Evolutionary paths to and from the red sequence: star formation and H i properties of transition galaxies atz∼ 0

Abstract

We investigate the properties of galaxies between the blue and the red sequence (i.e. the transition region, 4.5 < NUV −H < 6 mag) by combining ultraviolet (UV) and near‐infrared imaging to 21‐cm H i line observations for a volume‐limited sample of nearby galaxies. We confirm the existence of a tight relation between colour and H i fraction across all the range of colours, although outside the blue cloud this trend becomes gradually weaker. Transition galaxies are divided into two different families, according to their atomic hydrogen content. ‘H i‐deficient’ galaxies are the majority of transition galaxies in our sample. They are found in high‐density environments and all their properties are consistent with a quenching of the star formation via gas stripping. However, while the migration from the blue cloud is relatively quick (i.e. ≲1 Gyr), a longer amount of time (a few Gyr at least) seems required to completely suppress the star formation and reach the red sequence. At all masses, migrating ‘H i‐deficient’ galaxies are mainly discs, implying that the mechanism responsible for today's migration in clusters cannot have played a significant role in the creation of the red sequence at high redshift. Conversely, ‘H i‐normal’ transition galaxies are a more heterogeneous population. A significant fraction of these objects show star formation in ring‐like structures and evidence for accretion/minor‐merging events suggesting that at least part of the H i reservoir has an external origin. The detailed evolution of such systems is still unclear, but our analysis suggests that, in at least two cases, galaxies might have migrated back from the red sequence after accretion events. Interestingly, the H i available may be sufficient to sustain star formation at the current rate for several billion years. Our study clearly shows the variety of evolutionary paths leading to the transition region and suggests that the transition galaxies may not be always associated with systems quickly migrating from the blue to the red sequence.

Hα3: an Hαimaging survey of HI selected galaxies from ALFALFA

“…The time scale for gas depletion is short enough (∼100−300 Myr) to quickly transform HI-rich LTGs into anemic LTGs, which end up as gas-free, passive dEs. This is precisely the mechanism invoked by Boselli et al (2008) and by Gavazzi et al (2010) to migrate dwarf starforming systems into dEs in the neighborhood of rich galaxy clusters (see also Cortese & Hughes 2009;and Cortese et al 2011). Meanwhile the ram pressure mechanism, owing to its short time scale, is also capable of producing a k + a signature in their spectra, thus enhancing the number of galaxies presenting some PSB phase toward rich clusters (Gavazzi et al 2010;Wong et al 2012) and making the number of active nuclei (both star forming and AGNs of various forms) decrease near the center of rich clusters .…”

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS)

“…We, thus, define the limit between the blue cloud and the green valley with the relation NUV − i = 0.47 log M star − 1.0. The slope of the NUV − i vs. M star relation described by the model galaxies used here to select objects that belong to the blue cloud is steeper than the one determined using different sets of data by Wyder et al (2007) or Cortese et al (2009). These definitions, as every definition of red sequence, green valley, and blue cloud galaxies, are still quite arbitrary.…”

NGC 4262: a Virgo galaxy with an extended ultraviolet ring

“…Considering the clear symmetry the HI ring around NGC 4262, is unlikely to come from a primordial cloud of neutral hydrogen such as discussed in detail by Thilker et al (2009) and Cortese and Hughes (2009). In contrast, Bekki et al (2005) model the stripping of rings and arcs of cold gas as the result of interaction with other galaxies.…”