Much of the intense star-formation (SF) and many supermassive black holes (SMBH) are heavily obscured by dense regions of dusty gas. For example, even normal SF zones are obscured in the FUV and the most intense areas can be totally obscured to any direct light from stellar sources or associated HII regions. While such zones are the birthplace of key astrophysical objects ranging from AGN to compact massive star clusters, it is difficult to probe their internal structures, a problem exacerbated by their small spatial scales of often <10 pc. Furthermore the study of compact star clusters and HII regions demonstrates that dense modes of SF can be found throughout the disks of galaxies. Thus while these types of processes are key evolutionary drivers, they are accessible in suitable spatial detail only within nearby samples of galaxies.
Radio emission offers a crucial means to understand dense SF regions and their relationships to AGN. Radio observations provides a direct view of dense SF regions and radio luminosity is an excellent star-formation rate (SFR) indicator. For the majority of supermassive black holes (SMBHs) in the nuclei of galaxies, accretion rates are low and radiation is produced inefficiently (Luminosity proportional to accretion rate), resulting in undetectably low optical through X-ray luminosities. However such SMBHs produce jets with easily detectable radio emission, providing a measure of their accretion rate.
All giant galaxies are hybrid objects having, to a greater or lesser extent, emission from SF regions and from accretion onto a SMBH. The strong correlation between the mass of the SMBH and the galaxy bulge mass (Magorrian et al 1998) indicates a strong correlation, averaged over the age of the universe, between SFR and black hole growth. However whether the two processes directly correlate, or whether one lags the other, or whether the galaxy type (spiral/elliptical, LINER/AGN/absorption line) affects the interaction is unknown. Does AGN feedback turn off SF in all galaxies, or only in those with very luminous AGN? Does it matter if the feedback energy comes in the form of a fast, highly collimated, jet or a less collimated Seyfert-type outflow? Is high SF activity required to fuel AGN accretion and are galactic bars efficient in terms of bringing fuel to SMBH and nuclear SF regions?
Better measures of the astrophysics of AGN-SF interactions in the local universe are essential for understanding our observations of hybrid objects at high redshifts. In this survey we will measure SFRs and accretion rates in a complete sample of galaxies of wide morphological and spectral type. Hence we will determine how the local galactic environment affects SFR and AGN activity and, in particular, the crucial interaction between those two fundamental processes. For example, what mechanism produces powerful jets and how do they relate to their surroundings?
This survey will also address many important specific points. For example, what mechanism produces powerful jets? Spin? There are hints from Balmaverde and Capetti (2006) that low luminosity AGN (LLAGN) are more radio loud if their galaxy bulges have a flat `core' type light profile. Such a profile could be produced in a major merger, which might also give rise to large black hole spin. Our survey will greatly improve on the earlier observations and should give a definite answer. Also, is there a real physical difference between type 1 (broad line) and type 2 (narrow line) AGN or are observed differences purely a function of orientation-dependent obscuration? There are suggestions (Stevens et al 2005; Strong et al 2004) of higher SF and molecular gas content around type 2 AGN, implying a larger obscuring torus. Our observations will definitively measure SF products in galaxies and will probe the neutral and molecular gas which fuels both SF and accretion.
Although observations of the distant universe can tell us about the average evolution of SF and accretion rate with cosmic time, they do not have the spatial resolution to unravel the interaction between SFR and accretion in individual galaxies. Observations of our own Galaxy tell us about SF in individual molecular clouds and about accretion onto solar-mass sized compact objects but reveal nothing about how SFR or accretion onto SMBHs varies in galaxies of different mass or morphology. Only by observations of a large sample of nearby galaxies can we sample a wide enough range of galactic properties, with high enough angular resolution to answer the questions raised above. Here, by way of its unique combination of high resolution and high sensitivity, e-MERLIN has a vital role to play.
The critical importance of e-MERLIN observations: Due to its high sensitivity and high angular resolution e-MERLIN is, and will remain for the foreseeable future, the best radio facility in the world for the study of SF and accretion and is perfectly matched to this programme. Our sample (described below) contains galaxies from ~3 to 100Mpc. Even at the median distance of our more distant, shallower, sub-sample (20Mpc), a linear resolution of <4pc will be achieved at C-band, easily resolving SNR and HII regions, and equivalent to sub-torus scales within the nuclei of nearby AGN. In our deeper subsample with a 5-sigma detection limit of 14microJy/bm we will detect sources down to ~10^16 W/Hz at C-band, tracing far down into the radio luminosity function of both accretion dominated and SF related sources. Moreover the field of view, will be >~3 arcmin for all of our observations, adequately covering all of the optical extent of the large majority of galaxies in the sample. The EVLA simply does not have the resolution, and VLBI does not have adequate sensitivity to extended structure, in the 1 to 7 GHz frequency range where synchrotron and free-free radio emission on physically important size scales within nearby galaxies is dominant.
In order to separately understand accretion and SF, we must be able to discriminate between signitures of the two. Radio morphology, particularly when, as here, coupled with spectral index information, is the single most important discriminator, and is especially strong in nearby galaxies where parsec-scale resolution is available. However no single diagnostic can provide unambiguous discrimination and other important tools such as radio/mid-IR ratio, near IR colours, X-ray luminosity and spectrum and optical spectral type are crucial. Taken together, these various diagnostics provide excellent discrimination (e.g. see Seymour et al 2008; Smolic et al 2008). Our sample has been designed to maximise the availability of these cross-wavelength discriminators.
Specific key science aims: Whilst the breadth and range of science that will come from these data will be extremely extensive, as is required from any legacy project, the key scientific aims of this project can be distilled into a few primary objectives:-
- To measure levels of SF activity in galaxies of all morphological, luminosity and spectral population mix. These measurements, and in particular the resulting systematic and unbiased census of individual SF products (RSNe, SNR, HII regions and alike), will be used as a direct extinction-free tracer of SF, and hence be used to help calibrate commonly used SFR indicators such as IR and Halpha.
- A complete census of AGN activity and jet structures in galaxies of all optical types, including LINERs and absorption line galaxies as well as broad line AGN, which will be cross-correlated against levels of ongoing SF.
- A serendipitous parsec-scale imaging survey of the cold ISM using HI absorption and maser emission. This survey will constrain the content and composition of cold gas present in the immediate nuclear region of galaxies as well as its kinematics. Both of which are basically unknown on these size scales and can be addressed with e-MERLIN well before ALMA or the SKA are fully operational.