3,933 research outputs found
The infrared-dark dust content of high redshift galaxies
We present a theoretical model aimed at explaining the IRX- relation
for high redshift (z >5) galaxies. Recent observations (Capak+2015;
Bouwens+2016) have shown that early Lyman Break Galaxies, although
characterized by a large UV attenuation (e.g. flat UV beta slopes), show a
striking FIR deficit, i.e. they are "infrared-dark". This marked deviation from
the local IRX-beta relation can be explained by the larger molecular gas
content of these systems. While dust in the diffuse ISM attains relatively high
temperatures (Td = 45 K for typical size a=0.1 um; smaller grains can reach Td
= 60 K), a sizable fraction of the dust mass is embedded in dense gas, and
therefore remains cold. If confirmed, the FIR deficit might represent a novel,
powerful indicator of the molecular content of high-z galaxies which can be
used to pre-select candidates for follow-up deep CO observations. Thus, high-z
CO line searches with ALMA might be much more promising than currently thought.Comment: 8 pages, 4 Figures, MNRAS Submitte
Linking glacial and future climates through an ensemble of GCM simulations
In this paper we explore the relationships between the modelled climate of the Last Glacial Maximum (LGM) and that for doubled atmospheric carbon dioxide compared to the pre-industrial climate by analysing the output from an ensemble of runs from the MIROC3.2 GCM. <br><br> Our results lend support to the idea in other recent work that the Antarctic is a useful place to look for historical data which can be used to validate models used for climate forecasting of future greenhouse gas induced climate changes, at local, regional and global scales. Good results may also be obtainable using tropical temperatures, particularly those over the ocean. While the greater area in the tropics makes them an attractive area for seeking data, polar amplification of temperature changes may mean that the Antarctic provides a clearer signal relative to the uncertainties in data and model results. Our result for Greenland is not so strong, possibly due to difficulties in accurately modelling the sea ice extent. <br><br> The MIROC3.2 model shows an asymmetry in climate sensitivity calculated by decreasing rather than increasing the greenhouse gases, with 80% of the ensemble having a weaker cooling than warming. This asymmetry, if confirmed by other studies would mean that direct estimates of climate sensitivity from the LGM are likely to be underestimated by the order of half a degree. Our suspicion is, however, that this result may be highly model dependent. Analysis of the parameters varied in the model suggest the asymmetrical response may be linked to the ice in the clouds, which is therefore indicated as an important area for future research
Chaotic diffusion of particles with finite mass in oscillating convection flows
Deterministic diffusion in temporally oscillating convection is studied for
particles with finite mass. The particles are assumed to obey a simple
dissipative dynamical system and the particle diffusion is induced by the
strange attractor. The diffusion constants are numerically calculated for
convection models with free and rigid boundary conditions.Comment: 5 figure
Where are the z=4 Lyman Break Galaxies? Results from Conditional Luminosity Function Models of Luminosity-dependent Correlation Functions
Using the conditional luminosity function (CLF) -- the luminosity distribution of galaxies in a dark matter halo -- as a way to model galaxy statistics, we study how z=4 Lyman Break Galaxies (LBGs) are distributed in dark matter halos. For this purpose, we measure luminosity-dependent clustering of LBGs in the Subaru/XMM-Newton Deep Field by separating a sample of 16,920 galaxies to three magnitude bins in i'-band between 24.5 and 27.5. Our models fits to data show a possible trend for more luminous galaxies to appear as satellites in more massive halos. The satellite fraction of galaxies at z=4 in these magnitude bins is 0.13 to 0.3, 0.09 to 0.22, and 0.03 to 0.14, respectively, where the 1 sigma ranges account for differences coming from two different estimates of the z=4 LF from the literature. To jointly explain the LF and the large-scale linear bias factor of z=4 LBGs as a function of rest-UV luminosity requires central galaxies to be brighter in UV at z =4 than present-day galaxies in same dark matter mass halos. Moreover, UV luminosity of central galaxies in halos with total mass greater than roughly 10^{12} M_sun must decrease from z=4 to today by an amount more than the luminosity change for galaxies in halos below this mass. This mass-dependent luminosity evolution is preferred at more than 3 sigma confidence level compared to a pure-luminosity evolution scenario where all galaxies decrease in luminosity by the same amount from z=4 to today. The scenario preferred by the data is consistent with the ``down-sizing'' picture of galaxy evolution
