1,124 research outputs found
The HI content of extremely metal-deficient blue compact dwarf galaxies
We have obtained new HI observations with the 100m Green Bank Telescope (GBT)
for a sample of 29 extremely metal-deficient star-forming Blue Compact Dwarf
(BCD) galaxies, selected from the Sloan Digital Sky Survey spectral data base
to be extremely metal-deficient (12+logO/H<7.6). Neutral hydrogen was detected
in 28 galaxies, a 97% detection rate. Combining the HI data with SDSS optical
spectra for the BCD sample and adding complementary galaxy samples from the
literature to extend the metallicity and mass ranges, we have studied how the
HI content of a galaxy varies with various global galaxian properties. There is
a clear trend of increasing gas mass fraction with decreasing metallicity, mass
and luminosity. We obtain the relation M(HI)/L(g)~L(g)^{-0.3}, in agreement
with previous studies based on samples with a smaller luminosity range. The
median gas mass fraction f(gas) for the GBT sample is equal to 0.94 while the
mean gas mass fraction is 0.90+/-0.15, with a lower limit of ~0.65. The HI
depletion time is independent of metallicity, with a large scatter around the
median value of 3.4 Gyr. The ratio of the baryonic mass to the dynamical mass
of the metal-deficient BCDs varies from 0.05 to 0.80, with a median value of
~0.2. About 65% of the BCDs in our sample have an effective yield larger than
the true yield, implying that the neutral gas envelope in BCDs is more
metal-deficient by a factor of 1.5-20, as compared to the ionized gas.Comment: 21 pages, 13 figures, accepted for publication in MNRA
Bipartite entangled stabilizer mutually unbiased bases as maximum cliques of Cayley graphs
We examine the existence and structure of particular sets of mutually
unbiased bases (MUBs) in bipartite qudit systems. In contrast to well-known
power-of-prime MUB constructions, we restrict ourselves to using maximally
entangled stabilizer states as MUB vectors. Consequently, these bipartite
entangled stabilizer MUBs (BES MUBs) provide no local information, but are
sufficient and minimal for decomposing a wide variety of interesting operators
including (mixtures of) Jamiolkowski states, entanglement witnesses and more.
The problem of finding such BES MUBs can be mapped, in a natural way, to that
of finding maximum cliques in a family of Cayley graphs. Some relationships
with known power-of-prime MUB constructions are discussed, and observables for
BES MUBs are given explicitly in terms of Pauli operators.Comment: 8 pages, 1 figur
Self-Modification of Policy and Utility Function in Rational Agents
Any agent that is part of the environment it interacts with and has versatile
actuators (such as arms and fingers), will in principle have the ability to
self-modify -- for example by changing its own source code. As we continue to
create more and more intelligent agents, chances increase that they will learn
about this ability. The question is: will they want to use it? For example,
highly intelligent systems may find ways to change their goals to something
more easily achievable, thereby `escaping' the control of their designers. In
an important paper, Omohundro (2008) argued that goal preservation is a
fundamental drive of any intelligent system, since a goal is more likely to be
achieved if future versions of the agent strive towards the same goal. In this
paper, we formalise this argument in general reinforcement learning, and
explore situations where it fails. Our conclusion is that the self-modification
possibility is harmless if and only if the value function of the agent
anticipates the consequences of self-modifications and use the current utility
function when evaluating the future.Comment: Artificial General Intelligence (AGI) 201
AGI and the Knight-Darwin Law: why idealized AGI reproduction requires collaboration
Can an AGI create a more intelligent AGI? Under idealized assumptions, for a certain theoretical type of intelligence, our answer is: “Not without outside help”. This is a paper on the mathematical structure of AGI populations when parent AGIs create child AGIs. We argue that such populations satisfy a certain biological law. Motivated by observations of sexual reproduction in seemingly-asexual species, the Knight-Darwin Law states that it is impossible for one organism to asexually produce another, which asexually produces another, and so on forever: that any sequence of organisms (each one a child of the previous) must contain occasional multi-parent organisms, or must terminate. By proving that a certain measure (arguably an intelligence measure) decreases when an idealized parent AGI single-handedly creates a child AGI, we argue that a similar Law holds for AGIs
How Dry Are Red Mergers?
The focus of current research in galaxy evolution has increasingly turned to
understanding the effect that mergers have on the evolution of systems on the
red sequence. For those interactions purported to occur dissipationlessly (so
called "dry mergers"), it would appear that the role of gas is minimal.
