24 research outputs found
The Kinematics of CIV in Star-Forming Galaxies at z~1.2
We present the first statistical sample of rest-frame far-UV spectra of
star-forming galaxies at z~1. These spectra are unique in that they cover the
high-ionization CIV{\lambda}{\lambda}1548, 1550 doublet. We also detect
low-ionization features such as SiII{\lambda}1527, FeII{\lambda}1608,
AlII{\lambda}1670, NiII{\lambda}{\lambda}1741, 1751 and SiII{\lambda}1808, and
intermediate-ionization features from AlIII{\lambda}{\lambda}1854, 1862.
Comparing the properties of absorption lines of lower- and higher- ionization
states provides a window into the multi-phase nature of circumgalactic gas. Our
sample is drawn from the DEEP2 survey and spans the redshift range 1.01 < z <
1.35 ( = 1.25). By isolating the interstellar CIV absorption from the
stellar P-Cygni wind profile we find that 69% of the CIV profiles are
blueshifted with respect to the systemic velocity. Furthermore, CIV shows a
small but significant blueshift relative to FeII (offset of the best-fit linear
regression -76 26 km/s). At the same time, the CIV blueshift is on
average comparable to that of MgII{\lambda}{\lambda}2796, 2803. At this point,
in explaining the larger blueshift of CIV absorption at the ~ 3-sigma level, we
cannot distinguish between the faster motion of highly-ionized gas relative to
gas traced by FeII, and filling in on the red side from resonant CIV emission.
We investigate how far-UV interstellar absorption kinematics correlate with
other galaxy properties using stacked spectra. These stacking results show a
direct link between CIV absorption and the current SFR, though we only observe
small velocity differences among different ionization states tracing the
outflowing ISM.Comment: 21 pages, 14 figures, ApJ, accepte
SkyLens: Visual analysis of skyline on multi-dimensional data
Skyline queries have wide-ranging applications in fields that involve
multi-criteria decision making, including tourism, retail industry, and human
resources. By automatically removing incompetent candidates, skyline queries
allow users to focus on a subset of superior data items (i.e., the skyline),
thus reducing the decision-making overhead. However, users are still required
to interpret and compare these superior items manually before making a
successful choice. This task is challenging because of two issues. First,
people usually have fuzzy, unstable, and inconsistent preferences when
presented with multiple candidates. Second, skyline queries do not reveal the
reasons for the superiority of certain skyline points in a multi-dimensional
space. To address these issues, we propose SkyLens, a visual analytic system
aiming at revealing the superiority of skyline points from different
perspectives and at different scales to aid users in their decision making. Two
scenarios demonstrate the usefulness of SkyLens on two datasets with a dozen of
attributes. A qualitative study is also conducted to show that users can
efficiently accomplish skyline understanding and comparison tasks with SkyLens.Comment: 10 pages. Accepted for publication at IEEE VIS 2017 (in proceedings
of VAST
Searching for z > 6.5 Analogs Near the Peak of Cosmic Star Formation
Strong [OIII]4959,5007+H emission appears to be
typical in star-forming galaxies at z > 6.5. As likely contributors to cosmic
reionization, these galaxies and the physical conditions within them are of
great interest. At z > 6.5, where Ly is greatly attenuated by the
intergalactic medium, rest-UV metal emission lines provide an alternative
measure of redshift and also constraints on the physical properties of
star-forming regions and massive stars. We present the first statistical sample
of rest-UV line measurements in z 2 galaxies selected as analogs of
those in the reionization era based on [OIII]4959,5007 EW or
rest-frame U-B color. Our sample is drawn from the 3D-HST Survey and spans the
redshift range 1.36 z 2.49. We find that the median
Ly and CIII]1907,1909 EWs of our sample are
significantly greater than those of z 2 UV-continuum-selected
star-forming galaxies. Measurements from both individual and composite spectra
indicate a monotonic, positive correlation between CIII] and [OIII], while a
lack of trend is observed between Ly and [OIII] at [OIII] EW <
1000\unicode{x212B}. At higher [OIII] EW, extreme Ly emission starts
to emerge. Using stacked spectra, we find that Ly and CIII] are
significantly enhanced in galaxies with lower metallicity. Two objects in our
sample appear comparable to z > 6.5 galaxies with exceptionally strong rest-UV
metal line emission. These objects have significant
CIV1548,1550, HeII1640, and
OIII]1661,1665 emission in addition to intense Ly or
CIII]. Detailed characterization of these lower-redshift analogs provides
