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The H-alpha Galaxy Survey


Knowledge of the star formation histories of both the Universe and of individual galaxies provides the foundation for our understanding of the evolution of the Universe we seetoday. Substantial advances have been made in our understanding of high-redshift star formation. This has resulted in an anomalous situation in which the star formation history of the Universe appears to be more fully quantified at high redshift than it is locally. This results at least partly from the relative ease with which a representative volume of the Universe can be observed at high redshift, since only a small area of the sky need be observed. Star formation rates have been determined for many local galaxies using a range of different techniques, e.g. emission-line fluxes, far-infrared luminosities or direct ultraviolet emission from hot stars. However, in the main these studies have looked at the brightest and most rapidly star-forming galaxies, and most spectroscopic studies are biased towards galaxies with large equivalent width in the line concerned.

The H-alpha Galaxy Survey (HαGS) attempts to quantify star formation activity across the full range of star-forming galaxies, down to the faintest dwarf irregular types, by using narrow-band imaging through filters centred on the redshifted Balmer H-alpha line. The advantages of this technique are that it is sensitive to low levels of star formation even in faint, low surface brightness galaxies, and that it traces high mass stars, and hence recent star formation. Given suitable assumptions, principally about extinction and the stellar initial mass function, it yields quantitative measurements of the star formation rate. The method can be applied to a sensitive level using relatively modest integration times on small telescopes, which is an important consideration given that this survey must be done one object at a time, to ensure that each target galaxy is observed with the correct filter. The resulting data not only give estimates of the total star formation rate for each galaxy, but also detailed information on the star formation distribution, enabling, for example, the separation of nuclear and disk activity in spiral galaxies. Finally, the large format of current CCDs gives a reasonable probability of detecting nearby star-forming companion galaxies which lie in the same field as target galaxies, and which have similar recession velocities.

The primary data for this study are narrow-band H-alpha+[NII] and Johnson R band imaging. These observations were made using the 1.0 metre Jacobus Kapteyn Telescope. This project was allocated 100 nights of observing time. The instrument used was the facility 2048×2048 pixel SITe CCD camera, with 0.33 arcseconds pixels, giving a total unvignetted field of view of 10×10 arcminutes.

The observed sample consists of 334 galaxies across all Hubble types from S0/a to Im and with recession velocities of between 0 and 3000 km s-1. The basic data for each galaxy are narrow band H-alpha+[N II] and R-band imaging, from which they derive star formation rates, H-alpha+[N II] equivalent widths and surface brightnesses, and R-band total magnitudes.

A strong correlation is found between total star formation rate and Hubble type, with the strongest star formation in isolated galaxies occurring in Sc and Sbc types. More surprisingly, no significant trend is found between H-alpha+[N II] equivalent width and galaxy R-band luminosity.

Sample images and analysis of UGC 6644, Sc (left) and UGC 3711, IBm (right). For each galaxy, the top frame shows the H-alpha+[N II] growth curve (circles), the R-band growth curve (asterisks) and the H-alpha+[N II] equivalent width (crosses) as a function of aperture size. The vertical scale relates to the equivalent width plots, and is in nm; the H-alpha+[N II] and R-band fluxes are normalised arbitrarily. The horizontal scale is the semi-major axis or radius of the apertures used, in units of 0.33" pixels. The images show the R-band (central frame) and continuum-subtracted H-alpha+[N II] images (bottom frame), with superposed ellipses to show the apertures used for total line fluxes and R magnitudes. (Extracted from P. A. James et al., 2004, A&A, 414, 23) [ GIF ].

The astronomers studied the two major corrections which need to be applied to narrow-band H-alpha fluxes from galaxies in order to convert them to star-formation rates, i.e. [NII] contamination removal and correction for extinction internal to the galaxy in question.

From an imaging study using carefully-chosen narrow-band filters, they find that the [NII] emission is generally very differently distributed to the H-alpha emission, and that in particular nuclear measurements (e.g. from slit spectroscopy) can significantly over-estimate the contribution of [NII] to total narrow-band fluxes. In most star formation regions in galaxy disks, the [NII] fraction is small or negligible, and [NII] corrections applied in most previous studies may significantly under-estimate disk star formation rates as a result. These findings support the idea that heavily dust-embedded star formation, which would be underestimated using the H-alpha technique, is not a dominant contributor to the total star formation rate of most galaxies.

Supernovae of types Ib and Ic do appear to trace star formation activity, with a much higher fraction coming from the centres of bright star formation regions than is the case for the type II supernovae. Type Ia supernovae overall show a weak correlation with locations of current star formation, but there is evidence that a significant minority, up to about 40%, may be linked to the young stellar population. The radial distribution of all core-collapse supernovae (types Ib, Ic and II) closely follows that of the line emission and hence star formation in their host galaxies, apart from a central deficiency which is less marked for supernovae of types Ib and Ic than for those of type II. Core-collapse supernova rates overall are consistent with being proportional to galaxy total luminosities and star formation rates; however, within this total the type Ib and Ic supernovae show a moderate bias towards more luminous host galaxies, and type II supernovae a slight bias towards lower-luminosity hosts.

The total star formation rate density of the local Universe is found to be between 0.016 and 0.023 solar masses per year and Mpc3 with the uncertainties dominated by the internal extinction correction used in converting measured H-alpha fluxes to star formation rates. Almost 60% of the star formation rate density comes from galaxies of types Sb, Sbc or Sc; 9% from galaxies earlier than Sb and 33% from galaxies later than Sc. 75-80% of the total star formation in the local Universe is shown to be occurring in disk regions, defined as being >1 kpc from the centres of galaxies. Even though they are numerous, dwarf galaxies contribute little to the star formation in the local Universe.

Late type, bulge-free galaxies have a predominantly continuous mode of star formation, and could have assembled their stellar masses through continued star formation over a Hubble time with the currently-observed rate and spatial distribution. There is little evidence of predominantly isolated field galaxies of significant star formation through brief but intense starburst phases.

Upper image: UGC 4574 and satellite galaxy (image size 318"). Lower images: Satellite galaxy in R-band (left) and continuum-subtracted H-alpha (right: image sizes 53"×53"). (Extracted from P. A. James et al., 2008, A&A, 486, 131) [ GIF ].

This study has identified 9 probable star-forming satellite galaxies with projected separations consistent with their being as close to their central galaxies as the Magellanic Clouds are to the Milky Way. Overall, the satellite galaxies (including the Magellanic Clouds) are currently forming stars at a rate comparable to field Sm and Im galaxies in the HαGS sample. The only evidence of a strong starburst is in the tidally-disturbed companion to UGC 4541. Considering the 9 probable satellites and the Magellanic Clouds together, the LMC and SMC are the brightest and 7th brightest in R-band luminosity, and the 2nd and 9th most rapidly star-forming. Thus the LMC is clearly a large satellite, whereas the SMC is close to or just below average amongst those found here. The astronomers find no cases of 2 satellites around any of the 119 central galaxies studied, so the Milky Way appears well-favoured in the number of large star-forming companions in its immediate neighbourhood.

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Last modified: 13 December 2010