The evolution of Lithium: Implications of a universal Spite plateau

The cosmological 7Li problem consists in explaining why the primordial Li abundance, as predicted by the standard Big Bang nucleosynthesis theory with constraints from WMAP and Planck, is a factor of 3 larger than the Li abundance measured in the stars of the Spite plateau defined by old, warm dwarf stars of the Milky Way halo. Several explanations have been proposed to explain this difference, including various Li depletion processes as well as non-standard Big Bang nucleosynthesis, but the main question remains unanswered. In this paper, we present detailed chemical evolution models for dwarf spheroidal and ultra faint galaxies, compute the galactic evolution of 7Li abundance in these objects, and compare it with observations of similar objects. In our models, Li is mainly produced by novae and cosmic rays, and to a minor extent, by low and intermediate mass stars. We adopt the yield combination that best fits the Li abundances in the Milky Way stars. It is evident that the observations of dwarf objects define a Spite plateau, identical to that observed in the Milky Way, thus suggesting that the Spite plateau could be a universal feature and its meaning should be discussed. The predictions of our models for dwarf galaxies are obtained by assuming as Li primordial abundance either the one detected in the atmospheres of the oldest halo stars (Spite plateau; A(Li) ~ 2.2 dex), or the one from cosmological observations (WMAP; A(Li) ~ 2.66 dex). Finally, we discuss the implications of the universality of the Spite plateau results

The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter

Chemical evolution models and Monte Carlo simulation techniques have been combined for the first time to study the distribution of blue compact galaxies on the N/O plateau. Each simulation comprises 70 individual chemical evolution models. For each model, input parameters relating to a galaxy's star formation history (bursting or continuous star formation, star formation efficiency), galaxy age, and outflow rate are chosen randomly from ranges predetermined to be relevant. Predicted abundance ratios from each simulation are collectively overplotted onto the data to test its viability. We present our results both with and without observational scatter applied to the model points. Our study shows that most trial combinations of input parameters, including a simulation comprising only simple models with instantaneous recycling, are successful in reproducing the observed morphology of the N/O plateau once observational scatter is added. Therefore simulations which include delay of nitrogen injection are no longer favored over those which propose that most nitrogen is produced by massive stars, if only the plateau morphology is used as the principal constraint. The one scenario which clearly cannot explain plateau morphology is one in which galaxy ages are allowed to range below 250 Myr. We conclude that the present data for the N/O plateau are insufficient by themselves for identifying the portion of the stellar mass spectrum most responsible for cosmic nitrogen production.Comment: 41 pages, 15 figures; accepted by ApJ, to appear Aug. 20, 200

The Paradoxical Forces for the Classical Electromagnetic Lag Associated with the Aharonov-Bohm Phase Shift

The classical electromagnetic lag assocated with the Aharonov-Bohm phase shift is obtained by using a Darwin-Lagrangian analysis similar to that given by Coleman and Van Vleck to identify the puzzling forces of the Shockley-James paradox. The classical forces cause changes in particle velocities and so produce a relative lag leading to the same phase shift as predicted by Aharonov and Bohm and observed in experiments. An experiment is proposed to test for this lag aspect implied by the classical analysis but not present in the currently-accepted quantum topological description of the phase shift.Comment: 8 pages, 3 figure

On Dwarf Galaxies as the Source of Intracluster Gas

Recent observational evidence for steep dwarf galaxy luminosity functions in several rich clusters has led to speculation that their precursors may be the source of the majority of gas and metals inferred from intracluster medium (ICM) x-ray observations. Their deposition into the ICM is presumed to occur through early supernovae-driven winds, the resultant systems reflecting the photometric and chemical properties of the low luminosity dwarf spheroidals and ellipticals we observe locally. We consider this scenario, utilising a self-consistent model for spheroidal photo-chemical evolution and gas ejection via galactic superwinds. Insisting that post-wind dwarfs obey the observed colour-luminosity-metallicity relations, we conclude that the bulk of the ICM gas and metals does not originate within their precursors.Comment: 43 pages, 8 figures, LaTeX, also available at http://msowww.anu.edu.au/~gibson/publications.html, to appear in ApJ, Vol 473, 1997, in pres

High Carbon in I Zwicky 18: New Results from Hubble Space Telescope Spectroscopy

We present new measurements of the gas-phase C/O abundance ratio in both the NW and SE components of the extremely metal-poor dwarf irregular galaxy I Zw 18, based on ultraviolet spectroscopy of the two H II regions using the Faint Object Spectrograph on the Hubble Space Telescope. We determine values of log C/O = -0.63 +/- 0.10 for the NW component and log C/O = -0.56 +/- 0.09 for the SE component. In comparison, log C/O = -0.37 in the sun, while log C/O = -0.85 +/- 0.07 in the three most metal-poor irregular galaxies measured by Garnett et al. (1995a). Our measurements show that C/O in I Zw 18 is significantly higher than in other comparably metal-poor irregular galaxies, and above predictions for the expected C/O from massive star nucleosynthesis. These results suggest that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass carbon producing stars; this idea is supported by stellar photometry of I Zw 18 and its companion, which demonstrate that the current bursts of massive stars were not the first. Despite its very low metallicity, it is likely that I Zw 18 is not a ``primeval'' galaxy.Comment: 14 pages including 4 figures; uses aaspp4.sty. Accepted for publication in ApJ. Postscript version also available by e-mail request to author at [email protected]