unique insights into the physical conditions in z > 6.5 star-forming regions,
motivating future observations of reionization-era analogs at lower redshifts.Comment: 22 pages, 9 figures, accepted for publication in Ap
The MOSDEF-LRIS Survey: Probing ISM/CGM Structure of Star-Forming Galaxies at z~2 Using Rest-UV Spectroscopy
The complex structure of gas, metals, and dust in the interstellar and
circumgalactic medium (ISM and CGM, respectively) in star-forming galaxies can
be probed by Ly emission and absorption, low-ionization interstellar
(LIS) metal absorption, and dust reddening E(B-V). We present a statistical
analysis of the mutual correlations among Ly equivalent width
(EW), LIS equivalent width (EW), and E(B-V) in a sample of
157 star-forming galaxies at . With measurements obtained from
individual, deep rest-UV spectra and spectral-energy distribution (SED)
modeling, we find that the tightest correlation exists between EW and
E(B-V), although correlations among all three parameters are statistically
significant. These results signal a direct connection between dust and
metal-enriched HI gas, and that they are likely co-spatial. By comparing our
results with the predictions of different ISM/CGM models, we favor a dusty
ISM/CGM model where dust resides in HI gas clumps and Ly photons escape
through the low HI covering fraction/column density intra-clump medium.
Finally, we investigate the factors that potentially contribute to the
intrinsic scatter in the correlations studied in this work, including
metallicity, outflow kinematics, Ly production efficiency, and slit
loss. Specifically, we find evidence that scatter in the relationship between
EW and E(B-V) reflects the variation in metal-to-HI covering
fraction ratio as a function of metallicity, and the effects of outflows on the
porosity of the ISM/CGM. Future simulations incorporating star-formation
feedback and the radiative transfer of Ly photons will provide key
constraints on the spatial distributions of neutral hydrogen gas and dust in
the ISM/CGM structure.Comment: 19 pages, 6 figures, 3 tables, accepted for publication in Ap
Exploring the Correlation between -to-UV Ratio and Burstiness for Typical Star-forming Galaxies at
The -to-UV luminosity ratio () is
often used to probe SFHs of star-forming galaxies and it is important to
validate it against other proxies for burstiness. To address this issue, we
present a statistical analysis of the resolved distribution of
as well as stellar age and their correlations with the
globally measured for a sample of 310 star-forming
galaxies in two redshift bins of and
observed by the MOSDEF survey. We use the multi-waveband CANDELS/3D-HST imaging
of MOSDEF galaxies to construct and stellar age maps. We
analyze the composite rest-frame far-UV spectra of a subsample of MOSDEF
targets obtained by the Keck/LRIS, which includes 124 star-forming galaxies
(MOSDEF-LRIS) at redshifts , to examine the average stellar
population properties, and the strength of age-sensitive FUV spectral features
in bins of . Our results show no significant evidence
that individual galaxies with higher are undergoing
a burst of star formation based on the resolved distribution of
of individual star-forming galaxies. We segregate the
sample into subsets with low and high . The
high- subset exhibits, on average, an age of
= 8.0, compared to = 8.4 for the
low- galaxies, though the difference in age is
significant at only the level. Furthermore, we find no variation in
the strengths of Siiv and Civ P-Cygni features from massive stars between the two subsamples.Comment: 16 pages, 10 figures, published by the Monthly Notices of the Royal
Astronomical Societ
The Redshift Evolution of Rest-UV Spectroscopic Properties to z~5
We perform a comprehensive analysis of the redshift evolution of the rest-UV
spectra of star-forming galaxies out to z~5. We combine new z~5 measurements of
HI Ly and low- and high-ionization interstellar metal absorption
features with comparable measurements at z~2-4. We measure the equivalent
widths of interstellar absorption features using stacked spectra in bins of
Ly equivalent width, performing corrections to Ly strengths
based on a model for the transmission of the intergalactic medium. We find a
strong correlation between decreasing low-ionization absorption strength and
increasing Ly emission strength over the redshift range z~2-5,
suggesting that both of these quantities are fundamentally linked to neutral
gas covering fraction. At the highest Ly equivalent widths, we observe
evolution at towards greater Ly emission strength at fixed
low-ionization absorption strength. If we interpret the non-evolving
relationship of Ly emission strength and low-ionization line strength
at z~2-4 as primarily reflecting the radiative transfer of Ly photons,
