Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites<sup>∗</sup>

Gennaro, Mario; Tchernyshyov, Kirill; Brown, Thomas M.; Geha, Marla; Avila, Roberto J.; Guhathakurta, Puragra; Kalirai, Jason; Kirby, Evan N.; Renzini, A.; Simon, Joshua D.; Tumlinson, Jason; Vargas, Luís Carlos Orozco

doi:10.3847/1538-4357/aaa973

Cited by 73 publications

(128 citation statements)

References 48 publications

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“…More recent analyses have complicated this picture. Gennaro et al (2018b) measured the IMFs for the full sample of six UFDs from Brown et al (2014), confirming that each galaxy has an IMF slope shallower than Salpeter when fit with a power law. However, when describing the IMF as a log-normal function (Chabrier 2003), the parameters for the UFDs are consistent with the Milky Way IMF.…”

Section: Initial Mass Functionsmentioning

confidence: 77%

The Faintest Dwarf Galaxies

Simon

2019

Annu. Rev. Astron. Astrophys.

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523

384

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The lowest luminosity ([Formula: see text] L[Formula: see text]) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter–dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales. In this review, we summarize the discovery of ultra-faint dwarfs in the Sloan Digital Sky Survey in 2005 and the subsequent observational and theoretical progress in understanding their nature and origin. We describe their stellar kinematics, chemical abundance patterns, structural properties, stellar populations, orbits, and luminosity function, as well as what can be learned from each type of measurement. We conclude the following: ▪ In most cases, the stellar velocity dispersions of ultra-faint dwarfs are robust against systematic uncertainties such as binary stars and foreground contamination. ▪ The chemical abundance patterns of stars in ultra-faint dwarfs require two sources of r-process elements, one of which can likely be attributed to neutron star mergers. ▪ Even under conservative assumptions, only a small fraction of ultra-faint dwarfs may have suffered significant tidal stripping of their stellar components. ▪ Determining the properties of the faintest dwarfs out to the virial radius of the Milky Way will require very large investments of observing time with future telescopes. Finally, we offer a look forward at the observations that will be possible with future facilities as the push toward a complete census of the Local Group dwarf galaxy population continues.

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Section: Initial Mass Functionsmentioning

confidence: 77%

The Faintest Dwarf Galaxies

Simon

2019

Annu. Rev. Astron. Astrophys.

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523

384

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“…Recent measurements with the Hubble Space Telescope by Gennaro et al (2018b) of UFD satellite galaxies, as an extension of the study by Geha et al (2013), have suggested a possible gwIMF variation in these galaxies in the stellar-mass range (0.4−0.8 M ). The authors, however, mentioned that a larger data sample is needed to improve the reliability of the presented results.…”

Section: Ultra-faint Dwarf Galaxiesmentioning

confidence: 80%

“…For the case of Leo P, the known extent and baryonic matter in Leo P and the flat (non-rising) part of the rotational curve are UFDs shown by the coloured points from Gennaro et al (2018b). The IGIMF1 models (given by two slopes α 1 = 1.3 and α 2 = 2.3) are represented by the horizontal green line.…”

Section: Implications For the Btfr And The Cmdmentioning

confidence: 99%

Impact of metallicity and star formation rate on the time-dependent, galaxy-wide stellar initial mass function

