Finding and characterising WHIM structures using the luminosity density method
J. Nevalainen, L. J. Liivamagi, E. Tempel, E. Branchini, M. Roncarelli, C. Giocoli, P. Heinamaki, E. Saar, M. Bonamente, M. Einasto, A. Finoguenov, J. Kaastra, E. Lindfors, P. Nurmi, Y. Ueda
aa r X i v : . [ a s t r o - ph . C O ] O c t The Zeldovich UniverseProceedings IAU Symposium No. 308, 2014R. van de Weygaert, S. Shandarin, E. Saar & and J. Einasto, eds. c (cid:13) Finding and characterising WHIM structuresusing the luminosity density method
Jukka Nevalainen and L. J. Liivam¨agi and E. Tempel and E.Branchini and M. Roncarelli and C. Giocoli and P. Hein¨am¨aki andE. Saar and M. Bonamente and M. Einasto and A. Finoguenov and J. Kaastra and E. Lindfors and P. Nurmi and Y. Ueda Tartu Observatory, Observatooriumi 1, 61602 T˜oravere, Estoniaemail: [email protected] University Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy University of Bologna, viale Berti Pichat 6/2, I-40127 Bologna, Italy Tuorla Observatory, V¨ais¨al¨antie 20, FI-21500 Piikki¨o, Finland University of Alabama in Huntsville, Huntsville, AL 35899, USA SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, theNetherlands Kyoto Observatory, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501 JAPAN
Abstract.
We have developed a new method to approach the missing baryons problem. Weassume that the missing baryons reside in a form of Warm Hot Intergalactic Medium, i.e. theWHIM. Our method consists of (a) detecting the coherent large scale structure in the spatialdistribution of galaxies that traces the Cosmic Web and that in hydrodynamical simulations isassociated to the WHIM, (b) map its luminosity into a galaxy luminosity density field, (c) usenumerical simulations to relate the luminosity density to the density of the WHIM, (d) applythis relation to real data to trace the WHIM using the observed galaxy luminosities in the SloanDigital Sky Survey and 2dF redshift surveys. In our application we find evidence for the WHIMalong the line of sight to the Sculptor Wall, at redshifts consistent with the recently reportedX-ray absorption line detections. Our indirect WHIM detection technique complements thestandard method based on the detection of characteristic X-ray absorption lines, showing thatthe galaxy luminosity density is a reliable signpost for the WHIM. For this reason, our methodcould be applied to current galaxy surveys to optimise the observational strategies for detectingand studying the WHIM and its properties. Our estimates of the WHIM hydrogen columndensity N H in Sculptor agree with those obtained via the X-ray analysis. Due to the additionalN H estimate, our method has potential for improving the constrains of the physical parametersof the WHIM as derived with X-ray absorption, and thus for improving the understanding ofthe missing baryons problem. Keywords. cosmology: large-scale structure of universe, galaxies: intergalactic medium
1. Introduction
At low redshifts (z <
2) all observations of the visible matter sum up only to ∼
70% ofthe expected cosmological mass density of baryons (e.g. Shull et al., 2012). Large scalestructure formation simulations suggest that these missing baryons reside in the formof Warm-Hot Intergalactic Matter (WHIM) in the filamentary structure connecting theclusters of galaxies and superclusters (e.g. Cen & Ostriker, 1999, 2006). At the predictedtemperatures of 10 − K and densities 10 − − − cm − the X-ray emission from singleWHIM structures is too faint to be detected with current instrumentation. However,1 J. Nevalainen et al.the column densities of the highly ionised WHIM metals along large–scale filamentarystructures can reach a level of 10 − cm − , imprinting detectable absorption featureson the soft X-ray spectra of background sources (e.g. Nicastro et al., 2010).While the X-ray absorption measurements are crucial in the analysis of WHIM, theyare sparse. The usual ”blind search” is based on observing random bright blazar flares,without a priori knowledge of the foreground structure. To improve this, we focus on known foreground large scale structures traced by galaxies, like the cosmic filaments,that most likely contain WHIM. We have applied the Bisous model (Stoica et al., 2005)to trace and extract the filamentary network in the presently largest galaxy redshiftsurvey, SDSS DR8 (Tempel et al., 2014) and 2dF. Thus we have an extensive data baseof potential WHIM structure locations, sizes and redshifts.We report distances and lengths in co-moving coordinates (unless stated otherwise),using Ω m = 0.3, Ω Λ = 0.7 and H = 70 km s − Mpc −
2. The method
Luminosity density fields
We have examined the above galaxy structures using the luminosity density (LD) method,pioneered by our group (e.g. Liivam¨agi et al., 2012). With this method we quantify the 3-dimensional galaxy distribution using the optical (R - band) light in galaxies. We assumethat every galaxy is a visible member of a density enhancement (group or cluster). Weplace the galaxies at the mean distance of the group or the cluster, to correct for theeffect of the dynamical velocities (finger-of-god). We correct the galaxy luminosities forthe observational magnitude limited samples by a weighting factor that accounts forthe group galaxies outside the visibility window. The galaxy luminosity distribution isthen smoothed with a B3-spline kernel function. The smoothing length determines thecharacteristic scale of the objects under study. We have adopted 1.4 Mpc as the smoothingscale, in order to match the filament widths. We then sample the LD distribution at thepoints of a uniform grid, encompassing the survey volume, with a sampling scale of 1.4Mpc, thus creating the LD field. For a given galaxy structure, we use the LD field toevaluate the LD profile (see Fig. 1).
