arXiv subjects

Andrea V. Macciò

Publications and source records attributed to Andrea V. Macciò.

At least 73 records · Page 4Linked to original sources

arXiv2018

NIHAO XVII: The diversity of dwarf galaxy kinematics and implications for the HI velocity function

We use 85 pairs of high resolution LCDM cosmological simulations from the NIHAO project to investigate why in dwarf galaxies neutral hydrogen (HI) linewidths measured at 50% of the peak flux W_50 /2 (from the hydrodynamical simulations) tend to underpredict the maximum circular velocity VmaxDMO (from the corresponding dark matter only simulations). There are two main contributing processes. 1) Lower mass galaxies are less rotationally supported. This is confirmed observationally from the skewness of linewidths in bins of HI mass in both ALFALFA and HIPASS observations. 2) The HI distributions are less extended (relative to the dark matter halo) in dwarf galaxies. Coupled to the lower baryon-to-halo ratio results in rotation curves that are still rising at the last measured data point, in agreement with observations from SPARC. Combining these two effects, in both simulations and observations lower mass galaxies have on average lower W_50 / W_20. Additionally, mass loss driven by supernovae and projection effects (dwarf galaxies are in general not thin disks) further reduce the linewidths. The implied HI linewidth velocity function from NIHAO is in good agreement with observations in the nearby Universe of dwarf galaxies: 10 < W_50 /2 < 80 km/s. The dark matter only slope of -2.9 is reduced to -1.0 in the hydro simulations. Future radio observations of unbiased samples with higher spatial resolution will enable stricter tests of the simulations, and thus of the LCDM model.

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arXiv2018

Quantifying inhomogeneities in the HI distributions of simulated galaxies

The NIHAO cosmological simulations form a collection of a hundred high-resolution galaxies. We used these simulations to test the impact of stellar feedback on the morphology of the HI distribution in galaxies. We ran a subsample of twenty of the galaxies with different parameterizations of stellar feedback, looking for differences in the HI spatial distribution and morphology. We found that different feedback models do leave a signature in HI, and can potentially be compared with current and future observations. These findings can help inform future modeling efforts in the parameterization of stellar feedback in cosmological simulations of galaxy formation and evolution.

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arXiv2018

A deeper look into the structure of ΛCDM haloes: correlations between halo parameters from Einasto fits

We used high resolution dark matter only cosmological simulations to investigate the structural properties of Lambda Cold Dark Matter ($Λ$CDM) haloes over cosmic time. The haloes in our study range in mass from $\sim 10^{10}$ to $\sim 10^{12} \mathrm{M}_\odot$, and are resolved with $10^5$ to $10^7$ particles. We fit the spherically averaged density profiles of DM haloes with the three parameter Einasto function. For our sample of haloes, the Einasto shape parameter, $α$, is uncorrelated with the concentration, $c$, at fixed halo mass, and at all redshifts. Previous reports of an anti-correlation are traced to fitting degeneracies, which our fits are less sensitive to due to our higher spatial resolution. However, for individual haloes the evolution in $α$ and $c$ is anti-correlated: at redshift $z=7$, $α\simeq 0.4$ and decreases with time, while $c\simeq 3$ and increases with time. The evolution in structure is primarily due to accretion of mass at larger radii. We suggest that $α$ traces the evolutionary state of the halo, with dynamically young haloes having high $α$ (closer to a top-hat: $α^{-1}=0$), and dynamically relaxed haloes having low $α$ (closer to isothermal: $α=0$). Such an evolutionary dependence reconciles the increase of $α$ vs peak height, $ν$, with the dependence on the slope of the power spectrum of initial density fluctuations found by previous studies.

