Coevolution

Published projects:

• The non-causal origin of the black hole-galaxy scaling relations (06/2010)
• A near-IR study of the host galaxies of 2Jy radio sources (05/2010)
• HE0450-2958: A quasar, a ULIRG, a merger, galaxy formation live and in color! (11/2009)
• The evolution of the M_BH-M_stellar-relation over the last 9 Gyrs (09/2009)

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The non-causal origin of the black hole-galaxy scaling relations

K. Jahnke & A. Maccio, submitted to ApJ Letters

Most, if not all massive galaxies contain a supermassive black hole in their centers. The black holes, who can have masse of a million or even more than a billion times the mass of our sun, have been suspected since a decade to be involved in the late-stage evolution of their host galaxies. How so?

If gas comes near the black hole it will be subject to very much friction and will heat up and emit large amounts of electromagnetic radiation before it falls into the black hole. This light from the then active galactic nucleus (AGN) contains over time more energy than the gravitational binding energy of the gas and stars in the surrounding galaxy. It was suspected and observed that some of that energy coupled to the gas in the galaxy and heated it up, preventing further star formations. This process was christened "AGN feedback".

Around 1998 it was recognized that not only do black holes exist in every galaxy, but the masses of the black holes scaled very tightly with the mass of the host galaxies bulge, and the velocity dispersion of its stars. These scaling relations were generally interpreted as a coupled evolution of black hole and stellar bulge - despite the black hole being smaller in linear size by up to 9 orders of magnitude. The main idea was that when galaxies were colliding, not only their components coalesced, but also that gas was funneled down to the black hole, causing growth, but also AGN feedback. Hence changes in the black hole mass would immediately imply changes in the galaxy.

We have now submitted an analysis, in which we test whether one actually needs a driving physical mechanism behind the formation of the observed scaling relations. It was suggested in a thought experiment that the merger cascade of galaxies alone should have a "central limit theorem"-like effect, meaning that any initial scatter in the relation between black hole and bulge mass would be averaged out with more and more successive galaxy mergers, while at the same time a mass function of galaxies was built up. We now tested this idea with a realistic set of "merger trees", i.e. we followed simulations of an initial 6.5 million galaxies from the early Universe to today, when they have successively merged into a final ~10.000 galaxies.

The results show that it is really the merger cascade itself that produces the scaling relations. Any of the star formation and black hole growth mechanisms that can be thought of might modify these relations, but they are not their cause.

This has implications for the importance of the proposed AGN feedback: While some incarnations of it are seen at work, it is not known how important it is on the global scale. Our results now take away one aspect where it was often thought to be required. There are other issues in galaxy formation theory (color-magnitude distribution of galaxies, top end of the mass function) which continue to require extra mechanisms beyond simple merging, but as we show, these are independent of the creation of the black hole-galaxy scaling relations.

kj, 04.06.2010

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A near-IR study of the host galaxies of 2Jy radio sources

K. Inskip, et al. 2010, MNRAS, in press

The 2Jy sample of southern radio sources at z<0.5 is one of the best-studied radio galaxy samples in existence; the wealth of imaging and spectroscopic data existing for this sample over a wide range of wavelengths make it ideally suited for addressing a number of different science goals. In this work, we apply GALFIT to near-IR imaging data in order to study the morphological characteristics of the host galaxies of powerful radio galaxies at (relatively) low redshifts. This study is part of a larger investigation into the links between galaxy growth and radio source/AGN activity, and in particular the nature of radio source triggering.

We find that while the majority of sources are hosted by apparently passively evolving massive elliptical galaxies well modelled by a Sérsic profile with an index of n=4, with scale sizes of the order of 10kpc, morphological fits to approximately one third of the sample require either a Sérsic index of n=1-2, or the presence of a disk-like component in addition to the main galactic bulge. Roughly 50% of the sample are either disturbed systems or clearly undergoing interactions with companion objects.

Subsequent papers in this series (in preparation) address the links between morphological status and the recent and ongoing star formation activity in these sources

[WIP - more text and figures to be added]

ki, 18.05.2010

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HE0450-2958: A quasar, a ULIRG, a merger - galaxy formation live and in color! Is it also a "Black Hole Caught Zapping Galaxy into Existence?"

