[Paper Review] The central region of the Fornax cluster -- III. Dwarf galaxies, globular clusters, and cD halo - are there interrelations?

M. Hilker, L. Infante+1 more|ArXiv.org|May 10, 1999

Spectroscopy and Laser Applications63 citations

TL;DR

This paper investigates whether the rich globular cluster system (GCS) and cD halo in the Fornax cluster's central galaxy NGC 1399 could have formed via accretion and dissolution of dwarf galaxies. It proposes that infall of gas-poor and gas-rich dwarfs—through stripping of globular clusters and gas, and efficient in-situ GC formation from stripped gas—can explain the observed bimodal metallicity and high specific frequency of the central GCS, though additional mechanisms are likely needed to satisfy all constraints.

ABSTRACT

In this paper we briefly review the properties of the dwarf galaxy population at the core of the Fornax cluster, as well as the properties of the extraordinary rich globular cluster system and the cD halo around the central galaxy NGC 1399. In turn, the question whether a scenario in which dwarf galaxies have been accreted and dissolved in the cluster center can explain the observed properties is addressed. The possibility of accretion of a certain number of dwarf galaxies, the stripping of their globular clusters and gas, and the formation of new globular clusters from stripped infalling gas are discussed. An increase in the specific frequency of the central globular cluster system is only possible, if the infalling gas from stripped dwarfs formed globulars very efficiently and/or accreted and stripped dwarf galaxies possessed a rich globular cluster system themselves. In conclusion, we argue that although the dwarf galaxy infall is a very attractive scenario to explain a number of properties in the system there are probably other physical mechanisms that also take place.

Motivation & Objective

• To determine whether the accretion of dwarf galaxies can explain the formation of the central globular cluster system (GCS) and cD halo in the Fornax cluster.
• To assess the viability of dwarf galaxy infall in producing the observed bimodal metallicity distribution and high specific frequency ($S_N$) of the GCS around NGC 1399.
• To evaluate the required conditions—such as GC formation efficiency and initial luminosity function slope—for the infall scenario to reproduce observed properties.
• To test whether the accretion of gas-poor dwarfs alone can account for the metal-poor GC population, and whether gas-rich infall is necessary for metal-rich GC formation.
• To explore the role of additional processes beyond dwarf infall, such as stripping of more massive galaxies, in enriching the central system.

Proposed method

• Modeling the accretion of gas-poor and gas-rich dwarf galaxies into the dense central potential of the Fornax cluster.
• Estimating the efficiency of globular cluster formation from stripped infalling gas, comparing it to starburst and merging galaxy conditions.
• Simulating the accretion and stripping of pre-existing globular cluster systems from early-type dwarfs to assess their contribution to the central GCS.
• Analyzing the required faint-end slope ($\alpha$) of the initial luminosity function (LF) of dwarfs to match observed $S_N$ values.
• Using metallicity constraints to infer age spreads and formation sequences: requiring metal-rich GCs to be at least 2 Gyr older than metal-poor ones.
• Evaluating the dynamical feasibility of high accretion rates and long-term evolution of stripped GCs in the cluster potential using theoretical and simulation-based reasoning.

Experimental results

Research questions

• RQ1 Can the observed bimodal metallicity distribution of the globular cluster system around NGC 1399 be explained by the accretion and dissolution of dwarf galaxies?
• RQ2 What level of globular cluster formation efficiency from stripped gas is required to account for the high specific frequency ($S_N$) of the central GCS?
• RQ3 Is the accretion of gas-poor dwarfs sufficient to explain the metal-poor GC population, or is additional formation from stripped gas necessary?
• RQ4 What constraints do the observed metallicity and $S_N$ values place on the initial faint-end slope of the dwarf galaxy luminosity function?
• RQ5 Are there observational or dynamical inconsistencies in the dwarf infall scenario that suggest the need for additional processes, such as stripping of more massive galaxies?

Key findings

• The accretion of gas-poor dwarfs alone cannot explain the observed metal-poor GC population unless the initial luminosity function has a very steep faint-end slope ($\alpha < -1.4$), which leads to a predicted mean metallicity 0.5 dex more metal-poor than observed.
• To achieve the observed $S_N \approx 10$ for the central GCS, approximately 6000 dwarfs would need to be accreted, with about 50% of faint dwarfs ($-12.5 < M_V < -10.5$) possessing at least one globular cluster.
• A significant increase in $S_N$ via in-situ GC formation from stripped gas is only possible if the cluster formation efficiency is as high as in starburst or merging galaxies.
• If metal-rich GCs formed primarily from stripped gas, the gas must have been pre-enriched to at least $[Fe/H] \approx -1.0$ dex, implying that metal-rich GCs are at least 2 Gyr older than metal-poor ones.
• The bimodal metallicity distribution of the GCS implies a significant age spread, with metal-rich clusters forming after a delay of at least 2 Gyr.
• While dwarf galaxy infall is a plausible mechanism for forming the cD halo and central GCS, the stringent constraints suggest that additional processes—such as stripping of massive galaxies like NGC 1380 and NGC 1404—are likely involved.