Environmental Mechanisms Shaping the Nature of Dwarf Spheroidal Galaxies: The View of Computer Simulations

We review numerical works carried out over the last decade on the role of environmental mechanisms in shaping nature of the faintest galaxies known, dwarf spheroidals (dSphs). The combination of tidally induced morphological transformation, termed tidal stirring, with mass loss due to tidal and ram-pressure stripping aided by heating due to the cosmic ionizing background can turn late-type dwarfs resembling present-day dIrrs into classic dSphs. The time of infall into the primary halo is shown to be a key parameter. Dwarfs accreting at when the cosmic ultraviolet ionizing flux was much higher than today, and was thus able to keep the gas in the dwarfs warm and diffuse, were rapidly stripped of their baryons via ram pressure and tidal forces, producing very dark-matter-dominated objects with truncated star-formation histories, such as the Draco dSph. The low star-formation efficiency expected in such low-metallicity objects prior to their infall was crucial for keeping their disks gas dominated until stripping took over.Therefore gas stripping along with inefficient star-formation provides a new feedback mechanism, alternative to photoevaporation or supernovae feedback, playing a crucial role in dwarf galaxy formation and evolution. We also discuss how the ultra-faint dSphs belong to a different population of lower-mass dwarf satellites that were mostly shaped by reionization rather than by environmental mechanisms (“reionization fossils”). Finally, we scrutinize the various caveats in the current understanding of environmental effects as well as other recent ideas on the origin of Local Group dSphs.

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NIHAO – XXV. Convergence in the cusp-core transformation of cold dark matter haloes at high star formation thresholds

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3028 ◽

2020 ◽

Vol 499 (2) ◽

pp. 2648-2661

Author(s):

Aaron A Dutton ◽

Tobias Buck ◽

Andrea V Macciò ◽

Keri L Dixon ◽

Marvin Blank ◽

...

Keyword(s):

Dark Matter ◽

Star Formation ◽

Galaxy Formation ◽

Cold Dark Matter ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Good Description ◽

Good Match ◽

Virial Mass ◽

Density Threshold

ABSTRACT We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the response of cold dark matter (CDM) haloes to baryonic processes. Previous work has shown that the halo response is primarily a function of the ratio between galaxy stellar mass and total virial mass, and the density threshold above which gas is eligible to form stars, n[cm−3]. At low n all simulations in the literature agree that dwarf galaxy haloes are cuspy, but at high n ≳ 100 there is no consensus. We trace halo contraction in dwarf galaxies with n ≳ 100 reported in some previous simulations to insufficient spatial resolution. Provided the adopted star formation threshold is appropriate for the resolution of the simulation, we show that the halo response is remarkably stable for n ≳ 5, up to the highest star formation threshold that we test, n = 500. This free parameter can be calibrated using the observed clustering of young stars. Simulations with low thresholds n ≤ 1 predict clustering that is too weak, while simulations with high star formation thresholds n ≳ 5, are consistent with the observed clustering. Finally, we test the CDM predictions against the circular velocities of nearby dwarf galaxies. Low thresholds predict velocities that are too high, while simulations with n ∼ 10 provide a good match to the observations. We thus conclude that the CDM model provides a good description of the structure of galaxies on kpc scales provided the effects of baryons are properly captured.

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EDGE: a new approach to suppressing numerical diffusion in adaptive mesh simulations of galaxy formation

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3645 ◽

2020 ◽

Vol 501 (2) ◽

pp. 1755-1765

Author(s):

Andrew Pontzen ◽

Martin P Rey ◽

Corentin Cadiou ◽

Oscar Agertz ◽

Romain Teyssier ◽

...

Keyword(s):

