Diverse stellar haloes in nearby Milky Way mass disc galaxies

Harmsen, Benjamin; Monachesi, Antonela; Bell, Eric F.; Jong, Roelof S. de; Bailin, Jeremy; Radburn-Smith, David J.; Holwerda, Benne W.

doi:10.1093/mnras/stw2992

Cited by 127 publications

(288 citation statements)

References 114 publications

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“…The first, most conservative, model simply takes the WFC3 CMD as observed, scaling up in number only to account for the different areas of the WFC3 and ACS fields. The second model shifts the WFC3 RGB redward in color by 0.11 mag to match the mean color of the ACS RGB, and also scales it up in number by a factor of 2.27, as might be expected if the halo surface density profile follows an r −3 form consistent with the outer halos of MW, M31, and other spiral galaxies (e.g., Ibata et al 2014;Harmsen et al 2017;Medina et al 2018 and references within). The third model keeps the r −3 density scaling, but shifts the color redward by only 0.075 mag to match the blue side of the ACS RGB color distribution.…”

Section: Old Stellar Populations In the Acs Disk And Wfc3 Halo Fieldsmentioning

confidence: 86%

“…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.…”

Section: Old Stellar Populations In the Acs Disk And Wfc3 Halo Fieldsmentioning

confidence: 96%

“…Adopting a typical halo value of [α/Fe]=+0.3, our metallicity estimate of [M/H]=−1.7 translates to [Fe/H]∼−1.9 (Salaris et al 1993;Streich et al 2014), which is nearly 0.5-1 dex lower than most spiral galaxy halos; the Milky Way's halo is the only one known to approach these low metallicities (Harmsen et al 2017). Some of the difference may be due to the different radial ranges probed; the Harmsen et al sample is measured at 30 kpc, while our WFC3 field is further out at 47 kpc.…”

Section: The Properties Of M101's Halomentioning

confidence: 99%

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

Mihos

Durrell

Feldmeier

et al. 2018

ApJ

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We use deep Hubble Space Telescope imaging in the outskirts of the nearby spiral M101 to study stellar populations in the galaxy's outer disk and halo. Our Advanced Camera for Surveys (ACS) field lies 17 6 (36 kpc) from the center of M101 and targets the blue "NE Plume" of M101ʼs outer disk, while the parallel Wide Field Camera 3 (WFC3) field lies at a distance of 23 3 (47 kpc) to sample the galaxy's stellar halo. The WFC3 halo field shows a well-defined red giant branch characterized by low metallicity ([M/H] =−1.7±0.2), with no evidence of young stellar populations. In contrast, the ACS disk field shows multiple stellar populations, including a young main sequence, blue and red helium-burning stars, and old RGB and asymptotic giant branch (AGB) populations. The mean metallicity of these disk stars is quite low: [M/H]=−1.3±0.2 for the RGB population, and −1.15±0.2 for the younger helium-burning sequences. Of particular interest is a bunching of stars along the BHeB sequence, indicative of an evolving cohort of massive young stars. We show that the young stellar populations in this field are well-described by a decaying burst of star formation that peaked ∼300-400 Myr ago, along with a more extended star formation history to produce the older RGB and AGB populations. These results confirm and extend the results from our previous deep surface photometry of M101ʼs outer disk, providing an important cross-check on stellar population studies using resolved stellar populations versus integrated light photometry. We discuss our results in the context of halo formation models and the interaction history of M101 and its companions.

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Section: Old Stellar Populations In the Acs Disk And Wfc3 Halo Fieldsmentioning

confidence: 86%

Section: Old Stellar Populations In the Acs Disk And Wfc3 Halo Fieldsmentioning

confidence: 96%

Section: The Properties Of M101's Halomentioning

confidence: 99%

See 1 more Smart Citation

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

Mihos

Durrell

Feldmeier

et al. 2018

ApJ

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“…The metal-rich component is concentrated more towards the inner regions and the metal-poor component becomes significant only in the outer regions. The metallicity of both the metal-rich and metal-poor components increase with increasing galaxy luminosity (Mouhcine 2006) and the stellar halo metallicity scales with stellar halo mass (Harmsen et al 2017) with a slope that is broadly similar to the stellar mass versus stellar metallicity relation for local galaxies (Gallazzi et al 2005;Kirby et al 2013). Forbes et al (1997) and Mouhcine (2006) suggested that the metal-poor component is due to the tidal disruption of dwarflike objects whereas the metal-rich population is related to the formation of the bulge and/or disk.…”

