471 research outputs found
Recommended from our members
The Clustering of Young Stellar Clusters in Nearby Galaxies
Star clusters form the basic building blocks of galaxies. They span a wide range of ages, from a few million years to billions of years, making them exceptional tracers of the star formation histories of their host galaxies. Star formation is the process by which galaxies build up their stellar populations and their visible mass and occurs in a continuous, hierarchical social fashion across a large dynamical range, from individual stars up to kiloparsec-scale ensembles of stellar aggregates. It is the formation, evolution, and eventual destruction of these large hierarchical star-forming complexes that provide an essential role in understanding the physical mechanism and dynamical evolution of star formation on sub-galactic scales.
First, using star clusters from local galaxies as part of the LEGUS (Legacy Extragalactic UV Survey) sample, we find that star formation is coherent over scales of a few hundred parsec up to a few kpc depending on the galaxy. In all cases, these hierarchies are short lived and unbound, dissolving in a few tens to a hundred Myr. The recovered correlations between the spatial separations and ages of star clusters contained within these structures are consistent with theoretical expectations of arising from a turbulence-driven ISM. We also find evidence that the maximum size of correlated star formation is driven by galactic shear.
Second, we combine our star cluster catalogs with exquisite molecular gas observations to connect the detailed stellar population information to the natal gas from which it formed. We find that the timescale for star clusters to lose association with their natal clouds is of order a few Myr, with their ages rising rapidly as they become spatially separated from their molecular clouds.
Third, we introduce initial work that employs the use of machine learning as a process to identify star clusters, a quicker and more homogeneous method than traditional visual classification techniques employed for most stellar cluster catalogs.
The work contained in this dissertation represents the first large-scale study of its kind outside of the Local Group to characterize turbulence as the physical driver of correlated star formation and the association timescale of star clusters with their molecular reservoirs, marking a turning point in the effort to link local star forming structures to those that are common at high redshift
Report of Selected Interview Schedule Data
Questions and Answers by Full-time Faculty and Adjunct Faculty
Staff Meetings
Staff Meetings: Summary of Dat
Use of a predictive protocol to measure the antimicrobial resistance risks associated with biocidal product usage
Background
In this study we assessed the propensity of biocide exposure in the development of antimicrobial resistance in bacteria.
Methods
Our protocol is based on reporting changes in established antimicrobial susceptibility profiles in biocides and antibiotics after during use exposure to a product. The during use exposure reflects worse conditions of product use during application. It differs from the term low concentration, which usually reflects a concentration below the minimal inhibitory concentration, but not necessarily a concentration that occurs in practice.
Results
Our results showed that exposure to triclosan (0.0004%) was associated with a high risk of developing resistance and cross-resistance in Staphylococcus aureus and Escherichia coli. This was not observed with exposure to chlorhexidine (0.00005%) or a hydrogen peroxide–based biocidal product (in during use conditions). Interestingly, exposure to a low concentration of hydrogen peroxide (0.001%) carried a risk of emerging resistance to antibiotics if the presence of the oxidizing agent was maintained. We observed a number of unstable clinical resistances to antibiotics after exposure to the cationic biocide and oxidizing agent, notably to tobramycin and ticarcillin–clavulanic acid.
