809 research outputs found
Rest-Frame Ultraviolet to Near Infrared Observations of an Interacting Lyman Break Galaxy at z = 4.42
We present the rest-frame ultraviolet through near infrared spectral energy
distribution for an interacting Lyman break galaxy at a redshift z=4.42, the
highest redshift merging system known with clearly resolved tidal features. The
two objects in this system - HDF-G4 and its previously unidentified companion -
are both B_{435} band dropouts, have similar V_{606}-i_{775} and
i_{775}-z_{850} colors, and are separated by 1", which at z=4.42 corresponds to
7 kpc projected nuclear separation; all indicative of an interacting system.
Fits to stellar population models indicate a stellar mass of M_\star =
2.6\times 10^{10} M_\odot, age of \tau_\star = 720 My, and exponential star
formation history with an e-folding time \tau_0 = 440 My. Using these derived
stellar populations as constraints, we model the HDF-G4 system using
hydrodynamical simulations, and find that it will likely evolve into a quasar
by z\sim3.5, and a quiescent, compact spheroid by z\sim 2.5 similar to those
observed at z > 2. And, the existence of such an object supports galaxy
formation models in which major mergers drive the high redshift buildup of
spheroids and black holes.Comment: 7 pages, 7 figures, accepted for publication in Ap
Vegetation and floristics of a lowland tropical rainforest in northeast Australia
Background: Full floristic data, tree demography, and biomass estimates incorporating non-tree life forms are seldom collected and reported for forest plots in the tropics. Established research stations serve as important repositories of such biodiversity and ecological data. With a canopy crane setup within a tropical lowland rainforest estate, the 42-ha Daintree Rainforest Observatory (DRO) in Cape Tribulation, northern Australia is a research facility of international significance. We obtained an estimate of the vascular plant species richness for the site, by surveying all vascular plant species from various mature-phase, remnant and open vegetation patches within the site. We also integrate and report the demography and basal areas of trees ≥ 10 cm diameter at breast height (dbh) in a new 1- ha core plot, an extension to the pre-existing forest 1-ha plot under the canopy crane. In addition, we report for the canopy crane plot new demography and basal areas for smaller size shrubs and treelets subsampled from nine 20 m quadrats, and liana basal area and abundance from the whole plot. The DRO site has an estimated total vascular plant species richness of 441 species, of which 172 species (39%) are endemic to Australia, and 4 species are endemics to the Daintree region. The 2 x 1-ha plots contains a total of 262 vascular plant species of which 116 (1531 individuals) are tree species ≥ 10 cm dbh. We estimate a stem basal area of 34.9 m ha, of which small stems (tree saplings and shrubs <10cm dbh) and lianas collectively contribute c.4.2%. Comparing the stem density-diversity patterns of the DRO forest with other tropical rainforests globally, our meta-analysis shows that DRO forests has a comparatively high stem density and moderate species diversity, due to the influence of cyclones. These data will provide an important foundation for ecological and conservation studies in lowland tropical forest.
New information: We present a floristic checklist, a life form breakdown, and demography data from two 1-ha rainforest plots from a lowland tropical rainforest study site. We also present a meta-analysis of stem densities and species diversity from comparable-sized plots across the tropics
High Affinity for Farnesyltransferase and Alternative Prenylation Contribute Individually to K-Ras4B Resistance to Farnesyltransferase Inhibitors
Farnesyltransferase inhibitors (FTIs) block Ras farnesylation, subcellular localization and activity, and inhibit the growth of Ras-transformed cells. Although FTIs are ineffective against K-Ras4B, the Ras isoform most commonly mutated in human cancers, they can inhibit the growth of tumors containing oncogenic K-Ras4B, implicating other farnesylated proteins or suggesting distinct functions for farnesylated and for geranylgeranylated K-Ras, which is generated when farnesyltransferase is inhibited. In addition to bypassing FTI blockade through geranylgeranylation, K-Ras4B resistance to FTIs may also result from its higher affinity for farnesyltransferase. Using chimeric Ras proteins containing all combinations of Ras background, CAAX motif, and K-Ras polybasic domain, we show that either a polybasic domain or an alternatively prenylated CAAX renders Ras prenylation, Ras-induced Elk-1 activation, and anchorage-independent cell growth FTI-resistant. The polybasic domain alone increases the affinity of Ras for farnesyltransferase, implying independent roles for each K-Ras4B sequence element in FTI resistance. Using microarray analysis and colony formation assays, we confirm that K-Ras function is independent of the identity of the prenyl group and, therefore, that FTI inhibition of K-Ras transformed cells is likely to be independent of K-Ras inhibition. Our results imply that relevant FTI targets will lack both polybasic and potentially geranylgeranylated methionine-CAAX motifs
Measuring space-time variation of the fundamental constants with redshifted submillimetre transitions of neutral carbon
We compare the redshifts of neutral carbon and carbon monoxide in the
redshifted sources in which the fine structure transition of neutral carbon,
[CI], has been detected, in order to measure space-time variation of the
fundamental constants. Comparison with the CO rotational lines measures gives
the same combination of constants obtained from the comparison fine structure
line of singly ionised carbon, [CII]. However, neutral carbon has the distinct
advantage that it may be spatially coincident with the carbon monoxide, whereas
[CII] could be located in the diffuse medium between molecular clouds, and so
any comparison with CO could be dominated by intrinsic velocity differences.
