The blue compact dwarf galaxy I Zw 18: A comparative study of its low-surface-brightness component

Abstract

Abstract. Using HST and ground-based optical and NIR imaging data , , we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 possesses an extended low-surface-brightness (LSB) old stellar population underlying its star-forming regions, as is the case in the majority of BCDs. This question is central to the long-standing debate on the evolutionary state of I Zw 18. We show that the exponential intensity decrease observed in the filamentary LSB envelope of the BCD out to > ∼ 18 ( > ∼ 1.3 kpc assuming a distance of 15 Mpc) is not due to an evolved stellar disc underlying its star-forming regions, but rather, due to extended ionized gas emission. Ionized gas accounts for more than 80% of the line-of-sight emission at a galactocentric distance of ∼0.65 kpc (∼3 effective radii), and for > ∼ 30% to 50% of the R light of the main body of I Zw 18. Broad-band images reveal, after subtraction of nebular line emission, a relatively smooth stellar host extending slightly beyond the star-forming regions. This unresolved stellar component, though very compact, is not exceptional for intrinsically faint dwarfs with respect to its structural properties. However, being blue over a radius range of ∼5 exponential scale lengths and showing little colour contrast to the star-forming regions, it differs strikingly from the red LSB host of standard BCDs. This fact, together with the comparably blue colours of the faint C component, ∼1.6 kpc away from the main body of I Zw 18, suggests that the formation of I Zw 18 as a whole has occurred within the last 0.5 Gyr, making it a young BCD candidate. Furthermore, we show that the ionized envelope of I Zw 18 is not exceptional among star-forming dwarf galaxies, neither by its exponential intensity fall-off nor by its scale length. However, contrary to evolved BCDs, the stellar LSB component of I Zw 18 is much more compact than the ionized gas envelope. In the absence of an appreciable underlying stellar population, extended ionized gas emission dominates in the outer parts of I Zw 18, mimicking an exponential stellar disc on optical surface brightness profiles.

Spiral-like star-forming patterns in CALIFA early-type galaxies

“…These were derived with the isophotal annuli (isan) technique (Papaderos et al 2002, see K12, P13 and G15b for applications to IFS data), whereby the emission-line free stellar continuum between 6390 Å and 6490 Å (panels g-i) was used for the isan definition. It can be seen that in all cases the mean EW(Hα) increases smoothly beyond R eff (shaded area) and reaches in the case of NGC 1349 and NGC 3106 values between ∼6 Å and > ∼ 12 Å.…”

On the properties of the interstellar medium in extremely metal-poor blue compact dwarf galaxies

“…If the exponential LSB host of BCDs/XBCDs forms in an inside-out manner through radial migration of stars, then the associated stellar mass filtering effect (Papaderos et al 2002) will result into a stellar age overestimate, if colours are interpreted in terms of extended SFHs. In fact, a consequence of galaxy build-up through stellar migration is that instantaneous star formation models yield a better approximation to the true stellar age of the LSB host than the usually adopted continuous star formation SFH models (see for an update and a further discussion Papaderos & Östlin 2012).…”

“…Figure 16 suggests that regions 1 and 2+3 were formed almost coevally some 3 Myr ago, assuming an instantaneous burst, whereas their surroundings and the arm-like features appear to be slightly older. It should be born in mind that the EW maximum of nebular emission lines in starburst galaxies does not necessarily spatially coincide with the location of ionizing YSCs (e.g., Papaderos et al 1998Papaderos et al , 2002Guseva et al 2004;Lagos et al 2007;Papaderos & Östlin 2012), consequently EW maps are not always convertible into stellar age maps via standard evolutionary synthesis models (e.g., Starburst99). The unknown SFH is another important source of uncertainty in converting Hα luminosities into SFRs for starburst galaxies (e.g., Weilbacher & Fritze von Alvensleben 2001), just like the fraction of ionizing Lyman continuum photons leaking out of SF regions and galaxies (e.g., see also Guseva et al 2007;Relaño et al 2012;Papaderos et al 2013;Bergvall et al 2013).…”

“…Evidence for coeval star formation on scales of several hundred pc has also been found through photometric studies of high-luminosity BCDs with the Hubble Space Telescope (e.g., Östlin et al 2003;Adamo et al 2011) and surface photometry studies. For example, the subtraction of the underlying LSB host emission from surface brightness profiles of the BCD Mrk 178 yields almost flat color gradients within the starburst component, in agreement with the picture of a The three GH iiRs, resolved in this study, are indicated in the figure. nearly coevally triggered SF episode on spatial scales of ∼1 kpc (Papaderos et al 2002).…”

On the physical properties ofz≈6–8 galaxies

“…The presence of both nebular lines and continua and their contribution to broad-band photometry is well known in nearby star-forming galaxies, such as very metal-poor objects (e.g., I Zw 18, SBS 0335-052, and others), blue compact dwarf galaxies and related objects (cf. Izotov et al 1997;Papaderos et al 2002;Pustilnik et al 2004;Papaderos et al 2006). The strongest evidence of a significant contribution of the nebular continuum in some nearby star-forming galaxies is the observational finding of a Balmer jump in emission (see Guseva et al 2007).…”