Free Access
Issue
A&A
Volume 609, January 2018
Article Number A48
Number of page(s) 9
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201731711
Published online 05 January 2018

© ESO, 2018

1. Introduction

The structure formation in the Universe is driven by mergers and accretion of small substructures into larger ones (White & Rees 1978), and these processes are still active at present. Nevertheless, a fraction of the structures present today could be relics of the primitive Universe. These systems are called fossil groups (FG) or, more generically, fossil systems (FS). These ancient galaxy aggregations were formed at high redshift (z> 1) with few or no subsequent accretion (D’Onghia et al. 2005). But, do these objects really exist today? Are there systems in the Universe for which the hierarchical formation is frozen since several Gyr ago? If this kind of system exists, they are important because they contain valuable information about the assembly of massive halos in the primitive Universe. We can therefore understand the formation processes that occurred several Gyr ago by analyzing the properties of nearby FSs.

Ponman et al. (1994) established that the group RX J1340.6+4018 was the remnant of an ancient system of galaxies. This system shows extended X-ray emission, indicating that the group is embedded in a massive dark matter halo. In addition, its light is dominated by a central early-type galaxy. The 2nd ranked luminous galaxy belonging to the group is more than 2 mag fainter than the brightest one. The lack of L galaxies in the system was interpreted as the result of the central galaxy cannibalizing the surrounding luminous objects over time. These characteristics made this system a candidate for an early-formed and relaxed structure, and it was classified as a fossil relic.

After the discovery of RX J1340.6+4018, other systems were observed with large magnitude gaps between their 1st and 2nd ranked galaxies (Δm12). The presence of a large Δm12 in those systems was interpreted as indication of fossilness. Jones et al. (2003) gave the first observational definition of FSs as those galaxy aggregations with Δm12> 2.0 mag in the r-band for the galaxies of the systems within half its virial radius. In addition, the central galaxy should be surrounded by an extended diffuse X-ray halo with erg s-1. This definition was slightly modified by Dariush et al. (2010) according to the analysis of cosmological simulations. They claim that the magnitude gap between the 1st and 4th ranked galaxies of the system (Δm1,4) was a better quantity to select old and dynamically relaxed galaxy aggregations. In particular, they proposed that fossil systems are those with Δm1,4> 2.5 for galaxies within half of their virial radius.

Several samples containing fossil candidates have been selected during the last decade in different galaxy surveys based on large values of their Δm12 (e.g., Khosroshahi et al. 2007; Santos et al. 2007; Voevodkin et al. 2010; Proctor et al. 2011; Harrison et al. 2012). Many works have obtained the observational properties of these systems. These properties can be grouped in (i) properties of the hot intracluster component; (ii) properties of the central dominant galaxy or the brightest group galaxy (hereafter BGG); and (iii) properties of the galaxy satellite population.

The global X-ray scaling relations of dark matter halos are related to their formation. Halos formed early should have some imprints in their X-ray properties. In particular, more concentrated dark matter halos collapse first in the hierarchical scenario, which can produce a greater compression of the gas in the central regions of the halo and enhance the X-ray temperature (TX) and luminosity (LX). Some works suggest that X-ray global scaling relations in FS and non-FS are different. In particular, FSs were found to have larger TX than non-fossil ones (e.g., Khosroshahi et al. 2007). These differences between FS and non-FS were also reported in scaling relations combining optical and X-ray quantities, such as the LXLopt or LXσ relations (e.g., Proctor et al. 2011; Khosroshahi et al. 2014); they were interpreted as an indication that the halos of FSs formed early in the high redshift Universe. Nevertheless, larger samples of FSs and non-FSs have obtained that those differences do not exist, indicating that according to the global scaling relations, fossil and non-fossil halos are similar (e.g., Harrison et al. 2012; Girardi et al. 2014; Kundert et al. 2015).

The expected early formation in FSs, with no later accretion, should also produce some peculiarities in the observational properties of their BGGs. In particular, their formation at high redshift could be driven by mergers with large amounts of gas (wet mergers). Khosroshahi et al. (2006) reported differences in the isophotes shape between the BGGs in FS and non-FS pointing towards a formation via wet mergers. They obtained that BGGs of FSs have more disky isophotes than those in non-FSs. Nevertheless, these differences in the isophotal shape were not confirmed by other samples of galaxies (Méndez-Abreu et al. 2012). Indeed, the scaling relations of BGGs in FSs show that they assembled a fraction of their mass by wet mergers probably at high redshift. However, a significant fraction of their mass was accreted via later dry mergers (Méndez-Abreu et al. 2012). The similar properties of the stellar populations of BGGs in FSs and non-FSs also point toward a similar formation (La Barbera et al. 2009; Eigenthaler & Zeilinger 2013; Trevisan et al. 2017). Nevertheless, the merging process in the BGGs of FSs has been especially effective because these galaxies are among the brightest galaxies observed in the Universe (Aguerri et al. 2011; Méndez-Abreu et al. 2012; Zarattini et al. 2014).

