figures, published in the Astrophysical Journal, volume
713, page 134
The ACS Nearby Galaxy Survey Treasury. X. Quantifying the Star Cluster Formation Efficiency of Nearby Dwarf Galaxies
We study the relationship between the field star formation and cluster
formation properties in a large sample of nearby dwarf galaxies. We use optical
data from the Hubble Space Telescope and from ground-based telescopes to derive
the ages and masses of the young (t_age < 100Myr) cluster sample. Our data
provides the first constraints on two proposed relationships between the star
formation rate of galaxies and the properties of their cluster systems in the
low star formation rate regime. The data show broad agreement with these
relationships, but significant galaxy-to-galaxy scatter exists. In part, this
scatter can be accounted for by simulating the small number of clusters
detected from stochastically sampling the cluster mass function. However, this
stochasticity does not fully account for the observed scatter in our data
suggesting there may be true variations in the fraction of stars formed in
clusters in dwarf galaxies. Comparison of the cluster formation and the
brightest cluster in our sample galaxies also provide constraints on cluster
destruction models.Comment: 16 pages, 9 figures, Accepted to Ap
Star Formation in the Milky Way and Nearby Galaxies
We review progress over the past decade in observations of large-scale star
formation, with a focus on the interface between extragalactic and Galactic
studies. Methods of measuring gas contents and star formation rates are
discussed, and updated prescriptions for calculating star formation rates are
provided. We review relations between star formation and gas on scales ranging
from entire galaxies to individual molecular clouds.Comment: 55 pages, 15 figures, in press for Annual Reviews of Astronomy and
Astrophysics; Updated with corrected equation 5, improved references, and
other minor change
Preparation of Unsymmetrical Disulfides from Thioacetates and Thiosulfonates
A method for the transformation of organic thioacetates, a widely used functionality for the preparation of self-assembled monolayers on gold surfaces, into unsymmetrical disulfides is reported. Disulfides are readily immobilized on gold in contrast to thioacetates, which usually require a deprotection step prior to bonding to the metal surface. The potential of the method for the controlled preparation of unsymmetrical disulfides has been demonstrated with model compounds comprising several thioacetates, which were readily converted into the corresponding unsymmetrical disulfides
The star cluster formation history of the LMC
The Large Magellanic Cloud is one of the nearest galaxies to us and is one of
only few galaxies where the star formation history can be determined from
studying resolved stellar populations. We have compiled a new catalogue of
ages, luminosities and masses of LMC star clusters and used it to determine the
age distribution and dissolution rate of LMC star clusters. We find that the
frequency of massive clusters with masses M>5000 Msun is almost constant
between 10 and 200 Myr, showing that the influence of residual gas expulsion is
limited to the first 10 Myr of cluster evolution or clusters less massive than
5000 Msun. Comparing the cluster frequency in that interval with the absolute
star formation rate, we find that about 15% of all stars in the LMC were formed
in long-lived star clusters that survive for more than 10 Myr. We also find
that the mass function of LMC clusters younger than 1 Gyr can be fitted by a
power-law mass function with slope \alpha=-2.3, while older clusters follow a
significantly shallower slope and interpret this is a sign of the ongoing
dissolution of low-mass clusters. Our data shows that for ages older than 200
Myr, about 90% of all clusters are lost per dex of lifetime. The implied
cluster dissolution rate is significantly faster than that based on analytic
estimates and N-body simulations. Our cluster age data finally shows evidence
for a burst in cluster formation about 1 Gyr ago, but little evidence for
bursts at other ages.Comment: 18 pages, 6 figures, MNRAS in pres
Super star clusters in Haro 11: Properties of a very young starburst and evidence for a near-infrared flux excess
We have used multi-band imaging to investigate the nature of the extreme
starburst environment in Haro 11 galaxy. The central starburst region has been
observed in 8 HST wavebands and at 2.16 micron at the ESO-VLT. We constructed
integrated spectral energy distributions (SEDs) for about 200 star clusters and
compared them with single stellar population models in order to derive ages,
masses and extinctions of thestar clusters. The present starburst has lasted
for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such
an extremely young cluster population, Haro 11 represents a unique opportunity
to investigate the youngest phase of the cluster formation process and
evolution in starburst systems. Extinction tends to diminish as function of the
cluster age, but the spread is large and for clusters in partial embedded
phases ( 10^4 Msun), very young (1-3 Myr)
clusters is missing, either because they are embedded inthe parental molecular
cloud and heavily extinguished, or because of blending. Almost half of the
cluster sample is affected by flux excesses at wavelengths 8000 \AA which
cannot be explained by simple stellar evolutionary models. Fitting SED models
over all wavebands leads to systematic overestimates of cluster ages and
incorrect masses for the stellar population supplying the light in these
clusters. We show that the red excess affects also the HST F814W filter, which
is typically used to constrain cluster physical properties. The clusters which
show the red excess are younger than 40 Myr; we propose possible physical
explanations for the phenomenon. Finally, we estimate that Haro 11 hasproduced
bound clusters at a rate almost a factor of 10 higher than the massive and
regular spirals, like the Milky Way. (Abriged)Comment: Accepted for publication in MNRAS. 23 pages, 23 figure
The Massive Star Clusters in the Dwarf Merger ESO 185-IG13: is the Red Excess Ubiquitous in Starbursts?
We have investigated the starburst properties of the luminous blue compact
galaxy ESO 185-IG13. The galaxy has been imaged with the high resolution
cameras onboard to the Hubble Space Telescope. From the UV to the IR, the data
reveal a system shaped by hundreds of young star clusters, and fine structures,
like a tidal stream and a shell. The presence of numerous clusters and the
perturbed morphology indicate that the galaxy has been involved in a recent
merger event. Using previous simulations of shell formation in galaxy mergers
we constrain potential progenitors of ESO 185-IG13. The analysis of the star
cluster population is used to investigate the properties of the present
starburst and to date the final merger event, which has produced hundreds of
clusters younger than 100 Myr. We have found a peak of cluster formation only
3.5 Myr old. A large fraction of these clusters will not survive after 10-20
Myr, due to the "infant mortality" caused by gas expulsion. However, this
sample of clusters represents an unique chance to investigate the youngest
phases of cluster evolution. As already observed in the analog blue compact
galaxy Haro 11, a fraction of young clusters are affected by a flux excess at
wavelengths longer than 8000 \AA. Ages, masses, and extinctions of clusters
with this NIR excess are estimated from UV and optical data. We discuss
similarities and differences of the observed NIR excess in ESO 185-IG13
clusters with other cases in the literature. The cluster ages and masses are
used to distinguish among the potential causes of the excess. We observe, as in
Haro 11, that the use of the IR and the (commonly used) I band data results in
overestimates of age and mass in clusters affected by the NIR excess. This has
important implications for a number of related studies of star clusters.Comment: 23 pages, 16 figures, accepted for publication in MNRA
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