50 research outputs found
The ALMaQUEST Survey: The Molecular Gas Main Sequence and the Origin of the Star-forming Main Sequence
The origin of the star forming main sequence ( i.e., the relation between
star formation rate and stellar mass, globally or on kpc-scales; hereafter
SFMS) remains a hotly debated topic in galaxy evolution. Using the ALMA-MaNGA
QUEnching and STar formation (ALMaQUEST) survey, we show that for star forming
spaxels in the main sequence galaxies, the three local quantities,
star-formation rate surface density (\sigsfr), stellar mass surface density
(\sigsm), and the \h2~mass surface density (\sigh2), are strongly correlated
with one another and form a 3D linear (in log) relation with dispersion. In
addition to the two well known scaling relations, the resolved SFMS (\sigsfr~
vs. \sigsm) and the Schmidt-Kennicutt relation (\sigsfr~ vs. \sigh2; SK
relation), there is a third scaling relation between \sigh2~ and \sigsm, which
we refer to as the `molecular gas main sequence' (MGMS). The latter indicates
that either the local gas mass traces the gravitational potential set by the
local stellar mass or both quantities follow the underlying total mass
distributions. The scatter of the resolved SFMS ( dex) is the
largest compared to those of the SK and MGMS relations ( 0.2 dex).
A Pearson correlation test also indicates that the SK and MGMS relations are
more strongly correlated than the resolved SFMS. Our result suggests a scenario
in which the resolved SFMS is the least physically fundamental and is the
consequence of the combination of the SK and the MGMS relations
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The ALMaQUEST Survey. VII. Star Formation Scaling Relations of Green Valley Galaxies
We utilize the ALMA-MaNGA QUEnch and STar formation (ALMaQUEST) survey to
investigate the kpc-scale scaling relations, presented as the resolved star
forming main sequence (rSFMS: vs. ), the
resolved Schmidt-Kennicutt relation (rSK: vs. ), and the resolved molecular gas main sequence (rMGMS: vs. ), for 11478 star-forming and 1414 retired spaxels
(oversampled by a factor of ) located in 22 green valley (GV) and 12
main sequence (MS) galaxies. For a given galaxy type (MS or GV), the retired
spaxels are found to be offset from the sequences formed by the star-forming
spaxels on the rSFMS, rSK, and rMGMS planes, toward lower absolute values of
sSFR, SFE, and by 1.1, 0.6, and 0.5 dex. The scaling
relations for GV galaxies are found to be distinct from that of the MS
galaxies, even if the analyses are restricted to the star-forming spaxels only.
It is found that for star-forming spaxels, sSFR, SFE, and in GV
galaxies are reduced by 0.36, 0.14, and 0.21 dex, respectively, compared
to those in MS galaxies. Therefore, the suppressed sSFR/SFE/ in
GV galaxies are associated with not only an increased proportion of retired
regions in GV galaxies but also a depletion of these quantities in star-forming
regions. Finally, the reduction of SFE and in GV galaxies
relative to MS galaxies is seen in both bulge and disk regions (albeit with
larger uncertainties), suggesting that statistically, quenching in the GV
population may persist from the inner to the outer regions
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ALMaQUEST. IV. The ALMA-MaNGA QUEnching and STar Formation (ALMaQUEST) Survey
The ALMaQUEST (ALMA-MaNGA QUEnching and STar formation) survey is a program
with spatially-resolved CO(1-0) measurements obtained with the Atacama
Large Millimeter Array (ALMA) for 46 galaxies selected from the Mapping Nearby
Galaxies at Apache Point Observatory (MaNGA) DR15 optical integral-field
spectroscopic survey. The aim of the ALMaQUEST survey is to investigate the
dependence of star formation activity on the cold molecular gas content at kpc
scales in nearby galaxies. The sample consists of galaxies spanning a wide
range in specific star formation rate (sSFR), including starburst (SB),
main-sequence (MS), and green valley (GV) galaxies. In this paper, we present
the sample selection and characteristics of the ALMA observations, and showcase
some of the key results enabled by the combination of spatially-matched stellar
populations and gas measurements. Considering the global (aperture-matched)
stellar mass, molecular gas mass, and star formation rate of the sample, we
find that the sSFR depends on both the star formation efficiency (SFE) and the
molecular gas fraction (), although the correlation with the
latter is slightly weaker. Furthermore, the dependence of sSFR on the molecular
gas content (SFE or ) is stronger than that on either the atomic
gas fraction or the molecular-to-atomic gas fraction, albeit with the small HI
sample size. On kpc scales, the variations in both SFE and
within individual galaxies can be as large as 1-2 dex thereby demonstrating
that the availability of spatially-resolved observations is essential to