Star Formation ◽

Galaxy Formation ◽

Adaptive Mesh Refinement ◽

Large Scale ◽

Initial Conditions ◽

Dwarf Galaxy ◽

High Redshift ◽

Morphological Structure ◽

Adaptive Mesh ◽

Numerical Diffusion

ABSTRACT We introduce a new method to mitigate numerical diffusion in adaptive mesh refinement (AMR) simulations of cosmological galaxy formation, and study its impact on a simulated dwarf galaxy as part of the ‘EDGE’ project. The target galaxy has a maximum circular velocity of $21\, \mathrm{km}\, \mathrm{s}^{-1}$ but evolves in a region that is moving at up to $90\, \mathrm{km}\, \mathrm{s}^{-1}$ relative to the hydrodynamic grid. In the absence of any mitigation, diffusion softens the filaments feeding our galaxy. As a result, gas is unphysically held in the circumgalactic medium around the galaxy for $320\, \mathrm{Myr}$, delaying the onset of star formation until cooling and collapse eventually triggers an initial starburst at z = 9. Using genetic modification, we produce ‘velocity-zeroed’ initial conditions in which the grid-relative streaming is strongly suppressed; by design, the change does not significantly modify the large-scale structure or dark matter accretion history. The resulting simulation recovers a more physical, gradual onset of star formation starting at z = 17. While the final stellar masses are nearly consistent ($4.8 \times 10^6\, \mathrm{M}_{\odot }$ and $4.4\times 10^6\, \mathrm{M}_{\odot }$ for unmodified and velocity-zeroed, respectively), the dynamical and morphological structure of the z = 0 dwarf galaxies are markedly different due to the contrasting histories. Our approach to diffusion suppression is suitable for any AMR zoom cosmological galaxy formation simulations, and is especially recommended for those of small galaxies at high redshift.

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The Herschel Dwarf Galaxy Survey

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834457 ◽

2019 ◽

Vol 626 ◽

pp. A23 ◽

Cited By ~ 20

Author(s):

D. Cormier ◽

N. P. Abel ◽

S. Hony ◽

V. Lebouteiller ◽

S. C. Madden ◽

...

Keyword(s):

Star Formation ◽

Dwarf Galaxy ◽

Spectral Synthesis ◽

Line Emission ◽

Ionized Gas ◽

Star Forming ◽

Gas Phases ◽

Physical Conditions ◽

Radiation Fields ◽

Low Metallicity

The sensitive infrared telescopes, Spitzer and Herschel, have been used to target low-metallicity star-forming galaxies, allowing us to investigate the properties of their interstellar medium (ISM) in unprecedented detail. Interpretation of the observations in physical terms relies on careful modeling of those properties. We have employed a multiphase approach to model the ISM phases (H II region and photodissociation region) with the spectral synthesis code Cloudy. Our goal is to characterize the physical conditions (gas densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel Dwarf Galaxy Survey. We are particularly interested in correlations between those physical conditions and metallicity or star-formation activity. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [C II]157 μm line, and the characterization of the porosity of the ISM. We find that the lower-metallicity galaxies of our sample tend to have higher ionization parameters and galaxies with higher specific star-formation rates have higher gas densities. The [C II] emission arises mainly from PDRs and the contribution from the ionized gas phases is small, typically less than 30% of the observed emission. We also find a correlation – though with scatter – between metallicity and both the PDR covering factor and the fraction of [C II] from the ionized gas. Overall, the low metal abundances appear to be driving most of the changes in the ISM structure and conditions of these galaxies, and not the high specific star-formation rates. These results demonstrate in a quantitative way the increase of ISM porosity at low metallicity. Such porosity may be typical of galaxies in the young Universe.

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A Gaseous Group with Unusual Remote Star Formation

Publications of the Astronomical Society of Australia ◽

10.1071/as11008 ◽

2011 ◽

Vol 28 (3) ◽

pp. 271-279 ◽

Cited By ~ 2

Author(s):

N. Santiago-Figueroa ◽

M. E. Putman ◽

J. Werk ◽

G. R. Meurer ◽

E. Ryan-Weber

Keyword(s):

Star Formation ◽

Spiral Galaxy ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Spiral Arms ◽

Gaseous Disk ◽

Ram Pressure ◽

The Galaxy ◽

Galaxy Center ◽

Region 10

AbstractWe present VLA 21-cm observations of the spiral galaxy ESO 481-G017 to determine the nature of remote star formation traced by an Hii region found 43 kpc and ∼800 km s−1 from the galaxy center (in projection). ESO 481-G017 is found to have a 120 kpc Hi disk with a mass of 1.2 × 1010M⊙ and UV GALEX images reveal spiral arms extending into the gaseous disk. Two dwarf galaxies with Hi masses close to 108M⊙ are detected at distances of ∼200 kpc from ESO 481-G017 and a Hi cloud with a mass of 6 × 107M⊙ is found near the position and velocity of the remote Hii region. The Hii region is somewhat offset from the Hi cloud spatially and there is no link to ESO 481-G017 or the dwarf galaxies. We consider several scenarios for the origin of the cloud and Hii region and find the most likely is a dwarf galaxy that is undergoing ram pressure stripping. The Hi mass of the cloud and Hi luminosity of the Hii region (1038.1 erg s−1) are consistent with dwarf galaxy properties, and the stripping can trigger the star formation as well as push the gas away from the stars.

