Insights from the outskirts: Chemical and dynamical properties in the outer parts of the Fornax dwarf spheroidal galaxy

Abstract

We present radial velocities and [Fe/H] abundances for 340 stars in the Fornax dwarf spheroidal from R ∼ 16 000 spectra. The targets were obtained in the outer parts of the galaxy, a region that has been poorly studied. Our sample shows a wide range in [Fe/H], between −0.5 and −3.0 dex, in which we detect three subgroups. Removal of stars belonging to the most metal-rich population produces a truncated metallicity distribution function that is identical to Sculptor, indicating that these systems shared a similar early evolution, except that Fornax experienced a late, intense period of star formation (SF). The derived age-metallicity relation shows a fast increase in [Fe/H] at early ages, after which the enrichment flattens significantly for stars younger than ∼8 Gyr. Additionally, the data indicate a strong population of stars around 4 Gyr, followed by a second rapid enrichment in [Fe/H]. A leaky-box chemical enrichment model generally matches the observed relation but predicts neither a significant population of young stars nor strong enrichment at late times. The young population in Fornax may therefore stem from an externally triggered SF event. Our dynamical analysis reveals an increasing velocity dispersion with decreasing [Fe/H] from σ sys ≈ 7.5 km s −1 to ≥14 km s −1 . The large velocity dispersion at low metallicities is possibly the result of a non-Gaussian velocity distribution among stars older than ∼8 Gyr. Our sample also includes members from the Fornax globular clusters H2 and H5. In agreement with past studies we find [Fe/H] = −2.04 ± 0.04 and a mean radial velocity RV = 59.36 ± 0.31 km s −1 for H2 and [Fe/H] = −2.02 ± 0.11 and RV = 59.39 ± 0.44 km s −1 for H5. Finally, we test different calibrations of the calcium triplet over more than 2 dex in [Fe/H] and find best agreement with the calibration equations provided by Carrera et al. (2013, MNRAS, 434, 1681. Overall, we find high complexity in the chemical and dynamical properties, with signatures that additionally vary with galactocentric distance. Detailed knowledge about the properties of stars at all radii is therefore needed to draw a conclusive picture about the star formation and chemical evolution in Fornax.

Evidence for a chemical enrichment coupling of globular clusters and field stars in the Fornax dSph

“…From the few outliers, one is very metal-poor ([Fe/H] ≤ −2.0 dex) and with high radial velocity. This combination may be typical given that previous studies already detected a trend for more extreme velocities amongst metal-deficient stars in the galaxy (Battaglia et al 2006;Hendricks et al 2014b). The remaining four all have [Fe/H] ≈ −1.5, and three of them show almost identical radial velocities of 47.2, 48.2, and 46.6 km s −1 .…”

“…The radial velocity of the cluster has been measured in integrated-light studies by Larsen et al (2012) and Dubath et al (1992), who consistently report values of 46.2 and 47.2 km s −1 , respectively. Therefore H4's radial velocity is lower by ∼9 km s −1 compared to the mean galactic motion of Fornax, which approximately corresponds to the velocity dispersion of the galaxy at this metallicity (see Hendricks et al 2014b). Additionally, H4's metallicity is measured around [Fe/H] = −1.4 (Larsen et al 2012;Strader et al 2003), and is therefore A86, page 7 of 17 The red line indicates the background level that we determine from the stellar density outside the tidal radius of the cluster.…”

“…It is one of the most massive Galactic satellites with a few 10 7 M (McConnachie 2012), an extended star formation history (de Boer et al 2012b;del Pino et al 2013), and consequently hosts stars of a broad range of metallicities (Battaglia et al 2006;Hendricks et al 2014b). Moreover, the galaxy hosts its own population of five GCs (see Fig.…”

Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?* † ‡

“…Carrera et al (2013) derived a relation between the [Fe/H], luminosity, and the EWs of the CaT lines using 500 RGB stars in clusters and 55 metal-poor field stars. To date, many studies have used this calibration relation to derive the metallicities for giant stars, especially in dwarf spheroidal galaxies (see, e.g., Hendricks et al 2014;Simon et al 2015). However, most of the RGB stars used in these calibration works have temperatures higher than our M giant sample.…”

A Chemical Evolution Model for the Fornax Dwarf Spheroidal Galaxy

“…Data are taken from Refs. [1,14,15] (circles), [16] (squares), and [17] (triangles). Colors indicate the formation time of stars except for the purple squares, for which no such information is available.…”