EIG - II. Intriguing characteristics of the most extremely isolated
O. Spector, and N. Brosch. (2017)cite arxiv:1706.01884Comment: 40 pages; 17 figures; 12 tables.
We have selected a sample of 41 extremely isolated galaxies (EIGs) from the
local universe using both optical and HI ALFALFA redshifts (Spector & Brosch
2016). Narrow band H$_\alpha$ and wide band imaging along with public data were
used to derive star formation rates (SFRs), star formation histories (SFHs),
and morphological classifications for the EIGs. We have found that the extreme
isolation of the EIGs does not affect considerably their star-formation
compared to field galaxies. EIGs are typically `blue cloud' galaxies that fit
the `main sequence of star forming galaxies' and may show asymmetric star
formation and strong compact star-forming regions.
We discovered surprising environmental dependencies of the HI content,
M$_HI$, and of the morphological type of EIGs; The most isolated galaxies (of
subsample EIG-1) have lower M$_HI$ on average (with $2.5\,\sigma$ confidence)
and a higher tendency to be early-types (with 0.94 confidence) compared to the
less isolated galaxies of subsample EIG-2. To the best of our knowledge this is
the first study that finds an effect in which an isolated sample shows a higher
fraction of early-types compared to a less isolated sample.
Both early-type and late-type EIGs follow the same colour-to-M$_*$,
SFR-to-M$_*$ (`main sequence') and M$_HI$-to-$M_*$ relations. This
indicates that the mechanisms and factors governing star formation, colour and
the M$_HI$-to-M$_*$ relation are similar in early-type and late-type EIGs,
and that the morphological type of EIGs is not governed by their M$_HI$
content, colour or SFR.