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Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038860 ◽

2020 ◽

Vol 643 ◽

pp. A141 ◽

Cited By ~ 1

Author(s):

S. C. Madden ◽

D. Cormier ◽

S. Hony ◽

V. Lebouteiller ◽

N. Abel ◽

...

Keyword(s):

Star Formation ◽

Mass Fraction ◽

Conversion Factor ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Molecular Gas ◽

Star Forming ◽

Forming Efficiency ◽

Low Metallicity ◽

Co Observations

Context. Molecular gas is a necessary fuel for star formation. The CO (1−0) transition is often used to deduce the total molecular hydrogen but is challenging to detect in low-metallicity galaxies in spite of the star formation taking place. In contrast, the [C II]λ158 μm is relatively bright, highlighting a potentially important reservoir of H2 that is not traced by CO (1−0) but is residing in the C+-emitting regions. Aims. Here we aim to explore a method to quantify the total H2 mass (MH2) in galaxies and to decipher what parameters control the CO-dark reservoir. Methods. We present Cloudy grids of density, radiation field, and metallicity in terms of observed quantities, such as [O I], [C I], CO (1−0), [C II], LTIR, and the total MH2. We provide recipes based on these models to derive total MH2 mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total MH2 for each galaxy, and compare this to the H2 determined from the observed CO (1−0) line. This allows us to quantify the reservoir of H2 that is CO-dark and traced by the [C II]λ158 μm. Results. We demonstrate that while the H2 traced by CO (1−0) can be negligible, the [C II]λ158 μm can trace the total H2. We find 70 to 100% of the total H2 mass is not traced by CO (1−0) in the dwarf galaxies, but is well-traced by [C II]λ158 μm. The CO-dark gas mass fraction correlates with the observed L[C II]/LCO(1−0) ratio. A conversion factor for [C II]λ158 μm to total H2 and a new CO-to-total-MH2 conversion factor as a function of metallicity are presented. Conclusions. While low-metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas that is not detected by CO can exist. We suggest a general recipe to quantify the total mass of H2 in galaxies, taking into account the CO and [C II] observations. Accounting for this CO-dark H2 gas, we find that the star-forming dwarf galaxies now fall on the Schmidt–Kennicutt relation. Their star-forming efficiency is rather normal because the reservoir from which they form stars is now more massive when introducing the [C II] measures of the total H2 compared to the small amount of H2 in the CO-emitting region.

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Weak CS emission in an extremely metal-poor galaxy DDO 70

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa086 ◽

2020 ◽

Vol 496 (1) ◽

pp. L38-L42

Author(s):

Kaiyi Du ◽

Yong Shi ◽

Zhi-Yu Zhang ◽

Junzhi Wang ◽

Yu Gao

Keyword(s):

Star Formation ◽

Milky Way ◽

Dwarf Galaxy ◽

Signal To Noise Ratio ◽

Formation Rate ◽

Far Infrared ◽

Molecular Gas ◽

Solar Metallicity ◽

Low Metallicity ◽

Co Emission

ABSTRACT In most galaxies like the Milky Way, stars form in clouds of molecular gas. Unlike the CO emission that traces the bulk of molecular gas, the rotational transitions of HCN and CS molecules mainly probe the dense phase of molecular gas, which has a tight and almost linear relation with the far-infrared luminosity and star formation rate (SFR). However, it is unclear whether dense molecular gas exists at very low metallicity, and if exists, how it is related to star formation. In this work, we report ALMA observations of the CS J = 5 → 4 emission line of DDO 70, a nearby gas-rich dwarf galaxy with $\sim \!7{{\ \rm per\ cent}}$ solar metallicity. We did not detect CS emission from all regions with strong CO emission. After stacking all CS spectra from CO-bright clumps, we find no more than a marginal detection of CS J = 5 → 4 transition, at a signal-to-noise ratio of ∼3.3. This 3σ upper limit deviates from the $L^\prime _{\rm CS}$–LIR and $L^\prime _{\rm CS}$–SFR relationships found in local star-forming galaxies and dense clumps in the Milky Way, implying weaker CS emission at given infrared luminosity and SFR. We discuss the possible mechanisms that suppress CS emission at low metallicity.

