Starburst in HS 0822+3542 induced by the very blue LSB dwarf SAO 0822+3545

Abstract

Abstract.One of the most metal-deficient blue compact galaxies (BCGs) HS 0822+3542 (Z = 1/34 Z ), is also one of the nearest such objects (D ∼ 11 Mpc). It is in addition well isolated from known bright galaxies. A trigger mechanism for its current star-formation (SF) burst has thus remained unclear. We report the discovery of a very blue ((B − V) 23.m 4 arcsec −2 ) dwarf irregular (dIrr) galaxy, which we have named SAO 0822+3545. Its small relative velocity and projected distance of only ∼11 kpc from the BCG imply their physical association. For this LSB dIrr galaxy, we present spectroscopic results, total B, V, R magnitudes, and the effective radii and surface brightness (SB), and we describe its morphological properties. We compare the very blue colours of this dwarf with PEGASE.2 models of the colour evolution of a Z = 1/20 Z stellar population, and combine this analysis with the data on the LSBD EW(Hα) values. The models best describing all available observational data depend on the relative fraction of massive stars in the IMF used. For a Salpeter IMF with M up = 120 M , the best model includes a "young" single stellar population (SSP) with an age of ∼10 Myr and an "old" SSP with the age of ∼0.2-10 Gyr. The mass ratio of the old to young components should be in the range of 10 to 30. If the age of the old component is more than ∼1 Gyr, an additional coeval component of "intermediate" age (∼100 Myr) with a mass comparable to that of the "young" population, although not required, provided a good fit to the current data. For the two options of a model IMF biased toward the low-mass end, the best match of the observed BVR and EW(Hα) is for continuous star-formation rate (SFR) single-component models, with SF durations in the range of ∼0.1 to ∼1 Gyr. However, only a longer time-scale SF gives the stellar mass, compatible with the LSB galaxy mass estimates. Nevertheless, such a scenario would be inconsistent with the recent encounter of these two dwarfs. The role of interaction between the LSBD and BCG HS 0822+3542 in triggering their major SF episodes during the last ∼100-200 Myr is emphasized and discussed. For the BCG, based on the results of new spectroscopy with the Russian 6 m telescope, we estimate the physical parameters of its SF region and present the first evidence of an ionized gas supershell. This pair of dwarfs lies deep within the nearby Lynx-Cancer void, with the nearest bright (L > L * ) galaxies at distances >3 Mpc. This is probably one of the main factors responsible for the unevolved state of HS 0822+3542.

H I study of extremely metal-deficient dwarf galaxies

“…Two more XMD galaxies are found within the same void (Pustilnik et al 2003b Lee et al 2004), for the adopted metallicity of z = 0.001 corresponds to an age of an instantaneous starburst of ∼10 Myr (Leitherer et al 1999). The origin of such a non-central SF "burst" is unclear.…”

“…This galaxy is situated at ∼6 Mpc, in the inner rim region of the nearby Lynx-Cancer void described in Pustilnik et al (2003b).…”

Study of DDO 68: nearest candidate for a young galaxy?

“…Another XMD galaxy HS 0822+3542 is situated near the centre of the same Lynx-Cancer void (Pustilnik et al 2003b). At least one more dwarf galaxy with very low metallicity, KISSB 23 = KUG 0937+298 (12 + log (O/H) = 7.65, Lee et al 2004) is also situated within the same void, only at ∼650 kpc from DDO 68.…”

“…In particular, the XMD blue compact dwarf (BCD) galaxy HS 0822+3542 with 12 + log (O/H) = 7.38, appeared near the centre of the nearby small Lynx-Cancer void (Kniazev et al 2000;Pustilnik et al 2003b). Voids delineated by luminous, massive galaxies are not absolutely empty.…”

Dynamical and Chemical Evolution of IZw18

“…This has been suggested by Izotov & Thuan (1999) Assuming a primary nitrogen production in massive stars, we can reproduce the N/O plateau, but the amount of nitrogen required (1/1000 of the progenitor mass) is far above the predictions of models (Woosley & Weaver 1995;Meynet & Maeder 2002, hereafter MM02). It is worth noticing that some new observations of metal-poor galaxies (Skillman et al 2003;Pustilnik et al 2003) show N/O ratios lying well above the plateau of log (N/O) = −1.60 found by Izotov & Thuan (1999); thus this result needs further confirmation.…”