Abstract
Transition type dwarf galaxies are thought to be systems undergoing the process of transformation from a star-forming into a passively evolving dwarf, which makes them particularly suitable to study evolutionary processes driving the existence of different dwarf morphological types. Here we present results from a spectroscopic survey of ~200 individual red giant branch stars in the Phoenix dwarf, the closest transition type with a comparable luminosity to "classical" dwarf galaxies. We measure a systemic heliocentric velocity V = -21.2 km/s. Our survey reveals the clear presence of prolate rotation, which is aligned with the peculiar spatial distribution of the youngest stars in Phoenix. We speculate that both features might have arisen from the same event, possibly an accretion of a smaller system. The evolved stellar population of Phoenix is relatively metal-poor (<[Fe/H]> = -1.49+/-0.04 dex) and shows a large metallicity spread ($\sigma_{\rm [Fe/H]} = 0.51\pm0.04$\,dex), with a pronounced metallicity gradient of -0.13+/-0.01 dex per arcmin similar to luminous, passive dwarf galaxies. We also report a discovery of an extremely metal-poor star candidate in Phoenix and discuss the importance of correcting for spatial sampling when interpreting the chemical properties of galaxies with metallicity gradients. This study presents a major leap forward in our knowledge of the internal kinematics of the Phoenix transition type dwarf galaxy, and the first wide area spectroscopic survey of its metallicity properties.
Highlights
Understanding the properties of dwarf galaxies is important to put them in their proper cosmological context, and toLocal Group dwarf galaxies can be divided into two main classes: gas-rich systems and gas-deficient, passively evolving ones (‘classical dwarf spheroidals’ or ‘dwarf ellipticals’, at the bright end; ‘ultra-faint’ at the low*luminosity end)
Notable efforts in this direction have recently taken place (e.g. Ho et al 2012; Collins et al 2013; Kirby et al 2013, 2014). Both the kinematics and metallicity properties of WLM, a dwarf irregular galaxy at the edge of the Local Group, has been extensively studied from a sample of ∼200 red giant branch (RGB) stars. This dIrr has a shallower metallicity gradient when compared with most well-studied Milky Way (MW) dwarf spheroidal galaxies (dSphs), but similar to those of larger and rotation supported systems as the Large and Small Magellanic Clouds (Leaman et al 2013), suggesting that mass and angular momentum might play a role in driving the presence/absence of metallicity gradients (Schroyen et al 2011)
We focus on the Phoenix (Phx) dwarf galaxy that belongs to the handful of Local Group dwarf galaxies displaying intermediate properties between dIrrs and dSphs
Summary
Understanding the properties of dwarf galaxies is important to put them in their proper cosmological context, and to. Notable efforts in this direction have recently taken place (e.g. Ho et al 2012; Collins et al 2013; Kirby et al 2013, 2014) Both the kinematics and metallicity properties of WLM, a dwarf irregular (dIrr) galaxy at the edge of the Local Group, has been extensively studied from a sample of ∼200 red giant branch (RGB) stars. This dIrr has a shallower metallicity gradient when compared with most well-studied MW dSphs, but similar to those of larger and rotation supported systems as the Large and Small Magellanic Clouds (Leaman et al 2013), suggesting that mass and angular momentum might play a role in driving the presence/absence of metallicity gradients (Schroyen et al 2011). Removing these duplicates from the photometric catalogue does not lead to any significant changes in terms of structural properties
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