232 research outputs found
The Large Ultraviolet/Optical/Infrared Surveyor
Astronomy crossed a threshold three decades ago with the discovery of planets around other stars. Compared to scientists' previous expectations set by the Solar System, exoplanets are wonderfully abundant and varied. Indirect planet discovery techniques have shown that small rocky planets residing in stellar habitable zones, where such planets may have liquid water on their surfaces, are not rare. This revelation drives us to ask more ambitious and fundamental questions, that fascinate scientists and the public alike: are there other truly Earth-like planets out there and do any of them harbour life? Today, exoplanets are largely small black shadows' to us, with measurements of orbits, sizes and masses (all three in the best cases).The upcoming James Webb Space Telescope and future 30-m-class ground-based telescopes will characterize the atmospheres of habitable planet candidates orbit in glow-mass M dwarf stars. However, deeply probing atmospheres of the exoplanets most similar to the Earth, those around Sun-like stars, remains out of reach for currently planned observatories. Bringing them within our grasp is a primary motivation for the Large UV/Optical/Infrared Surveyor(LUVOIR) mission concept, currently the focus of a three-year NASA study
Detecting dark matter substructure spectroscopically in strong gravitational lenses
The Cold Dark Matter (CDM) model for galaxy formation predicts that a
significant fraction of mass in the dark matter haloes that surround L L*
galaxies is bound in substructures of mass 1E4-1E7Msun. The number of
observable baryonic substructures (such as dwarf galaxies and globular
clusters) falls short of these predictions by at least an order of magnitude.
We present a method for searching for substructure in the haloes of
gravitational lenses that produce multiple images of QSOs, such as 4-image
Einstein Cross lenses. Current methods based on broadband flux ratios cannot
cleanly distinguish between substructure, differential extinction, microlensing
and, most importantly, ambiguities in the host lens model. These difficulties
may be overcome by utilizing the prediction that when substructure is present,
the magnification will be a function of source size. QSO broad line and narrow
line emission regions are approximately ~1pc and >100pc in size, respectively.
When narrow line region (NLR) features are used as a normalisation, the
relative intensity and equivalent width of broad line region (BLR) features
will respectively reflect substructure-lensing and microlensing effects.
Spectroscopic observations of just a few image pairs would probably be able to
cleanly extract the desired substructure signature and distinguish it from
microlensing. In the rest-optical, the Hbeta/[OIII] region is ideal, since the
narrow wavelength range also largely eliminates differential reddening
problems. Simulations of Q2237+030 are done as an example to determine the
level of substructure that is detectable in this way, and possible systematic
difficulties are discussed. This is an ideal experiment to be carried out with
near-infrared integral field unit spectrographs on 8-m class telescopes.Comment: 9 pages, 8 figures, submitted to MNRAS, uses LaTeX2e mn2e.cl
The Effects of Ram-pressure Stripping and Supernova Winds on the Tidal Stirring of Disky Dwarfs: Enhanced Transformation into Dwarf Spheroidals
A conclusive model for the formation of dwarf spheroidal (dSph) galaxies
still remains elusive. Owing to their proximity to the massive spirals Milky
Way (MW) and M31, various environmental processes have been invoked to explain
their origin. In this context, the tidal stirring model postulates that
interactions with MW-sized hosts can transform rotationally supported dwarfs,
resembling present-day dwarf irregular (dIrr) galaxies, into systems with the
kinematic and structural properties of dSphs. Using N-body+SPH simulations, we
investigate the dependence of this transformation mechanism on the gas
fraction, fgas, in the disk of the progenitor dwarf. Our numerical experiments
incorporate for the first time the combined effects of radiative cooling,
ram-pressure stripping, star formation, supernova (SN) winds, and a cosmic UV
background. For a given orbit inside the primary galaxy, rotationally supported
dwarfs with gas fractions akin to those of observed dIrrs (fgas >= 0.5),
demonstrate a substantially enhanced likelihood and efficiency of
transformation into dSphs relative to their collisionless (fgas = 0)
counterparts. We argue that the combination of ram-pressure stripping and SN
winds causes the gas-rich dwarfs to respond more impulsively to tides,
augmenting their transformation. When fgas >= 0.5, disky dwarfs on previously
unfavorable low-eccentricity or large-pericenter orbits are still able to
transform. On the widest orbits, the transformation is incomplete; the dwarfs
retain significant rotational support, a relatively flat shape, and some gas,
naturally resembling transition-type systems. We conclude that tidal stirring
constitutes a prevalent evolutionary mechanism for shaping the structure of
dwarf galaxies within the currently favored CDM cosmological paradigm.Comment: Accepted for publication in ApJ Letters, 8 pages, 2 figures, LaTeX
(uses emulateapj.cls
Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite Accretion
(Abridged) We conduct a series of high-resolution, dissipationless N-body
simulations to investigate the cumulative effect of substructure mergers onto
thin disk galaxies in the context of the LCDM paradigm of structure formation.
Our simulation campaign is based on a hybrid approach. Substructure properties
are culled directly from cosmological simulations of galaxy-sized cold dark
matter (CDM) halos. In contrast to what can be inferred from statistics of the
present-day substructure populations, accretions of massive subhalos onto the
central regions of host halos, where the galactic disk resides, since z~1
should be common occurrences. One host halo merger history is subsequently used
to seed controlled numerical experiments of repeated satellite impacts on an
initially-thin Milky Way-type disk galaxy. We show that these accretion events
produce several distinctive observational signatures in the stellar disk
including: a ring-like feature in the outskirts; a significant flare; a central
bar; and faint filamentary structures that (spuriously) resemble tidal streams.
The final distribution of disk stars exhibits a complex vertical structure that
is well-described by a standard ``thin-thick'' disk decomposition. We conclude
that satellite-disk encounters of the kind expected in LCDM models can induce
morphological features in galactic disks that are similar to those being
discovered in the Milky Way, M31, and in other disk galaxies. These results
highlight the significant role of CDM substructure in setting the structure of
disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys
and astrometric satellites may be able to distinguish between competing
cosmological models by testing whether the detailed structure of galactic disks
is as excited as predicted by the CDM paradigm.Comment: Accepted version to appear in ApJ, 24 pages, 8 figures, LaTeX (uses
emulateapj.cls). Comparison between the simulated ring-like features and the
Monoceros ring stellar structure in the Milky Way performed; conclusions
unaltere
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