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PhD Thesis project in preparation by Toni Paradell:
A systematic survey of "missing satellites" around Milky Way-like galaxies in the local Universe with robotic small telescopes
Objectives
The main objective is to contribute to the census and study of dwarf satellite galaxies beyond the Local Group, a very active field of study with a growing number of publications (Javanmardi et al. 2016; Henkel et al. 2017; Tanaka et al 2018; Karachentsev et al 2020; Mutlu-Pakdil et al 2021).
The objective is, therefore, to identify and characterize dwarf galaxies around Milky Way-like galaxies in the local Universe.
Theoretical Framework
The standard cosmological model ΛCDM has proven to be very effective in describing the structure and evolution of the Universe at large scales and in galaxy formation processes. However, at smaller scales, such as host galaxies with their dwarf satellite galaxies, the simulations based on this model applied to the Local Group show divergences regarding the number and distribution of those dwarf satellite galaxies. These divergences have motivated an intense activity both in the theoretical field to improve the models, and in the observational field to increase the number and quality of observations of dwarf galaxies, extending them to groups of nearby galaxies beyond the Local Group.
Cosmological models and predictions at small scale
The analysis of the satellite galaxies that have been identified in the Local Group has revealed that their number is lower than expected, the so-called problem of missing satellites (Strigari et al. 2007), and they are distributed in planes (Müller et al. 2018).
Milky Way satellites in radius < 250 kpc: prediction (left) and observed (right)Source: Bullock, J. S. y Boylan-Kolchin, M.: 2017, Annual Review of A&A, 55(1), 343
The problem of missing satellites
According to the simulations carried out with the ΛCDM model (Garrison-Kimmel et al. 2014), in the Local Group there should be about 1000 galaxies with a mass ≥ 103 M⊙, however the observed satellites of the Milky Way and Andromeda do not reach a hundred according to the updated version of McConnachie (2012) survey. This discrepancy could be explained with the hypothesis that sub-halos of dark matter below a certain limit would not form stellar mass (Bullock and Boylan-Kolchin 2017). The introduction of new baryonic physics models in the simulations seems to confirm this hypothesis (Tanaka et al. 2018).
The anisotropic distribution of satellites in the Local Group
Simulations of the ΛCDM model also show that the distribution of satellites around their galaxies should be essentially isotropic (Kroupa et al. 2005; Müller et al. 2018). However, the known satellites in the Milky Way have a completely anisotropic distribution, mostly concentrated in a polar plane (Pawlowski et al. 2015), and a similar structure is observed in Andromeda (Ibata et al. 2013). Nevertheless, current simulations show that also in this case, introducing baryonic physics makes ΛCDM produce structures similar to those observed (Dutton et al. 2016).
In order to validate these theories and simulations, wide-field, deep observations of nearby massive galaxies are needed to obtain a complete census of satellite galaxies. These observations must be extended beyond the Local Group, otherwise there is the risk that we are adapting the models to the singularities of the Local Group, while such singularities may not be applicable to other systems (Carlsten et al. 2021). This survey can be directly compared with the cosmological simulation predictions. Although the results seem to agree with the observations for dwarf galaxies with a mass greater than 107 M⊙, this is not so clear if this is the case for galaxies with a lower mass (Fattahi et al. 2020).
Dwarf galaxies in the Local Group and in the Local Universe
There are already many dwarf galaxy surveys, and new instruments and telescopes are continually increasing them. The most referenced surveys that specifically addresses the Local Group is McConnachie (2012), which the author keeps updated on his website.
Many other surveys have addressed dwarf galaxies, such as, for example, DGSAT (Javanmardi et al. 2016; Henkel et al. 2017; Martinez-Delgado 2020), Karachentsev (Karachentseva and Karachentsev 1998; Karachentsev et al. 2013, 2015, 2020), KMNET (Park et al. 2017, 2019; Byun et al. 2020), Vera Rubin/Subaru (Mutlu-Pakdil et al. 2021), CFHT (van Dokkum et al. 2015; Muñoz et al 2018; Carlsten et al. 2020), MATLAS (Habas et al. 2020; Poulain et al. 2021), and SMUDGes (Zaritsky et al. 2019).
Methodology and resources
The search for dwarf galaxies starts with the selection of the host target galaxies, considering criteria such as coverage, morphology, distance to the host galaxy, and galactic latitude. Once the host galaxies have been selected, we search for dwarf galaxies in the images obtained through small robotic telescopes, following the same approach to those of the DGSAT project (Javanmardi et al. 2016; Henkel et al. 2017; Martinez-Delgado 2020). An example of such robotic telescope that we are considering is the Telescope Fabra ROA Montsec (TFRM) at the Montsec Observatory (Fors et al. 2013).
We also take advantage of surveys with large coverage and deep images, such as the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys (Dey et al. 2019), in short the Legacy Surveys (LS). The current official data release of the LS is DR10, which was made available to the scientific community in December 2022.
Anticipated results
Once we have selected the host galaxies, we will identify candidate satellite dwarf galaxies inside their virial radii. For each identified candidate, we will gather as much information as possible to verify that it is indeed a satellite of the host galaxy, including object geometry, photometric information (magnitudes, surface brightness, half-light radius), and if possible spectrometric information. While the census and characterization of dwarf satellite galaxies is an objective by itself, the survey will also contribute to a better understanding of dwarf satellite galaxies in the local Universe and whether the previously mentioned problems seem to still persist or not at this scale.
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