The galaxy population of FSs and non-FSs are clearly different in the bright-end of the galaxy luminosity function (LF). In particular, FSs show a lack of L galaxies (e.g., Khosroshahi et al. 2006; Mendes de Oliveira et al. 2006; Aguerri et al. 2011; Adami et al. 2012; Lieder et al. 2013; Khosroshahi et al. 2014; Zarattini et al. 2014). Additionally, there is also evidence for differences in the number of low-mass galaxies – FSs show LFs with shallower faint-end than non-FSs (Zarattini et al. 2015). This lack of substructure in fossil halos has been interpreted as a challenge for the hierarchical formation (D’Onghia & Lake 2004); nevertheless, this is still a puzzle and a matter of debate (Zibetti et al. 2009). The galaxy population in FSs also shows evidence of substructure similar to non-FSs, arguing against the relaxed and old dynamical age of FSs (Zarattini et al. 2016).

The previously reported properties indicate that selection of systems by only a large magnitude gap does not guarantee the selection of galaxy aggregations that are dynamically old, early forming, and with few subsequent accretions. This has also been pointed out by some numerical cosmological simulations that analyzed the properties of systems with large magnitude gaps. von Benda-Beckmann et al. (2008) showed that the fossilness of a system is a transition phase, whereby galaxy groups and clusters could pass several times through a fossil phase along their evolution. Recently, Kundert et al. (2017) found that, on average, the fossilness of a system changes over 23 Gyr. Some simulations also show that the last major merger in BGGs in FSs has occurred more recently than in similar galaxies in non-FSs (Díaz-Giménez et al. 2008; Kundert et al. 2017). The orbital structure of the galaxies in FSs increases the efficiency of mergers into the BGGs in FSs more than in non-FSs (Sommer-Larsen 2006). This produces FSs BGGs that are more massive on average than those in non-FSs (Kundert et al. 2017). Simulations also show that there are differences at the faint-end of the LF between FSs and non-FSs. Thus, Gozaliasl et al. (2014) obtained that there was almost no evolution in the faint-end LF in FSs since z ~ 1. In contrast, non-FSs show a strong evolution of the faint-end. Kundert et al. (2017) demonstrated that the most important difference between FSs and non-FSs is related with the halo mass accretion history over the past few Gyr. This difference in the last fraction of the accretion of the mass could be related with the surrounding large-scale environment (Díaz-Giménez et al. 2011). It is likely that a combination of the magnitude gap with other parameters provides a better selection of dynamically old systems. According to Raouf et al. (2014) the combination of the magnitude gap and the luminosity of the central BGG is one of the solutions to obtain dynamically old structures.

The present paper is part of the Fossil Group Origins (FOGO) project. This project aims at a multiwavelength characterization of a large sample of FSs (see Aguerri et al. 2011). We have analyzed so far the properties of their (i) dark matter halos (Girardi et al. 2014; Kundert et al. 2015), (ii) the central BGGs (Méndez-Abreu et al. 2012; Zarattini et al. 2014), and (iii) the galaxy populations (Zarattini et al. 2015, 2016). These observational properties have been compared with state-of-the-art cosmological simulations (Kundert et al. 2017). In the present work we have obtained deep spectroscopic data of the fossil group RX J075243.6+455653 down to . We have analyzed the properties of this group focusing on the faint-end of its galaxy LF and its large-scale nearby structure. As reported by Raouf et al. (2014), this group has the properties to be an old and evolved galaxy aggregation. In particular, it shows Δm12> 2.0 and one of the faintest BGGs (Mr = −22.67) observed in FSs (Zarattini et al. 2014).

The paper is organized as follows. Section 2 shows the data used. The results are presented in Sect. 3. The discussion and conclusions are given in Sects. 4 and 5, respectively. Through this paper we have used the cosmology given by H0 = 70 km s-1 Mpc-1, ΩΛ = 0.7, and Ωm = 0.3. Using this cosmology, RX J075243.6+455653 is located at a distance of 230.6 Mpc, resulting in a scale of 1.01 kpc arcsec-1.

2. The data for RX J075243.6+455653

We downloaded the photometric and velocity information for galaxy selected objects from Sloan Digital Sky Survey data release 6 (SDSS-DR6; Adelman-McCarthy et al. 2008) within a radius of 4 Mpc around the coordinates α(J2000) = 07h52m44.2s and δ(J2000) = + 45°56′57.4′′. These central coordinates correspond to the peak of X-ray emission detected from the ROSAT satellite and are associated with the group RX J075243.6+455653. This central X-ray peak has a luminosity of LX = 2.20 × 1043 erg s-1 (Zarattini et al. 2014).

The catalog turned out to have 89 galaxies brighter than mr ~ 18.0 with measured radial velocities. Using this data-set from SDSS-DR6, Zarattini et al. (2016) reported that the mean velocity of group (Vc) was 15498 km s-1, and the velocity dispersion σc = 259 km s-1. The estimate of LX allowed us to calculate r200 = 0.96 Mpc (Zarattini et al. 2014) using the relations from Böhringer et al. (2007) and Arnaud et al. (2005). The mass of the system was obtained following Girardi et al. (1998) and Girardi & Mezzetti (2001) resulting in M = 4.2 × 1013M (Zarattini et al. 2014).

