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Publications and source records attributed to Brad K. Gibson.
A highly colloquialized summary of the Stromlo Workshop on High-Velocity Clouds, held Aug 14-15, 1998, at Mount Stromlo Observatory. The proceedings will be edited by Brad Gibson and Mary Putman, and published as part of the ASP Conference Series.
The distance to NGC 4725 has been derived from Cepheid variables, as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen F555W (V) and four F814W (I) epochs of cosmic-ray-split Wide Field and Planetary Camera 2 observations were obtained. Twenty Cepheids were discovered, with periods ranging from 12 to 49 days. Adopting a Large Magellanic Cloud distance modulus and extinction of 18.50+/-0.10 mag and E(V-I)=0.13 mag, respectively, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 30.50 +/- 0.16 (random) +/- 0.17 (systematic) mag was determined for NGC 4725. The corresponding of distance of 12.6 +/- 1.0 (random) +/- 1.0 (systematic) Mpc is in excellent agreement with that found with an independent analysis based upon the DoPHOT photometry package. With a foreground reddening of only E(V-I)=0.02, the inferred intrinsic reddening of this field in NGC 4725, E(V-I)=0.19, makes it one of the most highly-reddened, encountered by the HST Key Project, to date.
We report the detection of Cepheids and a new distance to the spiral galaxy NGC 2541, based on data obtained with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope (HST). A total of 25 exposures (divided into 13 epochs) are obtained using the F555W filter (transformed to Johnson V), and nine exposures (divided into five epochs) using the F814W filter (transformed to Cousins I). Photometric reduction of the data is performed using two independent packages, DoPHOT and DAOPHOT II/ALLFRAME, which give very good agreement in the measured magnitudes. A total of 34 bona fide Cepheids, with periods ranging from 12 to over 60 days, are identified based on both sets of photometry. By fitting V and I period-luminosity relations, apparent distance moduli are derived assuming a Large Magellanic Cloud distance modulus and mean color excess of 18.50 +/- 0.10 mag and E(B-V) = 0.10 mag respectively. Adopting A(V)/E(V-I)=2.45, we obtain a true distance modulus to NGC 2541 of 30.47 +/- 0.11 (random) +/- 0.12 (systematic) mag (D = 12.4 +/- 0.6 (random) +/- 0.7 (systematic) Mpc), and a total (Galactic plus internal) mean color excess E(B-V) = 0.08 +/- 0.05 (internal error) mag.
We examine the origin of heavy elements in the intracluster medium (ICM) of galaxy clusters, concentrating upon the roles played by supernovae (SNe) Types Ia and II. The most accurately determined elemental abundances, Si and Fe, imply a mild predominance of Type II SNe as the source of ICM Fe, contributing approximately 60-80% of its total (and approximately 100% of the alpha-elements). (Currently) intractable uncertainties in measuring X-ray alpha-element ICM abundances, the initial mass function (IMF), and stellar evolution ingredients, make a more precise determination of the Ia:II ICM iron ``ratio'' impossible.
The distance to NGC 7331 has been derived from Cepheid variables observed with HST/WFPC2, as part of the Extragalactic Distance Scale Key Project. Multi-epoch exposures in F555W (V) and F814W (I), with photometry derived independently from DoPHOT and DAOPHOT/ALLFRAME programs, were used to detect a total of 13 reliable Cepheids, with periods between 11 and 42 days. The relative distance moduli between NGC 7331 and the LMC, imply an extinction to NGC 7331 of A_V = 0.47+-0.15 mag, and an extinction-corrected distance modulus to NGC 7331 of 30.89+-0.14(random) mag, equivalent to a distance of 15.1 Mpc. There are additional systematic uncertainties in the distance modulus of +-0.12 mag due to the calibration of the Cepheid Period-Luminosity relation, and a systematic offset of +0.05+-0.04 mag if we applied the metallicity correction inferred from the M101 results of Kennicutt et al 1998.
Recent determinations of the upper mass limit to the local initial mass function (IMF) claim a value of m(upper)=50+/-10 solar masses, based upon direct comparisons of the observed oxygen and iron abundances in metal-poor stars with the predicted stellar yields from Type II supernovae (SNe). An unappreciated uncertainty in these analyses is the input physics intrinsic to each SNe grid, and its effect upon stellar nucleosynthesis. We demonstrate how such uncertainties, coupled with the uncertain metal-poor halo star normalization, while allowing us to set a lower bound to m(upper) of approximately 40 solar masses, nullifies any attempt at constraining the upper bound.
