A supergroup in a supercluster, through the lens of the low-surface brightness Universe

Maria Angela Raj 1 , Enrica  Iodice 2 , Marilena  Spavone 2

  • 1 INAF Astronomical Observatory of Rome, Rome
  • 2 INAF Astronomical Observatory of Capodimonte, Naples

Abstract

According to the Lambda-Cold Dark Matter cosmological model for galaxy formation, the densest part of the large-scale structure (LSS) is built over time by accreting smaller clusters and groups of galaxies; recent simulations (Illustris, Eagle) have further supported this scenario. The infalling galaxies which contribute to the cluster mass can experience environmental mechanisms before they reach the core of a cluster, otherwise known as “pre-processing”. To constrain the formation history of the galaxies in the LSS, deep photometry is required to explore their faint outskirts as these regions are the relics of past processes. Additionally, some studies have shown that deep images can change the way we morphologically classify galaxies due to the detections of several LSB features.

The Fornax-Eridanus filament is at a distance < 20 Mpc, comprising several subgroups and clusters. Within this supercluster, it was previously established that the Eridanus supergroup is composed of three groups (NGC 1332, NGC 1395, NGC 1407) and will eventually merge to form a cluster with NGC 1407 as its brightest cluster galaxy. A supergroup of this nature in the local Universe had raised questions pertaining to its formation history. Therefore, we trace the evolutionary path of the bright galaxies (mB < 16 mag) in the Eridanus supergroup observed with VEGAS (VST Early-type GAlaxy Survey). We use current tools based on Max-Tree Objects to detect faint objects and features. The preliminary results of the Eridanus supergroup include analysis (morphology, structural parameters) of the galaxies in the dynamically evolved group NGC 1407, by mapping their light distribution down to 28-30 mag/arcsec². We also study the galaxies’ faint features which show evidence of past interactions and merger histories and detect disc-breaks, disc-truncations, which are vital clues to understanding the evolutionary phase of the group. In the effort to map the accretion history of this group in the supercluster, we estimate the intra-group light.

As future perspectives involve the analysis of the other two (NGC 1332, NGC 1395) groups part of the Eridanus supergroup, we connect the properties of this supergroup to those of other subsystems (e.g. Fornax cluster) and provide a new perspective of this structure in the Fornax-Eridanus supercluster, visualised in the LSB Universe. In a larger framework, our goal is to study the mass assembly history of the brightest parts of the LSS, which would further broaden our understanding of structure formation. With this in mind, we also aim to develop tools that can define the filaments in superclusters, which are only detectable in the ultra-LSB regime.

The Fornax-Eridanus Supercluster

Fig. 1. Top left panel: The Fornax-Eridanus Supercluster. Black triangles represent the brightest cluster/group member; Teal dots represent LTGs mB < 16 mag (mB=16 at 20 Mpc corresponds to MB of -15.5 mag), red dots are S0 (mB < 16 mag), and peach dots are early-type galaxies (ETGs mB < 16 mag) taken from Brough et al. (2006). Teal crosses are galaxies mB < 16 mag (Ferguson 1989), purple dots are mJ < 8 mag with velocities V < 2000 km/s (2MASSX, Huchra et al. 2012).  Middle panel: Eridanus supergroup with coloured squares indicating the 1x1 deg2 (FoV of OmegaCAM @VST) for observations with VEGAS. Right panel: NGC1407 group with bright and dwarf galaxies indicated. 

The Fornax-Eridanus supercluster (Fig. 1: left panel) is at a distance ~ 20 Mpc, comprising several subgroups and clusters. It extends from the NGC1407 group in the north to the Dorado group in the south, and in front of the "Great Wall". The Eridanus supergroup (Fig 1.: middle panel) is the densest region in the north of the supercluster, made up of three individual groups (NGC1332, NGC1395, NGC1407) of galaxies and other neighbouring triplets or pairs of galaxies (Brough et al. 2006; Nasonova et al. 2011) which will likely merge to form a cluster with NGC1407 as its BCG (brightest cluster galaxy).

 

NGC1407 group

This group (Fig. 1: right panel) is centered on the elliptical galaxy NGC1407 and is the brightest  (mB = 11 mag; Ferguson 1990) galaxy in the Eridanus supergroup. This group is dynamically evolved and has been a subject of interest. NGC1400 (the second brightest member in the group) and NGC1407 are located very close in projection (~ 12 arcmin) but have a large difference in velocity (Δν ∼ 1200 km/s; Su et al. 2014).  