Figure 1.
Luminosity density (LD) profile along the Sculptor Wall galaxy structure at z ∼ σ uncertainties of the redshift of the X-ray absorber (Fang et al., 2010) are shown in (red) dashedand dotted lines. Note that the redshifts and distances are reported in CMB rest frame. Theluminosity density value corresponding to baryon overdensity δ b = 10, as estimated with Eq.2.1, is denoted with (green) symbol on the right axis. ooking for WHIM Large Scale Structure simulations
If the galaxies follow the underlying dark matter (DM) potential, similarly as the WHIM,then the luminosity density field in filaments can be used to trace the missing baryons.To test the validity of this assumption we have applied our filament–finding and LDfield algorithms to a distribution of mock galaxies, DM and the WHIM (i.e. gas attemperatures 10 - 10 K) at z = 0 in the hydrodynamical simulations (Cui et al., 2012).These simulations make use of smoothed particle hydrodynamics in GADGET-3 codeto produce dark matter and diffuse baryonic components within a box with a size of570 Mpc. The simulations involve radiative cooling, star formation and feedback fromsupernova remnants. The galaxies are created by populating the DM haloes using ahalo occupation distribution constrained with the SDSS data (Zehavi et al., 2011). Thesimulated data are adequate to follow the different density components with a resolutionof ∼ ρ W HIM ) in the above simulations correlate ratherwell (Pearson correlation coefficient = 0.85). We used this correlation to derive a quanti-tative relation between these, i.e. the LD - ρ WHIM relation, within the filaments identifiedby the mock galaxies (see Eq. 2.1 and Fig. 2). ρ W HIM ≈ × LD . , when LD < . ρ W HIM ≈ × LD . , when LD > . , (2.1)where LD and ρ W HIM are expressed in units 10 h L ⊙ M pc − and 10 h M ⊙ M pc − . Figure 2.
The luminosity density (LD) - WHIM density ( ρ WHIM ) relation, and the 1 σ uncer-tainties derived from the cosmological hydrodynamical simulations (Cui et al., 2012) are indi-cated with (red) dashed line and the (blue) shaded region, respectively. The black dotted linesindicate the power-law approximations to the relation (Eq. 2.1). The (green) symbols indicatedifferent levels of overdensity of baryons and luminosity. We use this relation to convert the observational (SDSS and 2dF) LD values into ρ W HIM estimates. We then integrate the ρ W HIM profile of a given filament to obtain theWHIM hydrogen column density N H . The observed LD profile values typically correspondto baryon overdensities of ∼
10 (see Fig. 1), consistent with those predicted by simulationsfor the WHIM filaments (e.g. Cen & Ostriker 1999, 2006). J. Nevalainen et al.
Table 1.
Redshifts and WHIM hydrogen column densities N H in Sculptor X-ray LD X-ray LD system redshift redshift log N H log N H Sculptor Wall (SW) 0.030–0.032 0.028–0.033 21.0[20.1–22.3] 20.0[19.6–20.6]Pisces-Cetus (PC) 0.060–0.063 0.060–0.063 20.1[19.9–20.3] 19.8[19.4–20.3]Farther Sculptor Wall (FSW) 0.125–0.127 0.128–0.129 20.8[20.0–21.2] 19.9[19.6–20.5]
Notes: Redshift centroid (Chandra) and WHIM N H estimates based on X-ray absorption measurements (SW: Fanget al., 2010 ; PC and FSW: Zappacosta et al., 2010). The log N H values are derived assuming O abundance 0.1Solar, and O/H ratio from Grevesse & Sauval, 1988. The estimate for SW is obtained here by assuming T =10 K. Our luminosity density - based estimates for the redshift range corresponding to the extent of the given systemintersected by the H2356-309 sightline, and WHIM N H of the given system. In the observational frame.
3. Results
In order to test the feasibility of our WHIM N H estimation method, we extractedthe LD profile along the line-of-sight to the blazar H2356-309 behind the Sculptor Wall,where X-ray measurements of WHIM absorption have been obtained (Fang et al., 2010;Buote et al., 2009, Zappacosta et al., 2010). We found ∼
10 Mpc long luminosity densitystructures at redshifts consistent with those measured with X-rays (see Fig. 1). OurLD-WHIM density relation yields WHIM N H values consistent with those measured inX-rays (see Table 1), proving the reliability of our column density estimation method.
4. Discussion
Our plan is to apply our method to current galaxy surveys (and e.g. to Euclid resultsin near future) to optimise the observational strategies for detecting and studying theWHIM and its properties. In particular, we will cross-correlate the significant WHIMstructures found from e.g. SDSS and 2dF with bright background blazars. We aim atobtaining XMM-Newton/RGS and Chandra/LETGS data of such blazars in a high fluxstate, which are located in the line-of-sight to WHIM structures with the highest WHIMcolumn density estimates. This will be useful also for next generation X-ray telescopes likeATHENA. We will also extend this work to include far-ultraviolet WHIM measurementswith e.g. FUSE and HTS of the low temperature WHIM.
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