The influence of nova nucleosynthesis on the chemical evolution of the Galaxy

We adopt up-to-date yields of 7Li, 13C, 15N from classical novae and use a well tested model for the chemical evolution of the Milky Way in order to predict the temporal evolution of these elemental species in the solar neighborhood. In spite of major uncertainties due to our lack of knowledge of metallicity effects on the final products of explosive nucleosynthesis in nova outbursts, we find a satisfactory agreement between theoretical predictions and observations for 7Li and 13C. On the contrary, 15N turns out to be overproduced by about an order of magnitude.Comment: 8 pages, latex, 3 figures. To appear in "The Chemical Evolution of the Milky Way: Stars versus Clusters", eds. F. Giovannelli and F. Matteucci (Kluwer: Dordrecht

Iron as a tracer in galaxy clusters and groups

Available X-ray data are collected and organized concerning the iron and gas content of galaxy clusters and groups, together with the optical luminosity, mass and iron abundance of cluster galaxies. Several astrophysical inferences are then drawn, including the evidence for rich clusters having evolved without much baryon exchange with their surrondings, and having experienced very similar star formation histories. Groups are much gas-poor compared to clusters, and appear instead to have shed a major fraction of their original cosmic share of baryons, which indicates that galaxy clusters cannot have formed by assembling groups similar to the present day ones. It is argued that this favors low-$\Omega$ universes, in which the growth of rich clusters is virtually complete at high redshifts. It is also argued that elemental abundance ratios in clusters are nearly solar, which is consistent with a similar proportion of supernovae of Type Ia and Type II having enriched both the solar neghborhood as well clusters as a whole. Much of the iron in clusters appears to reside in the intracluster medium rather than inside galaxies. It appears that the baryon to star conversion in clusters has been nearly as efficient as currently adopted for the universe as a whole. Yet the metallicity of the clusters is $\sim 5$ times higher than the global metallicity adopted for the nearby universe. It is concluded that the intergalactic medium should have a metallicity $\sim 1/3$ solar if stellar nucleosynthesis has proceeded in stars within field galaxies with the same efficiency as in stars within clusters of galaxies.Comment: AASTex Latex, 29 pages, 6 figure

VLT Spectroscopy of Globular Clusters in Low Surface Brightness Dwarf Galaxies

We present VLT/FORS2 spectroscopic observations of globular clusters (GCs) in five low surface brightness (LSB) dwarf galaxies: KK211 and KK221, which are both dwarf spheroidal satellites (dSph) of NGC 5128, dSph KK84 located close to the isolated S0 galaxy NGC 3115, and two isolated dwarf irregular (dIrr) galaxies UGC 3755 and ESO 490-17. Our sample is selected from the Sharina et al. (2005) database of Hubble Space Telescope WFPC2 photometry of GC candidates in dwarf galaxies. For objects with accurate radial velocity measurements we confirm 26 as genuine GCs out of the 27 selected candidates from our WFPC2 survey. Lick absorption line indices in the spectra of confirmed GCs and the subsequent comparison with SSP model predictions show that all confirmed GCs in dSphs are old, except GC KK211-3-149 (6 +/- 2 Gyr), which we consider to be the nucleus of KK211. GCs in UGC 3755 and ESO 490-17 show a large spread in ages ranging from old objects (t > 10 Gyr) to clusters with ages around 1 Gyr. Most of our sample GCs have low metallicities [Z/H] <= -1. Two relatively metal-rich clusters with [Z/H] ~ -0.3 are likely to be associated with NGC 3115. Our sample GCs show in general a complex distribution of alpha-element enhancement with a mean [alpha/Fe]=0.19 +/-0.04 derived with the chi2 minimization technique and 0.18+/-0.12 dex computed with the iterative approach. These values are slightly lower than the mean [alpha/Fe]=0.29+/-0.01 for typical Milky Way GCs. We compare other abundance ratios with those of Local Group GCs and find indications for systematic differences in N and Ca abundance. The specific frequencies, S_N, of our sample galaxies are in line with the predictions of a simple mass-loss model for dwarf galaxies and compare well with S_N values found for dwarf galaxies in nearby galaxy clusters.Comment: accepted for publication in Ap

Evolutionary Status of Dwarf ``Transition'' Galaxies

We present deep B, R and Halpha imaging of 3 dwarf galaxies: NGC3377A, NGC4286, and IC3475. Based on previous broadband imaging and HI studies, these mixed-morphology galaxies were proposed by Sandage & Hoffman (1991) to be, respectively, a gas-rich low surface brightness Im dwarf, a nucleated dwarf that has lost most of its gas and is in transition from Im to dS0,N, and the prototypical example of a gas-poor ``huge low surface brightness'' early-type galaxy. From the combination of our broadband and Halpha imaging with the published information on the neutral gas content of these three galaxies, we find that (1) NGC3377A is a dwarf spiral; (2) NGC3377A and NGC4286 have comparable amounts of ongoing star formation, as indicated by their Halpha emission, while IC3475 has no detected HII regions to a very low limit; (3) the global star formation rates are at least a factor of 20 below that of 30 Doradus for NGC3377A and NGC4286; (4) while the amount of star formation is comparable, the distribution of star forming regions is very different between NGC3377A and NGC4286; (5) given their current star formation rates and gas contents, both NGC3377A and NGC4286 can continue to form stars for more than a Hubble time; (6) both NGC3377A and NGC4286 have integrated total B-R colors that are redder than the integrated total B-R color for IC3475, and thus it is unlikely that either galaxy will ever evolve into an IC3475 counterpart; and (7) IC3475 is too blue to be a dE. We thus conclude that we have not identified potential precursors to galaxies such as IC3475, and unless signifcant changes occur in the star formation rates, neither NGC3377A nor NGC4286 will evolve into a dwarf elliptical or dwarf spheroidal within a Hubble time.Comment: 34 pages, 6 jpg figures, 3 postscript figures, and 4 tables, uses AASTeX, ApJ, in pres