Galaxy Formation

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A image of M81 and M82, showing regions of active star formation.

Galaxy formation is very inefficient: only a small fraction of the baryons in the universe have been converted into stars. What mechanism or mechanisms prevents gas from cooling and forming stars?

The interacting M51 system.

The globally averaged properties of galaxies change with time. Is this due to a gradual change in the efficiency of star formation in all galaxies? Or does it involve sudden, drastic transformations of certain types of galaxies, in particular environments? We expect that galaxy mergers and close-interactions should be common at early times, and these could produce such transformations, under the right conditions.

Selected Publications

Part of a image of one of the distant groups (courtesy R. Henderson).	Balogh, M.L., Wilman, D., Henderson, R.D.E., Bower, R.G., Gilbank, D.G., Whitaker, R.J., Morris, S.L., Hau, G., Mulchaey, J.S., Oemler Jr., A., & Carlberg, R.G. The stellar mass content of distant galaxy groups 2006, MNRAS, in press, astro-ph/0610839 In this paper, we combine spectroscopy from the and telescopes, with near-infrared data from and the to compute the stellar fraction of distant galaxy groups. The lookback time to these groups is about 4.5 billion years, a third of the age of the Universe. However, we find that they have already formed most of their stars. This is somewhat surprising, as we had expected that these distant groups would be ideal environments to promote active star formation. It implies that there has been little environment-driven evolution in the past few billion years. (More results from this project can be found here).
Fig. 2 of Swinbank et al.: An image of our IFU target	Balogh, M., Miller, C., Nichol, R., Zabludoff, A. & Goto, T. Near-infrared images of 222 nearby, Hdelta-strong galaxies from the SDSS 2005, MNRAS, 360, 587, astro-ph/0503639 Swinbank, M., Balogh, M.L., Bower, R., Hau, G., Allington-Smith, J., Nichol, R. & Miller, C. GMOS integral field spectroscopy of a merging system with enhanced Balmer absorption, March 2005, ApJ, 622, 260 A good candidate for galaxies in the process of transformation are those known as "E+A" or "K+A" galaxies. Their spectra strongly suggest there has been a recent, sudden change in their star formation rate. In the first paper we study the near-infrared properties of over 200 of these galaxies, selected from the . We find that, in the nearby Universe, this unusual spectrum most likely results from a minor merger of a small, gas-rich galaxy with a larger, elliptical galaxy. Importantly, we can rule out the possibility that nearby E+A galaxies result from the truncation of star formation in a spiral galaxy. In the second paper we use the integral field unit on to study the spatial distribution of young stars in one of these galaxies.
Figure 1	Balogh, M.L., Baldry, I., Nichol, R., Miller, C., Bower, R. & Glazebrook K. The bimodal galaxy colour distribution: dependence on luminosity and environment Nov 2004, ApJ Letters, 615, L101 ADS abstract We analyse the colour distribution of galaxies in the as a function of their environment. Using a Gaussian--fitting model, we can separate the blue and red galaxy population, and we find that the average colour of each of these populations is only a weak function of environment. This means that the properties of a galaxy do not change gradually as they move into denser environments. On the other hand, the fraction of red galaxies increases strongly with increasing density. Either the transition from blue to red colours occurs very quickly, or preferentially at high redshift (so we do not observe the galaxies in transition today). The fitting parameters for the colour distribution as a function of environment and luminosity can be found here.
	Balogh, M.L., Christlein, D., Zabludoff, A. & Zaritsky, D. The Environmental Dependence of the Infrared Luminosity and Stellar Mass Functions 2001, ApJ, 557, 117 We use redshifts from the and near-infrared photometry from the to compute the infrared galaxy luminosity function in different environments, from the field to massive clusters. We use spectral information (the 4000 A break) to convert the luminosity functions to mass functions. We find that the mass function of cluster galaxies without emission lines is very similar to that of field galaxies with emission lines. This is consistent with a picture in which the bulk of the cluster population is made up of field galaxies in which star formation has ceased.
	Balogh, M.L., Pearce, F.R., Bower, R. G. & Kay, S. Revisiting the Cosmic Cooling Crisis 2001, MNRAS, 326, 1228 PDF Preprint (See this NOTE) From the stellar mass function of Cole et al. (2000) we compute the fraction of baryons in the universe which have cooled, and find it to be very low, less than about 5%. There is no good evidence that this fraction is higher in clusters or groups, though the uncertainty is larger. We compare this with recent numerical simulations which include cooling and show that this limit is easily and greatly exceeded unless an effective feedback scheme is implemented. We discuss the difficulty of matching this constraint simultaneously with the galaxy mass function.