Using [CI], we obtain a mean variation of dF/F = (-3.6 +/- 8.5) x 10^-5, over z
= 2.3 - 4.1, for the eight [CI] systems, which degrades to (-1.5+/- 11) x
10^-5, over z = 2.3 - 6.4 when the two [CII] systems are included. That is,
zero variation over look-back times of 10.8-12.8 Gyr. However, the latest
optical results indicate a spatial variation in alpha, which describes a dipole
and we see the same direction in dF/F. This trend is, however, due to a single
source for which the [CI] spectrum is of poor quality. This also applies to one
of the two [CII] spectra previously used to find a zero variation in
alpha^2/mu. Quantifying this, we find an anti-correlation between |dF/F| and
the quality of the carbon detection, as measured by the spectral resolution,
indicating that the typical values of >50 km/s, used to obtain a detection, are
too coarse to reliably measure changes in the constants. From the fluxes of the
known z > 1 CO systems, we predict that current instruments are incapable of
the sensitivities required to measure changes in the constants through the
comparison of CO and carbon lines. We therefore discuss in detail the use of
ALMA for such an undertaking ... ABRIDGEDComment: Accepted for publication in Section 3 - Cosmology (including clusters
of galaxies) of Astronomy and Astrophysic
A population of z> 2 far-infrared Herschel-spire-selected starbursts
We present spectroscopic observations for a sample of 36 Herschel-SPIRE
250-500um selected galaxies (HSGs) at 2<z<5 from the Herschel Multi-tiered
Extragalactic Survey (HerMES). Redshifts are confirmed as part of a large
redshift survey of Herschel-SPIRE-selected sources covering ~0.93deg^2 in six
extragalactic legacy fields. Observations were taken with the Keck I Low
Resolution Imaging Spectrometer (LRIS) and the Keck II DEep Imaging
Multi-Object Spectrograph (DEIMOS). Precise astrometry, needed for
spectroscopic follow-up, is determined by identification of counterparts at
24um or 1.4GHz using a cross-identification likelihood matching method.
Individual source luminosities range from log(L_IR/Lsun)=12.5-13.6
(corresponding to star formation rates 500-9000Msun/yr, assuming a Salpeter
IMF), constituting some of the most intrinsically luminous, distant infrared
galaxies yet discovered. We present both individual and composite rest-frame
ultraviolet spectra and infrared spectral energy distributions (SEDs). The
selection of these HSGs is reproducible and well characterized across large
areas of sky in contrast to most z>2 HyLIRGs in the literature which are
detected serendipitously or via tailored surveys searching only for high-z
HyLIRGs; therefore, we can place lower limits on the contribution of HSGs to
the cosmic star formation rate density at (7+-2)x10^(-3)Msun/yr h^3Mpc^(-3) at
z~2.5, which is >10% of the estimated total star formation rate density (SFRD)
of the Universe from optical surveys. The contribution at z~4 has a lower limit
of 3x10^(-3)Msun/yr h^3 Mpc^(-3), ~>20% of the estimated total SFRD. This
highlights the importance of extremely infrared-luminous galaxies with high
star formation rates to the build-up of stellar mass, even at the earliest
epochs.Comment: 25 pages, 10 figures; ApJ accepte
Star Formation in Galaxy Mergers with Realistic Models of Stellar Feedback & the Interstellar Medium
We use simulations with realistic models for stellar feedback to study galaxy
mergers. These high resolution (1 pc) simulations follow formation and
destruction of individual GMCs and star clusters. The final starburst is
dominated by in situ star formation, fueled by gas which flows inwards due to
global torques. The resulting high gas density results in rapid star formation.
The gas is self gravitating, and forms massive (~10^10 M_sun) GMCs and
subsequent super-starclusters (masses up to 10^8 M_sun). However, in contrast
to some recent simulations, the bulk of new stars which eventually form the
central bulge are not born in superclusters which then sink to the center of
the galaxy, because feedback efficiently disperses GMCs after they turn several
percent of their mass into stars. Most of the mass that reaches the nucleus
does so in the form of gas. The Kennicutt-Schmidt law emerges naturally as a
consequence of feedback balancing gravitational collapse, independent of the
small-scale star formation microphysics. The same mechanisms that drive this
relation in isolated galaxies, in particular radiation pressure from IR
photons, extend over seven decades in SFR to regulate star formation in the
most extreme starbursts (densities >10^4 M_sun/pc^2). Feedback also drives
super-winds with large mass loss rates; but a significant fraction of the wind
material falls back onto the disks at later times, leading to higher
post-starburst SFRs in the presence of stellar feedback. Strong AGN feedback is
required to explain sharp cutoffs in star formation rate. We compare the
predicted relic structure, mass profile, morphology, and efficiency of disk
survival to simulations which do not explicitly resolve GMCs or feedback.
