The Fornax Deep Survey with the VST

Abstract

Context. Low surface brightness (LSB) dwarf galaxies in galaxy clusters are an interesting group of objects as their contribution to the galaxy luminosity function and their evolutionary paths are not yet clear. Increasing the completeness of our galaxy catalogs is crucial for understanding these galaxies, which have effective surface brightnesses below 23 mag arcsec−2 (in optical). Progress is continuously being made via the performance of deep observations, but detection depth and the quantification of the completeness can also be improved via the application of novel approaches in object detection. For example, the Fornax Deep Survey (FDS) has revealed many faint galaxies that can be visually detected from the images down to a surface brightness level of 27 mag arcsec−2, whereas traditional detection methods, such as using Source Extractor (SE), fail to find them. Aims. In this work we use a max-tree based object detection algorithm (Max-Tree Objects, MTO) on the FDS data in order to detect previously undetected LSB galaxies. After extending the existing Fornax dwarf galaxy catalogs with this sample, our goal is to understand the evolution of LSB dwarfs in the cluster. We also study the contribution of the newly detected galaxies to the faint end of the luminosity function. Methods. We test the detection completeness and parameter extraction accuracy of MTO using simulated and real images. We then apply MTO to the FDS images to identify LSB candidates. The identified objects are fitted with 2D Sérsic models using GALFIT and classified as imaging artifacts, likely cluster members, or background galaxies based on their morphological appearance, colors, and structure. Results. With MTO, we are able to increase the completeness of our earlier FDS dwarf catalog (FDSDC) 0.5–1 mag deeper in terms of total magnitude and surface brightness. Due to the increased accuracy in measuring sizes of the detected objects, we also add many small galaxies to the catalog that were previously excluded as their outer parts had been missed in detection. We detect 265 new LSB dwarf galaxies in the Fornax cluster, which increases the total number of known dwarfs in Fornax to 821. Using the whole cluster dwarf galaxy population, we show that the luminosity function has a faint-end slope of α = −1.38 ± 0.02. We compare the obtained luminosity function with different environments studied earlier using deep data but do not find any significant differences. On the other hand, the Fornax-like simulated clusters in the IllustrisTNG cosmological simulation have shallower slopes than found in the observational data. We also find several trends in the galaxy colors, structure, and morphology that support the idea that the number of LSB galaxies is higher in the cluster center due to tidal forces and the age dimming of the stellar populations. The same result also holds for the subgroup of large LSB galaxies, so-called ultra-diffuse galaxies.

The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter

“…Even if we conservatively assume that the FDS dwarfs have only 10× as much dark matter as stars within their optical radius, we still get a 2.29σ tension with expectations (Equation 11). Therefore, our results reliably show that the ΛCDM paradigm is in serious tension with observations of perturbed dwarf galaxies in the Fornax Cluster (observations which are strongly suggestive of tidal effects, see also section 7.4 of Venhola et al 2022).…”

“…This implies at face value that in ΛCDM, the observed signs of tidal disturbance (section 7.4 of Venhola et al 2022) cannot be assigned to either cluster tides or to harassment. Since we explore the impact of cluster tides more carefully later in this contribution, we briefly reconsider our calculation of η har .…”

“…The FDS catalogue also provides information about other properties of the dwarfs. The ones which are relevant for this project are: M , the effective radius, the right ascension and declination, the apparent surface brightness in the r band, the Sérsic index of the surface brightness profile (Sérsic 1963), the morphological type, the nucleated flag indicating if the dwarf is nucleated or non-nucleated, and the tidal morphology (undisturbed, possibly/mildly disturbed, very disturbed, or unclear; Venhola et al 2022). The effective radius, the Sérsic index, and the apparent brightness in the r -band are obtained by fitting the data to a 2D Sérsic profile (Venhola et al 2018) using and in the r band with a filter enhancing the dwarf's low surface brightness features (right column).…”

The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter

“…The FDS catalogue also provides information about other properties of the dwarfs. The ones which are rele v ant for this project are: M , the ef fecti ve radius, the right ascension (RA) and declination (Dec), the apparent surface brightness in the r band, the S érsic index of the surface brightness profile (S érsic 1963 ), the morphological type, the nucleated flag indicating if the dwarf is nucleated or nonnucleated, and the tidal morphology (undisturbed, possibly/mildly disturbed, very disturbed, or unclear; Venhola et al 2022 ). The ef fecti ve radius, the S érsic index, and the apparent brightness in the r -band are obtained by fitting the data to a two-dimensional (2D) S érsic profile (Venhola et al 2018 ) using the GALFIT software (Peng et al 2002 ).…”

“…The morphological classifications such as the nucleated flags, the Hubble type (Venhola et al 2018(Venhola et al , 2019, and the tidal morphologies are done visually. The tidal morphology is classified in Venhola et al ( 2022 ) based on the following criteria: (i) Undisturbed: Dwarf galaxies that do not present irregularities, distortions to their shape, or tidal tails;…”