The GHOSTS survey – II. The diversity of halo colour and metallicity profiles of massive disc galaxies

Abstract

We study the stellar halo colour properties of six nearby massive highly inclined disk galaxies using HST ACS and WFC3 observations in both F 606W and F 814W filters from the GHOSTS survey. The observed fields probe the stellar outskirts out to projected distances of ∼ 50 -70 kpc from their galactic centre along the minor axis. The 50% completeness levels of the colour magnitude diagrams are typically at two mag below the tip of the red giant branch. We find that all galaxies have extended stellar haloes out to ∼ 50 kpc and two out to ∼ 70 kpc. We determined the halo colour distribution and colour profile for each galaxy using the median colours of stars in the RGB. Within each galaxy we find variations in the median colours as a function of radius which likely indicates population variations, reflecting that their outskirts were built from several small accreted objects. We find that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear negative colour gradient in their haloes, reflecting a declining metallicity; the other have no significant colour or population gradient. In addition, notwithstanding the modest sample size of galaxies, there is no strong correlation between their halo colour/metallicity or gradient with galaxy's properties such as rotational velocity or stellar mass. The diversity in halo colour profiles observed in the GHOSTS galaxies qualitatively supports the predicted galaxy-to-galaxy scatter in halo stellar properties; a consequence of the stochasticity inherent in the assembling history of galaxies.

Stellar Populations in the Outer Disk and Halo of the Spiral Galaxy M101

“…We note that either of the color shifts implies a strong metallicity gradient in the M101 halo. For example, for old (8-10 Gyr) stellar populations in the PARSEC 1.2S stellar isochrones (Bressan et al 2012;Marigo et al 2017; see below for details), a color shift of 0.11 (0.075) mag over 11 kpc would imply metallicity gradients of ∼−0.035 (−0.02) dexkpc −1 , larger than observed in spiral galaxy halos to date (e.g., Gilbert et al 2014;Monachesi et al 2016;Harmsen et al 2017). Figure 7 shows the effect of subtracting these different halo RGB models from the observed color distribution of stars within 0.5 mag of the TRGB in the WFC3 field.…”

“…While the small FOV of the WFC3 field precludes us from giving a global view of the entire M101 halo, we attempt here to place our results in context with other spiral galaxy halos. The stellar halos of spiral galaxies (including the Milky Way and M31) have very diverse properties, with a wide range of mean metallicities, number density profiles, and metallicity gradients (e.g., Mouhcine et al 2005aMouhcine et al , 2005bMonachesi et al 2013Monachesi et al , 2016Ibata et al 2014;Merritt et al 2016;Harmsen et al 2017) While there is no clear correlation of these properties with total galaxy mass or luminosity, there is a trend with the stellar halo mass, in the sense that lower-mass stellar halos have lower metallicities and weaker metallicity gradients (Harmsen et al 2017). These trends show broad agreement with models in which the outer halos of galaxies are shaped by accretion.…”

“…A complementary approach is available for nearby galaxies (10 Mpc): the study of the discrete stellar populations via deep Hubble Space Telescope (HST) imaging of individual stars in a galaxy's disk. The distribution of stars along the color-magnitude diagram (CMD) is sensitive to the age and metallicity of the underlying stellar population, and can be used to trace the star-forming history and metallicity evolution of the galaxy (e.g., Williams et al 2009;Rejkuba et al 2005;Dalcanton et al 2009;McQuinn et al 2010McQuinn et al , 2011McQuinn et al , 2012Radburn-Smith et al 2011;Bernard et al 2015;Bruzzese et al 2015;Monachesi et al 2016).…”

Globular Cluster Systems in Brightest Cluster Galaxies. Iii. Beyond Bimodality

“…Many authors have adopted the view that this two-part structure of the MDF is evidence of two major and perhaps distinct starforming epochs in the formation histories of large galaxies, a view that has persisted for many years (e.g., Zepf & Ashman 1993;Forbes et al 1997Forbes et al , 2011Arnold et al 2011;Blom et al 2012b;Brodie et al 2014;Cantiello et al 2014;Kartha et al 2016, among many others). However, bimodal MDFs are not characteristic of the field-halo stars in their parent galaxies, in the few cases where it has been possible to directly compare the GCs and halo stars in the same galaxy (e.g., Harris et al 2007;Durrell et al 2010;Rejkuba et al 2011Rejkuba et al , 2014Monachesi et al 2016). Reconciling this apparent mismatch between field stars and GCs presents an intriguing challenge for quantitative formation modeling.…”

The central spheroids of Milky Way mass-sized galaxies

“…Gilbert et al (2013) reports a significant decrease of metallicity as the projected galactocentric distance increases, going from [Fe/H]∼ −0.4 in the inner-most fields, at r < 20 kpc, to [Fe/H] ∼ −1.4 in the outer-most fields, at r > 90 kpc. Outside the Local Group, observation of six nearby massive disc galaxies with the Hubble Space Telescope have revealed their stellar haloes with sufficient precision to estimate that, in half of them, there exists a clear negative color gradient, reflecting declining metallicity profiles (Monachesi et al 2016;Harmsen et al 2017).…”