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Register a new userAngular momentum-related probe of cold gas deficiencies
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Recent studies of neutral atomic hydrogen (HI) in nearby galaxies found that all field disk galaxies are HI saturated, in that they carry roughly as much HI as permitted before this gas becomes gravitationally unstable. By taking this HI saturation for granted, the atomic gas fraction $f_{rm atm}$ of galactic disks can be predicted as a function of the stability parameter $q=jsigma/(GM)$, where $M$ and $j$ are the baryonic mass and specific angular momentum of the disk and $sigma$ is the HI velocity dispersion Obreschkow et al. 2016. The log-ratio $Delta f_q$ between this predictor and the observed atomic fraction can be seen as a physically motivated `HI deficiency. While field disk galaxies have $Delta f_q approx0$, objects subject to environmental removal of HI are expected to have $Delta f_q>0$. Within this framework, we revisit the HI deficiencies of satellite galaxies in the Virgo cluster and in clusters of the EAGLE simulation. We find that observed and simulated cluster galaxies are HI deficient and that $Delta f_q$ slightly increases when getting closer to the cluster centres. The $Delta f_q$ values are similar to traditional HI deficiency estimators, but $Delta f_q$ is more directly comparable between observations and simulations than morphology-based deficiency estimators. By tracking the simulated HI deficient cluster galaxies back in time, we confirm that $Delta f_qapprox0$ until the galaxies first enter a halo with $M_{rm halo}>10^{13} {rm M_{odot}}$, at which moment they quickly lose HI by environmental effects. Finally, we use the simulation to investigate the links between $Delta f_q$ and quenching of star formation.
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We show that the mass fraction f_atm = 1.35*MHI/M of neutral atomic gas (HI and He) in isolated local disk galaxies of baryonic mass M is well described by a straightforward stability model for flat exponential disks. In the outer disk parts, where gas at the characteristic dispersion of the warm neutral medium is stable in the sense of Toomre (1964), the disk consists of neutral atomic gas; conversely the inner part where this medium would be Toomre-unstable, is dominated by stars and molecules. Within this model, f_atm only depends on a global stability parameter q=j*sigma/(GM), where j is the baryonic specific angular momentum of the disk and sigma the velocity dispersion of the atomic gas. The analytically derived first-order solution f_atm = min{1,2.5q^1.12} provides a good fit to all plausible rotation curves. This model, with no free parameters, agrees remarkably well (+-0.2 dex) with measurements of f_atm in isolated local disk galaxies, even with galaxies that are extremely HI-rich or HI-poor for their mass. The finding that f_atm increases monotonically with q for pure stability reasons offers a powerful intuitive explanation for the mean variation of f_atm with M: in a cold dark matter universe galaxies are expected to follow j~M^(2/3), which implies the average scaling q~M^(-1/3) and hence f_atm~M^(-0.37), in agreement with observations.
We study the z=0 gas kinematics, morphology, and angular momentum content of isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE project spanning $M_{star}=10^{6-11}M_{odot}$. Gas becomes increasingly rotationally supported with increasing galaxy mass. In the lowest-mass galaxies ($M_{star}<10^{8}M_{odot}$), gas fails to form a morphological disk and is primarily dispersion and pressure supported. At intermediate masses ($M_{star}=10^{8-10}M_{odot}$), galaxies display a wide range of gas kinematics and morphologies, from thin, rotating disks, to irregular spheroids with negligible net rotation. All the high-mass ($M_{star}=10^{10-11}M_{odot}$) galaxies form rotationally supported gas disks. Many of the halos whose galaxies fail to form disks harbor high angular momentum gas in their circumgalactic medium. The ratio of the specific angular momentum of gas in the central galaxy to that of the dark-matter halo increases significantly with galaxy mass, from $j_{rm gas}/j_{rm DM}sim0.1$ at $M_{star}=10^{6-7}M_{odot}$ to $j_{rm gas}/j_{rm DM}sim2$ at $M_{star}=10^{10-11}M_{odot}$. The reduced rotational support in the lowest-mass galaxies owes to (a) stellar feedback and the UV background suppressing the accretion of high-angular momentum gas at late times, and (b) stellar feedback driving large non-circular gas motions. We broadly reproduce the observed scaling relations between galaxy mass, gas rotation velocity, size, and angular momentum, but may somewhat underpredict the incidence of disky, high-angular momentum galaxies at the lowest observed masses ($M_{star}=(10^{6}-2times10^{7})M_{odot}$). In our simulations, stars are uniformly less rotationally supported than gas. The common assumption that stars follow the same rotation curve as gas thus substantially overestimates galaxies stellar angular momentum, particularly at low masses.