understand the details of both star formation and quenching processes.STFC
ER
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Catchment-scale biogeography of riverine bacterioplankton
Lotic ecosystems such as rivers and streams are unique in that they represent a continuum of both space and time during the transition from headwaters to the river mouth. As microbes have very different controls over their ecology, distribution and dispersion compared with macrobiota, we wished to explore biogeographical patterns within a river catchment and uncover the major drivers structuring bacterioplankton communities. Water samples collected across the River Thames Basin, UK, covering the transition from headwater tributaries to the lower reaches of the main river channel were characterised using 16S rRNA gene pyrosequencing. This approach revealed an ecological succession in the bacterial community composition along the river continuum, moving from a community dominated by Bacteroidetes in the headwaters to Actinobacteria-dominated downstream. Location of the sampling point in the river network (measured as the cumulative water channel distance upstream) was found to be the most predictive spatial feature; inferring that ecological processes pertaining to temporal community succession are of prime importance in driving the assemblages of riverine bacterioplankton communities. A decrease in bacterial activity rates and an increase in the abundance of low nucleic acid bacteria relative to high nucleic acid bacteria were found to correspond with these downstream changes in community structure, suggesting corresponding functional changes. Our findings show that bacterial communities across the Thames basin exhibit an ecological succession along the river continuum, and that this is primarily driven by water residence time rather than the physiochemical status of the river
Madagascar corals track sea surface temperature variability in the Agulhas Current core region over the past 334 years
The Agulhas Current (AC) is the strongest western boundary current in the Southern Hemisphere and is key for weather and climate patterns, both regionally and globally. Its heat transfer into both the midlatitude South Indian Ocean and South Atlantic is of global significance. A new composite coral record (Ifaty and Tulear massive Porites corals), is linked to historical AC sea surface temperature (SST) instrumental data, showing robust correlations. The composite coral SST data start in 1660 and comprise 200 years more than the AC instrumental record. Numerical modelling exhibits that this new coral derived SST record is representative for the wider core region of the AC. AC SSTs variabilities show distinct cooling through the Little Ice Age and warming during the late 18th, 19th and 20th century, with significant decadal variability superimposed. Furthermore, the AC SSTs are teleconnected with the broad southern Indian and Atlantic Oceans, showing that the AC system is pivotal for inter-ocean heat exchange south of Africa
Indicators of river system hydromorphological character and dynamics: understanding current conditions and guiding sustainable river management
The work leading to this paper received funding from the EU’s FP7 programme under Grant Agreement No. 282656 (REFORM). The Indicators were developed within the context of REFORM deliverable D2.1, therefore all partners involved in this deliverable contributed to some extent to their discussion and development
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The ALMaQUEST Survey. XII. Dense Molecular Gas as Traced by HCN and HCO<sup>+</sup> in Green Valley Galaxies
Abstract
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of two dense gas tracers, HCN (1−0) and HCO+ (1-0) for three galaxies in the green valley and two galaxies on the star-forming main sequence with comparable molecular gas fractions as traced by the CO (1−0) emissions, selected from the ALMaQUEST survey. We investigate whether the deficit of molecular gas star formation efficiency (SFEmol) that leads to the low specific star formation rate (sSFR) in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction f
dense) or the low star formation efficiency of dense gas (SFEdense). We find that SFEmol as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFEdense and depends only weakly on f
dense. The sSFR on kiloparsec scales is primarily driven by SFEmol and SFEdense, followed by the dependence on f
mol, and is least correlated with f
dense or the dense-gas-to-stellar mass ratio (R
dense). When compared with other works in the literature, we find that our green valley sample shows lower global SFEmol and lower SFEdense while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the three green valley galaxies with a normal abundance of molecular gas is suppressed, mainly due to the reduced SFEdense rather than the lack of dense gas.</jats:p