Young, Blue, and Isolated Stellar Systems in the Virgo Cluster. II. A New Class of Stellar System

Abstract

We discuss five blue stellar systems in the direction of the Virgo cluster, analogous to the enigmatic object SECCO 1 (AGC 226067). These objects were identified based on their optical and UV morphology and followed up with H i observations with the Very Large Array (and Green Bank Telescope), Multi Unit Spectroscopic Explorer (on the Very Large Telescope) optical spectroscopy, and Hubble Space Telescope imaging. These new data indicate that one system is a distant group of galaxies. The remaining four are extremely low mass (M * ∼ 105 M ⊙), are dominated by young blue stars, have highly irregular and clumpy morphologies, are only a few kiloparsecs across, yet host an abundance of metal-rich, 12 + log ( O / H ) > 8.2 , H ii regions. These high metallicities indicate that these stellar systems formed from gas stripped from much more massive galaxies. Despite the young age of their stellar populations, only one system is detected in H i, while the remaining three have minimal (if any) gas reservoirs. Furthermore, two systems are surprisingly isolated and have no plausible parent galaxy within ∼30′ (∼140 kpc). Although tidal stripping cannot be conclusively excluded as the formation mechanism of these objects, ram pressure stripping more naturally explains their properties, in particular their isolation, owing to the higher velocities, relative to the parent system, that can be achieved. Therefore, we posit that most of these systems formed from ram-pressure-stripped gas removed from new infalling cluster members and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs and that they thus represent a new type of stellar system.

Dark No More: The Low-luminosity Stellar Counterpart of a Dark Cloud in the Virgo Cluster*

“…Here we show (Section 4) that ALFALFA Virgo 7 has a definite stellar counterpart that is over 20,000 times less massive than the H I and is made up exclusively of stars that recently formed in situ. The stellar properties of this object closely match those of the recently identified blue blobs (Jones et al 2022b). BC6 differs only in its exceptional H I content.…”

“…The blue blob candidates presented in Jones et al (2022b) arose from a cursory search (Sand et al 2015) of the Virgo cluster, and there are likely additional examples. In an attempt to identify additional objects, we began a visual search of the available imaging within the cluster from the Next Generation Virgo cluster Survey (NGVS; Ferrarese et al 2012), the Dark Energy Camera Legacy Survey (DECaLS; Dey et al 2019), and the Galaxy Evolution Explorer (GALEX; Martin et al 2005).…”

“…Sand et al (2015) searched for other objects with similar optical and UV morphology to SECCO 1, identifying five additional candidates in Virgo. With Hubble Space Telescope (HST) imaging, plus Jansky Very Large Array (VLA), GBT, and Multi Unit Spectroscopic Explorer observations (Jones et al 2022a(Jones et al , 2022bBellazzini et al 2022), we demonstrated that four 11 of these are analogous to SECCO 1 (though not all are H I-rich) and argued that they represent a new class of object, which we will refer to simply as "blue blobs." The only viable mechanism to explain both the isolation and young ages of blue blobs is high-speed ram pressure stripping of H Ibearing galaxies falling into the cluster (Jones et al 2022b).…”

“…With Hubble Space Telescope (HST) imaging, plus Jansky Very Large Array (VLA), GBT, and Multi Unit Spectroscopic Explorer observations (Jones et al 2022a(Jones et al , 2022bBellazzini et al 2022), we demonstrated that four 11 of these are analogous to SECCO 1 (though not all are H I-rich) and argued that they represent a new class of object, which we will refer to simply as "blue blobs." The only viable mechanism to explain both the isolation and young ages of blue blobs is high-speed ram pressure stripping of H Ibearing galaxies falling into the cluster (Jones et al 2022b). Even so, it is remarkable that they survived traversing hundreds of kpc through the hot ICM and may be the first examples of isolated star-forming clouds supported (in part) by external pressure (Burkhart & Loeb 2016;Bellazzini et al 2018;Calura et al 2020;Jones et al 2022b).…”

“…The only viable mechanism to explain both the isolation and young ages of blue blobs is high-speed ram pressure stripping of H Ibearing galaxies falling into the cluster (Jones et al 2022b). Even so, it is remarkable that they survived traversing hundreds of kpc through the hot ICM and may be the first examples of isolated star-forming clouds supported (in part) by external pressure (Burkhart & Loeb 2016;Bellazzini et al 2018;Calura et al 2020;Jones et al 2022b).…”