612 research outputs found
The role of environmental perceptions in migration decision-making: evidence from both migrants and non-migrants in five developing countries
© 2016, Springer Science+Business Media New York. Research has demonstrated that, in a variety of settings, environmental factors influence migration. Yet much of the existing work examines objective indicators of environmental conditions as opposed to the environmental perceptions of potential migrants. This paper examines migration decision-making and individual perceptions of different types of environmental change (sudden vs. gradual environmental events) with a focus on five developing countries: Vietnam, Cambodia, Uganda, Nicaragua, and Peru. The survey data include both migrants and non-migrants, with the results suggesting that individual perceptions of long-term (gradual) environmental events, such as droughts, lower the likelihood of internal migration. However, sudden-onset events, such as floods, increase movement. These findings substantially improve our understanding of perceptions as related to internal migration and also suggest that a more differentiated perspective is needed on environmental migration as a form of adaptation
ALMA observations of atomic carbon in z~4 dusty star-forming galaxies
We present ALMA [CI]() (rest frequency 492 GHz) observations for a
sample of 13 strongly-lensed dusty star-forming galaxies originally discovered
at 1.4mm in a blank-field survey by the South Pole Telescope. We compare these
new data with available [CI] observations from the literature, allowing a study
of the ISM properties of extreme dusty star-forming galaxies spanning
a redshift range . Using the [CI] line as a tracer of the molecular
ISM, we find a mean molecular gas mass for SPT-DSFGs of
M. This is in tension with gas masses derived via low- CO
and dust masses; bringing the estimates into accordance requires either (a) an
elevated CO-to-H conversion factor for our sample of and a gas-to-dust ratio , or (b) an high carbon abundance . Using observations of a range of additional atomic
and molecular lines (including [CI], [CII], and multiple transitions of CO), we
use a modern Photodissociation Region code (3D-PDR) to assess the physical
conditions (including the density, UV radiation field strength, and gas
temperature) within the ISM of the DSFGs in our sample. We find that the ISM
within our DSFGs is characterised by dense gas permeated by strong UV fields.
We note that previous efforts to characterise PDR regions in DSFGs may have
significantly underestimated the density of the ISM. Combined, our analysis
suggests that the ISM of extreme dusty starbursts at high redshift consists of
dense, carbon-rich gas not directly comparable to the ISM of starbursts in the
local Universe.Comment: 21 pages, 12 figures. Accepted for publication in MNRA
Sub-kiloparsec Imaging of Cool Molecular Gas in Two Strongly Lensed Dusty, Star-Forming Galaxies
We present spatially-resolved imaging obtained with the Australia Telescope
Compact Array (ATCA) of three CO lines in two high-redshift gravitationally
lensed dusty star-forming galaxies, discovered by the South Pole Telescope.
Strong lensing allows us to probe the structure and dynamics of the molecular
gas in these two objects, at z=2.78 and z=5.66, with effective source-plane
resolution of less than 1kpc. We model the lensed emission from multiple CO
transitions and the dust continuum in a consistent manner, finding that the
cold molecular gas as traced by low-J CO always has a larger half-light radius
than the 870um dust continuum emission. This size difference leads to up to 50%
differences in the magnification factor for the cold gas compared to dust. In
the z=2.78 galaxy, these CO observations confirm that the background source is
undergoing a major merger, while the velocity field of the other source is more
complex. We use the ATCA CO observations and comparable resolution Atacama
Large Millimeter/submillimeter Array dust continuum imaging of the same objects
to constrain the CO-H_2 conversion factor with three different procedures,
finding good agreement between the methods and values consistent with those
found for rapidly star-forming systems. We discuss these galaxies in the
context of the star formation - gas mass surface density relation, noting that
the change in emitting area with observed CO transition must be accounted for
when comparing high-redshift galaxies to their lower redshift counterparts.Comment: 14 pages, 7 figures; accepted for publication in Ap
Observations of neutral carbon in 29 high-z lensed dusty star forming galaxies and the comparison of gas mass tracers
The nature and evolution of high-redshift dusty star-forming galaxies (high-z
DSFGs) remain an open question. Their massive gas reservoirs play an important
role in driving the intense star-formation rates hosted in these galaxies. We
aim to estimate the molecular gas content of high-z DSFGs by using various gas
mass tracers such as the [CI], CO, [CII] emission lines and the dust content.
