X-ray emission from hot gas and XRBs in the NGC 5846 galaxy

3.1 Hardness ratio and luminosity function

Understanding spectral properties of the discrete sources is crucial in revealing their emission characteristics. However, its spectral treatment requires a sufficient number of counts to constrain these properties. In majority of the cases, source counts were insufficient to perform spectral analysis of individual sources. Under such circumstances, X-ray hardness or X-ray color–color plot of the resolved sources provides a potential way to investigate their spectral properties. Measured value of the X-ray color of individual source places it at an unique place in the color–color plot and hence enable us to understand their crude properties. This technique is advantageous and has been used by several researchers for sources with poor statistics (Sarazin et al., 2001, Irwin et al., 2000, Fabbiano 2006, Vagshette et al., 2013). Following this, we have also made an attempt to measure the hardness ratios (HR) of the resolved sources assuming the definitions of H 21 = ( M − S ) ∕ ( M + S ) and H 31 = ( H − S ) ∕ ( H + S ) , where S , M , and H represent net X-ray counts in the soft (0.3–1 keV), medium (1–2 keV), and hard (2–10 keV) bands, respectively. Details of all the spatially resolved sources within NGC 5846 along with their hardness ratios are presented in Table 1, while their positions in the X-ray colour-color (H31 versus H21) plot are shown by green circles (Figure 2, right panel). A careful look at this figure reveals that the majority of sources occupy positions along the diagonal swath with the center approximately in the range of ( − 0.2 , − 0.5 ) to ( + 0.7 , + 0.7 ) and are consistent with those reported by Irwin et al., (2000), Blanton et al., (2001), and Vagshette et al., (2013). On the same plot, we also show positions of point sources from the XMM-Newton Serendipitous Source Catalogue of extragalactic nonnuclear X-ray sources (Earnshaw et al., 2019). Here, the X-ray color definition followed by these researchers is HR2 = (0.5–1 keV)/(0.5 +1 keV), similar to our H21 = (1–2 keV)/(1 + 2 keV), while the definition of HR3 is identical to that of our H31. The diamond shaped points (red color) in this plot represent the X-ray sources with luminosities less than 1 0 38 erg s − 1 , while star-shaped (black) represent the ULXs as reported in Table A1 of the study by Earnshaw et al., (2019). The green circles in this plot depict XRBs, while magenta triangles represent ULXs from our study. A separation is evident in the positions of LMXBs and may be due to slightly different definitions of HR2 and H21. This plot reveals a large population of soft sources relative to those studied by Earnshaw et al., (2019).

To estimate X-ray luminosity of individual sources, we perform spectroscopic analysis of these sources and hence estimate their energy conversion factors in units of erg/counts. The luminosity of all the resolved sources are listed in Table 1. This study reveals one source (source no. 4) as the ultra-luminous X-ray (ULX) with luminosity 1.28 × 1 0 39 erg s − 1 (source no. 24 in the Table 1 of Trinchieri and Goudfrooij (2002)) and has hardness ratio of (0.33, 0.11). These type of ULXs can be a neutron star accreting at super-Eddington rates or a more massive black hole with luminosity close to their Eddington limit (Irwin et al., 2003). This study reveals a total of 49 sources that have luminosity in the range of 1.0 × 1 0 38 erg s − 1 to 8.3 × 1 0 38 erg s − 1 and exhibit luminosities equal to or higher than the Eddington limit for 1 M ⊙ neutron star accreting sources, 38 sources has luminosity of the order 1 0 37 erg s − 1 , while two of the remainder (source no. 47 and 50) have luminosity of the order 1 0 36 erg s − 1 . Four source, source number 8, 11, 12 and 13 in Table 1 of Trinchieri and Goudfrooij (2002), have luminosities of the order 1 0 39 erg s − 1 are found in the nuclear region; therefore, were not considered in this study. However, from the source number 4, 10, and 16 that have luminosities ≈ 1 0 39 in the Table 1 of Trinchieri and Goudfrooij (2002), for the same sources, we found the luminosities of 6.14 × 1 0 38 , 8.31 × 1 0 38 , and 8.31 × 1 0 38 , respectively, in the present study. The discrepancy may be found due to source region selection and background treatment. Source no. 75 from our study that exhibit hardness ratio near (+1, +1) and is located at about 3 arcmin from the center could potentially be a background object (Sarazin et al., (2000)). The sources with hardness ratios close to ( − 0.9 − 0.9 ) are marked as either super-soft or very-soft sources (Sarazin et al., 2000, Vagshette et al., 2013) due to the reason that they do not show counts over 1 keV. We do not find any of such sources in the current investigation.