The photometric catalog acquired from SDSS-DR6 was used to select targets for additional multi-object spectroscopy with different instruments. In particular, we selected those galaxies with gr< 1.1, mr< 19.5 and within 0.5° (~1.8 Mpc radius at the distance of the group) from the group center to be observed with the AF2 at WHT instrument. These objects will be referred to as spectroscopic targets. A total of 129 spectroscopic targets were selected and located in two AF2 fiber configurations. We obtained low-resolution spectra (R = 280 and grism R158B) in three exposures of 1800 s per pointing, with the spectra reaching S/N> 5 for a secure redshift determination (see e.g., Agulli et al. 2016). We also selected targets with gr< 1.1 and 19.5 <mr< 21.0 to be observed with the OSIRIS at GTC instrument. A total of 121 spectroscopic targets were located in nine different OSIRIS pointings within r200. In this case the low-resolution spectra (R = 360) of these objects were obtained using the grism R300B in three exposures of 1200 s for each pointing. These new observations consisted of a total of 250 new spectra.

The new spectroscopic data was reduced using different pipelines. The reduction of the data from AF2 at WHT was achieved with the instrument pipeline, version 3.0 (see Domínquez Palmero et al. 2014). In contrast, the data from OSIRIS at GTC was reduced by using the GTCMOS pipeline1.

2.1. Velocity determination and spectroscopic completeness

The recessional velocities of the new spectra were obtained using rvsao.xcsao IRAF task (see Kurtz et al. 1992). This task cross-correlates a template spectrum library with the observed galaxy spectrum. In this case we have used as template library the one given by Kennicutt (1992). A total of 191 new velocities were determined by this technique. This makes a total of 280 galaxies with measured radial velocities within 4 Mpc from the group center.

Figure 1 shows the velocity histogram of the galaxies in the direction of RX J075243.6+455653 and within an aperture of 4 Mpc. A prominent peak can be observed at Vc ~ 15 550 km s-1 which corresponds to the mean velocity of the group (see Zarattini et al. 2014). The velocity histogram also shows that the peak in velocity corresponding to the group is not isolated. In contrast, there is a continuum of background galaxies at larger velocities. The lack of isolation of this group can also be seen by the large number of galaxies with velocities Vc ± 3σc out from r200 (see Fig. 1). This suggest that the group is surrounded by other galaxies or groups.

thumbnail Fig. 1

Velocity histogram of the galaxies in the direction of RX J075243.6+455653. The black histogram shows the radial velocities of the galaxies within 4 Mpc from the cluster center. The gray filled histogram corresponds to galaxies within r200. The vertical line shows the mean velocity of the group. The dashed vertical lines are located at ± 3σc from Vc.

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The group members were determined by using a simple σ-clipping algorithm. Thus, we computed several times the mean velocity (Vc) and velocity dispersion σc by using the galaxies with recessional velocities within Vc ± 3 × σc and inside r200. This iterative process stopped when no changes in Vc and σc from one step to the other was obtained. This process returns a total of 26 groups members with mr< 21.0 and within r200. Using these group members we obtain: Vc = 15 551 ± 65 km s-1 and σc = 333 ± 46 km s-1. The remaining 254 objects with radial velocities out from the Vc ± 3 × σc range and within 4 Mpc from the group center will be called background galaxies.

Figure 2 shows the spectroscopic completeness (C) as a function of the r-band apparent magnitude (mr). This completeness was obtained by C = Nvel/Nphot, where Nvel and Nphot represents the number of galaxies with measured velocities and the number of spectroscopic targets per magnitude bin, respectively (see Agulli et al. 2014). In addition, Fig. 2 also shows the fraction of cluster members (fmem) as a function of mr. This fraction is given by: fmem = Nmem/Nvel, where Nmem shows the number of galaxy group members per magnitude bin (Agulli et al. 2014). The spectroscopic completeness is 100% for galaxies brighter than mr = 16.6. This completeness slowly decreases to ~ 80% at mr ~ 19, and ~ 25% at mr ~ 20.5. The fraction of members decreases steeply with the magnitude. At mr fainter than 18.0, less than 15% of the observed galaxies turned out to be group members. This indicates the low efficiency at faint magnitude for the selection of group members by a simple color cut.

thumbnail Fig. 2

Spectroscopic completeness (black) and fraction of group members (red dashed) as a function of the r-band magnitude.

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Figure 3 shows the sky distribution of the spectroscopic targets, background galaxies, and member galaxies within r200 around the center of RX J075243.6+455653.

thumbnail Fig. 3

Sky position of the spectroscopic targets (gray), background objects (black), and galaxy group members (red) within r200. The largest red point represents the position of the brightest galaxy in the group.

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3. Results

3.1. The fossilness of RX J075243.6+455653

Figure 4 shows the distance of the galaxies from the BGG as a function of their r-band apparent magnitude. According to the definition of fossil groups given by Jones et al. (2003), RX J075243.6+455653 is a fossil system. It shows a X-ray luminosity larger than 1042 erg s-1, and the magnitude gap between the 1st and 2nd ranked galaxies within 0.5r200 is Δm12 = 2.1. Furthermore, the magnitude gap between the 1st and 4th ranked galaxies of the system within 0.5r200 is Δm14 = 2.55 (Zarattini et al. 2014), meeting the fossil criteria of Dariush et al. (2010).

