The Local Group: the ultimate deep field Boylan-Kolchin, Michael; Weisz, Daniel R; Bullock, James S ...
Monthly Notices of the Royal Astronomical Society Letters,
10/2016, Volume:
462, Issue:
1
Journal Article
Peer reviewed
Open access
Near-field cosmology – using detailed observations of the Local Group and its environs to study wide-ranging questions in galaxy formation and dark matter physics – has become a mature and rich field ...over the past decade. There are lingering concerns, however, that the relatively small size of the present-day Local Group (∼2 Mpc diameter) imposes insurmountable sample-variance uncertainties, limiting its broader utility. We consider the region spanned by the Local Group's progenitors at earlier times and show that it reaches 3 arcmin ≈ 7 comoving Mpc in linear size (a volume of ≈350 Mpc3) at z = 7. This size at early cosmic epochs is large enough to be representative in terms of the matter density and counts of dark matter haloes with M
vir(z = 7) ≲ 2 × 109 M⊙. The Local Group's stellar fossil record traces the cosmic evolution of galaxies with 103 ≲ M
⋆(z = 0)/M⊙ ≲ 109 (reaching M
1500 > −9 at z ∼ 7) over a region that is comparable to or larger than the Hubble Ultra-Deep Field (HUDF) for the entire history of the Universe. In the JWST era, resolved stellar populations will probe regions larger than the HUDF and any deep JWST fields, further enhancing the value of near-field cosmology.
We map the star formation history across M31 by fitting stellar evolution models to color-magnitude diagrams of each 83″ × 83″ (0.3 × 1.4 kpc, deprojected) region of the Panchromatic Hubble Andromeda ...Treasury (PHAT) survey outside of the innermost portion. We find that most of the star formation occurred prior to ∼8 Gyr ago, followed by a relatively quiescent period until ∼4 Gyr ago, a subsequent star formation episode about 2 Gyr ago, and a return to relative quiescence. There appears to be little, if any, structure visible for populations with ages older than 2 Gyr, suggesting significant mixing since that epoch. Finally, assuming a Kroupa initial mass function from 0.1 to 100 M , we find that the total amount of star formation over the past 14 Gyr in the area over which we have fit models is 5 × 1010 M . Fitting the radial distribution of this star formation and assuming azimuthal symmetry, (1.5 0.2) × 1011 M of stars has formed in the M31 disk as a whole, (9 2) × 1010 M of which has likely survived to the present after accounting for evolutionary effects. This mass is about one-fifth of the total dynamical mass of M31.
Increasingly powerful and multiplexed spectroscopic facilities promise detailed chemical abundance patterns for millions of resolved stars in galaxies beyond the Milky Way (MW). Here, we employ the ...Cramér-Rao lower bound (CRLB) to forecast the precision to which stellar abundances for metal-poor, low-mass stars outside the MW can be measured for 41 current (e.g., Keck, MMT, the Very Large Telescope, and the Dark Energy Spectroscopic Instrument) and planned (e.g., the Maunakea Spectroscopic Explorer, the James Webb Space Telescope (JWST), and Extremely Large Telescopes (ELTs)) spectrograph configurations. We show that moderate-resolution (R 5000) spectroscopy at blue-optical wavelengths (λ 4500 ) (i) enables the recovery of two to four times as many elements as red-optical spectroscopy (5000 λ 10000 ) at similar or higher resolutions (R ∼ 10,000) and (ii) can constrain the abundances of several neutron-capture elements to 0.3 dex. We further show that high-resolution (R 20,000), low signal-to-noise ratio (∼10 pixel−1) spectra contain rich abundance information when modeled with full spectral fitting techniques. We demonstrate that JWST/NIRSpec and ELTs can recover (i) ∼10 and 30 elements, respectively, for metal-poor red giants throughout the Local Group and (ii) Fe/H and /Fe for resolved stars in galaxies out to several Mpc with modest integration times. We show that select literature abundances are within a factor of ∼2 (or better) of our CRLBs. We suggest that, like exposure time calculators, CRLBs should be used when planning stellar spectroscopic observations. We include an open-source Python package, Chem-I-Calc, that allows users to compute CRLBs for spectrographs of their choosing.