However, if these mergers are not completely dry, then even low levels of gas
may be detectable. The purpose of our study is to test whether early type
galaxies with HI in or around them, or "wet" ellipticals, would have been
selected as dry mergers by the criteria in van Dokkum (2005, AJ, 130, 2647). To
that end, we examine a sample of 20 early types from the HI Rogues Gallery with
neutral hydrogen in their immediate environs. Of these, the 15 brightest and
reddest galaxies match the optical dry merger criteria, but in each case, the
presence of HI means that they are not truly dry.Comment: 8 pages plus 1 table and 5 figures; accepted for publication in A
ESO 381-47, an early-type galaxy with extended HI and a star forming ring
ESO 381-47 is an early type galaxy with an extended HI disk. GALEX and very
deep optical images reveal a distinct stellar ring far outside the optical body
with a diameter of ~30 kpc, which has undergone recent star formation at 1.8 x
10^-4 Msun/yr/kpc^-2, consistent with other new results which detect low level
star formation below the traditional Kennicutt relation in the outer parts of
spiral galaxies. The morphology of this galaxy resembles the recently
identified class of ultraviolet objects called extended ultraviolet disks, or
XUV-disks. New HI observations of this galaxy taken at the ATCA and in the CnB
array at the VLA show that the cold gas lies in an extended (diameter ~90 kpc)
ring around the central S0 galaxy. The HI data cube can be well modeled by a
warped ring. The faint ionized gas in the inner parts of the galaxy is
kinematically decoupled from the stars and instead appears to exhibit
velocities consistent with the rotation of the HI ring at larger radius. The
peak of the stellar ring, as seen in the optical and UV, is slightly displaced
to the inside relative to the peak of the HI ring. We discuss the manner in
which this offset could be caused by the propagation of a radial density wave
through an existing stellar disk, perhaps triggered by a galaxy collision at
the center of the disk, or possibly due to a spiral density wave set up at
early times in a disk too hot to form a stellar bar. Gas accretion and
resonance effects due to a bar which has since dissolved are also considered to
explain the presence of the star forming ring seen in the GALEX and deep
optical data.Comment: 48 pages, 16 figures, 4 tables. Accepted for publication in the
Astronomical Journa
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A Flexure-Based Tool Holder for Sub-(micro)m Positioning of a Single Point Cutting Tool on a Four-axis Lathe
A tool holder was designed to facilitate the machining of precision meso-scale components with complex three-dimensional shapes with sub-{micro}m accuracy on a four-axis lathe. A four-axis lathe incorporates a rotary table that allows the cutting tool to swivel with respect to the workpiece to enable the machining of complex workpiece forms, and accurately machining complex meso-scale parts often requires that the cutting tool be aligned precisely along the axis of rotation of the rotary table. The tool holder designed in this study has greatly simplified the process of setting the tool in the correct location with sub-{micro}m precision. The tool holder adjusts the tool position using flexures that were designed using finite element analyses. Two flexures adjust the lateral position of the tool to align the center of the nose of the tool with the axis of rotation of the B-axis, and another flexure adjusts the height of the tool. The flexures are driven by manual micrometer adjusters, each of which provides a minimum increment of motion of 20 nm. This tool holder has simplified the process of setting a tool with sub-{micro}m accuracy, and it has significantly reduced the time required to set a tool
A New Distance to The Antennae Galaxies (NGC 4038/39) Based on the Type Ia Supernova 2007sr
Traditionally, the distance to NGC 4038/39 has been derived from the systemic
recession velocity, yielding about 20 Mpc for H_0 = 72 km/s/Mpc. Recently, this
widely adopted distance has been challenged based on photometry of the presumed
tip of the red giant branch (TRGB), which seems to yield a shorter distance of
13.3+-1.0 Mpc and, with it, nearly 1 mag lower luminosities and smaller radii
for objects in this prototypical merger. Here we present a new distance
estimate based on observations of the Type Ia supernova (SN) 2007sr in the
southern tail, made at Las Campanas Observatory as part of the Carnegie
Supernova Project. The resulting distance of D(SN Ia) = 22.3+-2.8 Mpc [(m-M)_0
= 31.74+-0.27 mag] is in good agreement with a refined distance estimate based
on the recession velocity and the large-scale flow model developed by Tonry and
collaborators, D(flow) = 22.5+-2.8 Mpc. We point out three serious problems
that a short distance of 13.3 Mpc would entail, and trace the claimed short
distance to a likely misidentification of the TRGB. Reanalyzing Hubble Space
Telescope (HST) data in the Archive with an improved method, we find a TRGB
fainter by 0.9 mag and derive from it a preliminary new TRGB distance of
D(TRGB) = 20.0+-1.6 Mpc. Finally, assessing our three distance estimates we
recommend using a conservative, rounded value of D = 22+-3 Mpc as the best
currently available distance to The Antennae.Comment: 8 pages, 5 figures, 1 table (emulateapj; uses amsmath package).
Accepted for publication in The Astronomical Journal, Vol. 136. Figs. 1 & 2
degraded to reduce file size
Extended Emission Line Gas in Radio Galaxies - PKS0349-27
PKS0349-27 is a classical FRII radio galaxy with an AGN host which has a
spectacular, spiral-like structure in its extended emission line gas (EELG). We
have measured the velocity field in this gas and find that it splits into 2
cloud groups separated by radial velocities which at some points approach 400
km/s Measurements of the diagnostic emission line ratios [OIII]5007/H-beta,
[SII]6716+6731/H-alpha, and [NII]6583/H-alpha in these clouds show no evidence
for the type of HII region emission associated with starburst activity in
either velocity system. The measured emission line ratios are similar to those
found in the nuclei of narrow-line radio galaxies, but the extended
ionization/excitation cannot be produced by continuum emission from the active
nucleus alone. We present arguments which suggest that the velocity
disturbances seen in the EELG are most likely the result of a galaxy-galaxy
collision or merger but cannot completely rule out the possibility that the gas
has been disrupted by the passage of a radio jet.Comment: 12 pages, 3 fig pages, to appear in the Astrophys.
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Development of a Manufacturing Process for High-Precision Cu EOS Targets
This document describes the development of a manufacturing process and the production of Cu EOS targets. The development of a manufacturing process for these targets required a great deal of research, because the specifications for the targets required a level of precision an order of magnitude beyond Target Fabrication's capabilities at the time. Strict limitations on the dimensions of the components and the interfaces between them required research efforts to develop bonding and deposition processes consistent with a manufacturing plan with a dimensional precision on the order of 0.1 {micro}m. Several months into this effort, the specifications for the targets were relaxed slightly as a result of discussions between the Target Fabrication Group and the physicists. The level of precision required for these targets remained an order of magnitude beyond previous capabilities, but the changes made it possible to manufacture targets to the specifications. The development efforts and manufacturing processes described in this document successfully produced a complete Cu EOS target that satisfied all of the fabrication and metrology specifications
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