There are several group members within 0.5 <R/r200< 1.0 with magnitudes similar to the BGG. Thus, these galaxies result in Δm12< 2 within r200. We can speculate that probably these galaxies are in the process of falling into the group, and will pass closer to the group center and merge with the BGG in the next few Gyr. This would imply that the fossilness of RX J075243.6+455653 is a transition state as reported by some cosmological simulations (von Benda-Beckmann et al. 2008; Kundert et al. 2017). The transitory nature of the fossil state has also been reported in some other observed systems in the literature (e.g., Zarattini et al. 2014).

thumbnail Fig. 4

Magnitude-distance diagram for the group RX J075243.6+ 455653. The distance to the BGG is shown as a function of the r-band magnitude for each galaxy within r200. The gray, black, and red points represent the spectroscopic targets, background galaxies, and group members, respectively. The horizontal and vertical full lines represent R/r200 = 0.5 and Δm12 = 2.0, respectively.

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According to the definition of fossil systems, the fossilness strongly depends on the adopted value of r200 of the structure. Figure 4 shows that changes in the value of r200 could change the fossillness of RX J075243.6+455653. The value of r200 for this group was determined from the X-ray luminosity obtained from RASS data. We have also computed r200 by using the velocity dispersion of the system and the relation: , where H(zc) is the value of the Hubble constant at the redshift of the group (zc) (e.g., Carlberg et al. 1997; Aguerri et al. 2007). In this case we have calculated r200 = 0.80 Mpc, which is ~ 17% smaller than the obtained from X-ray data. This difference could be related to the luminous AGN that the central galaxy of RX J075243.6+455653 hosts (see Kundert et al. 2015). Nevertheless, the fossilness of the system does not change with the new r200 computed (see Fig. 4).

We can also investigate whether or not the X-ray luminosity of the AGN would affect the fossilness definition of RX J075243.6+455653. Girardi et al. (2014) obtained a relation between the X-ray luminosity and the optical luminosity (Lopt) for a sample of clusters and fossil systems. According to this work, RX J075243.6+455653 shows an optical luminosity of . Taking into account the Girardi’s relation, RX J075243.6+455653 would have a X-ray luminosity of erg s-1. This value is larger than 10 erg s-1 chosen as limit to define fossil systems. Therefore, we can say that the fossillness of RX J075243.6+455653 is not compromised by the X-ray luminosity of the central AGN. We also note that the limit in the X-ray luminosity used to define fossil systems is taken in order to avoid isolated galaxies. In this case, the velocity dispersion of the system shows us that RX J075243.6+455653 is a group-like halo.

3.2. The color–magnitude diagram

Figure 5 shows the color–magnitude diagram of the galaxies located within r200 around the center of RX J075243.6+455653. We have fit the red sequence (RS) for those group members with mr< 18.0 and with gr> 0.8. The best fit is (gr)RS = −0.043 ± 0.01mr + 1.606 ± 0.20 and σRS = 0.036. Taking into account the errors of the fit of the red sequence, the intrinsic scatter is σRS,i = 0.16. The red sequence of the group is well defined down to the dwarf regime (Mr ~ −16.0).

We have divided the group members into red and blue galaxies. We have considered blue galaxies as those group members with gr< (gr)RS − 3σRS, where (gr)RS and σRS are the color and the dispersion of the red sequence (see e.g., Agulli et al. 2014). We have obtained six groups members classified as blue galaxies, implying that the blue fraction of this group is 0.23. These blue galaxies are located in the external regions of the group, at a mean distance of 0.7r200. This number of blue galaxies can be considered as an upper limit. No blue galaxies are obtained if we consider blue galaxias as those group members with gr< (gr)RS−3σRS,i.

We also note the absence of blue galaxies fainter than Mr ~ −18.0. Although some blue galaxies have been spectroscopically observed in this magnitude range, they turned out be background objects. Due to the small number of observed galaxies, it is difficult to be certain of a lack of blue dwarf galaxies in this group.

thumbnail Fig. 5

Color–magnitude diagram of the galaxies within r200 from the group center. The gray points represent all the galaxies in the direction of the group. Red and black points show the group members and the background galaxies, respectively. The horizontal dashed line represents the color limit chosen for the selection of the spectroscopic targets. The full line shows the fitted red sequence.

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thumbnail Fig. 6

Luminosity function of the galaxies within r200 from the group center (red points). The Schechter fits of the composite luminosity functions for systems with Δm12< 0.5 and Δm12> 1.5 from Zarattini et al. (2015) are also overplotted with filled and empty black circles, respectively. The gray full line represents the Schechter fit of the LF for groups with log (M) = 13.56 from Zandivarez & Martínez (2011). All LFs have been normalized in the magnitude interval 14 <mr< 18.

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3.3. The spectroscopic galaxy luminosity function

Figure 6 shows the spectroscopic galaxy LF of the group within r200. This LF was computed as Φ(Mr) = Nphot(Mr) × fmem(Mr) /A, where A is the area of the observations, Nphot is the number of photometric targets per magnitude bin, and fmem is the fraction of group members per magnitude bin (see e.g., Agulli et al. 2014). The uncertainties were obtained by Poisson statistics.

thumbnail Fig. 7

ROSAT image centered at the position of RX J075243.6+455653. The open circles show the position of other groups and galaxy clusters reported by NED within 2 degrees from the group center. The red symbols represent objects with radial velocities larger than 18 000 km s-1 or smaller than 12 000 km s-1. Blue circles show groups with radial velocities in the range 12 000 <V< 18 000 km s-1.

