The NGC 672 and 784 galaxy groups: evidence for galaxy formation and growth along a nearby dark matter filament

Abstract

We present U, B, V, R, I, Hα and NUV photometry of 14 galaxies in the very local Universe (within 10 Mpc). Most objects are dwarf irregular galaxies (dIrr) and are probably associated with the NGC 672/IC 1727 and NGC 784 galaxy groups. The galaxies are at low redshift (51 ≤ v⊙≤ 610 km s−1) and most appear projected on the sky as a 6° long linear filament. We show that the galaxy positions along this filament correlate with their radial velocity, hinting to an interpretation as a single kinematic entity. Our CCD photometry indicates that all objects qualify as ‘dwarf galaxies’ with MB≥−18 mag. We examine the star formation (SF) properties of individual objects in the context of their immediate environment. The current SF rate (SFR) is derived directly from the Hα line flux. An approximate SF history is derived by comparing the multiband photometry with results of galaxy evolution models from Bruzual & Charlot (2003a,b), assuming short SF bursts separated by long quiescence periods. Relations between the current SFR and the H i mass or the absolute B magnitude for the galaxies in these groups indicate that these objects behave like normal galaxies. A comparison of the photometric measurements with evolutionary synthesis model predictions indicates that most objects can be understood as containing at least one ‘old’ stellar population (≥1–10 Gyr) and one ‘young’ population (≤30 Myr). For both groups, the recent SF bursts appear to have occurred at similar times, a few to a few tens of Myr ago, arguing for synchronicity in SF in these objects. In an attempt to evaluate the possible role of galaxy–galaxy interaction, we investigate the trend of the SFR with an object's projected distance from the brightest and most massive galaxies of each group. We do not find a steadily decreasing SF as function of this distance; such a result could be expected if the SF would have been triggered by interactions. We propose that one possible explanation of the nearly synchronous SF in all objects is accretion of cold gas from intergalactic space on to dark matter haloes arranged along a filament threading the void where these dwarf galaxies reside. We point out this galaxy sample as an ideal target to study hierarchical clustering and galaxy formation among very nearby objects.