Several speakers at IAU Symposium #187 (Cosmic Chemical Evolution) alluded to the zeroth-order agreement between Type II supernovae (SNe) stellar yield compilations, as predicted by the models of those most responsible for driving progress in the field - i.e., Arnett (1991,1996); Maeder (1992); Woosley & Weaver (1995); Langer & Henkel (1995); Thielemann et al. (1996). It is important though for those entering (or indeed, already involved in!) the chemical evolution field to be cognizant of the fact that there are important first- and second-order differences between these compilations. In the next few pages, I provide a qualitative comparison of the currently available Type II SNe yield grids. The strengths and weaknesses of a given grid, demonstrated by comparing against relevant observations, are noted. Some simple chemical evolution models are shown which graphically demonstrate the effect of yield grid selection.
Chemical and photometric models of elliptical galaxies with infall of primordial gas, and subsequent ejection of processed material via galactic winds, are described. Ensuring that these models are consistent with the present-day colour-luminosity relation and the measured intracluster medium (ICM) abundances, we demonstrate that the initial mass function (IMF) must be significantly flatter (i.e., x<=0.80) than the canonical Salpeter slope (i.e., x=1.35). Such a ``top-heavy'' IMF is in agreement with the earlier conclusions based upon closed-box models for elliptical galaxy evolution. On the other hand, the top-heavy IMF, in conjunction with these semi-analytic infall models, predicts an ICM gas mass which exceeds that observed by up to a factor three, in contrast with the canonical closed-box models. Time and position-dependent IMF formalisms may prove to be a fruitful avenue for future research, but those presently available in the literature are shown to be inconsistent with several important observational constraints.
We re-examine the respective roles played by supernovae (SNe) Types Ia and II in enriching the intracluster medium (ICM) of galaxy clusters, in light of the recent downward shift of the ASCA abundance ratios of alpha-elements to iron favoured by Ishimaru & Arimoto (1997, PASJ, 49, 1). Because of this shift, Ishimaru & Arimoto conclude that >50% of the ICM iron must have originated from within Type Ia SNe progenitors. A point not appreciated in their study, nor in most previous analyses, is the crucial dependence of such a conclusion upon the adopted massive star physics. Employing several alternative Type II SN yield compilations, we demonstrate how uncertainties in the treatment of convection and mass-loss can radically alter our perception of the relative importance of Type Ia and II SNe as ICM polluters. If mass-loss of the form favoured by Maeder (1992, A&A, 264, 105) or convection of the form favoured by Arnett (1996, Supernovae and Nucleosynthesis) is assumed, the effect upon the oxygen yields would lead us to conclude that Type Ia SNe play no part in polluting the ICM, in contradiction with Ishimaru & Arimoto. Apparent dichotomies still exist (e.g. the mean ICM neon-to-iron ratio implies a 100% Type II Fe origin, while the mean sulphur ratio indicates a 100% Type Ia origin) that cannot be reconciled with the currently available yield tables.
We consider the simultaneous chemical, photometric, and gaseous thermal energy evolution of elliptical galaxies. The evolution of chemical abundances in the intracluster medium (ICM) is set by the differing timescales for gas ejection, via supernovae (SNe)-driven winds, from dwarf, normal, and giant ellipticals, and is monitored concurrently. Emphasis is placed upon the influence of, and sensitivity to, the underlying stellar initial mass function (IMF), star formation efficiency, supernovae Type Ia rates, supernovae remnant (SNR) dynamics, and the most recent advances in stellar nucleosynthesis. Unlike many previous studies, we adhere to a wide range of optical (e.g. colour-metallicity-luminosity relationship) and x-ray (e.g. recent ASCA ICM abundance measurements) observational constraints. IMFs biased toward high mass stars, at least during the early phases of star formation, are implicated in order to satisfy all the observational constraints.
The enrichment of the intracluster medium (ICM) of galaxy clusters, within the framework of the supernovae-driven wind model for elliptical galaxies, has been considered under two extreme assumptions regarding the binding energy-mass-radius relations. Unless the binding energy at the time of wind commencement, for a given galactic mass, was a factor of approximately five lower than that predicted by the present-day relation, at most 20% of the ICM gas could have originated within the cluster ellipticals themselves, the remainder being of primordial origin.
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.
Halo initial mass functions (IMFs), heavily-biased toward white dwarf (WD) precursors (i.e. 1->8 M_sun), have been suggested as a suitable mechanism for explaining microlensing statistics along the line of sight to the LMC. Such IMFs can apparently be invoked without violating the observed present-day WD luminosity function. By employing a simple chemical evolution argument, we demonstrate that reconciling the observed halo Population II dwarf abundances (i.e. [C,N/O]=-0.5), with that expected from the postulated ``WD-heavy'' IMF (i.e. [C,N/O]=+0.5), is difficult.