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Although the uncertainties of the LF are large, we can see that the faint-end is flat. We have fit the observed spectroscopic LF by a Schechter function. The reported faint-end slope was α = −1.08 ± 0.33. The large uncertainty in α results from the large observational errors in the LF. This slope is less steep than others from massive clusters (α ~ −1.5;Agulli et al. 2014, 2016) or the field (Blanton et al. 2005). Nevertheless, the slope is similar within the uncertainties with the LF in groups of galaxies (α ~ −1.16; Zandivarez & Martínez 2011) or some dynamically young massive clusters ((α ~ −1.13; Agulli et al. 2017).

The large uncertainty in the α parameter of the fitted Schechter function makes comparison with other samples difficult. In particular, we aim to compare the LF of the group with the composite LFs of Zarattini et al. (2015) for systems with small (Δm12< 0.5) and large (Δm12> 1.5) magnitude gaps. To avoid this problem, we have used the Pearson test. This statistical test reports that the LF of RX J075243.6+455653 can be modeled by the Schechter function fitted for systems with large magnitude gap (Δm12> 1.5;χ2 = 0.16). However, the LF of systems with small Δm12 is not a good representation of the LF of RX J075243.6+455653 (χ2 = 23.03). In addition, the Pearson test also suggests that the Zandivarez & Martínez (2011) LF for systems with similar mass as RX J075243.6+455653 is not a good representation of our data (χ2 = 10.05). This analysis indicates that the LF of this group is similar to the LF of systems showing flat faint-end slopes, such as the systems with large magnitude gaps reported by Zarattini et al. (2015).

The flatness of the LF can also be reflected in the dwarf-to-giant ratio (DGR). This quantity measures the ratio between the number of dwarf and bright galaxies in clusters and groups. Following the definition from Popesso et al. (2006), we have considered dwarf galaxies as those members with −18.0 <Mr< −16.5, and bright group members as those with Mr< −20.0. The obtained DGR for the members within r200 for RX J075243.6+455653 was 0.99 ± 0.49, indicating that this group contains about one dwarf galaxy per giant.

3.4. The large-scale environment

We have analyzed the degree of isolation of RX J075243.6+ 455653 by searching in the NASA Extragalactic Database (NED) for galaxy systems within 2 degrees of the group center; corresponding to a search radius of ~ 7.2 Mpc at the distance of the group. Sixteen galaxy aggregations with radial velocity in the range 12 000 <V< 18 000 km s-1 were found in this search, with 15 of them located within ± 3σc from the central radial velocity of RX J075243.6+455653. In addition, two of these groups are located at a projected distance from the group center smaller than r200. The data obtained from NED are heterogeneous. Nevertheless, NED has one of the largest datasets on clusters and groups of galaxies. The 16 galaxy aggregations retrieved from the NED database come from six sources in the literature. They were obtained from data of large galaxy surveys as SDSS (Smith et al. 2012; Berlind et al. 2006; Miller et al. 2005; McConnachie et al. 2009) and the Two Micron All-Sky Survey (2MASS; Díaz-Giménez & Zandivarez 2015; Crook et al. 2007). We have not identified groups or clusters within 2 degrees of the group center and with V< 20 000 km s-1 in other large cluster surveys as maxBCG (Koester et al. 2007), WHL (Wen et al. 2009), or RedMaPPer (Rykoff et al. 2016).

thumbnail Fig. 8

Conical diagrams for the RX J075243.6+455653 group (green point) and other galaxy groups (blue points). The black points represent galaxies with V< 22 000 km s-1. The gray lines shows the iso-velocity curves corresponding to 5000, 10 000, 15 000, and 20 000 km s-1.

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Figure 7 shows the position on the sky of the galaxy groups and clusters obtained from NED in a search radius of 2 degrees around the group center. The objects with radial velocities in the range 12 000 <V< 18 000 km s-1 are located in a filamentary structure which extends for several Mpc. In contrast, the background groups are more randomly distributed. Figure 8 represents the conical diagrams with the position of RX J075243.6+455653 and the other groups. This plot shows clearly that most of the groups have a similar radial velocity as RX J075243.6+455653. We can conclude that RX J075243.6+455653 is not an isolated structure. Indeed, this group forms part of a large and rich structure of groups of galaxies located within a radius of ~ 7.2 Mpc from the group center. The filamentary distribution of the nearby groups could indicate that they are falling into the potential well of RX J075243.6+455653.

Figure 9 shows the absolute magnitude histogram of the galaxies located within 2 degrees from the center of RX J075243.6+455653 and with | VVc | < 3σc. The central galaxy of RX J075243.6+455653 is the second brightest object in this region of the sky. There is another galaxy 0.14 mag brighter and located at 2.5 Mpc from the center of RX J075243.6+455653. We can consider that these two galaxies are the dominant ones in the field. Figure 7 shows the X-ray emission image from the ROSAT satellite in which it can be seen that the group RX J075243.6+455653 is located in the X-ray peak in the center of the image. No other galaxy group from NED with 12 000 <V< 18 000 km s-1 is associated with an X-ray peak that can be observed in the ROSAT image.