The LGM surface climate and atmospheric circulation over East Asia and the North Pacific in the PMIP2 coupled model simulations
International audienceThe surface climate and atmospheric circulation over East Asia and the North Pacific at the last glacial maximum has been investigated using the outputs from several coupled atmosphere-ocean general circulation model in PMIP2 database. In boreal summer, the weakening of high pressure over the North Pacific and less precipitation over East Asia are analyzed in most models. The reduced moisture transport seems to result in the less precipitation over East Asia. In boreal winter, the intensification of the Aleutian low and southward shift of the upper-level jet are analyzed in most models. Some of these results are consistent with geological records such as pollen, lake status and dust transport
Climatic Conditions for modelling the Northern Hemisphere ice sheets throughout the ice age cycle
International audienceThe ice sheet-climate interaction as well as the climatic response to orbital parameters and atmospheric CO2 concentration are examined in order to drive an ice sheet model throughout an ice age cycle. Feedback processes between ice sheet and atmosphere are analyzed by numerical experiments using a high resolution General Circulation Model (GCM) under different conditions at the Last Glacial Maximum. Among the proposed processes, the ice albedo feedback, the elevation-mass balance feedback and the desertification effect over the ice sheet were found to be the dominant processes for the ice-sheet mass balance. For the elevation-mass balance feedback, the temperature lapse rate over the ice sheet is proposed to be weaker than assumed in previous studies. Within the plausible range of parameters related to these processes, the ice sheet response to the orbital parameters and atmospheric CO2 concentration for the last glacial/interglacial cycle was simulated in terms of both ice volume and geographical distribution, using a three-dimensional ice-sheet model. Careful treatment of climate-ice sheet feedback is essential for a reliable simulation of the ice sheet changes during ice age cycles
Average Metallicity and Star Formation Rate of Lya Emitters Probed by a Triple Narrow-Band Survey
We present the average metallicity and star-formation rate of Lya emitters
(LAEs) measured from our large-area survey with three narrow-band (NB) filters
covering the Lya, [OII]3727, and Ha+[NII] lines of LAEs at z=2.2. We select 919
z=2.2 LAEs from Subaru/Suprime-Cam NB data in conjunction with Magellan/IMACS
spectroscopy. Of these LAEs, 561 and 105 are observed with KPNO/NEWFIRM
near-infrared NB filters whose central wavelengths are matched to redshifted
[OII] and Ha nebular lines, respectively. By stacking the near-infrared images
of the LAEs, we successfully obtain average nebular-line fluxes of LAEs, the
majority of which are too faint to be identified individually by narrow-band
imaging or deep spectroscopy. The stacked object has an Ha luminosity of
1.7x10^{42} erg s^{-1} corresponding to a star formation rate (SFR) of 14
M_{sun} yr^{-1}. We place, for the first time, a firm lower limit to the
average metallicity of LAEs of Z>~0.09 Z_{sun} (2sigma) based on the
[OII]/(Ha+[NII]) index together with photo-ionization models and empirical
relations. This lower limit of metallicity rules out the hypothesis that LAEs,
so far observed at z~2, are extremely metal poor (Z<2x10^{-2} Z_{sun}) young
galaxies at the 4sigma level. This limit is higher than a simple extrapolation
of the observed mass-metallicity relation of z~2 UV-selected galaxies toward
lower masses (5x10^{8} M_{sun}), but roughly consistent with a recently
proposed fundamental mass-metallicity relation when the LAEs' relatively low
SFR is taken into account. The Ha and Lya luminosities of our NB-selected LAEs
indicate that the escape fraction of Lya photons is ~12-30 %, much higher than
the values derived for other galaxy populations at z~2.Comment: 21 pages, 15 figures, 8 tables. Accepted for publication in Ap
Evidence for Low Extinction in Actively Star Forming Galaxies at z6.5
We present a search for the [CII] 158micron fine structure line (a main
cooling line of the interstellar medium) and the underlying far-infrared (FIR)