Working within the classic supernovae-driven wind framework for elliptical galaxy evolution, we perform a systematic investigation into the discrepancies between the predictions of three contemporary codes -- Arimoto & Yoshii (1987, A&A, 173, 23), Bressan et al. (1994, ApJS, 94, 63), and Gibson (1996a, MNRAS, 278, 829; 1996b, MNRAS, submitted). By being primarily concerned with reproducing the present-day colour-metallicity-luminosity (CML) relations amongst ellipticals, the approaches taken in the theoretical modelling have managed to obscure many of the hidden differences between the codes. Targeting the timescale for the onset of the initial galactic wind t_GW as a primary ``difference'' indicator, we demonstrate exactly how and why each code is able to claim successful reproduction of the CML relations, despite possessing apparently incompatible input ingredients.
We consider the present-day photometric and chemical properties of elliptical galaxies, adopting the bimodal star formation scenario of Elbaz, Arnaud & Vangioni-Flam (1995). These models utilise an initial mass function (IMF) biased heavily toward massive stars during the early phases of galactic evolution, leading to early Type II supernovae-driven galactic winds. A subsequent lengthy, milder, star formation phase with a normal IMF ensues, responsible supposedly for the stellar population observed today. Based upon chemical evolution arguments alone, this scenario has been invoked to explain the observed metal mass, and their abundance ratios, in the intracluster medium of galaxy clusters. Building upon the recent compilations of metallicity-dependent isochrones for simple stellar populations, we have constructed a coupled photometric and chemical evolution package for composite stellar populations in order to quantify the effects of such a model upon the photo-chemical properties of the resultant elliptical galaxies. We demonstrate that these predicted properties are incompatible with those observed at the current epoch.
We study the origin of iron and alpha-elements (O, Mg, Si) in clusters of galaxies. In particular, we discuss the [O/Fe] ratio and the iron mass-to-luminosity ratio in the intracluster medium (ICM) and their link to the chemical and dynamical evolution of elliptical and lenticular galaxies. We adopt a detailed model of galactic evolution incorporating the development of supernovae- driven galactic winds which pollute the ICM with enriched ejecta. We demonstrate \it quantitatively \rm the crucial dependence upon the assumed stellar initial mass function in determining the evolution of the mass and abundances ratios of heavy elements in typical model ICMs. We show that completely opposite behaviours of [alpha/Fe] ratios (\ie positive versus negative ratios) can be obtained by varying the initial mass function without altering the classic assumptions regarding type Ia supernovae progenitors or their nucleosynthesis. Our results indicate that models incorporating somewhat flatter-than-Salpeter initial mass functions (ie x approx 1, as opposed to x=1.35) are preferred, provided the intracluster medium iron mass-to-luminosity ratio, preliminary [alpha/Fe]>0 ASCA results, and present-day type Ia supernovae rates, are to be matched. A simple Virgo cluster simulation which adheres to these constraints shows that approx 70% of the measured ICM iron mass has its origin in type II supernovae, with the remainder being synthesized in type Ia systems.
The fundamental tenet of the classical supernovae-driven wind model of elliptical galaxies is that the residual thermal energy of all supernovae remnants (SNRs) provide sufficient energy to overcome the binding energy of the remaining interstellar gas, thereby driving a global galactic wind. We re-examine model predictions of this epoch of wind ejection t_GW, highlighting a heretofore underappreciated sensitivity to the adopted remnant thermal energy formalism, and illustrating cases in which previous work may have substantially overestimated t_GW. Arguments based upon chemical evolution alone, put forth to reject the hypothesis of dark matter distributions similar to the luminous component in spheroids, are shown to be tenuous. Finally, the predicted enrichment of intracluster gas during the wind phase of cluster ellipticals, and its relation to the selected SNR interior thermal energy evolutionary scheme, is addressed. Despite the success of previous wind models, our results still call into question the correctness of the simple analytical approach used thus far, and imply that a more appropriate technique should be adopted in the future.
Recent claims in the literature (Bressan, Chiosi & Fagotto 1994) that the epoch describing the onset of galactic winds in spheroidal star systems has been severely overestimated in the past, due to the neglect of energy deposited in the interstellar medium from stellar winds, is evaluated in the light of a more conservative approach to modeling the input of thermal energy to the system from massive stars undergoing mass loss. Applying the most recent models of stellar kinetic energy deposition, coupled with reasonable assumptions regarding the efficiency of thermalisation, it is shown that contrary to the aforementioned study, the influence of stellar winds in driving the galactic winds in ellipticals of mass M_G>1e9 M_Sun is most likely negligible.