Supermassive black holes and nuclear stellar disks, nuclear keys to galaxy formation and evolution

Lead Research Organisation: University of Hertfordshire
Department Name: Science and Technology RI


Like the pieces of a Cosmic Jigsaw, galaxies come in different shapes and colours. Understanding how galaxies evolved their present-day properties is one of the most important and longstanding issues in astronomy. Among the scientists in this field, common wisdom has it that galaxies formed from a combination of star-formation episodes, during which gas turns into young blue stars, and merging events, whereby galaxies collide and reassemble into different galactic configurations. If galaxies formed all at once, they would only become redder as their stellar population ages, without changing their shape. Whereas the star-formation history of galaxies has been measured by many observations, the pace at which galaxies merge through cosmic times remains largely unknown. In this respect, astronomers are left only with the predictions of their models. According to the widely accepted model of a Universe dominated by cold dark matter, because galaxies grow in mass over time by merging together, the largest galaxies have assembled just recently, whereas the smallest galaxies were already in place early on in the history of the Universe. Furthermore, galaxies in groups or clusters should have assembled before their counterparts in less densely populated environments. Galaxies can indeed find it difficult to merge once they crowd together. This is because they tend to swing by each other too fast for them to coalesce. As an Advanced Fellow I will test these theoretical predictions. For this investigation I will adopt as a record of the time when galaxies merged together the age of the stars in the small disks that are often found in the centre of galaxies. Numerical simulations show that such nuclear disks are easily destroyed when galaxies collides, so that their presence in a given galaxy indicates that no such a catastrophic event occurred since the disk formed. If the cold dark matter model is correct, I should find that nuclear disks are made of older stars in smaller galaxies, or in those living in the most crowded galactic environments. By the turn of the century, another puzzling component of galaxies turned out to have a key role in galaxy formation and evolution: Supermassive Black Holes. Once considered by astronomers only as a theoretical possibility, supermassive black holes have now been found to lurk at the center of many galaxies, including our Milky Way. Most importantly, the mass of the central black holes appears to scale with the mass of their host galaxies, or only to that of the bulge component of disk galaxies. Roughly speaking, central black holes have been found to be almost one thousand times less massive than their host galaxy or bulge. This striking coincidence between objects that are vastly different in size, with supermassive black holes extending at most to the size of our Solar system, strongly suggests that the evolution of supermassive black holes and galaxies is intimately related. The universality of this relationship, however, is not yet established. This is because the mass of supermassive black holes has been measured only in a limited number of galaxies, and almost exclusively in certain kinds. It would be surprising if galaxies that formed in different ways were found to share the same black-hole mass content. As an Advanced Fellow, I will complete the demography of supermassive black holes using large ground-based telescopes that can correct for the atmospheric blurring with the aid of bright reference stars or point sources created in the upper atmosphere by powerful lasers. This will allow me to measure the mass of supermassive black holes as if I were using the Hubble space telescope, and to observe precisely those galaxies for which there are very few black-hole mass measurements. Using these new data I will test the universality of the relationship between supermassive black holes and galaxies, and shed more light on the nature of this link.


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Baes M (2014) An extremely low gas-to-dust ratio in the dust-lane lenticular galaxy NGC 5485? in Monthly Notices of the Royal Astronomical Society: Letters

Description There were several key findings from this grant. On Nuclear Stellar disks I eventually published papers demonstrating both their fragility and that their stellar age can be measured with high precision (Sarzi et al. 2015, 2016b). This allows therefore to use them as robust clocks for the assembly history of their host galaxies. During my fellowship I also explored further the nature of the ionised- and hot-gas in early type galaxies. In the first case I found that hot UV-bright stars are most likely responsible for ionising their case (Sarzi et al. 2010), whereas in the latter I found that the hot-gas content of early-type galaxies correlates with the degree of rotational support of these systems (Sarzi et al. 2013), in the sense that fast-rotating galaxies are less capable of retaining their hot gas. Related to the ionised-gas emission of early-type galaxies, I have also investigated the presence of the prime stellar candidates for powering such a nebular emission, and that is post-Asymptotic Red Giant branch (pAGB) stars, which are also known as the central stars of Planetary Nebulae (PNe). In this respect, in Sarzi et al. (2011) I have demonstrated for the first time that Integral-Field Spectroscopic data allow to detect PNe in the optical region of early-type galaxies - where the ionised-gas emission is observed. Furthermore, in Pastorello, Sarzi et al. (2013) I have found that the brightest PNe could be notably absent in metal-rich environment such as galactic nuclei and the most massive early-type galaxies, which in turn may go back to pose some problem for powering the nebular emission of early-type galaxies with pAGB stars.
Exploitation Route I would hope that my results on Nuclear Stellar Disk (NSD) would set the basis for using these common sub-components in galaxies to help constraining more directly the assembly history of galaxies. Similarly, my work on PNes should provide some more constraints on our understanding of the late stages of stellar evolution, in particular in galactic environments unlike that of the Milky Way. Most recently the scietific justification for the part of this project concenrning NSDs has led to the award of an Erasmus+ student fellowship that allowed an MSc student (Andrea Incatasciato, working under the co-supervision of my colleague Massimo Dotti at Milano Bicocca) to work for six months on tailoring our numerical assessement for the fragility of NSDs at specific galaxies with state-of-the-art MUSE integral-field spectroscopic data. The results of this work should be published within the next year.
Sectors Education,Other

