Marcello Giroletti - Research

The early steps of my research activity were taken for the Master Thesis, which was based on VLBI and VSOP (Space VLBI) observations of the BL Lacertae objects and TeV emitters Markarian 421 and Markarian 501. This job has raised an interest for the physical properties of jets on the smallest observable scales and the actual viability of the unification of Active Galactic Nuclei (AGN). In the meanwhile, I undertook my first international collaboration, thanks to a summer studentship at the Joint Institute for VLBI in Europe: under the supervision of Dr. Mike Garrett, I considered deep radio observations of the Hubble Deep Field taken with the Westerbork Radio Synthesis Telescope. I produced a catalogue of radio sources, complete down to 45 µJy, with optical and far infrared identifications, and discussed the properties of the optical matches, which are typically starburst galaxies (see publication A1).

Following the completion of my Master Thesis, I entered the Ph.D. studies in Astronomy at the University of Bologna. In the three years as a Ph.D. student, I devoted myself mainly to the unification of radio loud AGN, both from an observational and interpretative point of view. In particular, I studied the parsec scale properties of compact radio sources of both high and low radio power. These are some of the main results:

i) physical properties of relativistic jets - The analysis of very high resolution, multi-epoch, and multi-frequency data reveals a dual velocity structure in the jet of the BL Lac Markarian 501, with a fast relativistic spine and a slower external shear. A model has been developed for the velocity and orientation of the jet, on the basis of components motion, adiabatic expansion (with parallel or perpendicular magnetic field), parsec scale and integrated spectral index study (see A3, B1, B2, B3, B4). Besides the properties of this peculiar source, the thesis focuses on a large sample of low redshift BL Lacs. This study extends to this population the unification scenario based on orientation, showing evidence for relativistic beaming and small viewing angle, requiring a parent population composed of FR I radio galaxies. Such evidence comes from arguments including: comparison of nuclear (pc) and extended (kpc) scale properties, study of superluminal motions, spectral indices, brightness temperatures, synchrotron models (self-absorption, SSC). The properties of Hubble Space Telescope have also been exploited to estimate the mass of the central black hole and to discuss the accretion mechanisms in these objects (A4, B6, B7, B8, B9, B10).

ii) evolutionary properties of radio sources - A natural and necessary extension of the unified models involves the time evolution from compact objects to classical extended radio sources. My contribution to this field consists of the measurement of the velocity advance of the hot spots of a large number of Compact Symmetric Objects. Since these motions are of order of a fraction of milliarcsecond, this study requires very high angular resolution observations. The analysis of multi-epoch VLBI data has significantly increased the number of objects with a dynamic estimate of the age. In the case of 4C 31.04, this age is remarkably low (about 500 years), even in comparison with a radiative estimate of the age, which is based on the high frequency steepening of the radio spectrum due to the radiative losses of the most energetic particles. The results in this area are of great importance for the formulation of evolutionary models of radio sources and the understanding of the mechanisms at the basis of the birth and diversification of extended radio galaxies (A2, A6, B2, B3, B4, B5, B12).

iii) low power sources - At the low end of the radio power range typical for radio loud objects, we find a large population of objects (Low Luminosity AGN, LINERs and Seyfert-like nuclear regions) whose properties are not well-known. In this area, we highlight the VLBI study of NGC 4278, which has revealed strong morphological and spectral similarities with more powerful radio sources. More observations of low power compact radio sources are useful to determine rates and peculiarities of the radio emission in this cores. It is interesting to understand whether these sources are compact because they are young or rather because they are weak and not capable of growing any larger. This field is of great interest at the moment, as it is expected to provide insight on the fundamental physical properties at the basis of the different observed kind of activity in the nuclei of galaxies (A5, B2, B3, B11).


this list is sooooo out of date. please check here for the latest publications.

Giroletti, M., Giovannini, G., Taylor, G. B., & Falomo, R. 2005, Astronomical Society of the Pacific Conference Series Radio Observations of Nearby HST BL Lacs

Giroletti, M., Giovannini, G., & Taylor, G. B. 2005, Astronomy and Astrophysics Low power compact radio galaxies at high angular resolution

Giroletti, M., Taylor, G. B., & Giovannini, G. 2005, Astrophysical Journal The Two-sided Parsec-Scale Structure of the Low-Luminosity Active Galactic Nucleus in NGC 4278

Gugliucci, N. E., Taylor, G. B., Peck, A. B., & Giroletti, M. 2005, Astrophysical Journal Dating COINS: Kinematic Ages for Compact Symmetric Objects

Giroletti, M. 2005, Baltic Astronomy Radio Observations of a Sample of BL Lac Objects

Gugliucci, N. E., Taylor, G. B., Peck, A. B., & Giroletti, M. 2004, American Astronomical Society Meeting Abstracts Dating COINS: Kinematic Ages for Compact Symmetric Objects

Giovannini, Gabriele, Giroletti, Marcello, & Taylor, Greg B. 2004, IAU Symposium Radio observations of a sample of nearby BL Lacs

Giroletti, M., Giovannini, G., Taylor, G. B., & Falomo, R. 2004, Astrophysical Journal A Sample of Low-Redshift BL Lacertae Objects. I. The Radio Data

Giroletti, M., Giovannini, G., Feretti, L., Cotton, W. D., Edwards, P. G., Lara, L., Marscher, A. P., Mattox, J. R., Piner, B. G., & Venturi, T. 2004, Astrophysical Journal Parsec-Scale Properties of Markarian 501

Giovannini, G., Giroletti, M., & Taylor, G. B. 2004, European VLBI Network on New Developments in VLBI Science and Technology The two sided parsec scale structure of the Low Luminosity Active Galactic Nucleus in NGC 4278

Giroletti, M., Giovannini, G., Taylor, G. B., & Conway, J. E. 2003, New Astronomy Review Ages of CSOs: the case of 4C 31.04

Giroletti, M., Giovannini, G., Taylor, G. B., Conway, J. E., Lara, L., & Venturi, T. 2003, Astronomy and Astrophysics Lobe advance velocities in the extragalactic compact symmetric object 4C 31.04

Giovannini, G., Giroletti, M., & Taylor, G. B. 2003, Astronomical Society of the Pacific Conference Series Parsec Scale Properties of Radio Sources

Giovannini, G., Giroletti, M., Taylor, G. B., Cotton, W. D., Lara, L., & Venturi, T. 2002, Proceedings of the 6th EVN Symposium Parsec Scale Properties of Radio Sources

Giroletti, M., Giovannini, G., & Edwards, P. G. 2002, Lighthouses of the Universe: The Most Luminous Celestial Objects and Their Use for Cosmology Proceedings of the MPA/ESO/, p. 286 Study of the BL Lac Object Markarian 501

Giroletti, M. 2002, Inflows, Outflows, and Reprocessing around Black Holes Motion and ages in compact radio sources

Garrett, M. A., de Bruyn, A. G., Giroletti, M., Baan, W. A., & Schilizzi, R. T. 2000, Astronomy and Astrophysics WSRT observations of the Hubble Deep Field region

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