146 research outputs found
Record linkage of Norwegian historical census data using machine learning
For errata and source code: https://github.com/uit-hdl/rhd-linking.The Historical Population Register (HPR) is a project to build the longitudinal life history of individuals by integrating the historical records of the people in Norway since the 19th century. This study attempted to improve the linking rate between the 1875-1900 censuses in HPR, which is currently low, using machine learning approaches. To this end, I developed a machine learning model for linking that is suitable for the Norwegian census and tested various algorithms, feature sets, and match selection options. I compared the results in terms of performance and match size, and also examined their representativeness to the entire population. The study results showed that the linking rate of HPR can be significantly improved by machine learning approaches while maintaining high accuracy. In addition, this study presented a reference for future use by demonstrating how the performance varies depending on the feature set and match selection. On the other hand, this study also revealed that linked data generally do not represent the population of the census, and the characteristics and degree of bias vary depending on the linking algorithm, suggesting that caution is needed when using linked data for research
A constraint on the formation timescale of the young open cluster NGC 2264: Lithium abundance of pre-main sequence stars
The timescale of cluster formation is an essential parameter in order to
understand the formation process of star clusters. Pre-main sequence (PMS)
stars in nearby young open clusters reveal a large spread in brightness. If the
spread were considered as a result of a real spread in age, the corresponding
cluster formation timescale would be about 5 -- 20 Myr. Hence it could be
interpreted that star formation in an open cluster is prolonged for up to a few
tens of Myr. However, difficulties in reddening correction, observational
errors, and systematic uncertainties introduced by imperfect evolutionary
models for PMS stars, can result in an artificial age spread. Alternatively, we
can utilize Li abundance as a relative age indicator of PMS star to determine
the cluster formation timescale. The optical spectra of 134 PMS stars in NGC
2264 have been obtained with MMT/Hectochelle. The equivalent widths have been
measured for 86 PMS stars with a detectable Li line (3500 < T_eff [K] <= 6500).
Li abundance under the condition of local thermodynamic equilibrium (LTE) was
derived using the conventional curve of growth method. After correction for
non-LTE effects, we find that the initial Li abundance of NGC 2264 is A(Li) =
3.2 +/- 0.2. From the distribution of the Li abundances, the underlying age
spread of the visible PMS stars is estimated to be about 3 -- 4 Myr and this,
together with the presence of embedded populations in NGC 2264, suggests that
the cluster formed on a timescale shorter than 5 Myr.Comment: 53 pages, 12 figures, 4 tables, Accepted for publication in the
Astrophysical Journa
What Makes Ly Nebulae Glow? Mapping the Polarization of LABd05
"Ly nebulae" are giant (100 kpc), glowing gas clouds in the
distant universe. The origin of their extended Ly emission remains a
mystery. Some models posit that Ly emission is produced when the cloud
is photoionized by UV emission from embedded or nearby sources, while others
suggest that the Ly photons originate from an embedded galaxy or AGN
and are then resonantly scattered by the cloud. At least in the latter
scenario, the observed Ly emission will be polarized. To test these
possibilities, we are conducting imaging polarimetric observations of seven
Ly nebulae. Here we present our results for LABd05, a cloud at =
2.656 with an obscured, embedded AGN to the northeast of the peak of Ly
emission. We detect significant polarization. The highest polarization
fractions are 10-20% at 20-40 kpc southeast of the Ly
peak, away from the AGN. The lowest , including upper-limits, are 5%
and lie between the Ly peak and AGN. In other words, the polarization
map is lopsided, with increasing from the Ly peak to the southeast.
