scientific abstract blue compact galaxies (bcgs) appear to be relatively unevolved and it is still not clear whether at least some of these are recently formed low-mass galaxies undergoing their first burst of star formation or whether they have experienced a series of bursts. under close examination at visible wavelengths they exhibit several distinctive morphologies. the evolutionary relationship between bcgs, other dwarf galaxies (such as low surface brightness dwarves) and other galaxy types is still open to speculation. in this proposal, lws grating, pht-s spectroscopy and isocam imaging are proposed for a sample of 16 bcgs selected from the catalogues of haro, markarian and zwickey. sws observations are proposed for two systems, to complement related work by the lws consortium. for those galaxies bright enough to yield measurements of the strengths of several spectral lines, emission line strengths and line ratios derived from lws and sws spectroscopy, along with the continuum spectra, will permit detailed study of the nature of, and the physical conditions in and around, the star- bursts, including elemental abundances, densities and temperatures and the gas-to-dust ratio and dust properties in the systems. for fainter galaxies, measurements of the 158 micron cii fine structure line will probe photodissoc- iation regions associated with massive star formation and also the quiescent diffuse hi component of the interstellar medium. cam imaging will allow the distribution of dust and related pah emission to be compared with the structures seen in the visible. pht-s spectra will support the identification of the unidentified feature (pah?) emission in the nir, tracing formation of carbon rich material around old stars. a correlation will be sought between distinc- tive visible and nir properties of the sample galaxies and their fir spectral properties that could help provide the key to identifying truly young galaxies experiencing their first starburst. observation summary the target sources for this proposal can be divided into two categories. those which have detections recorded in the iras psc and those which have not. estimates have been made of the fluxes which can be expected from the systems not seen by iras, based upon available data at other wavelengths and assuming a visible to fir spectrum similar to that of their brighter (though not necess- arily more luminous) counterparts. cam observations (aot cam01): the target galaxies are typically 1 to 2 arcminutes in diameter in the visible. therefore the cam 6 arcsecond per pixel f.o.v. has been chosen because it matches well the diffraction spot size for the lw3 and 6 filters used and because it gives an overall f.o.v. (3 arcminutes diameter) well matched to the outer halo size of the target galaxies. each target observed with cam is observed in both the lw3 (high sensitivity dust - 12 to 18 micron) filter and the lw6 (unidentified ir feature - 7 to 8.5 micron ) filter. the iras upper limit for the 12 micron flux of those included galaxies not detected by iras is as low as 0.1 jansky. if this flux were spread uniformly over the typical area of one of these targets (about 1 arcminute sq.) that would imply a surface brightness of about 30 micro jy per square arcsecond. of course, any ir emmission which does occur is likely to be more concentrated towards the centres of the galaxies, implying higher surface brightness there. consequently, an observation tuned to detect with s/n of 3 at the 3 microjy per square arcsecond contour should comfortably record any ir emission from the full extent of the galaxies, unless their ir flux is dramatically below the iras detection limit. this is highly unlikely since about half of the sample were detected by iras. the required observation then is a 4x4 step micro-scan (aot cam01) in both the lw3 and lw6 filters in the 6 arcsecond per pixel f.o.v. with a fundamental on-chip integration time (tint) of 5 seconds and a total observation time, including all instrumental and spacecraft overheads, of about 650 seconds per filter on source. the micro-scanning step size is 16 pixels. note that the chosen on-chip integration time is a compromise between longer on-chip integration times, giving lower total read noise, and shorter integration times, giving better immunity to responsive transients in the detector which may follow a movement of the source over the array during the micro-scan. since the zodiacal light is bright relative to the target sources in the filters used, such scan-induced transients should not be too severe (the array illumination is quite uniform). it has therefore been assumed that the worst case stabilisation time for the conditions of these observations will not apply, so that dwell times at each position of the microscan can be minimised allowing relatively short overall exposure times. since we are not forced to use long exposure times to avoid transients, the observation time has been fixed at a duration close to that at which flat-fielding noise causes the s/n to
Instrument
CAM01 , CAM03 , LWS01 , PHT40
Temporal Coverage
1996-02-13T15:40:59Z/1998-02-25T19:51:14Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, Metcalfe et al., 1999, 'ISO Observations of Blue Compact Galaxies - openParAutumn ListclosePar: ', 1.0, European Space Agency, https://doi.org/10.5270/esa-1nwjwfw
