A bottom-heavy initial mass function for the likely-accreted blue-halo stars of the Milky Way

Abstract

We use Gaia DR2 to measure the initial mass function (IMF) of stars within 250 pc and masses in the range 0.2 < m/Msun < 1.0, separated according to kinematics and metallicity, as determined from Gaia transverse velocity, v_T, and location on the Hertzsprung-Russell diagram (HRD). The predominant thin-disc population (v_T < 40 km/s) has an IMF similar to traditional (e.g. Kroupa 2001}) stellar IMFs, with star numbers per mass interval dN/dm described by a broken power law, m^(-alpha), and index alpha_high=2.03 +0.14/-0.05 above m~0.5, shallowing to alpha_low=1.34 +0.11/-0.22 at m~<0.5. Thick-disc stars (60 km/s < v_T < 150 km/s) and stars belonging to the "high-metallicity" or "red-sequence" halo (v_T > 100 km/s or v_T > 200 km/s, and located above the isochrone on the HRD with metallicity [M/H] > -0.6) have a somewhat steeper high-mass slope, alpha_high=2.35 +0.97/-0.19 (and a similar low-mass slope alpha_low=1.14 +0.42/-0.50). Halo stars from the "blue sequence", which are characterised by low-metallicity ([M/H] < -0.6), however, have a distinct, bottom-heavy IMF, well-described by a single power law with alpha=1.82 +0.17/-0.14 over most of the mass range probed. The IMF of the low-metallicity halo is reminiscent of the Salpeter-like IMF that has been measured in massive early-type galaxies, a stellar population that, like Milky-Way halo stars, has a high ratio of alpha elements to iron, [alpha/Fe]. Blue-sequence stars are likely the debris from accretion by the Milky Way, ~10 Gyrs ago, of the Gaia-Enceladus dwarf galaxy, or similar events. These results hint at a distinct mode of star formation common to two ancient stellar populations -- elliptical galaxies and galaxies possibly accreted early-on by ours.

Accepted to MNRAS; Stellar-Track-based Assignment of Mass (STAM) code is available at https://github.com/naamach/stam

References in corpus (20)

• The NumPy array: a structure for efficient numerical computation
• The Gaia mission
• Improving PARSEC models for very low mass stars
• A systematic variation of the stellar initial mass function in early-type galaxies
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star forming dwarf galaxies
• Gaia Data Release 1: Catalogue validation
• The influence of residual gas expulsion on the evolution of the Galactic globular cluster system and the origin of the Population II halo
• Variations of the stellar initial mass function in the progenitors of massive early-type galaxies and in extreme starburst environments
• Gaia DR2 reveals a star formation burst in the disc 2-3 Gyr ago
• The brightness of the Red Giant Branch tip: Theoretical framework, a set of reference models, and predicted observables
• IMF shape constraints from stellar populations and dynamics from CALIFA
• The Stellar Initial Mass Function in Early-Type Galaxies From Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the center of NGC 1407 from Non-Parametric Models
• Spatially resolved variations of the IMF mass normalisation in early-type galaxies as probed by molecular gas kinematics
• Sub one percent mass fractions of young stars in red massive galaxies
• The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2
• Towards a fully consistent Milky Way disc model - III. Constraining the initial mass function
• The delay time distribution of Type-Ia supernovae in galaxy clusters: the impact of extended star-formation histories
• The Salpeter IMF and its descendants
• Self-similarity in the chemical evolution of galaxies and the delay time distribution of SNe Ia

Cited by in corpus (16)

• Feedback-Free Starbursts at Cosmic Dawn: Observable Predictions for JWST
• The homogeneity of the star forming environment of the Milky Way disk over time
• Exploring the Extremes: Characterizing a New Population of Old and Cold Brown Dwarfs
• The chemical DNA of the Magellanic Clouds -- I. The chemical composition of 206 Small Magellanic Cloud red giant stars
• Dissecting the Gaia HR diagram within 200 pc
• The family pictures of our neighbours: investigating the mass function and dynamical parameters of nearby open clusters
• The Stellar "Snake" I: Whole Structure and Properties
• Searching for Anomalies in the ZTF Catalog of Periodic Variable Stars
• Population demographics of white dwarf binaries with intermediate separations: Gaia constraints on post-AGB mass transfer
• Faint Stars in a Faint Galaxy: II. The Low Mass Stellar Initial Mass Function of the Boötes I Ultrafaint Dwarf Spheroidal Galaxy
• Towards a fully consistent Milky Way disk model V. The disk model for 4 $-$ 14 kpc
• The Earliest Stage of Galactic Star Formation
• Using Binary Population Synthesis to Calculate the Yields of Low- and Intermediate-Mass Binary Populations at Low Metallicity
• Do Red Galaxies Form More Stars Than Blue Galaxies?
• Stellar initial mass function in the 100-pc solar neighbourhood
• Neutron-capture elements record the ordered chemical evolution of the disc over time