248 research outputs found
Ultrafaint Dwarfs - Star Formation and Chemical Evolution in the Smallest Galaxies
In earlier work we showed that a dark matter halo with a virial mass of
M can retain a large percentage of its baryons in the face of
the pre-ionization phase and supernova explosion from a 25 star. Here
we expand on the results of that work, investigating the star formation and
chemical evolution of the system beyond the first supernova. In a galaxy with a
mass M, sufficient gas is retained by the potential for
a second period of star formation to occur. The impact of a central explosion
is found to be much stronger than that of an off-centred explosion both in
blowing out the gas and in enriching it, as in the off-centered case most of
the supernova energy and metals escape into the IGM. We model the star
formation and metallicity given the assumption that stars form for 100, 200,
400 and 600 Myr and discuss the results in the context of recent observations
of very low mass galaxies. We show that we can account for most features of the
observed relationship between [/Fe] and [Fe/H] in ultra-faint dwarf
galaxies with the assumption that the systems formed at a low mass, rather than
being remnants of much larger systems
Segue 1 - A Compressed Star Formation History Before Reionization
Segue 1 is the current best candidate for a "first galaxy", a system which
experienced only a single short burst of star formation and has since remained
unchanged. Here we present possible star formation scenarios which can explain
its unique metallicity distribution. While the majority of stars in all other
ultra-faint dwarfs (UFDs) are within 0.5 dex of the mean [Fe/H] for the galaxy,
5 of the 7 stars in Segue 1 have a spread of [Fe/H] dex. We show
that this distribution of metallicities canot be explained by a gradual
build-up of stars, but instead requires clustered star formation. Chemical
tagging allows the separate unresolved delta functions in abundance space to be
associated with discrete events in space and time. This provides an opportunity
to put the enrichment events into a time sequence and unravel the history of
the system. We investigate two possible scenarios for the star formation
history of Segue 1 using Fyris Alpha simulations of gas in a M
dark matter halo. The lack of stars with intermediate metallicities
[Fe/H] can be explained either by a pause in star formation caused by
supernova feedback, or by the spread of metallicities resulting from one or two
supernovae in a low-mass dark matter halo. Either possibility can reproduce the
metallicity distribution function (MDF), as well as the other observed
elemental abundances. The unusual MDF and the low luminosity of Segue 1 can be
explained by it being a first galaxy that originated with
~M at .Comment: 12 pages, 6 figures, ApJ, Accepte
Ultrafaint Dwarf Galaxies - the lowest mass relics from before reionization
New observations indicate that ultrafaint dwarf galaxies (UFD) -- the least
luminous systems bound by dark matter halos (<10^5 Lsun) -- may have formed
before reionization. The extrapolated virial masses today are uncertain with
estimates ranging from 10^8 Msun to 10^9 Msun. We show that the progenitor halo
masses of UFDs can be as low as Mvir = 10^7 Msun. Under the right conditions,
such a halo can survive the energy input of a supernova and its radiative
progenitor. A clumpy medium is much less susceptible to both internal and
external injections of energy. It is less prone to SN sweeping because the
coupling efficiency of the explosive energy is much lower than for a diffuse
ISM. With the aid of the 3D hydro/ionization code Fyris, we show that
sufficient baryons are retained to form stars following a single supernova
event in dark matter halos down to Mvir ~ 10^7 Msun with radiative cooling. The
gas survives the SN explosion, is enriched with the abundance yields of the
discrete events, and reaches surface densities where low mass stars can form.
