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May 25, 2017, 12:47 pm
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SECCO 1: a very dark stellar system lost in Virgo

chart_subLeft panel: Hα images of the two bodies of SECCO 1 (projected separation ~6 kpc ) extracted from the MUSE data cube. Right panels: HST -ACS images of the same region.  At the distance of  the object 10 arcsec correspond to 825 pc.

Many models of structure formation within the Lambda-CDM cosmological paradigm predict the existence of a large number of small dark matter haloes (called “mini-halos”, with mass lower than 1 billion solar masses) that should contain 0.5 − 10 million solar masses of baryons in the form of neutral hydrogen, with or without an associated (small) stellar component.  Internal and external feedback effects (ionizing background radiation, supernovae, etc.) are expected to inhibit star formation in these systems, thus originating dark or almost dark galaxies. Identifying mini-halos is extremely difficult but it would be crucial to understand the process of galaxy formation in the lowest mass regime and to validate the cosmological model at the smallest scales.

In the last years various HI surveys have provided lists of Ultra Compact High Velocity HI clouds that are candidate  minihalos. The only way to directly confirm these candidates as genuine dwarf galaxies would be to identify a concomitant stellar counterpart whose distance could be estimated.  The SECCO survey, an international research project led by M. Bellazzini (INAF-OABo), is aimed at  searching for these stellar counterparts.

SECCO 1 is an enigmatic star-forming system that we identified as the stellar counterpart of one of the candidate mini-halos targeted in the SECCO survey and  it is located within the Virgo cluster of galaxies. 

We have used the very powerful new panoramic integral-field spectrograph  MUSE, mounted at one of the 8m ESO-VLT telescopes in the Chilean Andes, to get a complete view of the velocity field and chemical abundance of SECCO1. These data, coupled with recent Hubble Space Telescope images , allowed us to establish that  SECCO1 has an extremey low total stellar mass (~160000 solar masses) , does not show any sign of a population older than ~1 Gyr, and it displays a star formation rate much higher than dwarf galaxies of similar HI mass. Moreover, all its HII regions have similar chemical composition and the mean metallicity is typical of galaxies having stellar mass of about 1 billion solar masses. This strongly suggest that SECCO1 stars born from a gas cloud  that was pre-enriched in a very different environment, possibly stripped from the disc of a large spiral within Virgo.

A paper describing these results has been accepted for publication on Monthly Notices of the Royal Astronomic Society (Beccari et al. 2016). Further observations and analyses to understand the nature of this extremely dark and unique stellar system are ongoing.