On the Recent History of Star Formation in the Blue Compact Dwarf Galaxy VII Z[CLC]w[/CLC] 403

Abstract

Here we attempt to infer the recent history of star formation in the BCD galaxy VII Zw 403, based on an analysis that accounts for the dynamics of the remnant generated either by an instantaneous burst or by a continuous star formation event. The models are restricted by the size of the diffuse X-ray-emitting region, the H luminosity from the star-forming region, and the superbubble diffuse X-ray luminosity. We have reobserved VII Zw 403 with a better sensitivity corresponding to the threshold H flux 8:15 Â 10 À17 ergs cm À2 s À1 . The total H luminosity derived from our data is much larger than reported before and presents a variety of ionized filaments and incomplete shells superimposed on the diffuse H emission. This result has a profound impact on the predicted properties of the starburst-blown superbubble. Numerical calculations based on the Hubble Space Telescope H data predict two different scenarios of star formation able to match simultaneously all observed parameters. These are an instantaneous burst of star formation with a total mass of 5 Â 10 5 M and a star-forming event with a constant star formation rate SFR ¼ 4 Â 10 À3 M yr À1 , which lasts for 35 Myr. The numerical calculations based on the energy input rate derived from our observations predict a short episode of star formation lasting less than 10 Myr with a total star cluster mass of $ð1 3Þ Â 10 6 M . However, the five main star-forming knots are sufficiently distant to form a coherent shell in a short timescale and still keep their energies blocked within local, spatially separated bubbles. The X-ray luminosities of these are here shown to be consistent with the ROSAT PSPC diffuse X-ray emission.

Structure and kinematics of the interstellar medium in the star-forming region in the BCD galaxy VII Zw 403 (UGC 6456)

“…We corrected the data from Martin (1998) in the same way; the luminosity is given for a distance of 4.5 kpc. Silich et al (2002) applied a correction only for the background extinction E(B − V ) = 0.02, as inferred by Burstein and Heiles (1984); the correction for internal extinction in VII Zw 403 increases further the flux and luminosity found by these authors. As follows from Table 3, our measured Hα luminosity of the galaxy is considerably lower than the value obtained by Silich et al (2002) and agrees well with the previous estimates by Lynds et al (1998) and Martin (1998).…”

“…Based on HST observations, Lynds et al (1998) identified a shell structure of several bright HII regions 70-150 pc in size. Silich et al (2002) found traces of the faint giant (D ≃ 500 pc) ring in the Hα emission from the diffuse gas. An extended region of diffuse X-ray emission inside the giant ring was reported, but these data have not yet been confirmed (Papaderos et al 1994;Lira et al 2000;Bomans 2001).…”

“…The results of ionized-gas observations in the galaxy are ambiguous. Thus, for example, the measurements of the total Hα luminosity of the galaxy, which is used as the main criterion for the star formation rate, differ in the papers of Lynds et al (1998) and Silich et al (2002) by more than a factor of 20. Based on long-slit spectra taken with the 4-m KPNO telescope, Lynds et al (1998) found an expansion of bright shells with a velocity V exp = 50 − 70 km s −1 , but the observations by Thuan et al (1987) and Martin (1998) with a higher spectral resolution revealed no such high velocities in the galaxy.…”

The gas emission spectrum in a star-forming region in the BCD galaxy VII Zw 403 (UGC 6456)

“…Lynds et al [2] used slit spectrograms obtained on the 4-m telescope of the Kitt Peak National Observatory to derive the shells expansion rate, 50−70 km s −1 , although these results were not confirmed by Lozinskaya et al [1] (see below). Silich et al [7] found traces of a giant (D ≃ 500 pc), faint ring in Hα emission produced by diffuse gas. Reports of the presence of an extended region of diffuse X-ray emission inside this giant ring remain, for the moment, unconfirmed [8][9][10][11][12].…”

Gas and dust in the BCD galaxy VII Zw 403 (UGC 6456)

“…The HST observations have disclosed a shell-like structure of several bright HII regions; Lynds et al [5] measured the shell's expansion velocity of 50 − 70 km/s, however, Lozinskaya et al [4] did not confirm these results. Silich et al [13] found the traces of a faint giant ring sized D ≃ 500 pc in the H α emission of diffuse gas.…”