Hubble Space Telescope Observations of Accretion-Induced Star Formation in the Tadpole Galaxy Kiso 5639

Abstract

The tadpole galaxy Kiso 5639 has a slowly rotating disk with a drop in metallicity at its star-forming head, suggesting that star formation was triggered by the accretion of metal-poor gas. We present multi-wavelength Hubble Space Telescope Wide Field Camera 3 images of UV through I band plus Hα to search for peripheral emission and determine the properties of various regions. The head has a mass in young stars of~ M 10 6 and an ionization rate of6.4 10 51 s −1 , equivalent to ∼2100 O9-type stars. There are four older star-forming regions in the tail, and an underlying disk with a photometric age of ∼1 Gyr. The mass distribution function of 61 star clusters is a power law with a slope of −1.73±0.51. Fourteen young clusters in the head are more massive than, suggesting a clustering fraction of 30%-45%. Wispy filaments of Hα emission and young stars extend away from the galaxy. Shells and holes in the head H II region could be from winds and supernovae. Gravity from the disk should limit the expansion of the H II region, although hot gas might escape through the holes. The star formation surface density determined from Hα in the head is compared to that expected from likely pre-existing and accreted gas. Unless the surface density of the accreted gas is a factor of ∼3 or more larger than what was in the galaxy before, the star formation rate has to exceed the usual Kennicutt-Schmidt rate by a factor of 5.

A GMOS-N IFU study of the central H ii region in the blue compact dwarf galaxy NGC 4449: kinematics, nebular metallicity and star formation★

“…Sánchez Almeida et al (2015) found low-metallicity starburst regions in nine out of ten extremely-metal poor galaxies. Elmegreen et al (2016) reported a drop in metallicity at the star-forming head in the tadpole galaxy Kiso 5639. All these previous works interpret their finding of metallicity drops in the star-forming regions as a result of accretion of metal-poor gas.…”

Kinematics of Extremely Metal-Poor Galaxies: Evidence for Stellar Feedback

“…In addition to having chemically primitive starforming regions, XMPs also show signs of being structurally primitive, as if they were disks in early stages of assembly, with massive off-center starbursts providing them with a characteristic tadpole shape (e.g., Papaderos et al 2008;Morales-Luis et al 2011;Elmegreen et al 2012bElmegreen et al , 2016, with slowly rotating bodies, and with large spatially-unresolved turbulent motions (e.g., Sánchez Amorín et al 2015).…”

A Study of Two Dwarf Irregular Galaxies with AsymmetricalStar Formation Distributions

“…Furthermore, at least one tadpole head has been found to have a much higher SFR for its probable gas density (Elmegreen et al 2016b), similar to the HSF region in NGC 3738 as shown in Figure 5. Measurements of the oxygen abundance in several tadpoles revealed that the heads are more metal-poor than the bodies (Sánchez Almeida et al 2014;Elmegreen et al 2016b), and this characteristic of the tadpoles suggests the possibility that the star-forming head is the result of infall of metal-poor gas (Elmegreen et al 2016b;Ceverino et al 2016). On the other hand, the oxygen abundances of DDO 187 and NGC 3738 are fairly high (see Table 1), which would be inconsistent with accretion of metal-poor gas from the cosmic web.…”

“…However, local tadpoles have been found that are comparable to DDO 187 and NGC 3738, and this morphology is relatively common among extremely metal-poor galaxies (Morales-Luis et al 2011). Furthermore, at least one tadpole head has been found to have a much higher SFR for its probable gas density (Elmegreen et al 2016b), similar to the HSF region in NGC 3738 as shown in Figure 5. Measurements of the oxygen abundance in several tadpoles revealed that the heads are more metal-poor than the bodies (Sánchez Almeida et al 2014;Elmegreen et al 2016b), and this characteristic of the tadpoles suggests the possibility that the star-forming head is the result of infall of metal-poor gas (Elmegreen et al 2016b;Ceverino et al 2016).…”