History of the Geology and Geography Department, Ohio Wesleyan University

Author Institution: Department of Geology and Geography, Ohio Wesleyan Universit

Environmental Effects in Clusters: Modified Far-Infrared--Radio Relations within Virgo Cluster Galaxies

(abridged) We present a study on the effects of the intracluster medium (ICM) on the interstellar medium (ISM) of 10 Virgo cluster spiral galaxies using {\it Spitzer} far-infrared (FIR) and VLA radio continuum imaging. Relying on the FIR-radio correlation within normal galaxies, we use our infrared data to create model radio maps which we compare to the observed radio images. For 6 of our sample galaxies we find regions along their outer edges that are highly deficient in the radio compared with our models. We believe these observations are the signatures of ICM ram pressure. For NGC 4522 we find the radio deficit region to lie just exterior to a region of high radio polarization and flat radio spectral index, although the total 20 cm radio continuum in this region does not appear strongly enhanced. These characteristics seem consistent for other galaxies with radio polarization data in the literature. The strength of the radio deficit is inversely correlated with the time since peak pressure as inferred from stellar population studies and gas stripping simulations, suggesting the strength of the radio deficit is good indicator of the strength of the current ram pressure. We also find that galaxies having {\it local} radio {\it deficits} appear to have {\it enhanced global} radio fluxes. Our preferred physical picture is that the observed radio deficit regions arise from the ICM wind sweeping away cosmic-ray (CR) electrons and the associated magnetic field, thereby creating synchrotron tails as observed for some of our galaxies. We propose that CR particles are also re-accelerated by ICM-driven shocklets behind the observed radio deficit regions which in turn enhances the remaining radio disk brightness.Comment: 19 pages, 10 figures; Astrophysical Journa

The role of the Rayleigh-Taylor instability in ram pressure stripped disk galaxies

Ram pressure stripping, i.e. the removal of a galaxy's gas disk due to its motion through the intracluster medium of a galaxy cluster, appears to be a common phenomenon. Not every galaxy, however, is completely stripped of its gas disk. If the ram pressure is insufficiently strong, only the outer parts of the gas disk are removed, and the inner gas disk is retained by the galaxy. One example of such a case is the Virgo spiral NGC 4402. Observations of NGC 4402 (Crowl et al. 2005) reveal structures at the leading edge of the gas disk, which resemble the characteristic finger-like structures produced by the Rayleigh-Taylor (RT) instability. We argue, however, that the RT instability is unlikely to be responsible for these structures. We demonstrate that the conditions under which a galaxy's disk gas experiences ram pressure stripping are identical to those that lead to RT instability. If the galaxy's gravity prevents ram pressure stripping of the inner disk, it also prevents the RT instability. In contrast, the stripped gas could still be subject to RT instability, and we discuss consequences for the stripped gas.Comment: accepted by A&

Induced defenses in response to an invading crab predator: An explanation of historical and geographic phenotypic change

The expression of defensive morphologies in prey often is correlated with predator abundance or diversity over a range of temporal and spatial scales. These patterns are assumed to reflect natural selection via differential predation on genetically determined, fixed phenotypes. Phenotypic variation, however, also can reflect within-generation developmental responses to environmental cues (phenotypic plasticity). For example, water-borne effluents from predators can induce the production of defensive morphologies in many prey taxa. This phenomenon, however, has been examined only on narrow scales. Here, we demonstrate adaptive phenotypic plasticity in prey from geographically separated populations that were reared in the presence of an introduced predator. Marine snails exposed to predatory crab effluent in the field increased shell thickness rapidly compared with controls. Induced changes were comparable to (i) historical transitions in thickness previously attributed to selection by the invading predator and (ii) present-day clinal variation predicted from water temperature differences. Thus, predator-induced phenotypic plasticity may explain broad-scale geographic and temporal phenotypic variation. If inducible defenses are heritable, then selection on the reaction norm may influence coevolution between predator and prey. Trade-offs may explain why inducible rather than constitutive defenses have evolved in several gastropod species

Stellar population and the origin of intra-cluster stars around brightest cluster galaxies: the case of NGC 3311

