- evn2024@mpifr.de
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- Dr. Felix POETZL
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- Dr. Felix POETZL (Insitute of Astrophysics - FORTH)
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Currently viable dark matter (DM) models, such as the standard cold dark matter (CDM) model and alternative models developed to address its shortcomings - including warm DM, fuzzy DM, and self-interacting DM - differ in their predictions for DM halos at critical sub-galactic scales. If sufficiently dense, these sub-galactic DM halos could form a population of supermassive compact objects (SMCOs). SMCOs could potentially also consist of a population of free-floating (primordial) supermassive black holes (SMBHs), that are DM candidates.
The DM halos are nearly devoid of stars, and so any SMCO may possibly be detected only through the gravitational effect they exert on ordinary matter. The SMILE (Search for MIlli LEnses) project aims at probing the number density of low mass (${\sim}10^6-10^9\,M_\odot$) DM halos searching for gravitational lens systems at milliarcsecond scales (milli-lenses), where the lens is expected to have a mass in the range of interest.
This is achieved by studying radio images of active galactic nuclei made with Very-Long-Baseline Interferometry (VLBI). In a recent pilot project, we have searched for milli-lens candidates in a sample consisting of 13,828 compact radio sources from the Astrogeo VLBI FITS image database. Forty candidates with compact double structures have been found using a citizen-science approach, for which I will present the final analysis of follow-up observations with the European VLBI Network at 5 and 22 GHz in phase-referencing mode. These observations with increased sensitivity and frequency coverage allow us to better constrain the nature of the lens candidates. We can confidently reject most systems as milli-lenses, given constrains such as surface brightness ratio, stability of flux density ratio of components over time, and their spectrum. Rejected candidates are still interesting as potential compact symmetric object (CSO) or even supermassive black hole binary (SMBHB) candidates. I will put the pilot study in the context of the SMILE project, which ultimately aims at using a sample of ~5,000 sources with redshift information to test predictions from theoretical DM models about the number of expected milli-lenses.