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arXiv2018

Angular momentum evolution of bulge stars in disc galaxies in NIHAO

We study the origin of bulge stars and their angular momentum (AM) evolution in 10 spiral galaxies with baryonic masses above $10^{10}$M$_\odot$ in the NIHAO galaxy formation simulations. The simulated galaxies are in good agreement with observations of the relation between specific AM and mass of the baryonic component and the stellar bulge-to-total ratio ($B/T$). We divide the star particles at $z=0$ into disc and bulge components using a hybrid photometric/kinematic decomposition method that identifies all central mass above an exponential disc profile as the `bulge'. By tracking the bulge star particles back in time, we find that on average 95\% of the bulge stars formed {\it in situ}, 3\% formed {\it ex situ} in satellites of the same halo, and only 2\% formed {\it ex situ} in external galaxies. The evolution of the AM distribution of the bulge stars paints an interesting picture: the higher the final $B/T$ ratio, the more the specific AM remains preserved during the bulge formation. In all cases, bulge stars migrate significantly towards the central region, reducing their average galactocentric radius by roughly a factor 2, independently of the final $B/T$ value. However, in the higher $B/T$ ($\gtrsim0.2$) objects, the velocity of the bulge stars increases and the AM of the bulge is almost conserved, whereas at lower $B/T$ values, the velocity of the bulge stars decreases and the AM of bulge reduces. The correlation between the evolution of the AM and $B/T$ suggests that bulge and disc formation are closely linked and cannot be treated as independent processes.

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arXiv2018

NIHAO XV: The environmental impact of the host galaxy on galactic satellite and field dwarf galaxies

We study the impact of the host on dwarf galaxy properties using four new Milky Way-like, ultra high-resolution simulations, ($N_{\rm part} >10^7$) from the NIHAO project. We split our sample in satellite ($R 2.5 R_{\rm 200}$) galaxies. Simulated galaxies from all three groups are in excellent agreement with Local Group dwarf galaxies in terms of: stellar mass-velocity dispersion, stellar mass-metallicity relation, star formation histories, and stellar mass functions. Satellites and nearby galaxies show lower velocity dispersions and gas fractions compared to field galaxies. While field galaxies follow global abundance matching relations, satellites and nearby galaxies deviate from them, showing lower dark matter masses for given stellar mass. The reason for this deficit in dark matter mass is substantial mass loss experienced by satellites and $\sim80$\% of the nearby galaxies, while orbiting inside $R_{200}$ at earlier times. However, both satellites and nearby objects fall back onto the relation for field galaxies if we use the maximum of their virial mass instead of the present-day value. This allows us to provide estimates for the peak masses of observed Local Group galaxies. Finally, using radial velocities, distances, and the velocity dispersion-stellar mass relation from our simulations, we derive a metric to distinguish between galaxies harassed by the central object and unaffected ones. Applying this metric to observed objects we find that even far away dwarf galaxies like Eri II ($D\approx$ 370 kpc) have a strong probability ($\approx 83$\%) of having been affected by the Milky Way in the past. This naturally explains the lack of gas and recent star formation seen in Eri II.

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arXiv2018

Stars behind bars I: The Milky Way's central stellar populations

We show for the first time, that a fully cosmological hydrodynamical simulation can reproduce key properties of the innermost region of the Milky Way. Our high resolution simulation matches the profile and kinematics of the Milky Way's boxy/peanut-shaped bulge, and hence we can use it to reconstruct and understand the bulge assembly. In particular, the age dependence of the X-shape morphology of the simulated bulge parallels the observed metallicity dependent split in the red clump stars of the inner Galaxy. We use this feature to derive an observational metric that allows us to quantify when the bulge formed from the disk. The metric we propose can be employed with upcoming survey data to constrain the age of the Milky Way bar. From the split in stellar counts we estimate the formation of the 4~kpc scale bar in the simulation to have happened $t^{\rm bar}_{\rm form}\sim8^{+2}_{-2}$ Gyr ago, in good agreement with conventional methods to measure bar formation in simulations. We test the prospects for observationally differentiating the stars that belong to the bulge/bar compared to the surrounding disk, and find that the inner disk and bulge are practically indistinguishable in both chemistry and ages.