D. Elbaz, K. Jahnke, et al. 2009, A&A, 507, 1359 and
K. Jahnke, D. Elbaz, et al. 2009, ApJ, 700, 1820

30. Nov. 2009: ESO press release 46/09 (english|deutsch)

Media exposure: Science | El Pais | Daily Mail | Telegraph | Xinhua | El Mundo | Wired | msnbc | DigitalJournal | Universe Today | physicsworld.com | Science News | Spiegel Online | DRadio

HE0450-2958 is a comparably nearby quasar at a redshift of z=0.186. It's part of a peculiar and strange system, in interaction with a companion galaxy only 6.5kpc away. In earlier studies it was speculated whether HE0450-2958 had an unusually low-mass host galaxy - up to now, no host galaxy was detected around the quasar. One of the possible explanations is a screen of dust that could absorb most of the host galaxy's emission, hiding it from our view.

For this reason we used new imaging data in the infrared, that both penetrates dust, but also allows to detect warm dust by its characteristic radiation, to test the nature of this system. We acquired new images with the Hubble Space Telescope and its NICMOS near-infrared camera at a wavelength of 1.6mu as well as a mid-infrared image at 11.3mu from the ESO Very Large Telescope and the VISIR instrument. By combining these with existing data in the optical, radio and millimeter wavelengths we looked for and behind a potential dust screen

While our observational limits with NICMOS still did not allow to see the bulk mass of the host galaxy, we found that while no large amounts of obscuring dust are present arount the quasar, the derived upper limit are in agreement with predictions made from estimates of its central black hole mass. Furthermore, we identified a potential extension, slightly removed from the quasar center that is likely a part of the host galaxy, possibly a remnant from a violent interaction with the companion galaxy. This black hole-galaxy system in this respect is quite normal.

More importantly our new infrared data allowed to characterize the dust distribution also in the companion galaxy. We found a layer of optically thick dust is obscuring the view into the heart of the companion, which is producing stars at a frantic rate of ~340 solar masses per year - 100 times more than ususal. The companion is a star-formation powered Ultraluminous Infrared Galaxy (ULIRG) while our 11.3mu image showed that the quasar nucleus itself emits similar amounts of infrared radiation, put powered by the quasar nucleus. This system shows impressively that black hole growth and star-formation do necessarily occur in the same geometric location.

As an interpretation of a strong radio jet emitted by the quasar towards the companion galaxy, we speculate whether some or a substantial amount of the star-formation in the companion could in fact we triggered by the jet. Since quasar host and companion galaxy with enavitably merge into a single system in the future, the quasar would in this case stimulate the build-up of its own host galaxy. This is a route of "quasar feedback" that has not yet been widely explored in theoretical works. If it is effective, it could play an important role in galaxy formation in the earlier Universe.

kj, 25.11.2009

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The evolution of the M_BH-M_stellar-relation over the last 9 Gyrs: Combining COSMOS HST/ACS and HST/NICMOS imaging

K. Jahnke, M. Cisternas, K. Inskip, et al. 2009, ApJL, 706, 215

We investigate the masses of the host galaxies of quasars at redshift around z=1.4, and relate them to the masses of their central black holes. Using a sample of 10 quasars from the COSMOS survey (of M_stellar~10^11.2 M_sun and M_BH~10^8.2 M_sun) we find that the black hole mass follows the same linear relation with the total stellar mass of the host galaxies at z=1.4 as we find for the bulge stellar mass in the local Universe.

An interpretation of this depends on whether the host galaxies are very much pure spheroids or if whether they have substantial disk components. In the former case our results mean that over the last 9Gyrs neither bulge and black hole did not grow with respect to each other. In the latter case we conclude that we see bulge growth from redistribution of preexisting disk stars through galaxy mergers or disk instabilities.

It is necessary in the future to test the second interpretation at different redshifts and stellar masses, since this has implications for the concept of "AGN feedback". In this case the existence of a tight relation between BH and bulge mass at z=0 is not due to a direct regulation loop including quasar activity that drives out gas from the galaxy. Instead, the connection can be much looser and possibly be due to the statistics of galaxy merging and disk-to-bulge conversion in this process.

kj, 06.11.2009