The First Systematic Survey of Stellar Halos in High-Inclination Galaxies Reveals Unusually Quiescent Merger Histories of Nearby Galaxies

Abstract

Stellar halos are the only major stellar component of disk galaxies that lack systematic observational characterization, yet they encode critical information about galaxy merger histories. We present the first systematic census of stellar halos in a large, flux-limited sample of 169 high-inclination central galaxies with stellar masses 7.3 <= log Mstar/Msun <= 11.0 and redshift z < 0.1, using HSC-SSP Deep optical images. Stellar halos are detected in 93 galaxies, primarily through their low isophotal ellipticities in the outskirts, improving upon conventional methods of stellar halo identification. The halo detection rate reaches ~ 50% at log Mstar/Msun > 9.9 and >= 70% for Milky Way (MW)-mass galaxies. We derive halo surface brightness profiles, colors, and masses, finding that stellar halos generally follow power-law radial profiles. Higher-mass galaxies, on average, exhibit smaller power-law indices and larger halo mass fractions, indicating more extended halos and more active merger histories. A significant stellar halo color-mass correlation, driven mainly by the mass-metallicity relation, suggests dominance by a few massive accretion events. MW-mass galaxies have a median stellar halo fraction of 10% +/- 5%. Among nearby galaxies with halo measurements within 25 Mpc, two thirds (including the MW) lie below the mean stellar halo fraction-galaxy mass relation. Overall, the nearby galaxies show a median halo deficit of ~ 0.3 dex, implying unusually quiescent merger histories. We show that this deficit follows a broader trend in which typical halo fractions increase with heliocentric distance, tracking the gradual rise in matter density toward the cosmic average by z <= 0.07.

Figures (15)

• Figure 1: Distribution of our galaxies on the stellar mass--redshift plane. The background gray dots show the parent spectroscopic sample from SDSS DR7 and GAMA DR4, limited to galaxies with $r<17.77$ mag. Blue open circles represent high-inclination central galaxies without stellar halo detections, while red open squares mark galaxies with stellar halo detections. The vertical dashed line indicates the upper redshift limit applied in our sample selection. The red dashed curve represents the 95% completeness limit of our parent sample of central galaxies. Stellar masses plotted here are all taken from the parent catalogs.
• Figure 2: HSC-SSP Deep/UltraDeep images of nine representative galaxies in our final sample of high-inclination central galaxies. For each galaxy, the $g$, $r$, and $i$-band tricolor image of the high surface brightness central region is combined with the white-light grayscale image to reveal the lower surface brightness outskirts. Images are re-binned to enhance visibility of the low-surface brightness outskirts. The first row shows galaxies with MW-like stellar masses, $\log(M_{\star}/M_{\odot}) \sim (10.5,\,11.0)$. The second row displays galaxies with $\log(M_{\star}/M_{\odot}) \sim (10.0,\,10.5)$. The last row represents galaxies with the lowest stellar masses, $\log(M_{\star}/M_{\odot}) < 10.0$. The first column depicts galaxies with high stellar halo fractions, $\log f_{\rm halo} > -1$. The middle column shows galaxies with moderate stellar halo fractions, $\log f_{\rm halo} \sim (-2,\,-1)$. The last column shows galaxies without stellar halo detections. The color bar of the grayscale image in the lower-left panel is in units of mag arcsec $^{-2}$.
• Figure 3: Radial profiles of PSFs constructed for the HSC-SSP Deep/UltraDeep images. Cyan triangles show the $r$-band PSF profile provided by HSC-SSP PDR3, which captures only the PSF core due to atmospheric turbulence but misses the extended PSF wings. The combined "normal" wide-PSF profiles constructed in this work are shown as red squares for the $i$ band, green squares for the $r$ band, and blue squares for the $g$ band. The combined "Ultra-Bright" $r$-band wide-PSF profile is shown as violet circles. For comparison, black triangles represent the $r$-band wide-PSF from Wang_2019. See Section \ref{['sec:widepsf']} for details.
• Figure 4: Similar to Figure \ref{['Fig:SampleSnap1']}, but after subtracting bright stars and masking contamination sources. Red ellipses show the isophotal contours in the low surface brightness outskirts. See Sections \ref{['sec:widepsf']} and \ref{['sec:mask']} for details.
• Figure 5: Comparison of the outskirts ellipticities measured on images with and without subtracting the PSF-induced scattered light. Dark-red circles represent galaxies satisfying the ellipticity threshold (after correcting for the PSF effect) for stellar halo identification, while blue circles represent galaxies with power-law surface brightness profiles in the outskirts but do not satisfy the ellipticity requirement for halo identification (see Section \ref{['sec:indenthalo']} for details). Gray circles indicate galaxies without stellar halo detection. This comparison demonstrates the necessity of subtracting PSF-induced scattered light for reliable ellipticity measurements and stellar halo selection.