The X-ray luminosity function (XLF) plotted for the discrete sources has been served as an effective tool for investigating properties of the XRB population and shape of which are well represented by a broken power-law with a break at (2–5)  × 1 0 38 erg s − 1 , a neutron star accreting binaries in ETGs (Irwin et al., 2000, Sarazin et al., 2001). With an objective to investigate luminosity distribution of the population of discrete sources detected in NGC 5846, we have obtained the cumulative X-ray luminosity function (XLF) of 52 sources whose X-ray luminosity is above 1 0 38 erg s − 1 . This limit was set due to the reason that the XLF gets flattened below this. Hence, we have used the broken power-law (bpl1d) model within sherpa. The best-fit parameters of XLF fitting are Γ 1 = 1.15 ± 0.09 , Γ 2 = 3.66 ± 1.2 , and break luminosity equal to 6.6 × 1 0 38 erg s − 1 , which is slightly higher than 5.3 × 1 0 38 erg s − 1 as reported by Randall et al., (2004). The resultant best-fit luminosity function for all the 52 sources is shown in Figure 3 and exhibits a “knee” at around 6.6 × 1 0 38 erg s − 1 resembling the Eddington luminosity of 2.8 solar mass neutron star (NS). The sources below break luminosity are considered to be a mix of neutron stars or black hole systems (Irwin et al., 2003). The X-ray luminosities (0.3–10 keV) of the resolved XRBs from the present study are found to span over 1 0 37 erg s − 1 to 8.8 × 1 0 38 erg s − 1 .

Figure 3

X-ray luminosity function of 52 discrete sources with luminosities more than 1 0 38 erg s − 1 . The solid line represents the best-fitted broken power-law model with a break at 6.6 × 1 0 38 erg s − 1 .

3.2 Contribution of different X-ray emitting sources

Though X-ray luminosity of an early-type galaxy primarily originates from the diffuse plasma distributed throughout the host, a nonnegligible component is emitted by the population of discrete sources hosted by it. To investigate relative contribution of both these components to the total X-ray luminosity of NGC 5846, we have extracted the X-ray spectrum of the diffuse gas alone within 2.3 arcmin (covering maximum extent of the diffuse hot gas distribution) region of NGC 5846 using script specextract of CIAO. For quantifying diffuse emission from NGC 5846, we use ObsID-788 due to the reason that the source in this observation was located at the CCD center, and as a result, it was possible to cover maximum extent of diffuse gas distribution, while that in the ObsID-7923 was located at the edge of the CCD. Therefore, we have not used this observation for investigating diffuse emission. We also extract a cumulative X-ray spectra of all the resolved sources within the entire chip (using ObsID7923) of Chandra detector. Background spectrum for the diffuse emission and resolved sources were determined by using the blank sky background. These spectra were then treated independently using XSPEC version 12.9.1 by fitting with appropriate models and fixing the absorption column density N H at the Galactic value of ∼ 4.29 × 1 0 20 cm − 2 (Dickey and Lockman 1990).

The spectrum of the diffuse component was first treated with the VAPEC (variant abundance APEC) model in 0.4–4 keV band to account for the emission from collisionally-ionized diffuse gas, where temperature, metallicity (O, Ne, Mg, Si, S, and Fe), and normalization parameters were allowed to vary and other parameters were frozen. Due to poor fitting statistics (higher value of χ 2 ∕ dof ), we added a power-law component to account for the emission from the unresolved source population (e.g., cataclysmic variables and coronally active binaries) that remained undetected in the wavelet detection algorithm. This yielded a relatively better fit (Figure 4, left panel) with the best-fit parameters kT ≈ 0.74 ± 0.001 keV and metallicity values of O = 0.63 ± 0.12 , Ne = 1.25 ± 0.35 , Mg = 0.58 ± 0.13 , Si = 0.64 ± 0.13 , and Fe = 0.39 ± 0.05 , while other components were not present in the emission feature, and hence, we fixed these at 0.3 times solar value (Werner et al., 2006). The photon index value of the hard component was frozen at Γ = 1.64 , the best-fitted value of resolved sources. The normalization value of VAPEC model is of 2.79 × 1 0 − 3 ± 2.68 × 1 0 − 4 and for the power-law component it is 5.23 × 1 0 − 5 ± 1.15 × 1 0 − 5 . The composite model was grouped such as to have at least 30 counts per spectral bin to yield the acceptable statistics and hence provided with the reduced χ 2 per DOF ≈ 166 ∕ 138 = 1.20 . The error bars in this figure represent the 90% confidence level. The quantified flux value resulting into the luminosity of diffuse gas plus unresolved sources was found to be equal to ∼ 2.85 × 1 0 41 erg s − 1 .