Context. We investigate the stellar population and the origin of diffuse light around brightest cluster galaxies. Aims. We study the stellar population of the dynamically hot stellar halo of NGC 3311, the brightest galaxy in the Hydra I cluster, and that of photometric substructures in the diffuse light to constrain the origin of these components. Methods. We analyze absorption lines in medium-resolution, long-slit spectra in the wavelength range 4800-5800 angstrom obtained with FORS2 at the Very Large Telescope. We measure the equivalent width of Lick indices out to 20 kpc from the center of NGC 3311 and fit them with stellar population models that account for the [alpha/Fe] overabundance. Results. Stars in the dynamically hot halo of NGC 3311 are old (age > 13 Gyr), metal-poor ([Z/H] ~ -0.35), and alpha-enhanced ([alpha/Fe] ~ 0.48). Together with the high velocity dispersion, these measurements indicate that the stars in the halo were accreted from the outskirts of other early-type galaxies, with a possible contribution from dwarf galaxies. We identify a region in the halo of NGC 3311 associated with a photometric substructure where the stellar population is even more metal-poor ([Z/H] ~ -0.73). In this region, our measurements are consistent with a composite stellar population superposed along the line of sight, consisting of stars from the dynamically hot halo of NGC 3311 and stars stripped from dwarf galaxies. The latter component contributes < 28% to the local surface brightness. Conclusions. The build-up of diffuse light around NGC 3311 is on-going. Based on the observed stellar population properties, the dominant part of these stars may have come from the outskirts of bright early-type galaxies, while stars from stripped dwarf galaxies are presently being added.Comment: 8 pages, 4 figures. Accepted for publication in Astronomy & Astrophysic

ACCESS III: The Nature of Star Formation in the Shapley Supercluster

We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX ultraviolet imaging of the Shapley supercluster at z=0.048. Combining this with spectra of 814 supercluster members and 1.4GHz radio continuum maps, this represents the largest complete census of star-formation (both obscured and unobscured) in local cluster galaxies to date, reaching SFRs~0.02Msun/yr. We take advantage of this comprehensive panchromatic dataset to perform a detailed analysis of the nature of star formation in cluster galaxies, using several quite independent diagnostics of the quantity and intensity of star formation to develop a coherent view of the types of star formation within cluster galaxies. We observe a robust bimodality in the infrared (f_24/f_K) galaxy colours, which we are able to identify as another manifestation of the broad split into star-forming spiral and passive elliptical galaxy populations seen in UV-optical surveys. This diagnostic also allows the identification of galaxies in the process of having their star formation quenched as the infrared analogue to the UV "green valley" population. The bulk of supercluster galaxies on the star-forming sequence have specific-SFRs consistent with local field specific-SFR-M* relations, and form a tight FIR-radio correlation confirming that their FIR emission is due to star formation. We show that 85% of the global SFR is quiescent star formation within spiral disks, as manifest by the observed sequence in the IRX-beta relation being significantly offset from the starburst relation of Kong et al. (2004), while their FIR-radio colours indicate dust heated by low-intensity star formation. Just 15% of the global SFR is due to nuclear starbursts. The vast majority of star formation seen in cluster galaxies comes from normal infalling spirals who have yet to be affected by the cluster environment.Comment: 17 pages, 9 figures. Accepted for publication in MNRA

Metal enrichment processes

There are many processes that can transport gas from the galaxies to their environment and enrich the environment in this way with metals. These metal enrichment processes have a large influence on the evolution of both the galaxies and their environment. Various processes can contribute to the gas transfer: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy interactions and others. We review their observational evidence, corresponding simulations, their efficiencies, and their time scales as far as they are known to date. It seems that all processes can contribute to the enrichment. There is not a single process that always dominates the enrichment, because the efficiencies of the processes vary strongly with galaxy and environmental properties.Comment: 18 pages, 8 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 17; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke

Toward an internally consistent astronomical distance scale

Accurate astronomical distance determination is crucial for all fields in astrophysics, from Galactic to cosmological scales. Despite, or perhaps because of, significant efforts to determine accurate distances, using a wide range of methods, tracers, and techniques, an internally consistent astronomical distance framework has not yet been established. We review current efforts to homogenize the Local Group's distance framework, with particular emphasis on the potential of RR Lyrae stars as distance indicators, and attempt to extend this in an internally consistent manner to cosmological distances. Calibration based on Type Ia supernovae and distance determinations based on gravitational lensing represent particularly promising approaches. We provide a positive outlook to improvements to the status quo expected from future surveys, missions, and facilities. Astronomical distance determination has clearly reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press (chapter 8 of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space Age