xGASS: Robust quantification of asymmetries in global H i spectra and their relationship to environmental processes

Abstract

We present an analysis of asymmetries in global Hi spectra from the extended GALEX Arecibo SDSS Survey (xGASS), a stellar mass-selected and gas fraction-limited survey which is representative of the Hi properties of galaxies in the local Universe. We demonstrate that the asymmetry in a Hi spectrum is strongly linked to its signal-tonoise meaning that, contrary to what was done in previous works, asymmetry distributions for different samples cannot be compared at face value. We develop a method to account for noise-induced asymmetry and find that the typical galaxy detected by xGASS exhibits higher asymmetry than what can be attributed to noise alone, with 37% of the sample showing asymmetry greater than 10% at an 80% confidence level. We find that asymmetric galaxies contain, on average, 29% less Hi mass compared to their symmetric counterparts matched in both stellar mass and signal-to-noise. We also present clear evidence that satellite galaxies, as a population, exhibit more asymmetric Hi spectra than centrals and that group central galaxies show a slightly higher rate of Hi asymmetries compared to isolated centrals. All these results support a scenario in which environmental processes, in particular those responsible for gas removal, are the dominant driver of asymmetry in xGASS.

H i asymmetries in LVHIS, VIVA, and HALOGAS galaxies

“…An explanation for the lower asymmetric fractions we find is that the majority of the spectra used in the published results have 10 < SNR < 50, hence their A flux are likely affected by the relatively lower SNRs and have a larger scatter compared with our values. Our trends of higher asymmetric fractions in denser environments is in agreement with the results from Watts et al (2020) for xGASS satellites and centrals, with satellites found to be more frequently asymmetric than centrals and similarly for isolated vs group centrals.…”

“…Recent studies of single dish spectra of Virgo and Abell 1367 cluster galaxies considered A flux > 2σ or 3σ as likely produced by external, environmental influences and found asymmetric fractions of ∼ 16-26% (Scott et al 2018) and 27% for close galaxy pairs (Bok et al 2019) using the AMIGA sample 3σ significance threshold. Watts et al (2020) classified xGASS (Catinella et al 2018) galaxies as satellites or centrals and measured A flux , finding that satellites show a higher frequency of asymmetries than centrals, which supports the previous findings of asymmetries being more common in higher density environments. However, taking this further and understanding the physical origin of spectral asymmetries requires knowledge of the spatial distribution of gas within galaxies and the gas kinematics.…”

“…However, we have not taken into account the differing signal to noise ratios (SNR) for the different studies. Watts et al (2020) show that symmetric spectra will appear increasingly asymmetric for lower SNRs with the distribution in measured A flux increasing by ∆A flux ∼ 0.05-0.3 for SNR < 50. Our spectra are more robust to the effect of SNR as the majority of our spectra have SNR > 50 (82%, 76% and 100% for LVHIS, VIVA and HALOGAS, respectively, Fig.…”

MeerKAT HI commissioning observations of MHONGOOSE galaxy ESO 302-G014

“…For example, Richter & Sancisi (1994) found that half of the 1700 galaxies they studied showed a strong lopsidedness in their global H i profiles. A recent study by Watts et al (2020) of 562 galaxies finds a proportion of 37%. Espada et al (2011) studied lopsidedness in H i and distinguished between isolated and field galaxies.…”

MIGHTEE-HI: The H I emission project of the MeerKAT MIGHTEE survey

“…Thus, with ≈ 150-200 resolved galaxies out to z < 0.09, MIGHTEE-HI will provide a large homogeneous sample beyond the local Universe, which will significantly contribute to the above-mentioned studies. Additional information can be gained from the shape of the resolved H i spectral profile (Watts et al 2020).…”