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Register a new userThe evolving starburst-AGN connection: Implications for SKA and its pathfinders
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How well is the modern-day starburst-AGN connection mirrored in the early Universe? This is starting to be answered by deep wide radio surveys such as ATLAS, which are giving us a new view of high redshift galaxies. For example, we find powerful radio-loud AGNs which look like star-forming spirals in the optical and infrared, a composite which is almost unknown in the modern Universe. We find radio-bright objects which are unexpectedly invisible in the infrared, and which may be very high redshift radio galaxies and quasars. And although the radio-far-infrared correlation for star-forming galaxies has now been extended down to microJy levels, we still cannot reliably distinguish between starburst and AGN. So what do we need to do to ensure that SKA and its pathfinders will be able to understand galaxy evolution in the early Universe?
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Observations at ultraviolet, optical and near-infrared wavelengths have shown the existence of recent star formation in the nuclear regions of Seyfert 2 (Sy2) galaxies that suggest a connection between the Starburst and the Seyfert phenomenon. According with the standard unified models of AGN circumnuclear starbursts also have to be present (and in the same numbers) in Sy1 as in Sy2 galaxies. This review discuss evidence in favor of the Starburst-AGN connection, as well as possible differences in terms of star formation activity between Sy1 and Sy2, that suggest an alternative interpretation of the Seyfert classification to that proposed by the standard unification model.
The mass of super massive black holes at the centre of galaxies is tightly correlated with the mass of the galaxy bulges which host them. This observed correlation implies a mechanism of joint growth, but the precise physical processes responsible are a matter of some debate. Here we report on the growth of black holes in 400 local galactic bulges which have experienced a strong burst of star formation in the past 600Myr. The black holes in our sample have typical masses of 10^6.5-10^7.5 solar masses, and the active nuclei have bolometric luminosities of order 10^42-10^44erg/s. We combine stellar continuum indices with H-alpha luminosities to measure a decay timescale of ~300Myr for the decline in star formation after a starburst. During the first 600Myr after a starburst, the black holes in our sample increase their mass by on-average 5% and the total mass of stars formed is about 1000 times the total mass accreted onto the black hole. This ratio is similar to the ratio of stellar to black hole mass observed in present-day bulges. We find that the average rate of accretion of matter onto the black hole rises steeply roughly 250Myr after the onset of the starburst. We show that our results are consistent with a simple model in which 0.5% of the mass lost by intermediate mass stars in the bulge is accreted by the black hole, but with a suppression in the efficiency of black hole growth at early times plausibly caused by supernova feedback, which is stronger at earlier times. We suggest this picture may be more generally applicable to black hole growth, and could help explain the strong correlation between bulge and black hole mass.
The merger of two spiral galaxies is believed to be one of the main channels for the production of elliptical and early-type galaxies. In the process, the system becomes an (ultra) luminous infrared galaxy, or (U)LIRG, that morphs to a quasar, to a K+A galaxy, and finally to an early-type galaxy. The time scales for this metamorphosis are only loosely constrained by observations. In particular, the K+A phase should follow immediately after the QSO phase during which the dust and gas remaining from the (U)LIRG phase are expelled by the AGN. An intermediate class of QSOs with K+A spectral signatures, the post-starburst QSOs or PSQ, may represent the transitional phase between QSOs and K+As. We have compiled a sample of 72 {bona fide} $z<0.5$ PSQ from the SDSS DR7 QSO catalogue. We find the intermediate age populations in this sample to be on average significantly weaker and metal poorer than their putative descendants, the K+A galaxies. The typical spectral energy distribution of PSQ is well fitted by three components: starlight; an obscured power-law; and a hot dust component required to reproduce the mid-IR fluxes. From the slope and bolometric luminosity of the power-law component we estimate typical masses and accretion rates of the AGN, but we find little evidence of powerful radio-loud or strong X-ray emitters in our sample. This may indicate that the power-law component originates in a nuclear starburst rather than in an AGN, as expected if the bulk of their young stars are still being formed, or that the AGN is still heavily enshrouded in dust and gas. We find that both alternatives are problematic and that more and better optical, X-ray, and mm-wave observations are needed to elucidate the evolutionary history of PSQ.
We stress the importance of Wolf-Rayet stars for the understanding of the AGN phenomenon in galaxies. WR stars provide an unique opportunity to explore from the ground whether non-thermal nuclear activity and circumnuclear starbursts are connected. We review the known reported WR signatures observed so far in AGNs and point out some intrincacies related to the analysis of the spectra, linked to reddening correction, the origin of the Hbeta line, etc. Finally, we advocate that integral field spectroscopy is a very promising tool to study this problem and present preliminary results of a long-term project that have been obtained at the CFHT in 1998.
We present the results of X-ray imaging and spectroscopic analysis of a sample of Seyfert 2 galaxies that contain starbursts, based on their optical and UV characteristics. These composite galaxies exhibit extended, soft, thermal X-ray emission, which we attribute to their starburst components. Comparing their X-ray and far-infrared properties with ordinary Seyfert and starburst galaxies, we identify the spectral characteristics of their various intrinsic emission sources. The observed far-infrared emission of the composite galaxies may be associated almost exclusively with star formation, rather than the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared and [O III] 5007 luminosity distinguishes most of these composite galaxies from ``pure Seyfert 2 galaxies, while their total observed hard X-ray luminosity distinguishes them from ``pure starbursts. The hard nuclear X-ray source is generally heavily absorbed (N_H > 10^{23} cm^{-2}) in the composite galaxies. Based on these results, we suggest that the interstellar medium of the nuclear starburst is a significant source of absorption. The majority of the sample are located in groups or are interacting with other galaxies, which may trigger the starburst or allow rapid mass infall to the central black hole, or both. We conclude that starbursts are energetically important in a significant fraction of active galaxies, and starbursts and active galactic nuclei may be part of a common evolutionary sequence.
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