Gaseous outflows from low mass galaxies: Understanding local laboratories for high redshift star formation
The evolution of galaxies and the intergalactic medium is undoubtedly affected by starburst-driven outflows. These galactic winds are responsible for regulating the growth of galaxies, as well as enriching the intergalactic medium with metals. To date, our understanding of outflows at early cosmic times is based strictly on features in the ultraviolet spectra of galaxies. For these high redshift objects, detailed analyses are often hampered by extreme distances. Therefore, we propose COS spectroscopy of a sample of nine local dwarf starburst galaxies. These objects, selected from their high equivalent width nebular line emission in the SDSS (i.e. the "Green Peas"), are some of the most active star-forming dwarfs in the local universe. With lower masses and metallicities than other nearby starbursts, these galaxies may represent the best high redshift analogs that can be found locally. The proposed observations will measure galactic winds and mass loss, and determine how the outflows scale with other galaxy properties. Furthermore, COS spectroscopy will constrain the dust content and elemental abundances in the ISM, as well as the covering fraction of the outflowing gas and its role in regulating the escape of Lyman alpha and hydrogen-ionizing photons. By comparing these observations to higher redshift samples, we will clarify the relation between these local analogs and their high redshift counterparts. These data will provide a lasting legacy for HST and COS, as existing samples of local high redshift analogs with quality UV spectra remain small.
We will obtain FUV spectra of nine compact, actively star-forming galaxies at z~0.2. The goal of these observations is to measure interstellar absorption lines in both metals and the hydrogen Lyman series.
We have opted for an ACQ/IMAGE acquisition. As a bonus, this method will provide the first high-resolution images of these galaxies.
In order to determine the required exposure time, we assume that the galaxies have 1" diameters (as they are unresolved in SDSS images).
The ACQ/IMAGE dwelltime is calculated by assuming a uniform surface brightness. For both the target acquistion and the science exposures (described below), we use the primary science aperture (PSA), and Mirror A.
The science exposures are taken in both G130M and G160M, so that we can measure a wide range of absorption and emission features.
Our exposure times are determined by the requirement for sufficient continuum S/N to measure absorption lines. We estimate that our effective spectral resolution will be around 100 km/s for galaxies with 1" diameters. We aim to achieve a S/N of 10 per 100 km/s, which corresponds to a S/N of 3.8 per native (point source) spectral resolution element adopted by the ETC. In G130M, we calculate the exposure time to meet this requirement at a wavelength of 1400A. Since the sensitivity curve is relatively flat for G130M, we expect to achieve adequate S/N over the entire bandpass. For G160M, we aim to meet our sensitivity threshold at the observed wavelength of CII 1334. The redder SiIV 1393, 1403 lines that we aim to detect are relatively strong and will be adequately detected with the continuum S/N of 7-8.
With the exception of one galaxy, observations are made with only one CENWAVE per grating. While this choice leaves gaps in the spectra, it greatly reduces overheads and simplifies data reduction. We have carefully examined FUV line lists from Grimes et al. (2009) and Leitherer et al. (2011) to avoid placing potentially interesting features in the gaps. In one case (GP 1054+5238), we have opted for two CENWAVEs in the G130 grating. This option was necessary to cover the OVI 1032 line and the Lyman Continuum (which is reached for this galaxy as well as GP1249+1234 and GP0911+1831).
Following the reccomendations in the COS instrument handbook, spectroscopic observations are made at all four FP-POS settings, in TIME-TAG mode. Since the buffer time estimated by the COS spectroscopic ETC is much longer than the exposure time, we have entered a buffer time that is equivalent to the exposure time. In some cases, we have slightly reduced the overhead by making the buffer time 100 seconds less than the exposure time (COS IHB 5.4.1).
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