High-velocity blueshifted Fe ii absorption in the dwarf star-forming galaxy PHL 293B: evidence for a wind driven supershell?

Abstract

X-shooter and ISIS WHT spectra of the starforming galaxy PHL 293B also known as A2228-00 and SDSS J223036.79-000636.9 are presented in this paper.We find broad (FWHM = 1000km/s) and very broad (FWZI = 4000km/s) components in the Balmer lines, narrow absorption components in the Balmer series blueshifted by 800km/s, previously undetected FeII multiplet (42) absorptions also blueshifted by 800km/s, IR CaII triplet stellar absorptions consistent with [Fe/H] < −2.0 and no broad components or blushifted absorptions in the HeI lines. Based on historical records, we found no optical variability at the 5 σ level of 0.02 mag between 2005 and 2013 and no optical variability at the level of 0.1mag for the past 24 years.The lack of variability rules out transient phenomena like luminous blue variables or SN IIn as the origin of the blue shifted absorptions of HI and FeII. The evidence points to either a young and dense expanding supershell or a stationary cooling wind, in both cases driven by the young cluster wind.

ON THE ORIGIN OF THE ABSORPTION AND EMISSION LINE COMPONENTS IN THE SPECTRA OFPHL 293B

“…While the observed broad wings in the Balmer lines may have originated in a very old supernova (SN) remnant (the SN rate of PHL 293B is about 3 × 10 −4 yr −1 ), in order to generate relatively narrow absorption profiles such as the ones detected in PHL 293B, it was argued that the absorbing material must cover a substantial fraction of the continuum source, which, in this case, is a young stellar cluster several parsecs in size (Terlevich et al 2014).…”

“…From the observed Balmer line luminosity and using the equivalent width of the Balmer emission lines as an age indicator, the resulting upper limit for the ionizing star cluster mass is M SC ≈ (2-4) × 10 5 M and the upper limit for the age is about (5-7) × 10 6 yr. Terlevich et al (2014) analyzed extensive published and their own photometric and spectroscopic data and found that at the 3σ level, any long-term variability, i.e., over a few years, cannot be larger than 0.02 mag and that any medium term variability, i.e., over a few months, should be less than 0.04 mag. Furthermore, there is no variability at the level of a few tenths of magnitude over a period of 25 yr.…”

“…From the ACIS-Chandra X-ray image of PHL 293B, Terlevich et al (2014) estimated a luminosity upper limit in the 0.4 to 6 keV energy range of about 2 × 10 38 erg s −1 . Analysis of the detected narrow absorption lines of Fe allows us to estimate that the column densities of ionized hydrogen (N (H ii)) are around 10 20 cm −2 if Fe/H is similar to that of the interestellar medium (ISM) in PHL 293B or substantially lower if Fe/H is that of the cluster wind.…”

“…Assuming that the cluster wind has solar abundance, the estimated (N (H ii)) column density is 10 19 cm −2 . From the equivalent width of the Balmer lines, Terlevich et al (2014) also estimated that the column density of neutral hydrogen is 10 13 cm −2 < N(H i) < 10 14 cm −2 . The true column density of neutral hydrogen, however, may be higher because only Balmer absorption was used in this estimate.…”

“…As discussed by Terlevich et al (2014), while the presence of broad Balmer lines and blueshifted absorptions may be explained by either an extremely LBV or an old SN type IIn, the lack of variability over at least a quarter of a century strongly argues against these scenarios. Thus, the understanding of this object constitutes a challenge.…”