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 $(2.69\pm0.05)\times10^{9}$ M$_{\odot}$ of H$_2$ associated with the inner ring (72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor ($\alpha_{\rm CO}$). The spokes and disk are not detected. Analysis of the inner ring's CO kinematics show it to be expanding ($V_{\rm exp}=68.9\pm4.9$ km s$^{-1}$) implying an $\approx70$ Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where HI surface density ($\Sigma_{\rm HI}$) is high, representing $M_{\rm H_2}=(7.5\pm0.8)\times10^{8}$ M$_{\odot}$ for a metallicity appropriate $\alpha_{\rm CO}$, giving small $\Sigma_{\rm H_2}$ ($3.7$ M$_{\odot}$ pc$^{-2}$), molecular fraction ($f_{\rm mol}=0.10$), and H$_2$ depletion timescales ($\tau_{\rm mol} \approx50-600$ Myr). Elsewhere in the outer ring $\Sigma_{\rm H_2}\lesssim 2$ M$_{\odot}$ pc$^{-2}$, $f_{\rm mol}\lesssim 0.1$ and $\tau_{\rm mol}\lesssim 140-540$ Myr (all $3\sigma$). The inner ring and nucleus are H$_2$-dominated and are consistent with local spiral SF laws. $\Sigma_{\rm SFR}$ in the outer ring appears independent of $\Sigma_{\rm H_2}$, $\Sigma_{\rm HI}$ or $\Sigma_{\rm HI+H_2}$. 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$_2$ component or an abundance of CO-faint low column density molecular clouds. The H$_2$ content of evolved starburst rings may therefore be substantially larger. Due to its lower $\Sigma_{\rm SFR}$ 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