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Register a new userNIHAO-LG: The uniqueness of Local Group dwarf galaxies
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Recent observational and theoretical studies of the Local Group (LG) dwarf galaxies have highlighted their unique star formation history, stellar metallicity, gas content, and kinematics. We investigate the commonality of these tantalizing features by comparing constrained LG and field central dwarf halo simulations in the NIHAO project. For the first time, constrained LG simulations performed with NIHAO hydrodynamics which track the evolution of MW and M31 along with ~100 dwarfs in the Local Group are presented. The total gas mass and stellar properties (velocity dispersion, evolution history, etc.) of present-day LG dwarfs are found to be similar to field systems. Overall, the simulated LG dwarfs show representative stellar properties to other dwarfs in the Universe. However, relative to fields, LG dwarfs have more cold gas in their central parts and more metal-rich gas in the halo stemming from interactions with MW/M31 and/or feedback. The larger gas metal content in LG dwarfs results in early star formation events that lead to strong feedback and subsequent quenching. We also test for the impact of metal diffusion on the chemical evolution of LG dwarfs, and find that metal diffusion does not affect the stellar or gaseous content of LG relative to field dwarfs; the largest differences are found with the gas metallicity (~0.1 dex). Our results show that properties from LG dwarfs may be used as general constraints for studying the overall dwarf population in the Universe, providing a powerful local laboratory for galaxy formation tests and comparisons.
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We study the Local Group (LG) dwarf galaxy population predicted by the apostle $Lambda$CDM cosmological hydrodynamics simulations. These indicate that: (i)~the total mass within $3$ Mpc of the Milky Way-Andromeda midpoint ($M_{rm 3Mpc}$) typically exceeds $sim 3$ times the sum of the virial masses ($M_{rm 200crit}$) of the two primaries and (ii)~the dwarf galaxy formation efficiency per unit mass is uniform throughout the volume. This suggests that the satellite population within the virial radii of the Milky Way and Andromeda should make up fewer than one third of all LG dwarfs within $3$ Mpc. This is consistent with the fraction of observed LG galaxies with stellar mass $M_*>10^7,M_{odot}$ that are satellites ($12$ out of $42$; i.e., $28$ per cent). For the apostle galaxy mass-halo mass relation, the total number of such galaxies further suggests a LG mass of $M_{rm 3 Mpc}sim 10^{13} , M_{odot}$. At lower galaxy masses, however, the observed satellite fraction is substantially higher ($42$ per cent for $M_*>10^5,M_{odot}$). If this is due to incompleteness in the field sample, then $sim 50$ dwarf galaxies at least as massive as the Draco dwarf spheroidal must be missing from the current LG {it field} dwarf inventory. The incompleteness interpretation is supported by the pronounced flattening of the LG luminosity function below $M_*sim 10^7, M_{odot}$, and by the scarcity of low-surface brightness LG field galaxies compared to satellites. The simulations indicate that most missing dwarfs should lie near the virial boundaries of the two LG primaries, and predict a trove of nearby dwarfs that await discovery by upcoming wide-field imaging surveys.
The gas content of the complete compilation of Local Group dwarf galaxies (119 within 2 Mpc) is presented using HI survey data. Within the virial radius of the Milky Way (224 kpc here), 53 of 55 dwarf galaxies are devoid of gas to limits of M$_{rm HI}<10^4$ M$_odot$. Within the virial radius of M31 (266 kpc), 27 of 30 dwarf galaxies are devoid of gas (with limits typically $<10^5$ M$_odot$). Beyond the virial radii of the Milky Way and M31, the majority of the dwarf galaxies have detected HI gas and have HI masses higher than the limits. When the relationship between gas content and distance is investigated using a Local Group virial radius, more of the non-detected dwarf galaxies are within this radius (85$pm1$ of the 93 non-detected dwarf galaxies) than within the virial radii of the Milky Way and M31. Using the Gaia proper motion measurements available for 38 dwarf galaxies, the minimum gas density required to completely strip them of gas is calculated. Halo densities between $10^{-5}$ and $5 times 10^{-4}$ cm$^{-3}$ are typically required for instantaneous stripping at perigalacticon. When compared to halo density with radius expectations from simulations and observations, 80% of the dwarf galaxies with proper motions are consistent with being stripped by ram pressure at Milky Way pericenter. The results suggest a diffuse gaseous galactic halo medium is important in quenching dwarf galaxies, and that a Local Group medium also potentially plays a role.