Description STFC Public Engagement Fellowship
Amount £120,000 (GBP)
Organisation Science and Technologies Facilities Council (STFC) 
Sector Academic/University
Country United Kingdom
Start 09/2014 
End 10/2016
Title The GandALF spectral fitting code 
Description During my award I had the time to perfect my GandALF fitting code, which I released in 2007 (before my award) and which is now fairly commonly used to analyse the optical spectra of galaxies. More specifically GandALF (Gas and Absorption Line Fitting) allows to simultaneously describe the nebular and stellar parts of the optical spectra of external galaxies. 
Type Of Material Data analysis technique 
Year Produced 2007 
Provided To Others? Yes  
Impact The code has now be used in over 100 refereed publications. A list of these publications (to May 2014) can be found at this link 
Description Yonsei collaboration 
Organisation Yonsei University
Country Korea, Republic of 
Sector Academic/University 
PI Contribution This is a collaboration with the group of Prof. Sukyoung Yi, at Yonsei University in Seoul. Much of my travel support from my STFC fellowship has been used to foster this collaboration, which most notably use the SDSS spectroscopic database to extract with my code "Gandalf" the basic properties of more than 600,000 galaxies within a redshift of 0.2.
Impact At the moment, the most important output of this work, a complete catalogue of spectral measurements based on my "Gandalf" code for the entire SDSS-DR7 spectroscopic release, is being reviewed for publication on ApJ. We expect this paper to be published by 2011.
Start Year 2009
Description Invited Speaker at the 2013 Norvegian Astronomy Society conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Type Of Presentation Keynote/Invited Speaker
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Similar to the London Astrofest, I was invited to give a talk on the subject of Supermassive Black Hole, in Tromso, Norway

A talk
Year(s) Of Engagement Activity 2013
Description Invited Speaker at the London Astrofest 2012 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Type Of Presentation Keynote/Invited Speaker
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact I was a plenary speaker at the London Astrofest, which is the biggest conference for Astronomy and Space Science enthusiasts, which takes place every year in Kensington, London.

I gave a talk on the subject of Supermassive Black Holes
Year(s) Of Engagement Activity 2012
Description Main Organiser of the Open Night at the Bayfordbury Observatory (since 2009) 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Since October 2009, I am the main organiser of the Open Night events that the Centre for Astrophysics Research organises at the Bayfordbury Observatory of the University of Hertfordshire.
During such event, our guest can visit our optical and radio telescope, enjoy our Planetarium and partecipate in a number of lab and computer activities.

Over the year, I have expanded the scope of these events, which are now routinely fully booked and see around 400 visitors per open night. With 5-6 events per year, this means that we reach more than 2000 visitors per year.
Year(s) Of Engagement Activity 2009,2010,2011,2012,2013
Description Radio Interviews on BBC Three Counties Radio (2013, 14) 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact On January 9, 2013 I was briefly interviewed by Roberto Perrone on BBC Three Counties Radio to describe the program of our Bayfordbury Open Night on that same day. Such an Open Night was indeed part of the BBC Stargazing activities that were promoted by the BBC during the months of January and February 2013.

On March 24, 2014 I was briefly interviewed by Roberto Perrone on BBC Three Counties Radio to describe the program of our Bayfordbury Open Night, and in particular to talk about our special Art & Astronomy event, on the previous friday the 21st.

This interviews certainly generated a great deal of publicity for our Open Night program at Bayfordbury, which are usually fully booked.
Year(s) Of Engagement Activity 2013,2014