Shape matters in black hole growth January 31, 2010
Posted by sarah in: astronomy, new astronomy , trackback
Fig. 1 (from Schawinski et al., 2010)
Active galaxies have gone by many names: active galactic nuclei, quasars, QSOs, Seyfert galaxies, radio galaxies. Astronomers used to think these were all distinct types of objects, unified by the observation of large amounts of energy emerging from a compact region at the centre of the galaxy. These days, despite a great variety in observational characteristics, active galaxies’ engines are generally thought to be driven by a single mechanism, the accretion of material onto a supermassive central black hole.
In a paper published to the Arxiv last week, Kevin Schawinski and collaborators have used Galaxy Zoo classifications of local Universe galaxies to show that active elliptical galaxies are markedly different from those with a more disk-like or spiral shapes, adding morphology as an additional factor to consider in our model of active galaxies.
Over the last few decades scientists have gradually converged on the idea that all galaxies have a massive black hole in their centre, and that the AGN characteristics we see in some are symptomatic of material being accreted onto the black hole. This causes vast amounts of energy to be spewed out across the Universe at all wavelengths from radio to X-ray, making active galaxies far more luminous than their passive counterparts of the same age and size. In this picture, “activeness” is simply a phase in the evolution of regular galaxies, rather than a distinct type. For a good early overview of the background to this, see Richstone et al (1997); more recently, Heckman (2008) summarises our current understanding and future prospects.
The growth and evolution of galaxies and their black holes appears to be tightly linked – that may seem to be a logical statement, but in fact it’s rather odd: black holes are tiny compared to the size of an entire galaxy, and their masses just a small fraction of its mass; why should their evolutions be so tightly linked, and through what mechanism? This is a major question in the study of galaxy evolution today.
The Sloan Digital Sky Survey is arguably one of the most important observational datasets in the history of astronomy for the study of galaxy evolution: with images and spectra of around a million galaxies, it’s allowing astronomers to carry out science on a large sample of objects all obtained under the same conditions, rather than a handful at a time. As well as the science data, SDSS also led to the creation of the Galaxy Zoo project, where citizen scientists are invited to vote on the apparent shape characteristics of SDSS galaxies – a task traditionally tough to automate reliably (Kevin Schawinski was in fact the astronomer who started the project). As every galaxy is seen by many Zoo visitors, the spread of responses automatically gives an indication of the confidence level of the classification.
The Zoo classifications allowed Schawinski and his collaborators to compare for the first time “traditionally used” indicators of galactic activity and black hole properties, such as emission line strength in the spectra, velocity dispersion or colour, to the observed shape of galaxies – for a sample of 47,675 galaxies, of which they identified 942 as active. When they compared the numbers of active galaxies as a fraction of the total in the different morphology classes, the results look strikingly different for early-type or spheroidal and late-type or disk-like galaxies (see Fig. 1). A third class, the “indeterminate” galaxies, are those for which Zooites did not provide an 80% consensus on the shape. The plot’s x-axis shows stellar mass, the y-axis colour in increasing redness; the contours show the location of the full sample of galaxies. Comparing the top-right panel, the early types, to the bottom-right, the late types, it’s clear that the highest fraction of early-type AGN is concentrated at lower mass and bluer colour than in the late types. Those of indeterminate type appear to largely follow the late types.
The reason this wasn’t known before is that the absolute number of early-type AGN is so small: out of the 942 AGN identified in the sample, only 109 or 11.6% were identified by the Zooites as being early type, or spheroidal. Looking at the top-left panel of the figure, showing the AGN fraction for all morphologies, the distribution mirrors that of the late types and indeterminates, of which there are far more. So unless you explicitly separate out the morphologies, the effect just doesn’t stand out.
Comparing the numbers of active galaxies per morphology as a function of the mass of the black hole with those of the regular passive ones (rather than the galaxies’ stellar mass that is shown in the plot above), the authors find that in early types, black holes with masses at the low end of the scale are more likely to be accreting, whereas in late types it’s the high mass black holes that seem to accrete more.
There are many ifs, buts and maybes to this paper, as most galaxy properties are derived using observational proxies from the images or spectra. This is the usual way in these kind of studies, and every proxy carries its own assumptions and provisos. Disentangling selection effects and biases is often the hardest part of these large statistical studies. But as Schawinski points out, none of their selection effects are substantial enough to negate the result. There’s lots of discussion in the paper of what this result means to our understanding of how galaxies and their black holes evolve, but no obvious explanation appears to be at hand.
Why do there appear to be different pathways for black hole growth in spheroidal and disk galaxies? And given the tight correlations we see between the evolution of black holes and their host galaxies, what does it mean for our models of galaxy evolution? The answers to these questions aren’t clear, but this work clearly shows that morphology is an important factor to consider in future observational studies.
Kevin Schawinski, C. Megan Urry, Shanil Virani, Paolo Coppi, Steven P. Bamford, Ezequiel Treister, Chris J. Lintott, Marc Sarzi, William C. Keel, Sugata Kaviraj, Carolin N. Cardamone, Karen L. Masters, Nicholas P. Ross, Dan Andreescu, Phil Murray, Robert C. Nichol, M. Jordan Raddick, Anze Slosar, Alex S. Szalay, Daniel Thomas, & Jan Vandenberg (2010). Galaxy Zoo: The fundamentally different co-evolution of supermassive black holes and their early- and late-type host galaxies accepted to ApJ arXiv: 1001.3141v1
D. Richstone, E. A. Ajhar, R. Bender, G. Bower, A. Dressler, S. M. Faber, A. V. Filippenko, K. Gebhardt, R. Green, L. C. Ho, J. Kormendy, T. Lauer, J. Magorrian, & S. Tremaine (1998). Supermassive Black Holes and the Evolution of Galaxies Nature 395 (1998) A14-A19 arXiv: astro-ph/9810378v1
T. M. Heckman (2008). The Co-Evolution of Galaxies and Black Holes: Current Status and Future Prospects Invited contribution to “Astrophysics in the Next Decade: JWST and Concurrent Facilities”, Astrophysics & Space Science Library, Eds. H. Thronson, A. Tielens, M. Stiavelli, Springer: Dordrecht (2008) arXiv: 0809.1101v1
Comments»
Good morning, Sarah. Thank you for great job of explaining the graph, and it’s implications. I’d been lost trying to decipher it myself.
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