Global galaxy properties are similar, but sub-galactic properties and star
formation rates can differ significantly.Comment: 17 pages, 13 figures (+appendices), MNRAS accepted (matches
published). Movies of the simulations are available at
http://www.tapir.caltech.edu/~phopkins/Site/Movies_sbw_mgr.htm
The Kinetics and Mechanism of the Organo-Iridium-Catalysed Enantioselective Reduction of Imines
The iridium complex of pentamethylcyclopentadiene and (S,S)-1,2-diphenyl-N′-tosylethane- 1,2-diamine is an effective catalyst for the asymmetric transfer hydrogenation of imine substrates under acidic conditions. Using the Ir catalyst and a 5:2 ratio of formic acid: triethylamine as the hydride source for the asymmetric transfer hydrogenation of 1-methyl-3,4- dihydroisoquinoline and its 6,7-dimethoxy substituted derivative, in either acetonitrile or dichloromethane, shows unusual enantiomeric excess (ee) profiles for the product amines. The reactions initially give predominantly the (R) enantiomer of the chiral amine products with >90% ee but which then decreases significantly during the reaction. The decrease in ee is not due to racemisation of the product amine, but because the rate of formation of the (R)- enantiomer follows first-order kinetics whereas that for the (S)-enantiomer is zero-order. This difference in reaction order explains the change in selectivity as the reaction proceeds - the rate formation of the (R)-enantiomer decreases exponentially with time while that for the (S)- enantiomer remains constant. A reaction scheme is proposed which requires rate-limiting hydride transfer from the iridium hydride to the iminium ion for the first-order rate of formation of the (R)-enantiomer amine and rate-limiting dissociation of the product for the zero-order rate of formation of the (S)-enantiomer
The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3
Stellar archeology shows that massive elliptical galaxies today formed
rapidly about ten billion years ago with star formation rates above several
hundreds solar masses per year (M_sun/yr). Their progenitors are likely the
sub-millimeter-bright galaxies (SMGs) at redshifts (z) greater than 2. While
SMGs' mean molecular gas mass of 5x10^10 M_sun can explain the formation of
typical elliptical galaxies, it is inadequate to form ellipticals that already
have stellar masses above 2x10^11 M_sun at z ~ 2. Here we report
multi-wavelength high-resolution observations of a rare merger of two massive
SMGs at z = 2.3. The system is currently forming stars at a tremendous rate of
2,000 M_sun/yr. With a star formation efficiency an order-of-magnitude greater
than that of normal galaxies, it will quench the star formation by exhausting
the gas reservoir in only ~200 million years. At a projected separation of 19
kiloparsecs, the two massive starbursts are about to merge and form a passive
elliptical galaxy with a stellar mass of ~4x10^11 M_sun. Our observations show
that gas-rich major galaxy mergers, concurrent with intense star formation, can
form the most massive elliptical galaxies by z ~ 1.5.Comment: Appearing in Nature online on May 22 and in print on May 30.
Submitted here is the accepted version (including the Supplementary
Information), see nature.com for the final versio
How to distinguish starbursts and quiescently star-forming galaxies: The `bimodal' submillimetre galaxy population as a case study
In recent work (arXiv:1101.0002) we have suggested that the high-redshift (z
~ 2-4) bright submillimetre galaxy (SMG) population is heterogeneous, with
major mergers contributing both at early stages, where quiescently star-forming
discs are blended into one submm source (`galaxy-pair SMGs'), and late stages,
where mutual tidal torques drive gas inflows and cause strong starbursts. Here
we combine hydrodynamic simulations of major mergers with 3-D dust radiative
transfer calculations to determine observational diagnostics that can
distinguish between quiescently star-forming SMGs and starburst SMGs via
integrated data alone. We fit the far-IR SEDs of the simulated galaxies with
the optically thin single-temperature modified blackbody, the full form of the
single-temperature modified blackbody, and a power-law temperature-distribution
model. The effective dust temperature, T_dust, and power-law index of the dust
emissivity in the far-IR, \beta, derived can significantly depend on the
fitting form used, and the intrinsic \beta\ of the dust is not recovered.
However, for all forms used here, there is a T_dust above which almost all
simulated galaxies are starbursts, so a T_dust cut is very effective at
selecting starbursts. Simulated merger-induced starbursts also have higher
L_IR/M_gas and L_IR/L_FUV than quiescently star-forming galaxies and lie above
the star formation rate-stellar mass relation. These diagnostics can be used to
test our claim that the SMG population is heterogeneous and to observationally
determine what star formation mode dominates a given galaxy population. We
comment on applicability of these diagnostics to ULIRGs that would not be
selected as SMGs. These `hot-dust ULIRGs' are typically starburst galaxies
lower in mass than SMGs, but they can also simply be SMGs observed from a
different viewing angle.Comment: 21 pages, 11 figures. Accepted for publication in MNRAS. Minor
changes to text but otherwise identical to v
Geographies of responsibility
Issues of space, place and politics run deep. There is a long history of the entanglement of the conceptualisation of space and place with the framing of political positions. The injunction to think space relationally is a very general one and, as this collection indicates, can lead in many directions. The particular avenue to be explored in this paper concerns the relationship between identity and responsibility, and the potential georaphies of both
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