These tracers need to be well calibrated as they are all limited by
uncertainties on factors such as aCO, XCI, aCII and GDR, thereby affecting the
determination of the gas mass accurately. The main goal of our work is to check
the consistency between the gas mass tracers and cross-calibrate the uncertain
factors. We observe the two [CI] line transitions for 29 SPT-SMGs with the
ALMA-ACA. Additionally, we also present new APEX observations of [CII] line for
9 of these galaxies. We find a nearly linear relation between the infrared
luminosity and [CI] luminosity if we fit the starbursts and main-sequence
galaxies separately. We measure a median [CI]-derived excitation temperature of
34.5+/-2.1 K. We probe the properties of the interstellar medium (ISM) such as
density and radiation field intensity using [CI] to mid- or high-J CO lines and
[CI] to infrared luminosity ratio, and find similar values to the SMG
populations in literature. Finally, the gas masses estimated from [CI], CO,
dust, and [CII] do not exhibit any significant trend with the infrared
luminosity or the dust temperature. We provide the various cross-calibrations
between these tracers. Our study confirms that [CI] is a suitable tracer of the
molecular gas content, and shows an overall agreement between all the classical
gas tracers used at high redshift. However, their absolute calibration and thus
the gas depletion timescale measurements remain uncertain.Comment: Accepted for publication in A&A, 25 pages, 11 figures, 6 table
SPT 0538-50: Physical conditions in the ISM of a strongly lensed dusty star-forming galaxy at z=2.8
We present observations of SPT-S J053816-5030.8, a gravitationally-lensed
dusty star forming galaxy (DSFG) at z = 2.7817, first discovered at millimeter
wavelengths by the South Pole Telescope. SPT 0538-50 is typical of the
brightest sources found by wide-field millimeter-wavelength surveys, being
lensed by an intervening galaxy at moderate redshift (in this instance, at z =
0.441). We present a wide array of multi-wavelength spectroscopic and
photometric data on SPT 0538-50, including data from ALMA, Herschel PACS and
SPIRE, Hubble, Spitzer, VLT, ATCA, APEX, and the SMA. We use high resolution
imaging from HST to de-blend SPT 0538-50, separating DSFG emission from that of
the foreground lens. Combined with a source model derived from ALMA imaging
(which suggests a magnification factor of 21 +/- 4), we derive the intrinsic
properties of SPT 0538-50, including the stellar mass, far-IR luminosity, star
formation rate, molecular gas mass, and - using molecular line fluxes - the
excitation conditions within the ISM. The derived physical properties argue
that we are witnessing compact, merger-driven star formation in SPT 0538-50,
similar to local starburst galaxies, and unlike that seen in some other DSFGs
at this epoch.Comment: 16 pages, 11 figures. Accepted for publication in Ap
Extragalactic millimeter-wave point source catalog, number counts and statistics from 771 square degrees of the SPT-SZ Survey
We present a point source catalog from 771 square degrees of the South Pole
Telescope Sunyaev Zel'dovich (SPT-SZ) survey at 95, 150, and 220 GHz. We detect
1545 sources above 4.5 sigma significance in at least one band. Based on their
relative brightness between survey bands, we classify the sources into two
populations, one dominated by synchrotron emission from active galactic nuclei,
and one dominated by thermal emission from dust-enshrouded star-forming
galaxies. We find 1238 synchrotron and 307 dusty sources. We cross-match all
sources against external catalogs and find 189 unidentified synchrotron sources
and 189 unidentified dusty sources. The dusty sources without counterparts are
good candidates for high-redshift, strongly lensed submillimeter galaxies. We
derive number counts for each population from 1 Jy down to roughly 9, 5, and 11
mJy at 95, 150, and 220 GHz. We compare these counts with galaxy population
models and find that none of the models we consider for either population
provide a good fit to the measured counts in all three bands. The disparities
imply that these measurements will be an important input to the next generation
of millimeter-wave extragalactic source population models.Comment: 23 pages, 8 figures, submitted to Ap
CO excitation of normal star forming galaxies out to z=1.5 as regulated by the properties of their interstellar medium
We investigate the CO excitation of normal star forming galaxies at z=1.5 using IRAM PdBI observations of the CO[2-1], CO[3-2] and CO[5-4] transitions for 4 galaxies, and VLA observations of CO[1-0] for 3 of them, measuring reliable line fluxes with S/N>4-7 for individual transitions. While the average CO Spectral Line Energy Distribution (SLED) has a sub-thermal excitation similar to the Milky Way (MW) up to CO[3-2], we show that the average CO[5-4] emission is 4 times stronger than assuming MW excitation. This demonstrates the presence of an additional component of more excited, denser and possibly warmer molecular gas. The ratio of CO[5-4] to lower-J CO emission is however lower than in local (U)LIRGs and high-redshift starbursting SMGs, and appears to correlate closely with the average intensity of the radiation field and with the star formation surface density, but not with SF efficiency (SFE). This suggests that the overall CO excitation is at least indirectly affected by the metallicity of the ISM. The luminosity of the CO[5-4] transition is found to correlate linearly with the bolometric infrared luminosity over 4 orders of magnitudes, with BzK galaxies following the same linear trend as local spirals and (U)LIRGs and high redshift star bursting sub-millimeter galaxies. The CO[5-4] luminosity is thus related to the dense gas, and might be a more convenient way to probe it than standard high--density tracers. We see excitation variations among our sample galaxies, linked to their evolutionary state and clumpiness in optical rest frame images. In one galaxy we see spatially resolved excitation variations, the more highly excited part corresponds to the location of massive SF clumps. This provides support to models that suggest that giant clumps are the main source of the high excitation CO emission in high redshift disk-like galaxies
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