The measured polarization angles are oriented northeast, roughly
perpendicular to the gradient. This unique polarization pattern suggests
that 1) the spatially-offset AGN is photoionizing nearby gas and 2) escaping
Ly photons are scattered by the nebula at larger radii and into our
sightline, producing tangentially-oriented, radially-increasing polarization
away from the photoionized region. Finally we conclude that the interplay
between the gas density and ionization profiles produces the observed central
peak in the Ly emission. This also implies that the structure of LABd05
is more complex than assumed by current theoretical spherical or cylindrical
models.Comment: 11 pages, 8 figure
To the Edge of M87 and Beyond: Spectroscopy of Intracluster Globular Clusters and Ultra Compact Dwarfs in the Virgo Cluster
We present the results from a wide-field spectroscopic survey of globular
clusters (GCs) in the Virgo Cluster. We obtain spectra for 201 GCs and 55
ultracompact dwarfs (UCDs) using the Hectospec on the Multiple Mirror
Telescope, and derive their radial velocities. We identify 46 genuine
intracluster GCs (IGCs), not associated with any Virgo galaxies, using the 3D
GMM test on the spatial and radial velocity distribution.They are located at
the projected distance 200 kpc R 500 kpc from the center
of M87. The radial velocity distribution of these IGCs shows two peaks, one at
= 1023 km s associated with the Virgo main body, and another
at = 36 km s associated with the infalling structure. The
velocity dispersion of the IGCs in the Virgo main body is 314 km s, which is smoothly connected to the velocity dispersion
profile of M87 GCs, but much lower than that of dwarf galaxies in the same
survey field, 608 km s. The UCDs are more
centrally concentrated on massive galaxies, M87, M86, and M84. The radial
velocity dispersion of the UCD system is much smaller than that of dwarf
galaxies. Our results confirm the large-scale distribution of Virgo IGCs
indicated by previous photometric surveys. The color distribution of the
confirmed IGCs shows a bimodality similar to that of M87 GCs. This indicates
that most IGCs are stripped off from dwarf galaxies and some from massive
galaxies in the Virgo.Comment: 19 pages, 20 figures, 8 tables, accepted for publication in Ap
Red supergiant stars in IC 1613 and metallicity-dependent mixing length in the evolutionary model
We report a spectroscopic study on red supergiant stars (RSGs) in the
irregular dwarf galaxy IC 1613 in the Local Group. We derive the effective
temperatures () and metallicities of 14 RSGs by synthetic
spectral fitting to the spectra observed with the MMIRS instrument on the MMT
telescope for a wavelength range from 1.16 m to 1.23 m. A weak
bimodal distribution of the RSG metallicity centered on the [Fe/H]= is
found, which is slightly lower than or comparable to that of the Small
Magellanic Cloud (SMC). There is no evidence for spatial segregation between
the metal rich ([Fe/H]) and poor ([Fe/H]) RSGs throughout the
galaxy. The mean effective temperature of our RSG sample in IC 1613 is higher
by about 250 K than that of the SMC. However, no correlation between
and metallicity within our RSG sample is found. We calibrate
the convective mixing length () by comparing stellar
evolutionary tracks with the RSG positions on the HR diagram, finding that
models with can best reproduce the
effective temperatures of the RSGs in IC 1613 for both Schwarzschild and Ledoux
convection criteria. This result supports our previous study that a metallicity
dependent mixing length is needed to explain the RSG temperatures observed in
the Local Group, but we find that this dependency becomes relatively weak for
RSGs having a metallicity equal to or less than the SMC metallicity.Comment: 15 pages, 7 figures, accepted in Ap
A constraint on the formation timescale of the young open cluster NGC 2264: Lithium abundance of pre-main sequence stars
The timescale of cluster formation is an essential parameter in order to understand the formation process of star clusters. Pre-main sequence (PMS) stars in nearby young open clusters reveal a large spread in brightness. If the spread were considered to be a result of a real spread in age, the corresponding cluster formation timescale would be about 5-20 Myr. Hence it could be interpreted that star formation in an open cluster is prolonged for up to a few tens of Myr. However, difficulties in reddening correction, observational errors, and systematic uncertainties introduced by imperfect evolutionary models for PMS stars can result in an artificial age spread. Alternatively, we can utilize Li abundance as a relative age indicator of PMS star to determine the cluster formation timescale. The optical spectra of 134 PMS stars in NGC 2264 have been obtained with MMT/Hectochelle. The equivalent widths have been measured for 86 PMS stars with a detectable Li line (). Li abundance under the condition of local thermodynamic equilibrium (LTE) was derived using the conventional curve of growth method. After correction for non-LTE effects, we find that the initial Li abundance of NGC 2264 is . From the distribution of the Li abundances, the underlying age spread of the visible PMS stars is estimated to be about 3-4 Myr and this, together with the presence of embedded populations in NGC 2264, suggests that the cluster formed on a timescale shorter than 5 Myr
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