Using Hubble Space Telescope photometry to measure star formation histories, we age-date the stellar populations surrounding supernova remnants (SNRs) in M31 and M33. We then apply stellar evolution ...models to the ages to infer the corresponding masses for their supernova progenitor stars. We analyze 33 M33 SNR progenitors and 29 M31 SNR progenitors in this work. We then combine these measurements with 53 previously published M31 SNR progenitor measurements to bring our total number of progenitor mass estimates to 115. To quantify the mass distributions, we fit power laws of the dN/dM is proportional to M super(- alpha ). Our new larger sample of M31 progenitors follows a distribution with alpha = 4.4 super(+0.4) sub(-0.4), and the M33 sample follows a distribution with alpha = 3.8 super(+0.4) sub(-0.5). Thus both samples are consistent within the uncertainties, and the full sample across both galaxies gives alpha = 4.2 super(+0.3) sub(-0.3). Both the individual and full distributions display a paucity of massive stars when compared to a Salpeter initial mass function, which we would expect to observe if all massive stars exploded as SN that leave behind observable SNR. If we instead fix alpha = 2.35 and treat the maximum mass as a free parameter, we find M sub(max) ~ 35-45 M sub(middot in circle) indicative of a potential maximum cutoff mass for SN production. Our results suggest that either SNR surveys are biased against finding objects in the youngest (< 10 Myr old) regions, or the highest mass stars do not produce SNe.
We present preliminary results from a new Hubble Space Telescope (HST) archival program aimed at tightly constraining the ancient (>4 Gyr ago) star formation histories (SFHs) of the field populations ...of the Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC). We demonstrate the quality of the archival data by constructing HST/Wide Field Planetary Camera 2-based colour-magnitude diagrams (CMDs; M
F555W
∼ +8) for seven spatially diverse fields in the SMC and eight fields in the LMC. The HST-based CMDs are >2 mag deeper than any from ground-based observations, and are particularly superior in high surface brightness regions, e.g. the LMC bar, which contain a significant fraction of star formation and are crowding limited from ground-based observations. To minimize systematic uncertainties, we derive the SFH of each field using an identical maximum likelihood CMD fitting technique. We then compute an approximate mass weighted average SFH for each galaxy. From the average SFHs, we find that both galaxies lack a dominant burst of early star formation, which suggests either a suppression or an underfuelling of ancient star formation in the Magellanic Clouds (MCs). From 10 to 12 Gyr ago, we find that the LMC experienced a period of enhanced stellar mass growth relative to the SMC. Similar to some previous studies, we find two notable peaks in the SFH of the SMC at ∼4.5 and 9 Gyr ago, which could be due to repeated close passages with the LMC, implying an interaction history that has persisted for at least 9 Gyr. We find little evidence for strong periodic behaviour in the lifetime SFHs of both MCs, suggesting that repeated encounters with the Milky Way are unlikely. Beginning ∼3.5 Gyr ago, both galaxies show sharp increases in their SFHs, in agreement with previous studies. Subsequently, the SFHs track either remarkably well. Spatial variations in the SFH of the SMC are consistent with a picture where gas was driven into the centre of the SMC ∼3.5 Gyr ago, which simultaneously shut down star formation in the outer regions while dramatically increasing the star formation rate in the centre. In contrast, the LMC shows little spatial variation in its ancient SFH. The planned additional analysis of HST pointings at larger galactocentric radii will allow us to make more confident statements about spatial variations in the SFHs of the SMC and LMC.
