[ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL]/WFPC2 and VLA Observations of the Ionized Gas in the Dwarf Starburst Galaxy NGC 4214

Abstract

We present new Hα and [O III] λ5007 narrow band images of the starbursting dwarf galaxy NGC 4214, obtained with the Wide Field and Planetary Camera (WFPC2) onboard the Hubble Space Telescope (HST), together with VLA observations of the same galaxy. The HST images resolve features down to physical scales of 2 − 5 pc, revealing several young (< 10 Myr) star forming complexes of various ionized gas morphologies (compact knots, complete or fragmentary shells) and sizes (∼ 10 − 200 pc). Our results are consistent with a uniform set of evolutionary trends: The youngest, smaller, filled regions that presumably are those just emerging from dense star forming clouds, tend to be of high excitation and are highly obscured. Evolved, larger shell-like regions have lower excitation and are less extincted due of the action of stellar winds and supernovae. In at least one case we find evidence for induced star formation which has led to a two-stage starburst. Age estimates based on W (Hα) measurements do not agree with those inferred from wind-driven shell models of expanding H II regions. The most likely explanation for this effect is the existence of an ≈ 2 Myr delay in the formation of superbubbles caused by the pressure exerted by the high density medium in which massive stars are born. We report the detection of a supernova remnant embedded in one of the two large H II complexes of NGC 4214. The dust in NGC 4214 is not located in a foreground screen but is physically associated with the warm ionized gas. OBSERVATIONS AND DATA REDUCTION HST/WFPC2 imagingWe obtained deep, high resolution, multiwavelength imaging of NGC 4214 with the WFPC2 instrument aboard HST (prop. ID 6569) on 1997 July 22. In Figure 1 we show a graphic representation of the WFPC2 field superimposed on a 13. ′ 6 × 13. ′ 6 greyscale reproduction of the Digitized Sky Survey Image 6 . The equatorial coordinates of the intersection point of the four camera fields are α = 12 h 15 m 40. s 53, δ = 36 • 19 ′ 37. ′′ 6 (J2000). This location was chosen in order to minimize Charge Transfer Efficiency (CTE) effects, since in this way no area of interest is separated from its collecting point by low signal areas (Whitmore et al. 1999). At a distance of 4.1 Mpc, 1 ′′ = 20 pc, so that one WF pixel corresponds to 2.0 pc and one PC pixel to 0.91 pc. Spatial resolution on these physical scales is within a factor of 2 − 4 of the best resolution that can be achieved from the ground for the LMC. Thus, the high spatial resolution, high sensitivity, wide dynamical range and faint magnitude limit (roughly 3 magnitudes fainter than typical ground-based limits, ∼ 26 in V) of WFPC2 permits one to identify regions structurally similar to those in the LMC (e.g. 30 Doradus), with ground-based resolution quality, for objects at nearly 80 times the distance to the LMC. Throughout the remainder of this paper, all conversions between pixels and arcseconds/physical scales are based on the plate scale of the WF chips, where most of NGC 4214 was imaged.We summarize the WFPC2 data set analyzed ...

A Hubble Space Telescope/NICMOS view of the prototypical giant Hii region NGC604 in M33

“…The morphology of these regions changes quite rapidly during the first few million years after the first massive generation is born. Excellent pictures showing these evolutionary morphologies are the H II regions in the irregular galaxy NGC 4214 (Maíz Apellániz 2000;MacKenty et al 2000).…”

The dust SED of dwarf galaxies

“…We used the measurements of the Hα emission from MacKenty et al (2000) to calculate the expected thermal radio emission from NW and SE. They presented an Hα narrowband map of the centre of NGC 4214 obtained with the HST Wide Field and Planetary Camera (WFPC2).…”

“…They used the spectroscopic values of the Balmer ratio Hα/Hβ presented by Maíz-Apellániz (2000) to correct for dust extinction and they reported intrinsic Hα fluxes of NW and SE. Assuming that the dust is mixed well with the gas, MacKenty et al (2000) calculated extinction-corrected Hα fluxes of 12.88 × 10 −12 erg cm −2 s −1 for the NW complex and 5.75 × 10 −12 erg cm −2 s −1 for the SE complex.…”

“…When resolved, the two complexes show smaller individual knots of SF (see the small field of Fig.1). One of the most striking features in these complexes is the large shell structure in the NW region, where most of the gas in front of the central star cluster seems to have been removed by the action of stellar winds and supernovae (SNe) (Maíz-Apellániz et al 1998;MacKenty et al 2000). NGC 4214-SE is more compact and shows no clear evidence of a decoupling between the star clusters and the gas.…”

“…NGC 4214-SE is more compact and shows no clear evidence of a decoupling between the star clusters and the gas. The morphological differences for the two complexes were interpreted as an evolutionary trend by MacKenty et al (2000). Using stellar synthesis models, Úbeda et al (2007) determined the age, the mass, the radius, and the extinction of the star clusters within the NW and SE complexes.…”