Our highest resolution simulations reveal why cooling is so effective in
retaining gas compared to any other factor. In the early stages, the super-hot
metal-enriched SN ejecta exhibit strong cooling, leading to much of the
explosive energy being lost. Consistent with earlier work, the baryons do *not*
survive in smooth or adiabatic models in the event of a supernova. The smallest
galaxies carry signatures of the earliest epochs of star formation, which may
distinguish a small primordial galaxy from one that was stripped down to its
present size through tidal interaction. We discuss these results in the context
of local UFDs and damped Ly-alpha systems (z~2) at very low metallicity ([Fe/H]
~ -3). We show that both classes of objects are consistent with primordial
low-mass systems that have experienced only a few enrichment events.Comment: 32 pages, 25 figures, accepted by Ap
Resonant X-Ray Scattering Investigations of Charge Density Wave and Nematic Orders in Cuprate Superconductors
In the cuprate superconductors, superconductivity often co-exists with other
types of order, including charge density wave and nematic orders. Over the past
decade, resonant x-ray scattering has emerged as a key tool to investigate
these competing/coexisting orders, providing valuable insights into their
microscopic character. In this report we provide a brief review of the
technique and highlight selected recent advances in study charge density wave
order and nematic order in the cuprates.Comment: 12 pages, 6 figure
Galaxy destruction and diffuse light in clusters
Deep images of the Centaurus and Coma clusters reveal two spectacular arcs of
diffuse light that stretch for over 100 kpc, yet are just a few kpc wide. At a
surface brightness of m_b \sim 27-28th arcsec^-2, the Centaurus arc is the most
striking example known of structure in the diffuse light component of a rich
galaxy cluster. We use numerical simulations to show that the Centaurus feature
can be reproduced by the tidal debris of a spiral galaxy that has been tidally
disrupted by the gravitational potential of NGC 4709. The surface brightness
and narrow dimensions of the diffuse light suggest that the disk was
co-rotating with its orbital path past pericentre. Features this prominent in
clusters will be relatively rare, although at fainter surface brightness levels
the diffuse light will reveal a wealth of structure. Deeper imaging surveys may
be able to trace this feature for several times its presently observed extent
and somewhere along the tidal debris, a fraction of the original stellar
component of the disk will remain bound, but transformed into a faint
spheroidal galaxy. It should be possible to confirm the galactic origin of the
Centaurus arc by observing planetary nebulae along its length with redshifts
close to that of NGC 4709.Comment: Replaced with version accepted by MNRAS (Dec. 1999): Added missing
reference (to pg. 4 & reference list). Section 3 shortened; removed three
figures. Now 8 pages long, with 8 figures. Low resolution images included,
high resolution version available at
http://star-www.dur.ac.uk:80/~calcaneo/cenarc.htm
Star formation in ultra-faint dwarfs: continuous or single-aged bursts?
We model the chemical evolution of six ultra-faint dwarfs (UFDs): Bootes I, Canes Venatici II, Coma Berenices,
Hercules, Leo IV, and Ursa Major I based on their recently determined star formation histories. We show that two
single-age bursts cannot explain the observed [α/Fe] versus [Fe/H] distribution in these galaxies and that some selfenrichment
is required within the first burst. An alternative scenario is modeled, in which star formation is
continuous except for short interruptions when one or more supernovae temporarily blow the dense gas out from
the center of the system. This model allows for self-enrichment and can reproduce the chemical abundances of the
UFDs in which the second burst is only a trace population. We conclude that the most likely star formation history
is one or two extended periods of star formation, with the first burst lasting for at least 100 Myr. As found in earlier
work, the observed properties of UFDs can be explained by formation at a low mass (Mvir ~ 107 Me), rather than
being stripped remnants of much larger systems.D.W. is funded by an Australian Postgraduate Award. J.B.H.
is funded by an ARC Laureate Fellowship
The Chemical Evolution of Very Metal-Poor Damped LYα Systems
In earlier work we showed that a dark matter halo with a virial mass of 107 M⊙ can survive feedback from its own massive stars and form stars for ≳100 Myr. We also found that our modeled systems were consistent with observations of ultrafaint dwarfs
Angular Fluctuations of a Multicomponent Order Describe the Pseudogap of
The hole-doped cuprate high-temperature superconductors enter the pseudogap regime as their superconducting critical temperature, , falls with decreasing hole density. Recent x-ray scattering experiments in observe incommensurate charge-density wave fluctuations whose strength rises gradually over a wide temperature range above , but then decreases as the temperature is lowered below . We propose a theory in which the superconducting and charge-density wave orders exhibit angular fluctuations in a six-dimensional space. The theory provides a natural quantitative fit to the x-ray data and can be a basis for understanding other characteristics of the pseudogap.Physic
DAzLE: The Dark Ages z (redshift) Lyman-alpha Explorer
DAzLE is an near infrared narrowband differential imager being built by the
Institute of Astronomy, Cambridge, in collaboration with the Anglo-Australian
observatory. It is a special purpose instrument designed with a sole aim; the
detection of redshifted Lyman-alpha emission from star forming galaxies at z>7.
DAzLE will use pairs of high resolution (R=1000) narrowband filters to exploit
low background `windows' in the near infrared sky emission spectrum. This will
enable it to reach sensitivities of ~2E-21 W/m^2, thereby allowing the
detection of z>7 galaxies with star formation rates as low as a few solar
masses per year. The design of the instrument, and in particular the crucial
narrowband filters, are presented. The predicted performance of DAzLE,
including the sensitivity, volume coverage and expected number counts, is
discussed. The current status of the DAzLE project, and its projected timeline,
are also presented.Comment: 11 pages, 7 figures, to appear in Proceedings of SPIE Vol. 5492,
Ground-based Instrumentation for Astronom
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