We can speculate that due to the bright luminosity of the BGG and its position at the peak of the X-ray, the RX J075243.6+455653 group is the dominant structure in this region of the sky. Then, the small groups of galaxies located around RX J075243.6+455653 would contribute to the hierarchical formation of this system. In addition, the bright galaxies could merge with the central galaxy of RX J075243.6+455653 increasing its luminosity, hence reinforcing its fossilness nature. The accretion of the different galaxy groups could also bring dwarf galaxies into the group and will increase the faint-end slope of the LF of RX J075243.6+455653.

thumbnail Fig. 9

Absolute r-band magnitude of the galaxies within 2 degrees from the center of RX J075243.6+455653 and with | VVc | < 3σc. The vertical dashed line represent the absolute magnitude of the BGG of RX J075243.6+455653.

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4. Discussion

The results obtained in Sect. 3 indicate that RX J075243.6+ 455653 is not the remnant of an evolved and ancient group. In contrast, this galaxy aggregation is still in the hierarchical process of the accretion of its mass. Several facts related with the large scale environment and the faint-end slope of the spectroscopic LF of this group confirm this main result.

4.1. The dwarf galaxy population of RX J075243.6+455653

Dwarf galaxies in groups and clusters have valuable information about the mass assembly of the systems. There is still an open debate over the formation of the dwarf population in galaxy aggregations. There is some dynamical evidence that at least a fraction of dwarf galaxies residing in galaxy clusters today were not formed within the cluster (e.g., Conselice et al. 2001). In contrast, they were accreted into the clusters over several Gyr (Adami et al. 2007; Aguerri et al. 2017), or could be produced by the disruption of brighter galaxies due to strong tidal interactions between galaxies or with the cluster potential (e.g., Popesso et al. 2006; Aguerri 2016).

The faint-end slope of the galaxy LF indicates the density of the dwarf galaxy population. It depends on many cluster properties such as the dynamical relaxation of the cluster (López-Cruz et al. 1997; Agulli et al. 2017) or the mass of the halo (Zandivarez & Martínez 2011). The LF in groups of galaxies seem to be flatter than those in clusters. Zandivarez & Martínez (2011) found, for a large sample of galaxy groups and clusters from SDSS-DR7, a dependence between the mass of the halo and the faint-end slope of their LFs. In particular, for halos of masses similar to RX J075243.6+455653 they obtained α = −1.16. The Pearson test indicated that the Zandivarez & Martínez (2011) LF was not a good representation of our data. In contrast, according to this statistical test, the LF of this group is similar to systems showing flatter faint-end slopes in their LFs as those with large magnitude gaps reported by Zarattini et al. (2015). This implies that RX J075243.6+455653 shows a flatter LF than groups with similar mass, and therefore, the density of dwarfs in RX J075243.6+455653 is smaller than in groups with similar mass.

The DGR is a statistic to compare the number of dwarfs per giant galaxy between different clusters or groups. Popesso et al. (2006) shows that the DGR measured within r200 is independent of the cluster or group mass. We have computed that the DGR of RX J075243.6+455653 is 0.99 ± 0.49. This number is one of the smallest values reported by Popesso et al. (2006) for a set of groups and clusters. This also points towards RX J075243.6+455653 having a lower density of dwarf galaxies in comparison with other systems with similar mass.

Several scenarios could explain the lack of dwarfs observed in RX J075243.6+455653. It could be related with the accretion status of the system. In particular, the low number of dwarfs in RX J075243.6+455653 might indicate that this system is still accreting the dwarf galaxy population. The accretion of the groups in its nearby environment will increase the number of dwarfs in the system.

RX J075243.6+455653 has a well defined red sequence down to the dwarf regime. Indeed, all dwarf galaxies of this system were found to be red with no detection of blue dwarf galaxies. Galaxy clusters at z ~ 0.8−1.0 have been found to show a lack of red dwarf galaxies (De Lucia et al. 2007); and this later formation of the red sequence in the dwarf regime in galaxy clusters indicates that the red dwarf population takes several Gyr to settle down. The fact that RX J075243.6+455653 already has red dwarf galaxies could indicate that this group was already formed several Gyr ago, where the red population of dwarf galaxies in the system are likely remnants of early accretion. Numerical simulations show that halos of groups of galaxies with masses similar to RX J075243.6+455653 have assembled half of their mass at z ~ 1 (Kundert et al. 2017). Nevertheless, the number of dwarfs of this system is low in comparison with other similar halos, which could indicate that this group is under the hierarchical process of its total mass accretion. The strong dynamical friction suffered by the luminous galaxies results in their merging with the group brightest galaxy on a short time scale. In contrast, the dynamical friction will be less effective on the dwarf accreted population and will then not merge. This accretion process could decrease the number of luminous galaxies and increase the number of dwarfs. Then, the DGR of the system can increase by the accretion of nearby satellites into the group.