this evolution at z~5 suggests a higher intrinsic production rate of Ly
photons than at lower redshift. Our conclusion is supported by the joint
evolution of the relationships among Ly emission strength, interstellar
absorption strength, and dust reddening. We perform additional analysis in bins
of stellar mass, star-formation rate, UV luminosity, and age, examining how the
relationships between galaxy properties and Ly emission evolve towards
higher redshift. We conclude that increasing intrinsic Ly photon
production and strong detection of nebular CIV emission (signaling lower
metallicity) at z~5 indicate an elevated ionized photon production efficiency
().Comment: 20 pages, 17 figures, accepted for publication in MNRA
HEALTH AND SAFETY MANAGEMENT PRACTICES OF ROAD CONSTRUCTION PROJECTS IN SRI LANKA
Health and Safety Management Practices of Road Construction Projects in Sri Lanka
Road construction workers are exposed to hazards. This study attempts to identify, health and safety practices prevailing in road construction projects, issues and propose measures to mitigate.The study was confined to seven contract packages implemented under Northern Road Connectivity Project (Additional Financing) funded by Asian Development Bank (ADB). Five of them were located in the North Central Province while the other two were in Northern Province. Data was gathered through questionnaires, interviews and observations. Questionnaire comprised six major criteria, namely, training and supervision, safe work procedures, communication, reporting safety, management commitment and injury and return to work. Study revealed that workers are exposed to hazards of frequent and heavy lifting, noise and vibration, hazardous substances such as oil, gas, hydraulic fluid, welding fumes, heavy vehicle operation, working in blasting faces, clearing operations, slips, trips and falls, struck by, being struck by falling objects, hand-arm-whole body vibration, dust, sources of noise, electrical hazards etc. In asphalting areas workers are exposed to high temperatures and Polycyclic Aromatic Hydro Carbons (PAHC) which are carcinogenic. Working in excavated areas is prone to collapse of slopes. Workers are also prone to ergonomic issues caused due to manual handling and awkward postures. Main causes of accidents have been identified as fall from height, fall of objects, struck on stationary objects, struck by moving objects, struck on moving objects, caught in between, exposure to heat, exposure to electricity and exposure to harmful substances. Information gathered from 250 respondents revealed a high positive response in overall health and safety management practices. Five of the six criteria concerned showed a high positive response of greater than 78 percent. Communication recorded the lowest. Contract package wise comparison shows that P3 and P4 had a low positive response in the criteria concerned other than management commitment. Interviews conducted with Project Managers revealed a high commitment. Safety Committees were nonexistent in six of the seven packages except (P2). Observations revealed safety lapses such as working without PPE, working under unsafe conditions, non barricading hazardous zones, lack of warning sign boards and deploying non competent flagmen etc. It is recommended to conduct awareness programmes through Labour Department Officials prior to commencement of construction work, qualified Safety Officers should be employed, sites should be thoroughly inspected to eradicate hazards, work zones should be safe for workers, consultant’s field staff should check adequacy of precautionary measures, importance of PPE should be emphasized, hazard areas should be properly barricaded, trained flagman should be deployed, records of toolbox meetings should be maintained, near misses should be recorded, Safety Policy should be accessible to all workers, officials from the Labour Department should visit the sites to check compliance
The Redshift Evolution of Rest-UV Spectroscopic Properties in Lyman-break Galaxies at z ~ 2–4
We present the first comprehensive evolutionary analysis of the rest-frame UV spectroscopic properties of star-forming galaxies at z ~ 2–4. We match samples at different redshifts in UV luminosity and stellar mass, and perform systematic measurements of spectral features and stellar population modeling. By creating composite spectra grouped according to Lyα equivalent width (EW) and various galaxy properties, we study the evolutionary trends among Lyα, low- and high-ionization interstellar (LIS and HIS) absorption features, and integrated galaxy properties. We also examine the redshift evolution of Lyα and LIS absorption kinematics, and fine-structure emission EWs. The connections among the strengths of Lyα, LIS lines, and dust