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arXiv2018

Introducing galactic structure finder: the multiple stellar kinematic structures of a simulated Milky Way mass galaxy

We present the first results of applying Gaussian Mixture Models in the stellar kinematic space of normalized angular momentum and binding energy on NIHAO high resolution galaxies to separate the stars into multiple components. We exemplify this method using a simulated Milky Way analogue, whose stellar component hosts: thin and thick discs, classical and pseudo bulges, and a stellar halo. The properties of these stellar structures are in good agreement with observational expectations in terms of sizes, shapes and rotational support. Interestingly, the two kinematic discs show surface mass density profiles more centrally concentrated than exponentials, while the bulges and the stellar halo are purely exponential. We trace back in time the Lagrangian mass of each component separately to study their formation history. Between z~3 and the end of halo virialization, z~1.3, all components lose a fraction of their angular momentum. The classical bulge loses the most (~95%) and the thin disc the least (~60%). Both bulges formed their stars in-situ at high redshift, while the thin disc formed ~98% in-situ, but with a constant SFR~1.5M$_{\rm\odot}$/yr$^{\rm-1}$ over the last ~11 Gyr. Accreted stars (6% of total stellar mass) are mainly incorporated to the thick disc or the stellar halo, which formed ex-situ 8% and 45% of their respective masses. Our analysis pipeline is freely available at https://github.com/aobr/gsf.

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arXiv2018

Is the dark-matter halo spin a predictor of galaxy spin and size?

The similarity between the distributions of spins for galaxies ($λ_{\rm g}$) and for dark-matter haloes ($λ_{\rm h}$), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via $R_{\rm e}\simeqλ_{\rm h} R_{\rm v}$, with $R_{\rm e}$ the half-mass radius of the galaxy and $R_{\rm v}$ the halo radius. Utilizing two different suites of zoom-in cosmological simulations, we find that $λ_{\rm g}$ and $λ_{\rm h}$ are in fact only barely correlated, especially at $z\geq 1$. A general smearing of this correlation is expected based on the different spin histories, where the more recently accreted baryons through streams gain and then lose significant angular momentum compared to the gradually accumulated dark matter. Expecting the spins of baryons and dark matter to be correlated at accretion into $R_{\rm v}$, the null correlation at the end reflects an anti-correlation between $λ_{\rm g}/λ_{\rm h}$ and $λ_{\rm h}$, which can partly arise from mergers and a compact star-forming phase that many galaxies undergo. On the other hand, the halo and galaxy spin vectors tend to be aligned, with a median $\cosθ=0.6$-0.7 between galaxy and halo, consistent with instreaming within a preferred plane. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. Following the null spin correlation, $λ_{\rm h}$ is not a useful proxy for $R_{\rm e}$. While our simulations reproduce a general relation of the sort $R_{\rm e}=AR_{\rm vir}$, in agreement with observational estimates, the relation becomes tighter with $A=0.02(c/10)^{-0.7}$, where $c$ is the halo concentration, which in turn introduces a dependence on mass and redshift.

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arXiv2017

Inspiraling Halo Accretion Mapped in Lyman-$α$ Emission around a $z\sim3$ Quasar

In an effort to search for Ly$α$ emission from circum- and intergalactic gas on scales of hundreds of kpc around $z\sim3$ quasars, and thus characterise the physical properties of the gas in emission, we have initiated an extensive fast-survey with the Multi Unit Spectroscopic Explorer (MUSE): Quasar Snapshot Observations with MUse: Search for Extended Ultraviolet eMission (QSO MUSEUM). In this work, we report the discovery of an enormous Ly$α$ nebula (ELAN) around the quasar SDSS~J102009.99+104002.7 at $z=3.164$, which we followed-up with deeper MUSE observations. This ELAN spans $\sim297$ projected kpc, has an average Ly$α$ surface brightness ${\rm SB}_{\rm Lyα}\sim 6.04\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ (within the $2σ$ isophote), and is associated with an additional four, previously unknown embedded sources: two Ly$α$ emitters and two faint active galactic nuclei (one Type-1 and one Type-2 quasar). By mapping at high significance the line-of-sight velocity in the entirety of the observed structure, we unveiled a large-scale coherent rotation-like pattern spanning $\sim300$ km s$^{-1}$ with a velocity dispersion of $<270$ km s$^{-1}$, which we interpret as a signature of the inspiraling accretion of substructures within the quasar's host halo. Future multiwavelength data will complement our MUSE observations, and are definitely needed to fully characterise such a complex system. None the less, our observations reveal the potential of new sensitive integral-field spectrographs to characterise the dynamical state of diffuse gas on large scales in the young Universe, and thereby witness the assembly of galaxies.