Jeřabková

Zonoozi

Kroupa

et al. 2018

A&A

146

192

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The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses. These initial stellar masses are generally represented by the canonical IMF, which is defined as the result of one star formation event in an embedded cluster. As a consequence, the galaxy-wide IMF (gwIMF) should also be invariant and of the same form as the canonical IMF; gwIMF is defined as the sum of the IMFs of all star-forming regions in which embedded clusters form and spawn the galactic field population of the galaxy. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation, it is useful to relate the observed IMF to the gwIMF. Starting with the IMF determined in resolved star clusters, we apply the IGIMF-theory to calculate a comprehensive grid of gwIMF models for metallicities, [Fe/H] ∈ (−3, 1), and galaxy-wide star formation rates (SFRs), SFR ∈ (10−5, 105) M⊙ yr−1. For a galaxy with metallicity [Fe/H] < 0 and SFR > 1 M⊙ yr−1, which is a common condition in the early Universe, we find that the gwIMF is both bottom light (relatively fewer low-mass stars) and top heavy (more massive stars), when compared to the canonical IMF. For a SFR < 1 M⊙ yr−1 the gwIMF becomes top light regardless of the metallicity. For metallicities [Fe/H] > 0 the gwIMF can become bottom heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top heavy within the first few hundred Myr of the life of the galaxy and that it evolves into a bottom heavy gwIMF in the metal-enriched galactic centre. Using the gwIMF grids, we study the SFR−Hα relation and its dependency on metallicity and the SFR. We also study the correction factors to the Kennicutt SFRK − Hα relation and provide new fitting functions. Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star-forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for gas-consumption timescales and for the main sequence of galaxies. We explicitly discuss Leo P and ultra-faint dwarf galaxies.

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“…Galaxies with a high star formation rate are known to be producing stellar populations overabundant in massive stars (Gunawardhana et al 2011), while galaxies with a low star formation rate show a deficit of massive stars (Lee et al 2009;Watts et al 2018). Old, dormant galaxies also show significant variations of their stellar populations: elliptical galaxies may be dominated by very low mass stars (van Dokkum & Conroy 2010), while faint diffuse dwarf galaxies have a deficit of low mass stars (Gennaro et al 2018) when compared to the canonical stellar population (Yan et al 2017;Jeřábková et al 2018).…”

Section: The Properties Of Ngc 1052-df2 At Different Distancesmentioning

confidence: 99%

A new formulation of the external field effect in MOND and numerical simulations of ultra-diffuse dwarf galaxies – application to NGC 1052-DF2 and NGC 1052-DF4

Haghi

Kroupa

Banik

et al. 2019

Monthly Notices of the Royal Astronomical Society

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The ultra-diffuse dwarf galaxy NGC 1052-DF2 (DF2) has ten (eleven) measured globular clusters (GCs) with a line-of-sight velocity dispersion of σ = 7.8 +5.2 −2.2 km/s (σ = 10.6 +3.9 −2.3 km/s). Our conventional statistical analysis of the original ten GCs gives σ = 8.0 +4.3 −3.0 km/s. The overall distribution of velocities agrees well with a Gaussian of this width. Due to the non-linear Poisson equation in MOND, a dwarf galaxy has weaker self-gravity when in close proximity to a massive host. This external field effect is investigated using a new analytic formulation and fully self-consistent live N-body models in MOND. Our formulation agrees well with that of Famaey and McGaugh (2012). These new simulations confirm our analytic results and suggest that DF2 may be in a deep-freeze state unique to MOND. The correctly calculated MOND velocity dispersion agrees with our inferred dispersion and that of van Dokkum et al. (2018b) if DF2 is within 150 kpc of NGC 1052 and both are 20 Mpc away. The GCs of DF2 are however significantly brighter and larger than normal GCs, a problem which disappears if DF2 is significantly closer to us. A distance of 10-13 Mpc makes DF2 a normal dwarf galaxy even more consistent with MOND and the 13 Mpc distance reported by Trujillo et. al. (2019). We discuss the similar dwarf DF4, finding good agreement with MOND. We also discuss possible massive galaxies near DF2 and DF4 along with their distances and peculiar velocities, noting that NGC 1052 may lie at a distance near 10 Mpc.

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Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites^∗

Cited by 73 publications

References 48 publications

The Faintest Dwarf Galaxies

The Faintest Dwarf Galaxies

Impact of metallicity and star formation rate on the time-dependent, galaxy-wide stellar initial mass function

A new formulation of the external field effect in MOND and numerical simulations of ultra-diffuse dwarf galaxies – application to NGC 1052-DF2 and NGC 1052-DF4

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Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites∗

Cited by 73 publications

References 48 publications

The Faintest Dwarf Galaxies

The Faintest Dwarf Galaxies

Impact of metallicity and star formation rate on the time-dependent, galaxy-wide stellar initial mass function

A new formulation of the external field effect in MOND and numerical simulations of ultra-diffuse dwarf galaxies – application to NGC 1052-DF2 and NGC 1052-DF4

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Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites^∗