extinction are redshift independent, as is the decoupling of the Lyα and HIS line strengths, and the bulk outflow kinematics as traced by the LIS lines. Stronger Lyα emission is observed at higher redshift at fixed UV luminosity, stellar mass, SFR, and age. Much of this variation in the average Lyα strength with redshift, and the variation in Lyα strength at fixed redshift, can be explained in terms of variations in the neutral gas covering fraction and/or dust content in the ISM and CGM. However, based on the connection between Lyα and C iii] emission strengths, we additionally find evidence for variations in the intrinsic production rate of Lyα photons at the highest Lyα EWs. The challenge now is to understand the observed evolution of the neutral gas covering fraction and dust extinction within a coherent model for galaxy formation, and make robust predictions for the escape of ionizing radiation at z > 6
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Characterizing the Interstellar and Circumgalactic Medium in Distant Star-Forming Galaxies
Rest-frame ultraviolet (UV) and optical spectroscopy provides valuable information on the physical properties of the neutral and ionized interstellar medium (ISM) in star-forming galaxies. Such observations probe both the systemic interstellar component originating from H II regions, and the multi-phase outflowing component that is associated with star-formation feedback and can ultimately contribute to the circumgalactic and even intergalactic medium (CGM and IGM, respectively). In this dissertation, I investigate the physical properties of ionized gas in star-forming regions, and the kinematics and evolution of the multi-phase outflowing ISM/CGM in distant star-forming galaxies spanning the redshift range z ~ 1-4. This work consists of three studies that examine different aspects of the ISM and CGM, which collectively improve our understanding of the gas content in galaxies and the processes associated with the formation of massive stars near the peak of the star-formation-rate (SFR) density in the universe. I present a comparison of kinematics between the low- and high-ionization absorption features at z ~ 1, and demonstrate that the apparent larger blueshift of interstellar C IV relative to the low-ionization features is likely a result from the nature of resonant transitions instead of an evidence of the faster motion of the highly ionized gas. I further investigate the origin of the highly ionized gas by examining the correlations between the spectral properties of C IV and various galaxy properties. Both the blueshift and equivalent width (EW) of C IV are modulated by SFR and specific SFR, suggesting a direct connection between the highly ionized gas and the formation of massive stars. Nebular emission features provide valuable insights into the physical conditions of the ionized gas in H II regions, as well as the properties of young, massive stars. I show that the nebular C III] emission at z ~ 1 is much weaker compared to the detections from galaxies observed during the epoch of reionization (z > 6), and explore the factors that modulate the strength of this nebular feature. In combination with the results from photoionization models, I further infer the gas-phase metallicity and abundance pattern in the z ~ 1 star-forming galaxies based on the observed rest-frame C III] EW. Studying the lower-redshift analogs of the z > 6 C III] emitters is an alternative way to obtain more detailed information on the physical properties of these extreme-emission-line-galaxies (EELGs). By assembling a sample of EELGs at z ~ 1-2 and examining C III] and other nebular emission features, I aim to characterize the physical conditions of the z > 6 galaxies that are likely responsible for the cosmic reionization. Finally, with carefully constructed samples and uniform measurements, I investigate the evolution of the ISM/CGM at z ~ 2-4 as probed by rest-UV spectroscopy. I discover redshift-independent correlations among Lya emission, low-ionization interstellar absorption lines, and dust extinction. I further show that the covering fraction of neutral gas decreases with increasing redshift at multiple fixed galaxy properties. Gaining a full understanding of galaxy evolution requires further studies of the ISM/CGM in a systemic manner at higher redshifts. With exceptional capabilities in the near-IR and excellent spectroscopic sensitivity, the next generation of large telescopes will enable rest-UV and rest-optical spectroscopic studies of star-forming galaxies out to z > 10. Answering key questions regarding the interplay among massive stars, their contribution to the ionizing background, and feedback will deliver a clear picture of the formation and evolution of these distant star-forming galaxies