continuum in three high-redshift (6.6<z<8.2) star-forming galaxies using the
IRAM Plateau de Bure interferometer. We targeted two Lyman-Alpha-selected
galaxies (Lyman-Alpha-Emitters, LAEs) with moderate UV-based star formation
rates (SFR~20 M_sun/yr; Himiko at z=6.6 and IOK-1 at z=7.0) and a Gamma Ray
Burst (GRB) host galaxy (GRB 090423 at z~8.2). Based on our 3 sigma rest-frame
FIR continuum limits, previous (rest-frame) UV continuum measurements and
spectral energy distribution (SED) fitting, we rule out SED shapes similar to
highly obscured galaxies (e.g. Arp220, M82) and less extreme dust-rich nearby
spiral galaxies (e.g. M51) for the LAEs. Conservatively assuming a SED shape
typical of local spiral galaxies we derive upper limits for the FIR-based star
formation rates (SFRs) of ~70 M_sun/yr, ~50 M_sun/yr and ~40 M_sun/yr for
Himiko, IOK-1 and GRB 090423, respectively. For the LAEs these limits are only
a factor ~3 higher than the published UV-based SFRs (uncorrected for
extinction). This indicates that the dust obscuration in the z>6 LAEs studied
here is lower by a factor of a few than what has recently been found in some
LAEs at lower redshift (2<z<3.5) with similar UV-based SFRs. A low obscuration
in our z>6 LAE sample is consistent with recent rest-frame UV studies of z~7
Lyman-Break-Galaxies (LBGs).Comment: Accepted for publication in the Astrophysical Journa
Very blue UV-continuum slopes of low luminosity z~7 galaxies from WFC3/IR: Evidence for extremely low metallicities?
We use the ultra-deep WFC3/IR data over the HUDF and the Early Release
Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband
spectral properties of candidate star-forming galaxies at z~7. We determine the
UV-continuum slope beta in these galaxies, and compare the slopes with galaxies
at later times to measure the evolution in beta. For luminous L*(z=3) galaxies,
we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to
the beta~-2 derived at similar luminosities at z~5-6. However, for the lower
luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is
substantially bluer than is found for similar luminosity galaxies at z~4, just
800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can
be matched by a very young, dust-free stellar population, but when nebular
emission is included the expected beta becomes >~-2.7. To produce these very
blue beta's (i.e., beta~-3), extremely low metallicities and mechanisms to
reduce the red nebular emission are likely required. For example, a large
escape fraction (i.e., f_{esc}>~0.3) could minimize the contribution from this
red nebular emission. If this is correct and the escape fraction in faint z~7
galaxies is >~0.3, it may help to explain how galaxies reionize the universe.Comment: 5 pages, 5 figures, accepted for publication in Astrophysical Journal
Letter
Linking glacial and future climates through an ensemble of GCM simulations
International audienceIn this paper we explore the relationships between the modelled climate of the Last Glacial Maximum (LGM) and that for doubled atmospheric carbon dioxide compared to the pre-industrial climate by analysing the output from an ensemble of runs from the MIROC3.2 GCM. Our results lend support to the idea in other recent work that the Antarctic is a useful place to look for historical data which can be used to validate models used for climate forecasting of future greenhouse gas induced climate changes, at local, regional and global scales. Good results may also be obtainable using tropical temperatures, particularly those over the ocean. While the greater area in the tropics makes them an attractive area for seeking data, polar amplification of temperature changes may mean that the Anatarctic provides a clearer signal relative to the uncertainties in data and model results. Our result for Greenland is not so strong, possibly due to difficulties in accurately modelling the sea ice extent. The MIROC3.2 model shows an asymmetry in climate sensitivity calculated by decreasing rather than increasing the greenhouse gases, with 80% of the ensemble having a weaker cooling than warming. This asymmetry, if confirmed by other studies would mean that direct estimates of climate sensitivity from the LGM are likely to be underestimated by the order of half a degree. Our suspicion is, however, that this result may be highly model dependent. Analysis of the parameters varied in the model suggest the asymmetrical response may be linked to the ice in the clouds, which is therefore indicated as an important area for future research
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