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Register a new userI Zw 18 as morphological paradigm for rapidly assembling high-z galaxies
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Added by
Polychronis Papaderos
Publication date
2011
fields
Physics
and research's language is
English
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IZw18, ever since regarded as the prototypical blue compact dwarf (BCD) galaxy, is, quite ironically, the most atypical BCD known. This is because its large exponential low-surface brightness envelope is not due to an old stellar host but entirely due to extended nebular emission (ne) (Papaderos et al. 2002; P02). We study IZw18 and IZw18C down to an unprecedently faint surface brightness level using HST ACS data. We argue that the properties of IZw18C can be consistently accounted for by propagating star formation over the past ~100 Myr, in combination with stellar diffusion and the associated radial stellar mass filtering effect (P02). As for IZw18, we find that ne extends out to ~16 stellar scale lengths and provides at least 1/3 of the total optical emission. The case of IZw18 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 that ne is not necessarily cospatial with the underlying ionizing and non-ionizing stellar background, neither has to scale with its 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 IZw18. Therefore, the morphological paradigm of IZw18, while probably unique in the nearby Universe, may be ubiquitous among high-SSFR galaxies at high redshift. Using IZw18 as reference, we show that extended ne may introduce substantial observational biases and significantly affect fundamental galaxy relations. Among others, we show that the surface brightness profiles of distant morphological analogs to IZw18 may be barely distinguishable from Sersic profiles with an exponent 2<n<5, thus mimicking the profiles of massive galaxy spheroids. (abridged)
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Ultraviolet and 21-cm observations suggest that the extremely low-metallicity galaxy, I Zw 18, is a stream-fed galaxy containing a pocket of pristine stars responsible for producing nebular He II recombination emission observed in I Zw18-NW. Far-UV spectra by Hubble/COS and the Far Ultraviolet Spectroscopic Explorer (FUSE) make this suggestion conclusive by demonstrating that the spectrum of I Zw 18-NW shows no metal lines like O VI 1032, 1038 of comparable ionization as the He II recombination emission.
Hubble Space Telescope (HST) colour - magnitude diagrams in B, V and R along with long-slit Multiple Mirror Telescope (MMT) spectrophotometric data are used to investigate the evolutionary status of the nearby blue compact dwarf (BCD) galaxy I Zw 18. We find that the distance to I Zw 18 should be as high as 20 Mpc, twice the previously accepted distance, to be consistent with existing observational data on the galaxy: colour-magnitude diagrams, the high ionization state of the gas and presence of WR stars in the main body, and the ionization state of the C component. The spectral energy distribution (SED) of the main body of I Zw 18 is consistent with that of a stellar population with age < 5 Myr. However, the presence of large-scale shells observed around the main body suggests that star formation began ~ 20 Myr at the NW end and propagated in the SE direction. Our analysis of colour-magnitude diagrams and of the spectral energy distribution of the C component implies that star formation in this component started < 100 Myr ago at the NW end, propagated to the SE and stopped ~ 15 Myr ago. Thus, I Zw 18 is likely to be one of the youngest nearby extragalactic objects.
We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet (W-R) bumps at the wavelengths $lambda$4650 and $lambda$5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II $lambda$4686 and C IV $lambda$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass loss rates. The WC stars in the northwest component of I Zw 18 can be responsible for the presence of the nebular He II $lambda$4686 emission line, however the observed intensity of this line is several times larger than model predictions, and other sources of ionizing radiation at wavelengths shorter than 228AA are necessary.
Herschel has opened new windows into studying the evolution of rapidly star-forming galaxies out to high redshifts. Todays massive starbursts are characterized by star formation rates (SFRs) of 100+ Mo/yr and display a chaotic morphology and nucleated star formation indicative of a major merger. At z~2, galaxies of similar mass and SFR are characterized by ordered rotation and distributed star formation. The emerging cold accretion paradigm provides an intuitive understanding for such differences. In it, halo accretion rates govern the supply of gas into star-forming regions, modulated by strong outflows. The high accretion rates at high-z drive more rapid star formation, while also making disks thicker and clumpier; the clumps are expected to be short-lived in the presence of strong galactic outflows as observed. Hence equivalently rapid star-formers at high redshift are not analogous to local merger-driven starbursts, but rather to local disks with highly enhanced accretion rates.
With a metallicity of 12 + Log(O/H) $approx$ 7.1-7.2, I Zw 18 is a canonical low-metallicity blue compact dwarf (BCD) galaxy. A growing number of BCDs, including I Zw 18, have been found to host strong, narrow-lined, nebular He II ($lambda$4686) emission with enhanced intensities compared to H$beta$ (e.g., He II($lambda$4686)/H$beta$ > 1%). We present new observations of I Zw 18 using the Keck Cosmic Web Imager. These observations reveal two nebular He II emission regions (or He III regions) northwest and southeast of the He III region in the galaxys main body investigated in previous studies. All regions exhibit He II($lambda4686$)/Hbeta greater than 2%. The two newly resolved He III regions lie along an axis that intercepts the position of I Zw 18s ultraluminous X-ray (ULX) source. We explore whether the ULX could power the two He III regions via shock activity and/or beamed X-ray emission. We find no evidence of shocks from the gas kinematics. If the ULX powers the two regions, the X-ray emission would need to be beamed. Another potential explanation is that a class of early-type nitrogen-rich Wolf-Rayet stars with low winds could power the two He III regions, in which case the alignment with the ULX would be coincidental.
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