Figure 4

(Left panel) The point sources removed spectral fit of the diffuse emission within 2.3 arcmin radius, which covers the maximum extent of diffuse hot gas from the center of NGC 5846 galaxy, (right panel) the best-fit spectra of cumulative resolved point sources within the entire chip of detector.

The cumulative spectrum of the emission from resolved sources was well constrained by a power-law plus APEC component (model = wabs  ×  wabs  ×  (APEC+powerlaw)) with the photon-index Γ = 1.64 ± 0.17 , kT = 0.28 ± 0.038 and the abundance frozen at the solar value. The first absorption column component was fixed at the Galactic value, while the intrinsic absorption was found to be N H = 5.4 × 1 0 21 cm − 2 . To result in better statistics, the spectrum was grouped such as to have a minimum of 30 counts per bin, which resulted in 146.39/112 = 1.36 χ 2 /DOF. Figure 4 (right panel) shows the best-fit power-law component of the cumulative spectrum with a luminosity of resolved sources alone equal to ∼ 1.66 × 1 0 40 erg s − 1 , while that of the diffuse (unresolved) component equal to ∼ 4.87 × 1 0 39 erg s − 1 . A comparison of the two components reveal that the diffuse emission is higher, roughly by two orders of magnitudes, over that of the discrete sources and is consistent with the findings of Vagshette et al., (2013). Further, higher values of heavier metals in contrast to the solar values imply that the diffuse plasma in the environment of NGC 5846 is enriched by the ejecta from supernovae. The measured values of the flux densities and luminosities of different components are presented in Table 2. Ever since LMXBs were first detected in the Milky Way, the issue of contribution from such sources has been debated (Giacconi et al., 1974). Chandra due to its high angular resolution have been instrumental in determining contribution of LMXBs to the total luminosity of their host. Combined optical and X-ray investigations of a sizable sample of ETGs have revealed that about 70% of the LMXBs are identified with the globular clusters (Sivakoff et al., 2003, Randall et al., 2004). As a result, overall X-ray luminosity of the LMXBs in a galaxy is likely scale with the number of globular clusters hosted by it, and therefore, it was assumed that all the LMXBs were created in GCs, independent of their locations (Irwin 2006). However, as NGC 5846 happens to be the brightest member in the group of 50; therefore, it is likely that it has acquired excess amount of plasma from the neighbors through a merger-like episode.

3.3 Temperature and metallicity maps

2D spatial distribution of plasma in a galaxy and its metallicity variation could be a potential way of looking at the evolutionary scenario of the hot gas within a galaxy environment. With this objective in mind, we have also derived 2D temperature and metallicity maps of the X-ray emitting gas in NGC 5846. This was achieved by following the contour binning contbin technique of Sanders (2006), which computes spatial bins of the X-ray surface brightness distribution until it reaches to predefined signal-to-noise ratio of 30. The identified regions from it were then used for the spectral extraction of the supplied image of NGC 5846. We extracted 0.5–3 keV spectra from individual bin, then treated them like earlier and were fitted with the WABS × APEC component within xspec. Before this spectra were grouped to have a minimum 20 counts in each bin, and temperature (kT) and metallicity (Z) (relative to the solar values) were allowed to vary. The resulting temperature and metallicity values quantified from the fitting of individual spatial bin were used to derive their 2D temperature and abundance maps, and the resultant maps are shown in the left and right panels of Figure 5. A careful inspection of both these maps revealed that the plasma distribution in the environment of NGC 5846 is not isotropic and homogeneous but exhibit structures. The jumps, arc-like discontinuities, and unusual elongation along the north-west to south-east are some of the interesting features. Interestingly, the spatial bins, where metallicity exhibit jumps, correspond to the relatively cooler gas.

Figure 5

2D temperature (left panel) and metallicity (right panel) maps of the hot gas distribution within 6 ′ × 6 ′ ( 43 × 43 kpc) of NGC 5846 obtained from the contour binning technique. The temperature units are in keV, while those of the metallicity are in Z ⊙ with a signal-to-noise of 30.