Section: Observed Metallicity Distribution Functions (Mdf) Of Clustermentioning

confidence: 99%

The difference in metallicity distribution functions of halo stars and globular clusters as a function of galaxy type

Lamers

Kruijssen

Bastian

et al. 2017

A&A

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Context. Observations of globular clusters (GCs) and field stars in the halos of the giant elliptical galaxy Cen A and the spiral galaxy M31 show a large range of cluster-to-star number ratios (or 'specific frequencies'). The cluster-to-star ratio decreases with increasing metallicity by over a factor of 100-1000, at all galactocentric radii and with a slope that does not seem to depend on radius. In dwarf galaxies, the GCs are also more metal-poor than the field stars on average. These observations indicate a strong dependence of either the cluster formation efficiency and/or the cluster destruction rate on metallicity and environment. Aims. We aim to explain the observed trends by considering the various effects that influence the cluster-to-star ratio as a function of metallicity, environment and cosmological history. Methods. We discuss the following effects that may influence the observed cluster-to-star ratio: (a) the formation efficiency of GCs; (b) the destruction of embedded GCs by gas expulsion; (c) the maximum masses of GCs; (d) the destruction of GCs by tidal stripping, dynamical friction, and tidal shocks as a function of environment; (e) the hierarchical assembly of GC systems during galaxy formation and the dependence on metallicity. Results. We show that both the cluster formation efficiency and the maximum cluster mass increase with metallicity, so they cannot explain the observed trend. Destruction of GCs by tidal stripping and dynamical friction destroy clusters mostly within the inner few kpc, whereas the cluster-to-star ratio trend is observed over a much larger range of galactocentric radii. We show that cluster destruction by tidal shocks from giant molecular clouds in the high-density formation environments of GCs becomes increasingly efficient towards high galaxy masses and, hence, towards high metallicities. The predicted cluster-to-star ratio decreases by a factor 100-1000 towards high metallicities and should only weakly depend on galactocentric radius due to orbital mixing during hierarchical galaxy merging, consistent with the observations. Conclusions. The observed, strong dependence of the cluster-to-star ratio on metallicity and the independence of its slope on galactocentric radius can be explained by cluster destruction and hierarchical galaxy growth. During galaxy assembly, GC metallicities remain a good tracer of the host galaxy masses in which the GCs formed and experienced most of their destruction. As a result, we find that the metallicity-dependence of the cluster-to-star ratio does not reflect a GC formation efficiency, but a survival fraction.

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“…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).…”

Section: Introductionmentioning

confidence: 99%

The central spheroids of Milky Way mass-sized galaxies

Tissera

Machado

Carollo

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We study the properties of the central spheroids located within 10 kpc of the centre of mass of Milky Way mass-sized galaxies simulated in a cosmological context. The simulated central regions are dominated by stars older than 10 Gyr, mostly formed in situ, with a contribution of ∼ 30 per cent from accreted stars. These stars formed in well-defined starbursts, although accreted stars exhibit sharper and earlier ones. The fraction of accreted stars increases with galactocentric distance, so that at a radius of ∼ 8−10 kpc a fraction of ∼ 40 per cent, on average, are detected. Accreted stars are slightly younger, lower metallicity, and more α-enhanced than in situ stars. A significant fraction of old stars in the central regions come from a few (2 − 3) massive satellites (∼ 10 10 M ). The bulge components receive larger contributions of accreted stars formed in dwarfs smaller than ∼ 10 9.5 M . The difference between the distributions of ages and metallicities of old stars is thus linked to the accretion histories -those central regions with a larger fraction of accreted stars are those with contributions from more massive satellites. The kinematical properties of in situ and accreted stars are consistent with the latter being supported by their velocity dispersions, while the former exhibit clear signatures of rotational support. Our simulations demonstrate a range of characteristics, with some systems exhibiting a co-existing bar and spheroid in their central regions, resembling in some respect the central region of the Milky Way.

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Diverse stellar haloes in nearby Milky Way mass disc galaxies

Cited by 127 publications

References 114 publications

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

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

The difference in metallicity distribution functions of halo stars and globular clusters as a function of galaxy type

The central spheroids of Milky Way mass-sized galaxies

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