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arXiv2017

The edge of galaxy formation II: evolution of Milky Way satellite analogues after infall

In the first paper we presented 27 hydrodynamical cosmological simulations of galaxies with total masses between $5 \times 10^8$ and $10^{10}\,\mathrm{M}_\odot$. In this second paper we use a subset of these cosmological simulations as initial conditions (ICs) for more than forty hydrodynamical simulations of satellite and host galaxy interaction. Our cosmological ICs seem to suggest that galaxies on these mass scales have very little rotational support and are velocity dispersion ($σ$) dominated. Accretion and environmental effects increase the scatter in the galaxy scaling relations (e.g. size - velocity dispersion) in very good agreement with observations. Star formation is substantially quenched after accretion. Mass removal due to tidal forces has several effects: it creates a very flat stellar velocity dispersion profiles, and it reduces the dark matter content at all scales (even in the centre), which in turn lowers the stellar velocity on scales around 0.5 kpc even when the galaxy does not lose stellar mass. Satellites that start with a cored dark matter profile are more prone to either be destroyed or to end up in a very dark matter poor galaxy. Finally, we found that tidal effects always increase the "cuspyness" of the dark matter profile, even for haloes that infall with a core.

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arXiv2017

The edge of galaxy formation I: formation and evolution of MW-satellites analogues before accretion

The satellites of the Milky Way and Andromeda represent the smallest galaxies we can observe in our Universe. In this series of papers we aim to shed light on their formation and evolution using cosmological hydrodynamical simulations. In this first paper we focus on the galaxy properties before accretion, by simulating twenty seven haloes with masses between $5\times 10^8$ and $10^{10} M_\odot$. Out of this set nineteen haloes successfully form stars, while eight remain dark. The simulated galaxies match quite well present day observed scaling relations between stellar mass, size and metallicity, showing that such relations are in place before accretion. Our galaxies show a large variety of star formation histories, from extended star formation periods to single bursts. As in more massive galaxies, large star formation bursts are connected with major mergers events, which greatly contribute to the overall stellar mass build up. The intrinsic stochasticity of mergers induces a large scatter in the stellar mass halo mass relation, up to two orders of magnitude. Despite the bursty star formation history, on these mass scales baryons are very ineffective in modifying the dark matter profiles, and galaxies with a stellar mass below $\approx 10^6 M_\odot$ retain their cuspy central dark matter distribution, very similar to results from pure N-body simulations.

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arXiv2017

CALIFA reveals Prolate Rotation in Massive Early-type Galaxies: A Polar Galaxy Merger Origin?

We present new evidence for eight early-type galaxies (ETGs) from the CALIFA Survey that show clear rotation around their major photometric axis ("prolate rotation"). These are LSBCF560-04, NGC 0647, NGC 0810, NGC 2484, NGC 4874, NGC 5216, NGC 6173 and NGC 6338. Including NGC 5485, a known case of an ETG with stellar prolate rotation, as well as UGC 10695, a further possible candidate for prolate rotation, we report ten CALIFA galaxies in total that show evidence for such a feature in their stellar kinematics. Prolate rotators correspond to ~9% of the volume-corrected sample of CALIFA ETGs, a fraction much higher than previously reported. We find that prolate rotation is more common among the most massive ETGs. We investigate the implications of these findings by studying N-body merger simulations, and show that a prolate ETG with rotation around its major axis could be the result of a major polar merger, with the amplitude of prolate rotation depending on the initial bulge-to-total stellar mass ratio of its progenitor galaxies. Additionally, we find that prolate ETGs resulting from this formation scenario show a correlation between their stellar line-of-sight velocity and higher order moment h_3, opposite to typical oblate ETGs, as well as a double peak of their stellar velocity dispersion along their minor axis. Finally, we investigate the origin of prolate rotation in polar galaxy merger remnants. Our findings suggest that prolate rotation in massive ETGs might be more common than previously expected, and can help towards a better understanding of their dynamical structure and formation origin.

