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I Zw 18 as morphological paradigm for rapidly assembling high-z galaxies
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Universidade do Porto, Portugal.ORCID iD: 0000-0002-3733-8174
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Number of Authors: 22012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 537, article id A126Article in journal (Refereed) Published
Abstract [en]

Context. I Zw 18, ever since regarded as the prototypical blue compact dwarf (BCD) galaxy, is, quite ironically, the most atypical BCD known. This is because its large low-surface brightness (LSB) envelope is not due to an old underlying stellar host, as invariably is the case for typical BCDs, but is entirely due to extended nebular emission.

Aims. Our goal is to explore I Zw 18 and its detached C component I Zw 18 C down to an unprecedently faint surface brightness mu (mag/square '') level in order to gain further insight into the structural properties and evolutionary history of this enigmatic galaxy pair.

Methods. We present a photometric analysis of the entire set of archival HST ACS V, R and I band data for I Zw 18.

Results. Radial color profiles for I Zw 18 C reveal blue and practically constant colors (0 +/- 0.05) down to mu similar to 27.6, and a previously undisclosed, slightly redder (V - I approximate to 0.2), stellar population in its extreme periphery (mu similar to 29). We argue that stellar diffusion over tau similar to 10(8) yr and the associated stellar mass filtering effect can consistently account for the observed properties of the stellar component in the outskirts of I Zw 18 C. This process, in combination with propagating star formation with a mean velocity of similar to 20 km s(-1) can reproduce all essential characteristics of I Zw 18 C within similar to tau. An extremely faint substrate of older stars cannot be ruled out but does not need to be postulated. As for I Zw 18, we find that nebular emission (ne) extends out to similar to 16 stellar scale lengths, shows a nearly exponential outer profile, and provides at least one third of the total optical emission. Nebular emission dominates already at mu similar to 23.5, as evident from e. g. the uniform and extremely blue (V - I approximate to -1, R - I approximate to -1.4) colors of the LSB envelope of I Zw 18.

Conclusions. The case of I Zw 18 suggests caution in studies of distant galaxies in dominant stages of their evolution, rapidly assembling their stellar mass at high specific star formation rates (SSFRs). It calls attention to the fact of ne not necessarily being cospatial with the underlying ionizing and non-ionizing stellar background. It also does not have to scale with the background surface density. The prodigious energetic output during dominant phases of galaxy evolution may result in large exponential ne envelopes, extending much beyond the still compact stellar component, just like in I Zw 18. Therefore, the morphological paradigm of I Zw 18, while probably unique in the nearby Universe, may be ubiquitous among high-SSFR galaxies at high redshift. Using I Zw 18 as reference, we show that extended ne may introduce substantial observational biases and affect several of the commonly studied fundamental galaxy relations. Among others, we show that the surface brightness profiles of distant morphological analogs to I Zw 18 may be barely distinguishable from Sersic profiles with an exponent 2 less than or similar to eta less than or similar to 5, thus mimicking the profiles of massive galaxy spheroids.

Place, publisher, year, edition, pages
2012. Vol. 537, article id A126
Keywords [en]
galaxies: structure, galaxies: evolution, galaxies: dwarf, galaxies: high-redshift, galaxies: halos, galaxies: starburst
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-162452DOI: 10.1051/0004-6361/201117551ISI: 000300416800126OAI: oai:DiVA.org:su-162452DiVA, id: diva2:1267697
Available from: 2018-12-03 Created: 2018-12-03 Last updated: 2018-12-03Bibliographically approved

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Papaderos, PolychronisÖstlin, Göran
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