3.4 Cold front

2D temperature map of the plasma distribution within this galaxy revealed unusual morphologies, particularly, arc shaped discontinuities along north-west and south-east. To explore them further and also to examine the thermal properties of the diffuse gas distributed within the galaxy, we derive its azimuthally averaged radial temperature and metallicity profiles. Hence, we extract 0.3–5 keV X-ray spectra from concentric elliptical annuli centered on the X-ray peak of NGC 5846. Source and background spectra, photon weighted response files, and effective area files were generated for each of the extraction using CIAO tool acisspec. Spectra from each of the annulus were fitted with a single temperature APEC thermal plasma code by allowing temperature, abundance, and normalization parameters to vary. The resultant projected temperature, metallicity, and best-fit χ 2 are shown in Figure 6. The temperature profile remains almost constant up to 5 kpc, which then increases sharply in the outer part up to 20 kpc and then flattens in outward direction and is consistent with the study by Machacek et al., (2011). The metallicity profile exhibits a small rise up to 20 kpc, which then decreases sharply in the outward direction. Break in both the profiles at about 20 kpc might be due to its association with the cold front. Investigation of cold front at 20 kpc in this system has been reported by Machacek et al., (2011). The rise in the temperature profile in inner region point toward the intermittent heating of the gas like that evidenced in the cooling flow galaxies (Gastaldello et al., 2007, Rasmussen and Ponman 2007, David et al., 2009, Sun et al., 2009, Pandge et al., 2013). The possible source of its heating have been identified as the feedback from AGN operative at its center (Kyadampure et al., 2021). As NGC 5846 shows the cooling flow nature, it is likely that the gas in the central region cools radially and hence to maintain a pressure balance outer gas falls in the core leading to a well-known scenario called as cooling flow. The inward falling gas condenses and hence may lead to the rise in the metal abundances.

Figure 6

Projected azimuthally averaged profiles of temperature and metallicity of the hot gas emission from NGC 5846. Notice the discontinuities at 20 kpc in the temperature and metallicity profiles.

To confirm presence of the cold front in this system, we further extract surface brightness and 0.3–5 keV spectral profiles of the X-ray emission by extracting photons from wedge-shaped sectors covering 0– 9 0 ∘ along NE of the X-ray peak (Figure 7, left panel). The resultant temperature and surface brightness distribution along this region are shown in Figure 7 (bottom and top panels, respectively). Both these profiles exhibit sharp breaks at about 20 kpc implying that it is due to a cold front, which might have formed due to the minor mergers (Machacek et al., 2011). Presence of such cold fronts have been evidenced in several of the cooling flow galaxy groups and clusters (Mazzotta et al., 2001, 2003, Markevitch et al., 2001, Churazov et al., 2003, Dupke and White 2003, Sanders et al., 2005, Vagshette et al., 2019, Kadam et al., 2024).

Figure 7

Left side: Surface brightness distribution overlaid with the wedge-shaped sectorial region used for detection of the cold-front. Right side: Temperature (lower panel) and surface brightness (top panel) profiles along the wedge-shaped region show a jump at 20 kpc.

3.5 Optical emission mechanism

Trinchieri and Goudfrooij (2002) employing optical and X-ray data from this system have reported complex morphologies of warm ionized gas centered on the H α emission and its strong association with the intricate X-ray emission and interstellar dust. Based on the spectral analysis of X-ray emission from this galaxy, they further demonstrate that the X-ray emitting gas eventually cools and gets deposited in the core of this galaxy. As a result, there is likely a core of the cool gas, which was revealed in the form of nebular emission at optical wavelengths, and may lead to the star formation. Therefore, to explore it further, we have made an attempt to quantify the star formation at the core of this system using the H α emission flux densities. The rate of star formation in a galaxy is often determined by the observation of emission at H α   line, which is related to the presence of short-lived massive stars. The optical spectra were used to quantify the H α luminosity equal to 2.2 × 1 0 40 erg s − 1 taken from Trinchieri (1647) and estimate the star formation using relation of Kennicutt (1998):

where L H α has unit of erg s − 1 . This yielded the star formation of 0.17 M ⊙ year − 1 and is comparable to that seen in other ETGs. Further to compare the SFR with the mass flow rate toward the center of the NGC 5846. We fit the “vcoolflow” model on Chandra X-ray data extracted within an arc-minute radius from center. The best-fit model estimate the cooling flow rate M ˙ ∼ 2.72 M ⊙ year − 1 . Which is much smaller then that of current SFR estimated form H α   luminosity. By using the optical emission line flux densities measured by Balmaverde and Capetti (2006), we calculate the line ratio log([N II]/H α ) ≈ 0.18 and log([O III]/ H β ) ≈ 0.09. The observed large values of line ratios log([N II]/H α ) indicate that the gas is ionized by a nonthermal source like low luminosity AGN or due to the shocks.