The lack of dwarfs shown by this group can also be explained as a consequence of a destruction of low-mass halos in this system. This galaxy disruption could be due to interactions with the group potential or other galaxy group members (e.g., Popesso et al. 2006; Aguerri 2016). The low velocity dispersion of the system would favor low-velocity interactions that make stripping of stars from low-mass galaxies more efficient, making them fainter (e.g., Mastropietro et al. 2005; Aguerri & González-García 2009; D’Onghia et al. 2009, 2010). In contrast, the low mass of the halo of RX J075243.6+455653 makes it difficult to follow the hypothesis of a massive destruction of dwarf galaxies by halo gravitational shocking (e.g., Mayer et al. 2002). It could also be that dwarf galaxies merge with the dominant galaxy of the system. The low-velocity dispersion of this group would favor this hypothesis. In addition, these minor mergers could transport some gas to the center of the main galaxy and contribute to the AGN feedback, which could explain the bright AGN associated with the central galaxy of RX J075243.6+455653. Indeed, Hess et al. (2012) found that the central galaxy of this group shows extended bipolar jets, as a consequence of this AGN activity.

4.2. The large-scale environment of fossil systems

Little is known about the large-scale environment around fossil and non-fossil systems. Only the environment of four fossil systems has been analyzed so far. The results are not conclusive because two of them seem to be embedded in dense large-scale environments and the other two appear isolated (Adami et al. 2007, 2012; Pierini et al. 2011). Numerical simulations show that FSs are embedded in denser environments at z ~ 0.4−0.7 than non-FSs. In addition, local FSs are located in slightly more under-dense regions than non-FSs (Díaz-Giménez et al. 2011; Dariush et al. 2010). Kundert et al. (2017) have shown that the difference between fossil and non-fossil systems identified at z = 0 is in the mass accretion of their dark matter halos over the past few Gyr. In particular, present-day FSs have assembled 80% of their mass at higher redshifts than non-FS. This suggests that the large-scale environment might be crucial for the evolution of the magnitude gap. Thus, systems embedded in dense environment could accrete a larger amount of their mass over smaller time-scales than those isolated. This accretion extended along several Gyr would make the fossilness a transitional stage. In addition, extended accretion would also allow for the central galaxies to increase their mass during a larger redshift interval.

The system RX J075243.6+455653 is embedded in a rich and complex large-scale structure formed by several groups of galaxies. There are 15 small galaxy groups within ~7 Mpc around the group and with radial velocities within Vc ± 3σc. This indicates that the region around RX J075243.6+455653 is rich in substructure. In addition, RX J075243.6+455653 is the dominant structure of this complex system as indicated by the brightest group galaxy located at a peak of X-ray emission with erg s-1 (Zarattini et al. 2014). No other surrounding group of galaxies is located at any other X-ray emission peak. This dominance indicates that RX J075243.6+455653 is surrounded by a massive dark matter halo, and the other nearby galaxy groups will probably fall into this group. This rich environment reveals that RX J075243.6+455653 is far from the end of the accretion of its mass.

The BGG of RX J075243.6+455653 has an absolute magnitude of Mr = −22.67, and is one of the less luminous BGGs in the sample of FSs analyzed by Zarattini et al. (2014). However, this galaxy can still grow by the accretion of the bright galaxies in the nearby environment due to dynamical friction. The galaxies 2 mag fainter than the BGG, located in the galaxy groups, and with | VVc | < 3σc, have a total luminosity of . All these stars around RX J075243.6+455653 could potentially produce a central BGG with Mr = −25.43. This new galaxy would be comparable in luminosity to the brightest galaxies observed in other fossil clusters and will produce in the future a system with a large magnitude gap (see Zarattini et al. 2014). Interestingly, this scenario for RX J075243.6+455653 is in contrast with the prediction from numerical simulations which find that FSs with faint central galaxies are dynamically old objects (see Raouf et al. 2014).

The accretion of galaxies as we propose here for RX J075243.6+455653 could be not so rare in fossil systems. Recent minor mergers have also been proposed to explain the complex structure of the hot intracluster medium of the fossil group NGC 1132. This system shows a disturbed and asymmetrical morphology of its hot gas. These observations can be explained considering a recent minor merger with a low impact parameter (Kim et al. 2017). The case of NGC 1132 shows that fossil systems can be rejuvenated by recent mergers of galaxies or groups. Other X-ray observations indicate that FSs show radial variation of their hot gas content and their gas clumping (Pratt et al. 2016). All these observations indicate that fossil systems are not dynamically evolved systems as was thought.

5. Conclusions

We have analyzed the properties of the fossil group RX J075243.6+455653. Our main results are as follows:

  • We have obtained 26 galaxy group members down toMr = −16.0 and within r200 from the group center. These members provide a mean radial velocity of the group of Vc = 1551 ± 65 km s-1 and a dispersion σc = 333 ± 46 km s-1.

  • We have obtained the spectroscopic LF of RX J075243.6+455653 down to Mr = −16.0 and within r200. The LF was fitted by a single Schechter function giving a flat faint-end slope α = −1.08 ± 0.33. We have used the Pearson test in order to compare the LF of our group with other LFs from the literature. In particular, the Pearson test reports that the LF of RX J075243.6+455653 is statistically similar to the LF of systems with large magnitude gaps (Δm12> 1.5) obtained by Zarattini et al. (2014). And furthermore, the LF of RX J075243.6+455653 is not statistically similar to the LF of systems with Δm12< 0.5 (Zarattini et al. 2014) or groups of galaxies with similar mass (Zandivarez & Martínez 2011).

  • The DGR within r200 is 0.99 ± 0.49. This value is low compared with the DGR of other groups and galaxy clusters. Both the faint-end of the LF and the DGR of RX J075243.6+455653 indicate that this group has a lack of faint galaxies compared with groups or clusters with larger Δm12 or similar halos mass.

