1,283 research outputs found
Why Are Ring Galaxies Interesting?
Compared with ordinary spirals, the ISM in ring galaxies experiences markedly
different physical conditions and evolution. As a result, ring galaxies provide
interesting perspectives on the triggering/quenching of large scale star
formation and the destructive effects of massive stars on molecular cloud
complexes. We use high resolution radio, sub-millimeter, infrared, and optical
data to investigate the role of gravitational stability in star formation
regulation, factors influencing the ISM's molecular fraction, and evidence of
peculiar star formation laws and efficiencies in two highly evolved ring
galaxies: Cartwheel and the Lindsay-Shapley ring.Comment: 6 pages with 4 figures (2 color). To appear in the conference
proceedings for "Galaxy Wars: Stellar Populations and Star Formation in
Interacting Galaxies"
Polygamous Marriage, Monogamous Divorce
Could the constitutional right to marry also encompass polygamy? That question, which has long intrigued legal scholars, has taken on even greater significance in the wake of Obergefell v. Hodges. This Article answers that question in a novel way by scrutinizing the practice of plural marriage through the lens of economic game theory, exploring the extreme harms that would befall the state should polygamy become law. More specifically, the Article delves into the ex ante consequences of legalization, not on practicing polygamists (as is typically the focus), but on sequential bigamists—that is, those who never intend to have more than one spouse at any given time but who nonetheless marry more than one person in their lifetime. The Article concludes that the state has a compelling economic interest in limiting marriage to two people. If polygamy were to become the law of the land, states could no longer prohibit bigamy. In turn, separating couples would lose one of the strongest incentives they currently have to choose formal divorce proceedings over the seemingly simpler option of mutual desertion: the threat of criminal charges for bigamy. In essence, a sequential bigamist could then marry multiple times in his lifetime without ever divorcing and, at the same time, without risking a criminal charge of bigamy. Such actions—dubbed “sequential polygamy”—would compromise the state’s interest in protecting its citizens from financial harms. After all, divorce proceedings provide the state with an opportunity to intercede into the process, thereby obtaining some assurance that those who are leaving a marriage are not doing so at their financial peril. With the legalization of polygamy, however, bigamy becomes a thing of the past, eroding the state’s ability to encourage divorce as a means of safeguarding the health and safety of its citizens. Most concerning is the impact this change would have on those living in poverty—the people likely to be hardest hit by any societal shift away from formal divorce. Finally, any attempts by the state to distinguish between bigamy and polygamy (for example, by permitting plural marriage but only if all spouses consent), would fail to ameliorate the resulting harm to its citizens
Star Formation and the Interstellar Medium In Nearby Tidal Streams (SAINTS): Spitzer Mid-infrared Spectroscopy and Imaging of Intergalactic Star-forming Objects
A spectroscopic analysis of 10 intergalactic star forming objects (ISFOs) and
a photometric analysis of 67 ISFOs in a sample of 14 interacting systems is
presented. The majority of the ISFOs have relative polycyclic aromatic
hydrocarbon (PAH) band strengths similar to those of nearby spiral and
starburst galaxies. In contrast to what is observed in blue compact dwarfs
(BCDs) and local giant HII regions in the Milky Way (NGC 3603) and the
Magellanic Clouds (30 Doradus and N 66), the relative PAH band strengths in
ISFOs correspond to models with a significant PAH ion fraction (<50%) and
bright emission from large PAHs (~100 carbon atoms). The [NeIII]/[NeII] and
[SIV]/[SIII] line flux ratios indicate moderate levels of excitation with an
interstellar radiation field that is harder than the majority of the Spitzer
Infrared Nearby Galaxies Survey and starburst galaxies, but softer than BCDs
and local giant HII regions. The ISFO neon line flux ratios are consistent with
a burst of star formation < 6 million years ago. Most of the ISFOs have
~million solar masses of warm molecular hydrogen with a likely origin in
photo-dissociation regions (PDRs). Infrared Array Camera photometry shows the
ISFOs to be bright at 8 um, with one third having [4.5] - [8.0] > 3.7, i.e.,
enhanced non-stellar emission, most likely due to PAHs, relative to normal
spirals, dwarf irregulars and BCD galaxies. The relative strength of the 8 um
emission compared to that at 3.6 um or 24 um separates ISFOs from dwarf
galaxies in Spitzer two color diagrams. The infrared power in two thirds of the
ISFOs is dominated by emission from grains in a diffuse interstellar medium.
