Barry F. Madore

Galaxy and Mass Assembly (GAMA): the star formation rate dependence of the stellar initial mass function

The stellar initial mass function (IMF) describes the distribution in stellar masses produced from a burst of star formation. For more than 50 yr, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the high-mass IMF slope for a sample of low-to-moderate redshift galaxies from the Galaxy and Mass Assembly survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMF–star formation rate dependency is discovered, which shows that highly star-forming galaxies form proportionally more massive stars (they have IMFs with flatter power-law slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF.

The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (I): Results

We present the first detection of the mean flux of the optical extragalactic background light (EBL) at 3000, 5500, and 8000 A. Diffuse foreground flux at these wavelengths comes from terrestrial airglow, dust-scattered sunlight (zodiacal light), and dust-scattered Galactic starlight (diffuse Galactic light). We have avoided the brightest of these, terrestrial airglow, by measuring the absolute surface brightness of the night sky from above the Earths atmosphere using the Wide Field Planetary Camera 2 (WFPC2) and Faint Object Spectrograph, both on board the Hubble Space Telescope (HST). On the ground we have used the du Pont 2.5 m Telescope at the Las Campanas Observatory (LCO) to obtain contemporaneous spectrophotometry of blank sky in the HST field of view to measure and then subtract foreground zodiacal light from the HST observations. We have minimized the diffuse Galactic light in advance by selecting the HST target field along a line of sight with low Galactic dust column density and then estimated the low-level Galactic foreground using a simple scattering model and the observed correlation between thermal, 100 μm emission, and optical scattered flux from the same dust. In this paper, we describe the coordinated LCO/HST program and the HST observations and data reduction and present the resulting measurements of the EBL. Galaxies brighter than V = 23 AB mag are not well sampled in an image the size of the WFPC2 field of view. We have therefore measured the EBL from unresolved and resolved galaxies fainter than V = 23 AB mag by masking out brighter galaxies. We write this as EBL23 to emphasize this bright magnitude cutoff. From absolute surface photometry using WFPC2 and ground-based spectroscopy, we find mean values for the EBL23 of 4.0 (±2.5), 2.7 (±1.4), and 2.2 (±1.0) in units of 10-9 ergs s-1 cm-2 sr-1 A-1 in the F300W, F555W, and F814W bandpasses, respectively. The errors quoted are 1 σ combined statistical and systematic uncertainties. The total flux measured in resolved galaxies with V > 23 AB mag by standard photometric methods is roughly 15% of the EBL23 flux in each band. We have also developed a new method of source photometry, uniquely suited to these data, with which we can measure the ensemble flux from detectable sources much more accurately than is possible with standard methods for faint galaxy photometry. Using this method, we have quantified systematic biases affecting standard galaxy photometry, which prevent light from being recovered in isophotes within a few percent of the sky level. These biases have a significant effect on faint galaxy counts. The flux from resolved sources as measured by our ensemble photometry method is 3.2 (±0.22), 0.89 (±0.01), and 0.76 (±0.01) in units of 10-9 ergs s-1 cm-2 sr-1 A-1 in the F300W, F555W, and F814W bandpasses, respectively, with 1 σ combined errors. These values, the total flux from resolved sources, represent absolute minima for the EBL23 in each band and are roughly 30% of the mean flux we measure for the total EBL23.

Galaxy and mass assembly (GAMA): dust obscuration in galaxies and their recent star formation histories

We present self-consistent star formation rates derived through pan-spectral analysis of galaxies drawn from the Galaxy and Mass Assembly (GAMA) survey. We determine the most appropriate form of dust obscuration correction via application of a range of extinction laws drawn from the literature as applied to H , [Oii] and UV luminosities. These corrections are applied to a sample of 31 508 galaxies from the GAMA survey at z < 0:35. We consider several dierent obscuration curves, including those of Milky Way, Calzetti (2001) and Fischera and Dopita (2005) curves and their eects on the observed luminosities. At the core of this technique is the observed Balmer decrement, and we provide a prescription to apply optimal obscuration corrections using the Balmer decrement. We carry out an analysis of the star formation history (SFH) using stellar population synthesis tools to investigate the evolutionary history of our sample of galaxies as well as to understand the eects of variation in the Initial Mass Function (IMF) and the eects this has on the evolutionary history of galaxies. We nd that the Fischera & Dopita (2005) obscuration curve with an