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arXiv2017

The tilting rate of the Milky Way's disc

We present tilting rates for galaxies comparable to the Milky Way (MW) in a $Λ$ cold dark matter cosmological hydrodynamical simulation, and compare these with the predicted tilting rate detection limit of the {\it Gaia} satellite $0.28\degrees$Gyr$^{-1}$. We first identify galaxies with mass comparable to the MW ($9 \times 10^{11} \le M_{200} \le 1.2 \times 10^{12} \Msun $) and consider the tilting rates between $z=0.3$ and $0$. This sample yields a tilting rate of $7.6\degrees \pm 4.5\degrees$Gyr$^{-1}$. We constrain our sample further to exclude any galaxies that have high stellar accretion during the same time. We still find significant tilting, with an average rate of $6.3\degrees$Gyr$^{-1}$. Both subsamples tilt with rates significantly above {\it Gaia}'s predicted detection limit. We show that our sample of galaxies covers a wide range of environments, including some similar to the MW's. We find galaxies in denser regions tilt with higher rates then galaxies in less dense regions. We also find correlations between the angular misalignment of the hot gas corona, and the tilting rate. {\it Gaia} is likely to be able to directly measure tilting in the MW. Such a detection will provide an important constraint on the environment of the MW, including the rate of gas cooling onto the disc, the shape and orientation of its dark matter halo, and the mass of the Large Magellanic Cloud. Conversely, failure to detect tilting may suggest the MW is in a very quiet configuration.

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arXiv2017

NIHAO XIII: Clumpy discs or clumpy light in high redshift galaxies?

Many massive star forming disc galaxies in the redshift range 3 to 0.5 are observed to have a clumpy morphology showing giant clumps of size $\sim$1 kpc and masses of about $10^7M_{\odot}$ to $10^{10} M_{\odot}$. The nature and fate of these giant clumps is still under debate. In this work we use 19 high-resolution simulations of disc galaxies from the NIHAO sample to study the formation and the evolution of clumps in the discs of high redshift galaxies. We use mock HST - CANDELS observations created with the radiative transfer code GRASIL-3D to carry out, for the first time, a quantitative comparison of the observed fraction of clumpy galaxies and its evolution with redshift with simulations. We find a good agreement between the observed clumpy fraction and the one of the NIHAO galaxies. We find that dust attenuation can suppress intrinsically bright clumps and enhance less luminous ones. In our galaxy sample we only find clumps in light (u-band) from young stars but not in stellar mass surface density maps. This means that the NIHAO sample does not show clumpy stellar discs but rather a clumpy light distribution originating from clumpy star formation events. The clumps found in the NIHAO sample match observed age/color gradients as a function of distance from the galaxy center but they show no sign of inward migration. Clumps in our simulations disperse on timescales of a about a hundred Myr and their contribution to bulge growth is negligible.

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arXiv2017

NIHAO XII: galactic uniformity in a ΛCDM universe

We use a sample of 83 high-resolution cosmological zoom-in simulations and a semi-analytic model (SAM) to study the stochasticity of galaxy formation in haloes ranging from dwarf to Milky Way masses. Our simulated galaxies reproduce the observed inefficiency of galaxy formation as expressed through the stellar, gas and baryonic Tully-Fisher relations. For HI velocities in the range (70 < V < 220 km/s), the scatter is just 0.08 to 0.14 dex, consistent with the observed intrinsic scatter at these scales. At low velocities (20 < V < 70 km/s), the simulated scatter is 0.2-0.25 dex, which could be tested with future observations. The scatter in the stellar mass versus dark halo velocity relation is constant for 30 < V < 180 km/s, and smaller (~0.17 dex) when using the maximum circular velocity of the dark matter only simulation, Vmax, compared to the virial velocity ($V_{200}$ or $V_{200}^{DMO}$). The scatter in stellar mass is correlated with halo concentration, and is minimized when using a circular velocity at a fixed fraction of the virial radius $\simeq 0.4 R_{200}$ or with $V_α=V_{200}^{DMO} (V_{max}^{DMO} / V_{200}^{DMO})^α$ with α~ 0.7, consistent with constraints from halo clustering. Using the SAM we show the correlation between halo formation time and concentration is essential in order to reproduce this result. This uniformity in galaxy formation efficiency we see in our hydrodynamical simulations and a semi-analytic model proves the simplicity and self-regulating nature of galaxy formation in a ΛCold Dark Matter universe.