One in six ISFOs have significant emission from PDRs, contributing ~30 % - 60 %
of the total power. ISFOs are young knots of intense star formation.Comment: Accepted in ApJ. 49 pages 9 figure
Interstellar turbulence, random density variations, and scintillation measurements
The presence of random electron variations suggests that the ionized interstellar medium is turbulent. In the interstellar plasma the presence of power spectra of such variations extending to spatial scales much less than a Coulomb mean free path, Lambda sub c, is required by analyses of measurements of scintillation and angular broadening of pulsar radio signals. The existence of corresponding variations in magnetic field strength could efficiently scatter cosmic rays and thus constrain cosmic-ray propagation. Unfortunately both the origin of the electron density variations and mechanisms by which these variations couple to fluctuations in magnetic field strength are unknown. It is conjectured that the small-scale density variations are generated by the convective distortion of initially large-scale isobaric entropy structures in the turbulent interstellar plasma. An investigation of the spectra of turbulent entropy structures, velocity, and magnetic fields at small spatial scales is made. The modifier small is employed to characterize length scales much less than the dimension, L, containing the bulk of the turbulent energy
Diffuse galactic annihilation radiation from supernova nucleosynthesis
The propagation of MeV positrons in the outer ejecta of type I supernovae was investigated. It was found that the positrons created at times of approx 100 days propagated along magnetic field lines in the outer ejecta without any appreciable pitch-angle scattering or excitation of hydromagnetic waves. The lack of significant pitch-angle scattering is well consistent with models of wave excitation and scattering by resonant interactions. This occurs because time periods to scatter the particles or to excite waves are significantly longer than escape times. Thus it is expected that, when positrons are not coupled to the ejecta by Coulomb collisions, they escape from the relatively cold, dense ejecta and reside predominantly in the tenuous, hotter, shock-heated interstellar gas. In the tenuous shock-heated gas the positron lifetime against annihilation is much greater than lifetimes in the dense ejectra. Thus the production of steady-state diffuse annihilation radiation by some fraction of these escaped positrons seems probable
Molecular Gas and Star Formation in the Cartwheel
Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(J=1-0) observations
are used to study the cold molecular ISM of the Cartwheel ring galaxy and its
relation to HI and massive star formation (SF). CO moment maps find
M of H associated with the inner ring
(72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor
(). The spokes and disk are not detected. Analysis of the
inner ring's CO kinematics show it to be expanding ( km
s) implying an Myr age. Stack averaging reveals CO emission
in the starburst outer ring for the first time, but only where HI surface
density () is high, representing M for a metallicity appropriate
, giving small ( M
pc), molecular fraction (), and H depletion
timescales ( Myr). Elsewhere in the outer ring
M pc,
and Myr (all ). The inner ring and
nucleus are H-dominated and are consistent with local spiral SF laws.
in the outer ring appears independent of ,
or . The ISM's long confinement in the
robustly star forming rings of the Cartwheel and AM0644-741 may result in
either a large diffuse H component or an abundance of CO-faint low column
density molecular clouds. The H content of evolved starburst rings may
therefore be substantially larger. Due to its lower and age
the Cartwheel's inner ring has yet to reach this state. Alternately, the outer
ring may trigger efficient SF in an HI-dominated ISM.Comment: 10-pages text; 5-figure
Redshifts from Spitzer Spectra for Optically Faint, Radio Selected Infrared Sources
Spectra have been obtained with the Infrared Spectrograph on the Spitzer
Space Telescope for 18 optically faint sources (R > 23.9,mag) having f(nu)
(24um) > 1.0,mJy and having radio detections at 20 cm to a limit of 115
microJy. Sources are within the Spitzer First Look Survey. Redshifts are
determined for 14 sources from strong silicate absorption features (12 sources)
or strong PAH emission features (2 sources), with median redshift of 2.1.
Results confirm that optically faint sources of ~1 mJy at 24um are typically at
redshifts z ~ 2, verifying the high efficiency in selecting high redshift
sources based on extreme infrared to optical flux ratio, and indicate that 24um
sources which also have radio counterparts are not systematically different
than samples chosen only by their infrared to optical flux ratios. Using the
parameter q = log[f(nu)(24um)/f(nu)(20 cm)] 17 of the 18 sources observed have
values of 0<q<1, in the range expected for starburst-powered sources, but only
a few of these show strong PAH emission as expected from starbursts, with the
remainder showing absorbed or power-law spectra consistent with an AGN
luminosity source. This confirms previous indications that optically faint
Spitzer sources with f(nu)(24um) > 1.0mJy are predominately AGN and represent
the upper end of the luminosity function of dusty sources at z ~ 2. Based on
the characteristics of the sources observed so far, we predict that the nature
of sources selected at 24um will change for f(nu)(24um) < 0.5 mJy to sources
dominated primarily by starbursts.Comment: Accepted ApJ 20 February 2006, v638 2 issue, 10pages including 3
figure
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