We present stellar metallicities in Leo I, Leo II, IC 1613, and Phoenix dwarf galaxies derived from medium (F390M) and broad (F555W, F814W) band photometry using the Wide Field Camera 3 (WFC3) instrument aboard the Hubble Space Telescope. We measured metallicity distribution functions (MDFs) in two ways, 1) matching stars to isochrones in color-color diagrams, and 2) solving for the best linear combination of synthetic populations to match the observed color-color diagram. The synthetic technique reduces the effect of photometric scatter, and produces MDFs 30-50 % narrower than the MDFs produced from individually matched stars. We fit the synthetic and individual MDFs to analytical chemical evolution models (CEM) to quantify the enrichment and the effect of gas flows within the galaxies. Additionally, we measure stellar metallicity gradients in Leo I and II. For IC 1613 and Phoenix our data do not have the radial extent to confirm a metallicity gradient for either galaxy. We find the MDF of Leo I (dwarf spheroidal) to be very peaked with a steep metal rich cutoff and an extended metal poor tail, while Leo II (dwarf spheroidal), Phoenix (dwarf transition) and IC 1613 (dwarf irregular) have wider, less peaked MDFs than Leo I. A simple CEM is not the best fit for any of our galaxies, therefore we also fit the `Best Accretion Model of Lynden-Bell 1975. For Leo II, IC 1613 and Phoenix we find similar accretion parameters for the CEM, even though they all have different effective yields, masses, star formation histories and morphologies. We suggest that the dynamical history of a galaxy is reflected in the MDF, where broad MDFs are seen in galaxies that have chemically evolved in relative isolation and narrowly peaked MDFs are seen in galaxies that have experienced more complicated dynamical interactions concurrent with their chemical evolution.
The near and mid-infrared characteristics of large amplitude, Mira, variables in Local Group dwarf irregular galaxies (LMC, NGC 6822, IC 1613, Sgr dIG) are described. Two aspects of these variables are discussed. First, the short period (P < 420 days) Miras are potentially powerful distance indicators, provided that they have low circumstellar extinction, or can be corrected for extinction. These are the descendants of relatively low mass stars. Secondly, the longer period stars, many of which undergo hot bottom burning, are poorly understood. These provide new insight into the evolution of intermediate mass stars during the high mass-loss phases, but their use as distance indicators depends on a much firmer understanding of their evolution. The change in slope of the K period luminosity relation for O-rich stars that is seen around 400 to 420 days in the LMC is due to the onset of hot bottom burning. It will be sensitive to metallicity and should therefore be expected at different periods in populations with significant differences from the LMC. The [4.5] period-luminosity relation splits into two approximately parallel sequences. The fainter one fits stars where the mid-infrared flux originates from the stellar photosphere, while the brighter one fits observations dominated by the circumstellar shell.
XMM-Newton and Chandra have ushered in a new era for the study of dwarf galaxies in the Local Group. We provide an overview of the opportunities, challenges, and some early results. The large number of background sources relative to galaxy sources is a major theme. Despite this challenge, the identification of counterparts has been possible, providing hints that the same mechanisms producing X-ray sources in larger galaxies are active in dwarf galaxies. A supersoft X-ray source within 2 of the supermassive black hole in M32 may be a remnant of the tidal disruption of a giant, although other explanations cannot be ruled out.
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