We derive the stellar-to-halo specific angular momentum relation (SHSAMR) of galaxies at $z=0$ by combining i) the standard $Lambda$CDM tidal torque theory ii) the observed relation between stellar mass and specific angular momentum (Fall relation) and iii) various determinations of the stellar-to-halo mass relation (SHMR). We find that the ratio $f_j = j_ast/j_{rm h}$ of the specific angular momentum of stars to that of the dark matter i) varies with mass as a double power-law, ii) it always has a peak in the mass range explored and iii) it is $3-5$ times larger for spirals than for ellipticals. The results have some dependence on the adopted SHMR and we provide fitting formulae in each case. For any choice of the SHMR, the peak of $f_j$ occurs at the same mass where the stellar-to-halo mass ratio $f_ast = M_ast/M_{rm h}$ has a maximum. This is mostly driven by the straightness and tightness of the Fall relation, which requires $f_j$ and $f_ast$ to be correlated with each other roughly as $f_jpropto f_ast^{2/3}$, as expected if the outer and more angular momentum rich parts of a halo failed to accrete onto the central galaxy and form stars (biased collapse). We also confirm that the difference in the angular momentum of spirals and ellipticals at a given mass is too large to be ascribed only to different spins of the parent dark-matter haloes (spin bias).
We use high-resolution HI data from the WHISP survey to study the HI and angular momentum properties of a sample of 114 late-type galaxies. We explore the specific baryonic angular momentum -- baryonic mass ($j_b - M_b$) relation, and find that an unbroken power law of the form $j_b propto M_b^{0.55 pm 0.02}$ fits the data well, with an intrinsic scatter of $sim 0.13 pm 0.01$ dex. We revisit the relation between the atomic gas fraction, $f_{atm}$, and the integrated atomic stability parameter $q$ (the $f_{atm} - q$ relation), originally introduced by Obreschkow et al., and probe this parameter space by populating it with galaxies from different environments, in order to study the influence of the environment on their $j_b$, $f_{atm}$ and $q$ values. We find evidence that galaxies with close neighbours show a larger intrinsic scatter about the $f_{atm} - q$ relation compared to galaxies without close-neighbours. We also find enhanced SFR among the deviating galaxies with close neighbours. In addition, we use the bulge-to-total (B/T) ratio as a morphology proxy, and find a general trend of decreasing B/T values with increasing disc stability and HI fraction in the $f_{atm} - q$ plane, indicating a fundamental link between mass, specific angular momentum, gas fraction and morphology of galaxies.
The total specific angular momentum j of a galaxy disk is matched with that of its dark matter halo, but the distributions are different, in that there is a lack of both low- and high-j baryons with respect to the CDM predictions. I illustrate how the probability density function PDF(j/j_mean) can inform us of a galaxys morphology and evolutionary history with a spanning set of examples from present-day galaxies and a galaxy at z~1.5. The shape of PDF(j/j_mean) is correlated with photometric morphology, with disk-dominated galaxies having more symmetric PDF(j/j_mean) and bulge-dominated galaxies having a strongly-skewed PDF(j/j_mean). Galaxies with bigger bulges have more strongly-tailed PDF(j/j_mean), but disks of all sizes have a similar PDF(j/j_mean). In future, PDF(j/j_mean) will be useful as a kinematic decomposition tool.
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