VST Early-type GAlaxy Survey (VEGAS)

VEGAS (P.I. E. Iodice; Iodice et al. 2020a) is a deep, multi-band (u, g, r, i) imaging survey observed with the VLT Survey Telescope (VST). The survey maps the galaxy structure down the low-surface brightness (LSB) regime (μg~30mag/arcsec2) thus enabling us to study the faint features of galaxies that are prone to environmental processes. VEGAS has provided detailed studies on galaxy stellar halos, intra-cluster light, globular clusters, dwarf galaxies, and features in different environments (e.g. groups, clusters). In the context of galaxy evolution in dense environments, previous results (see Iodice et al. 2019a and references therein), provided insights into pre-processing of galaxies before cluster infall. VEGAS will observe the unexplored regions (Fig. 1.: middle panel) connecting the densest parts of the large-scale structure of the Fornax-Eridanus supercluster- in particular, the Eridanus supergroup. In this work, we present the preliminary results on NGC1407. 

Preliminary results

Fig. 3. Top right panel: LSB features detected in the NGC1407 group are indicated. Bottom panel: The surface brightness level contours at μg= 30 mag/arcsec2 are shown in blue.  X-ray contour levels at 0.5-2.0 keV (taken from XMM-Newton image; Su et al. 2014) are shown in black.

NGC1407 and NGC1400: possible tidal interaction? 

By visual inspection of the images, we detect asymmetries in two of the brightest galaxies in the NGC1407 group. Spolaor et al. (2008) ruled out the possibility of an interaction between NGC1400 and NGC1407 due to the absence of tidal disturbances in the outskirts of these galaxies. However, their surface photometry reached a limit of μg~25mag/arcsec2, r~100 arcsec. Given the depth of the images analysed here (μg~30mag/arcsec2), we find several faint features (e.g loop, tidal tail; see Fig. 3) and therefore, speculate that there is an ongoing interaction between NGC1400 and NGC1407, possibly due to NGC1400's first infall. Previously, Su et al. (2014) detected a region of enhanced X-ray surface brightness east side of NGC1400 to be that of a hot gas tail, stripped from NGC1400. Given the large relative velocity (~1200km/s) of NGC1400, they concluded that it was a ram-pressure stripping event and is likely due to this galaxy recently entering the group medium. The tidal tail of NGC1400 detected here (μr~29mag/arcsec2) also coincides with the X-ray detection by Su et al. (2014) (see Fig. 3) and therefore, we suspect that it is the result of the ongoing interaction with NGC1407. The bridge (μr~30mag/arcsec2) between these galaxies is about 3 arcmin (~19.5kpc) long. A similar bridge (between NGC1399 and NGC1387) was also found in the Fornax cluster core (Iodice et al. 2016), however as this work is still in progress, we do not draw any conclusions here. 

Photometry

The preliminary step in our robust methodology is to employ Max-Tree Objects (Teeninga et al. 2013, 2016) to detect faint features of galaxies (code developed by Haigh et al. 2021) followed by deriving observables from deep photometry. For all the galaxies in our sample, we: (i) characterise their morphology: 1D fitting of their surface brightness (SB) profiles,  2D models out to their faint regions to study asymmetries and detect remnants of past merger events (e.g. tidal tails); (ii) detect disc-breaks/disc-truncations using novel methods (Raj et al. 2019; Trujillo et al. 2020); (ii) derive colour gradients, e.g. colours of the inner versus outer discs for galaxies with disc-breaks and/or disc-truncations, which provide evidence of the ongoing/past processes (see Fig. 2)

NGC1390

Fig. 2. MTObjects LSB feature detection (left panel) and surface photometry (right panel) of NGC1390. 


Right panel: Surface brightness contour levels at 30 mag/arcsec2 are shown in blue in the r-band image (top left);  g-r colour map (top right); SB profiles in the g and r bands (bottom left) and g-r colour profile (bottom right). 

NGC1390 is an SBa galaxy located southwest of the NGC1407 group. This galaxy appears to be asymmetric and is more extended on the east side.  The sources on the southeast side of this galaxy are background, which is evident in the colour map (g-r >1.1 mag) and could be a possible explanation for the reddening in the outskirts seen in the g-r profile (Fig. 2). As this work is in progress, we do not draw conclusions here on the formation of the observed asymmetry. 

Work in progress

  • We are currently studying the structure and evolution of all galaxy members in the NGC1407 group and will also provide an estimate of the intra-group light. 
  • Our complete analysis of the Eridanus supergroup will elucidate the physics of the mass assembly in large-scale structures and give an in-depth understanding of pre-processing of galaxies before cluster infall. 

In order to define the filamentary structure, we are currently looking into algorithms so if you have any suggestions, do write to me.

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