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Effects of reionization on dwarf galaxy formation

Proceedings of the International Astronomical Union ◽

10.1017/s174392130802471x ◽

2008 ◽

Vol 4 (S255) ◽

pp. 142-146

Author(s):

Massimo Ricotti

Keyword(s):

Star Formation ◽

Galaxy Formation ◽

High Velocity ◽

Dwarf Galaxy ◽

Late Phase ◽

Star Formation History ◽

Formation History ◽

High Velocity Clouds ◽

Gas Accretion ◽

In Virtue Of

AbstractIn this talk I revisit the problem of gas accretion onto minihalos after reionization. I show that primordial minihalos with vcir < 20 km s−1 stop accreting gas after reionization, as is usually assumed, but in virtue of their increasing concentration and the decreasing temperature of the intergalactic medium as redshift decreases, they have a late phase (at redshift z<2) of gas accretion and possibly star formation. As a result we expect that pre-reionization fossils have a more complex star formation history than previously envisioned. A signature of this model is a bimodal star formation history. The dwarf spheroidal galaxy Leo T, that inspired the present work, fits with this scenario. Another prediction of the model is the existence of a population of gas rich minihalos that never formed stars. A subset of compact high-velocity clouds may be identified as such objects but the bulk of them may still be undiscovered.

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Cosmic archaeology with massive stellar black hole binaries

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa063 ◽

2020 ◽

Vol 495 (1) ◽

pp. L81-L85 ◽

Cited By ~ 2

Author(s):

L Graziani ◽

R Schneider ◽

S Marassi ◽

W Del Pozzo ◽

M Mapelli ◽

...

Keyword(s):

Black Hole ◽

Star Formation ◽

Gravitational Wave ◽

Galaxy Formation ◽

Population Synthesis ◽

Formation Model ◽

Black Hole Binaries ◽

The Galaxy ◽

Low Metallicity ◽

Black Hole Masses

ABSTRACT The existence of massive stellar black hole binaries (MBHBs), with primary black hole masses $\ge 31 \, \mathrm{ M}_\odot$, was proven by the detection of the gravitational wave (GW) event GW150914 during the first LIGO/Virgo observing run (O1), and successively confirmed by seven additional GW signals discovered in the O1 and O2 data. By adopting the galaxy formation model gamesh coupled with binary population synthesis (BPS) calculations, here we investigate the origin of these MBHBs by selecting simulated binaries compatible in mass and coalescence redshifts. We find that their cosmic birth rates peak in the redshift range 6.5 ≤ z ≤ 10, regardless of the adopted BPS. These MBHBs are then old systems forming in low-metallicity ($Z \sim [0.01\!-\!0.1] \, Z_{\odot }$), low-stellar-mass galaxies, before the end of cosmic reionization, i.e. significantly beyond the peak of cosmic star formation. GW signals generated by coalescing MBHBs open up new possibilities to probe the nature of stellar populations in remote galaxies, at present too faint to be detected by available electromagnetic facilities.

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EDGE: the mass–metallicity relation as a critical test of galaxy formation physics

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3053 ◽

2019 ◽

Vol 491 (2) ◽

pp. 1656-1672 ◽

Cited By ~ 14

Author(s):

Oscar Agertz ◽

Andrew Pontzen ◽

Justin I Read ◽

Martin P Rey ◽

Matthew Orkney ◽

...

Keyword(s):

Star Formation ◽

Galaxy Formation ◽

Dwarf Galaxy ◽

Self Regulation ◽

Stellar Mass ◽

Dark Matter Halo ◽

V Band ◽

The Galaxy ◽

Unique Constraint ◽

Low Mass

ABSTRACT We introduce the ‘Engineering Dwarfs at Galaxy Formation’s Edge’ (EDGE) project to study the cosmological formation and evolution of the smallest galaxies in the Universe. In this first paper, we explore the effects of resolution and sub-grid physics on a single low-mass halo ($M_{\rm halo}=10^{9}{\, \rm M}_\odot$), simulated to redshift z = 0 at a mass and spatial resolution of $\sim 20{\, \rm M}_\odot$ and ∼3 pc. We consider different star formation prescriptions, supernova feedback strengths, and on-the-fly radiative transfer (RT). We show that RT changes the mode of galactic self-regulation at this halo mass, suppressing star formation by causing the interstellar and circumgalactic gas to remain predominantly warm (∼104 K) even before cosmic reionization. By contrast, without RT, star formation regulation occurs only through starbursts and their associated vigorous galactic outflows. In spite of this difference, the entire simulation suite (with the exception of models without any feedback) matches observed dwarf galaxy sizes, velocity dispersions, V-band magnitudes, and dynamical mass-to-light-ratios. This is because such structural scaling relations are predominantly set by the host dark matter halo, with the remaining model-to-model variation being smaller than the observational scatter. We find that only the stellar mass–metallicity relation differentiates the galaxy formation models. Explosive feedback ejects more metals from the dwarf, leading to a lower metallicity at a fixed stellar mass. We conclude that the stellar mass–metallicity relation of the very smallest galaxies provides a unique constraint on galaxy formation physics.

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