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arXiv2017

NIHAO VII: Predictions for the galactic baryon budget in dwarf to Milky Way mass haloes

We use the NIHAO galaxy formation simulations to make predictions for the baryonic budget in present day galaxies ranging from dwarf to Milky Way masses. The sample is made of 88 independent high resolution cosmological zoom-in simulations. NIHAO galaxies reproduce key properties of observed galaxies, such as the stellar mass vs halo mass and cold gas vs stellar mass relations. Thus they make plausible predictions for the baryon budget. We present the mass fractions of stars, cold gas ($T<10^4$K), cool gas ($10^4 < T < 10^5$K), warm-hot gas ($10^5 < T < 5\times10^6$K), and hot gas (T$> 5\times10^6$K) inside the virial radius, $R_{200}$. Compared to the predicted baryon mass, using the dark halo mass and the universal baryon fraction, $f_{\rm b}\equiv Ω_{\rm b}/Ω_{\rm m}=0.15$, we find that all of our haloes are missing baryons. The missing mass has been relocated past 2 virial radii, and cool gas dominates the corona at low mass (M$_{200} < 3 \times 10^{11} M_\odot$) while the warm-hot gas dominates at high mass (M$_{200} > 3 \times 10^{11} M_\odot$). Haloes of mass $\sim 10^{10} M_\odot$ are missing $\sim 90\%$ of their baryons. More massive haloes ($\sim 10^{12} M_\odot$) retain a higher fraction of their baryons, with $\sim 30\%$ missing, consistent with recent observational estimates. Moreover, these more massive haloes reproduce the observed fraction of cold, warm-hot and hot gas. The fraction of cool gas we predict ($0.11\pm0.06$) is significantly lower than the observation from COS-HALOs (0.3-0.47), but agrees with the alternative analysis of Stern et al. 2016.

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arXiv2017

Quenching vs. Quiescence: forming realistic massive ellipticals with a simple starvation model

The decrease in star formation (SF) and the morphological change necessary to produce the $z=0$ elliptical galaxy population are commonly ascribed to a sudden quenching event, which is able to rid the central galaxy of its cold gas reservoir in a short time. Following this event, the galaxy is able to prevent further SF and stay quiescent via a maintenance mode. We test whether such a quenching event is truly necessary using a simple model of quiescence. In this model, hot gas (all gas above a temperature threshold) in a $\sim10^{12} M_{\odot}$ halo mass galaxy at redshift $z\sim3$ is prevented from cooling. The cool gas continues to form stars at a decreasing rate and the galaxy stellar mass, morphology, velocity dispersion and position on the color magnitude diagram (CMD) proceed to evolve. By $z=0$, the halo mass has grown to $10^{13} M_{\odot}$ and the galaxy has attained characteristics typical of an observed $z=0$ elliptical galaxy. Our model is run in the framework of a cosmological, smooth particle hydrodynamic code which includes SF, early stellar feedback, supernova feedback, metal cooling and metal diffusion. Additionally, we post-process our simulations with a radiative transfer code to create a mock CMD. In contrast to previous assumptions that a pure "fade away" model evolves too slowly to account for the sparsity of galaxies in the "green valley", we demonstrate crossing times of $<$1 Gyr. We conclude that no sudden quenching event is necessary to produce such rapid colour transitions.

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arXiv2016

NIHAO XI: Formation of Ultra-Diffuse Galaxies by outflows

We address the origin of Ultra-Diffuse Galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and whether they are failed $\rm L_{\star}$ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the NIHAO project, we show that UDG analogues form naturally in medium-mass haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses $10^{10-11}\rm M_{\odot}$, have stellar masses of $10^{7-8.5}\rm M_{\odot}$, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colors, an average Sérsic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible HI gas mass of $10^{7-9}\rm M_{\odot}$, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes that form UDGs: feedback driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more HI gas than less extended dwarfs of similar $\rm M_{\star}$.

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