探索遺失重子：堆疊全天宇宙絲狀結構中的康普頓 y 圖

Abstract

宇宙中有相當比例的重子物質尚未被直接觀測到，此即所謂的「失蹤重子問題」。理論模擬預測這些失蹤的重子主要存在於星系間的絲狀結構中，形成溫熱星系間介質（WHIM），其溫度約為 10^5 至 10^7 K。本研究利用熱蘇尼亞耶夫-澤爾多維奇效應（thermal Sunyaev–Zel'dovich effect, tSZ）來探測絲狀結構中的重子分布，透過將 Planck 衛星所提供的 Compton-y 圖像堆疊於星系對的位置以增強微弱訊號。

我們建立了一個全天的星系目錄，透過交叉比對 Gaia DR3 星系候選資料與 WISExSuperCOSMOS 光度紅移星表，並使用 SDSS DR16 的光譜紅移資料進行機器學習模型的訓練，以提升紅移估計的準確度。最終修正後的紅移在 z = 0.117 之處的相對誤差為 1.58%。

根據修正後的紅移資料，我們選出超過 700 萬個具有物理關聯性的星系對，其橫向距離在 7 至 20 Mpc 之間，視線方向（LOS）距離小於 25 Mpc。同時建立一組包含 2480 萬個非物理關聯性的星系對作為控制樣本，以消除來自星系暈的訊號。我們對 Planck Compton-y 圖像進行堆疊分析，並從星系對堆疊結果中扣除控制樣本的堆疊圖像。扣除後的剩餘圖顯示一個連接星系對的橋狀結構，代表可能存在絲狀氣體的訊號。

進一步假設絲狀結構為等溫圓柱模型，我們估算出絲狀結構中的重子過密度為 0.28±0.03。考慮星系分布在紅移範圍 z=0.06 到 0.18，該氣體貢獻約為宇宙總重子密度的 8.7%，即 0.087 Ω_b。本研究結果支持絲狀結構中存在低密度重子的觀測證據，並展示光度紅移資料結合堆疊分析於探測宇宙網絡中失蹤重子之潛力。

A significant fraction of the Universe baryonic matter remains undetected, known as the ``missing baryon problem.” Cosmological observation and simulations support that a large portion of these baryons reside in the warm-hot intergalactic medium (WHIM) within filamentary structures connecting galaxies. In this work, we investigate the thermal Sunyaev–Zel'dovich (tSZ) effect as a probe of baryonic gas in cosmic filaments by stacking the Planck Compton-y map at the locations of galaxy pairs.

We construct an all-sky photometric galaxy catalogue by cross-matching Gaia DR3 galaxy candidates with the WISExSuperCOSMOS catalogue. To improve redshift precision, we train a machine learning model using photometric data and extinction maps, calibrated against SDSS spectroscopic redshifts. The resulting redshift estimates yield an uncertainty of 1.58\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\% at z = 0.117.

Using this refined catalogue, we select over 7 million galaxy pairs with transverse separations between 7 and 20 Mpc and line of sight separations less than 25 Mpc. A control sample of 24.8 million non-physical pairs with large line of sight separations is also constructed to remove contamination from individual halos. After performing a stacking analysis of the Planck y-map and subtracting the control signal, we detect a residual signal consistent with a filament connecting the galaxy pairs.

Assuming an isothermal cylindrical model, we estimate the baryon over-density in the filament to be 0.28±0.03. The corresponding baryonic mass in these filaments accounts for approximately 8.7% of the total cosmic baryon density. Our results provide further evidence for the existence of diffuse baryons in large-scale filamentary structures and demonstrate the effectiveness of stacking analyses based on photometric redshift surveys.