Search for Faint Lone Double-Peaked H$α$ Lines as IMBH Signatures in the MUSE Deep Field

TL;DR

This study tests whether faint, off-nuclear wandering IMBHs can be identified through isolated double-peaked H$\alpha$ emission in the deep MUSE MXDF field. It develops an automated detection pipeline that cross-correlates observed spectra with a Gaussian kernel tuned to the H$\alpha$ rest wavelength and a separations threshold, then applies flux-based and spatial cross-matching filters to distinguish genuine astrophysical signals from artifacts. Despite the MXDF’s depth, the analysis yields no new IMBH candidates, with many initially detected peaks attributable to known emission-line doublets, cosmic rays, sky residuals, or flat-field artifacts, even after spatial binning. The results underscore the practical challenges of blind IMBH searches in deep IFU data, while providing a robust, scalable method that can be deployed on future datasets (e.g., DESI, JWST, 4MOST) to constrain IMBH populations."

Abstract

Double-peaked H$α$ emission profiles can serve as potential signatures of accreting intermediate-mass black holes (IMBHs), particularly those residing outside galactic nuclei. Such features are expected to arise from rotating disk-like structures around black holes and can be used to identify elusive IMBH candidates. \citet{Almeida2022ApJ...934..100S} reported a sample of such double-peaked H$α$ sources in the MUSE-Wide survey, interpreting them as potential signatures of wandering IMBHs after systematically excluding alternative explanations. Their method relied on constructing H$α$ maps around central galaxies and visually identifying compact emission clumps in the surrounding halo regions. In this work, we revisit the analysis using the deeper MUSE Extremely Deep Field (MXDF) data and an automated detection algorithm tailored to identify such features. However, we do not recover any candidate population in MXDF, resulting in a null detection. This outcome is nevertheless informative, as it (1) highlights the inherent challenges in detecting IMBHs, and (2) demonstrates the potential of automated approaches for future systematic searches, even though it did not yield a positive outcome in this case.

Figures (10)

• Figure 1: Top panels show the false negative (left) and false positive (right) flags as a function of threshold cutoff, with the gaussian kernel width ($\sigma_{gauss}$) fixed at 2.36 Å (the minimum allowed by spectral resolution of MUSE). These flags are generated during testing of the algorithm at varying S/N ratios in the artificial spectrum. Bottom panels shows the false negative (left) and false positive (right) flags as a function of kernel width while fixing the cutoff at 3.7$\sigma$ at varying S/N ratios in the artificial spectrum. The fraction of false flags represents the number of false positives or negatives per Monte Carlo run. Since multiple points overlap, we introduce a random jitter with standard deviation of 0.05 in the x axis.
• Figure 2: Illustration of the flux estimation of an emission line, with the peak wavelength indicated in magenta. The continuum is estimated using the green points, which lie below 3$\times$ the standard deviation. Red points are excluded from the continuum fit due to significant deviations. The blue arrow away from line indicates the wavelength range on either side of the emission line used to estimate the continuum based on the median flux. The grey shaded region marks the integrated flux of the line.
• Figure 3: Example of a detected double peak in the observed spectrum (top panel) and the corresponding correlation spectrum (bottom panel). Data points exceeding the 3.7$\sigma$ threshold are shown in orange, while the detected double peaks in the correlation are marked with green crosses. Grey shaded region in the top panel shows the standard deviation of the flux.
• Figure 4: The distribution of detected double peaks in MUSE-MXDF.
• Figure 5: Wavelength versus R.A. for double peaks above the 3F$_{pl}$ threshold. Blue and red represents the first and second peak. Grey shows all the detected peaks.