  • Our search of groups or clusters of galaxies in the nearby environment of RX J075243.6+455653 finds that this group is not isolated. In contrast, it is embedded in a dense region with up to 15 groups of galaxies located in a thin velocity layer of ± 1000 km s-1 and within ~ 7 Mpc from the group center.

  • RX J075243.6+455653 is the dominant structure of its complex and rich nearby environment. This is confirmed by the evidence that the central BGG of the group is located at the peak of an extended X-ray emission. No other surrounding galaxy groups are located in any other X-ray peak. This could imply that the surrounding galaxy groups will probably merge with RX J075243.6+455653 in the next few Gyr.

The results obtained in this paper indicate that RX J075243.6+455653 is still in the process of assembling its mass. The groups of galaxies located in its nearby environment will merge in the next few Gyr. These mergers will produce a growth of the BGG and will increase the number of dwarf galaxies of the system. This indicates that although today RX J075243.6+455653 is a fossil system, it is not a remnant of an evolved and ancient group. The fossil phase of this system could just be a transition state.

RX J075243.6+455653 is a fossil system according to the observational definition of this type of system. Nevertheless, it is not a relic of a structure formed at high redshift, and the halo of this group is still accreting mass from the groups located in its dense nearby environment. RX J075243.6+455653 is an example showing that the selection of fossil systems by only large magnitude gaps between the first two ranked galaxies does not guarantee the selection of dynamically old galaxy aggregations. New criteria should be obtained in order to find real relics of the structure formation at high redshift. In the near future, we will continue analyzing the properties of the large-scale environment and the faint-end of the LF of other nearby fossil systems. These properties in combination with state-of-the-art cosmological numerical simulations will allow us to find new observables in order to obtain remnants of ancient galaxy aggregations.


Acknowledgments

J.A.L.A. and S.Z. want to thank the support of this work by the Spanish Ministerio de Economia y Competitividad (MINECO) under the grant AYA2013-43188-P. We thank Dr. R. Barrena for the assistance provided for the data reduction and analysis of the OSIRIS data. This research has made use of the Thirteen Data Release of SDSS, and of the NASA/IPAC Extragalactic Database which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The WHT and its service program are operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

References

All Figures

thumbnail Fig. 1

Velocity histogram of the galaxies in the direction of RX J075243.6+455653. The black histogram shows the radial velocities of the galaxies within 4 Mpc from the cluster center. The gray filled histogram corresponds to galaxies within r200. The vertical line shows the mean velocity of the group. The dashed vertical lines are located at ± 3σc from Vc.

Open with DEXTER
In the text
thumbnail Fig. 2

Spectroscopic completeness (black) and fraction of group members (red dashed) as a function of the r-band magnitude.

Open with DEXTER
In the text
thumbnail Fig. 3

Sky position of the spectroscopic targets (gray), background objects (black), and galaxy group members (red) within r200. The largest red point represents the position of the brightest galaxy in the group.

Open with DEXTER
In the text
thumbnail Fig. 4

Magnitude-distance diagram for the group RX J075243.6+ 455653. The distance to the BGG is shown as a function of the r-band magnitude for each galaxy within r200. The gray, black, and red points represent the spectroscopic targets, background galaxies, and group members, respectively. The horizontal and vertical full lines represent R/r200 = 0.5 and Δm12 = 2.0, respectively.

Open with DEXTER
In the text
thumbnail Fig. 5

Color–magnitude diagram of the galaxies within r200 from the group center. The gray points represent all the galaxies in the direction of the group. Red and black points show the group members and the background galaxies, respectively. The horizontal dashed line represents the color limit chosen for the selection of the spectroscopic targets. The full line shows the fitted red sequence.

Open with DEXTER
In the text
thumbnail Fig. 6

Luminosity function of the galaxies within r200 from the group center (red points). The Schechter fits of the composite luminosity functions for systems with Δm12< 0.5 and Δm12> 1.5 from Zarattini et al. (2015) are also overplotted with filled and empty black circles, respectively. The gray full line represents the Schechter fit of the LF for groups with log (M) = 13.56 from Zandivarez & Martínez (2011). All LFs have been normalized in the magnitude interval 14 <mr< 18.

Open with DEXTER
In the text
thumbnail Fig. 7

ROSAT image centered at the position of RX J075243.6+455653. The open circles show the position of other groups and galaxy clusters reported by NED within 2 degrees from the group center. The red symbols represent objects with radial velocities larger than 18 000 km s-1 or smaller than 12 000 km s-1. Blue circles show groups with radial velocities in the range 12 000 <V< 18 000 km s-1.

Open with DEXTER
In the text
thumbnail Fig. 8

Conical diagrams for the RX J075243.6+455653 group (green point) and other galaxy groups (blue points). The black points represent galaxies with V< 22 000 km s-1. The gray lines shows the iso-velocity curves corresponding to 5000, 10 000, 15 000, and 20 000 km s-1.

Open with DEXTER
In the text
thumbnail Fig. 9

Absolute r-band magnitude of the galaxies within 2 degrees from the center of RX J075243.6+455653 and with | VVc | < 3σc. The vertical dashed line represent the absolute magnitude of the BGG of RX J075243.6+455653.

Open with DEXTER
In the text

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