Conclusions
Using a decision tree based on the change in antimicrobial susceptibility test results, we were able to provide information on the effect of biocide exposure on the development of bacterial resistance to antimicrobials. Such information should address the call from the U.S. Food and Drug Administration and European Union Biocidal Products Regulation for manufacturers to provide information on antimicrobial resistance and cross-resistance in bacteria after the use of their product
A Comprehensive Comparative Test of Seven Widely-Used Spectral Synthesis Models Against Multi-Band Photometry of Young Massive Star Clusters
We test the predictions of spectral synthesis models based on seven different
massive-star prescriptions against Legacy ExtraGalactic UV Survey (LEGUS)
observations of eight young massive clusters in two local galaxies, NGC 1566
and NGC 5253, chosen because predictions of all seven models are available at
the published galactic metallicities. The high angular resolution, extensive
cluster inventory and full near-ultraviolet to near-infrared photometric
coverage make the LEGUS dataset excellent for this study. We account for both
stellar and nebular emission in the models and try two different prescriptions
for attenuation by dust. From Bayesian fits of model libraries to the
observations, we find remarkably low dispersion in the median E(B-V) (~0.03
mag), stellar masses (~10^4 M_\odot) and ages (~1 Myr) derived for individual
clusters using different models, although maximum discrepancies in these
quantities can reach 0.09 mag and factors of 2.8 and 2.5, respectively. This is
for ranges in median properties of 0.05-0.54 mag, 1.8-10x10^4 M_\odot and
1.6-40 Myr spanned by the clusters in our sample. In terms of best fit, the
observations are slightly better reproduced by models with interacting binaries
and least well reproduced by models with single rotating stars. Our study
provides a first quantitative estimate of the accuracies and uncertainties of
the most recent spectral synthesis models of young stellar populations,
demonstrates the good progress of models in fitting high-quality observations,
and highlights the needs for a larger cluster sample and more extensive tests
of the model parameter space.Comment: Accepted for publication in MNRAS (14 Jan. 2016). 30 pages, 16
figures, 9 table
Localised HI 21-cm absorption towards a double-lobed z=0.24 radio galaxy
We present the results of a mini-survey for associated HI 21-cm absorption at
z < 0.42 with the Giant Metrewave Radio Telescope. Our targets are radio
galaxies, selected on the basis that the 1216 Angstrom luminosities are below
10^23 W/Hz, above which there has never been a detection of 21-cm absorption.
Of the three sources for which we obtained good data, two are unclassified
active galactic nuclei (AGN) and one is type-2. Being a non-detection, the
type-2 object is consistent with our previous result that 21-cm absorption in
radio sources is not dictated by unified schemes of AGN. In the case of the
detection, the absorption only occurs towards one of the two resolved radio
lobes in PKS 1649-062. If the absorption is due to an another intervening
galaxy, or cool HI gas in the intergalactic medium, covering only the
south-west lobe, then, being at the same redshift, this is likely to be
gravitationally bound to the optical object identified as PKS 1649-062. If the
absorption is due to an inclined disk centred between the lobes, intervening
the SW lobe while being located behind the NE lobe, by assuming that it covers
the emission peak at 150 kpc from the nucleus, we estimate a dynamical mass of
~3 x 10^12 solar masses for the disk.Comment: 5 pages accepted by MNRAS Letter
Star Formation Histories of the LEGUS dwarf galaxies. II. Spatially resolved star formation history of the Magellanic irregular NGC 4449
We present a detailed study of the Magellanic irregular galaxy NGC 4449 based
on both archival and new photometric data from the Legacy Extragalactic UV
Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys
and Wide Field Camera 3. Thanks to its proximity ( Mpc) we
reach stars 3 magnitudes fainter than the tip of the red giant branch in the
F814W filter. The recovered star formation history spans the whole Hubble time,
but due to the age-metallicity degeneracy of the red giant branch stars, it is
robust only over the lookback time reached by our photometry, i.e.
Gyr. The most recent peak of star formation is around 10 Myr ago. The average
surface density star formation rate over the whole galaxy lifetime is
M yr kpc. From our study it emerges that NGC 4449 has
experienced a fairly continuous star formation regime in the last 1 Gyr with
peaks and dips whose star formation rates differ only by a factor of a few. The
very complex and disturbed morphology of NGC 4449 makes it an interesting
galaxy for studies of the relationship between interactions and starbursts, and
our detailed and spatially resolved analysis of its star formation history does
indeed provide some hints on the connection between these two phenomena in this
peculiar dwarf galaxy.Comment: 16 pages, 25 figures. Accepted for publication in the Astrophysical
Journa
- …