Abstract We measure the metallicities of 374 red giant branch (RGB) stars in the isolated, quenched dwarf galaxy Tucana using Hubble Space Telescope narrowband (F395N) calcium H and K imaging. Our ...sample is a factor of ∼7 larger than what is available from previous studies. Our main findings are as follows. (i) A global metallicity distribution function (MDF) with 〈 Fe/H 〉 = − 1.55 − 0.04 + 0.04 and σ Fe/H = 0.54 − 0.03 + 0.03 . (ii) A metallicity gradient of −0.54 ± 0.07 dex R e − 1 (−2.1 ± 0.3 dex kpc −1 ) over the extent of our imaging (∼2.5 R e ), which is steeper than literature measurements. Our finding is consistent with predicted gradients from the publicly available FIRE-2 simulations, in which bursty star formation creates stellar population gradients and dark matter cores. (iii) Tucana’s bifurcated RGB has distinct metallicities: a blue RGB with 〈 Fe/H 〉 = − 1.78 − 0.06 + 0.06 and σ Fe/H = 0.44 − 0.06 + 0.07 and a red RGB with 〈 Fe/H 〉 = − 1.08 − 0.07 + 0.07 and σ Fe/H = 0.42 − 0.06 + 0.06 . (iv) At fixed stellar mass, Tucana is more metal-rich than Milky Way satellites by ∼0.4 dex, but its blue RGB is chemically comparable to the satellites. Tucana’s MDF appears consistent with star-forming isolated dwarfs, though MDFs of the latter are not as well populated. (v) About 2% of Tucana’s stars have Fe/H < −3% and 20% have Fe/H > −1. We provide a catalog for community spectroscopic follow-up.
ABSTRACT We present novel constraints on the underlying galaxy formation physics (e.g. mass-loading factor, star formation history, and metal retention) at z ≳ 7 for the low-mass (M* ∼ 105 M⊙) Local ...Group ultrafaint dwarf galaxy (UFD) Eridanus ii (Eri ii). Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri ii’s CaHK-based photometric metallicity distribution function (MDF; Fe/H) and find that the evolution of Eri ii is well characterized by a short, exponentially declining star formation history ($\tau _\text{SFH}=0.39\pm _{0.13}^{0.18}$ Gyr), a low star formation efficiency ($\tau _\text{SFE}=27.56\pm _{12.92}^{25.14}$ Gyr), and a large mass-loading factor ($\eta =194.53\pm _{42.67}^{33.37}$). Our results are consistent with Eri ii forming the majority of its stars before the end of reionization. The large mass-loading factor implies strong outflows in the early history of Eri ii and is in good agreement with theoretical predictions for the mass scaling of galactic winds. It also results in the ejection of >90 per cent of the metals produced in Eri ii. We make predictions for the distribution of Mg/Fe–Fe/H in Eri ii as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri ii (Fe/H > −2) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the reionization era.
Abstract We present the elemental abundances and ages of 19 massive quiescent galaxies at z ∼ 1.4 and z ∼ 2.1 from the Keck Heavy Metal Survey. The ultradeep LRIS and MOSFIRE spectra were modeled ...using a full-spectrum stellar population fitting code with variable abundance patterns. The galaxies have iron abundances between Fe/H = −0.5 and −0.1 dex, with typical values of −0.2 −0.3 at z ∼ 1.4 z ∼ 2.1. We also find a tentative log σ v –Fe/H relation at z ∼ 1.4. The magnesium-to-iron ratios span Mg/Fe = 0.1–0.6 dex, with typical values of 0.3 0.5 dex at z ∼ 1.4 z ∼ 2.1. The ages imply formation redshifts of z form = 2–8. Compared to quiescent galaxies at lower redshifts, we find that Fe/H was ∼0.2 dex lower at z = 1.4–2.1. We find no evolution in Mg/Fe out to z ∼ 1.4, though the z ∼ 2.1 galaxies are 0.2 dex enhanced compared to z = 0–0.7. A comparison of these results to a chemical evolution model indicates that galaxies at higher redshift form at progressively earlier epochs and over shorter star formation timescales, with the z ∼ 2.1 galaxies forming the bulk of their stars over 150 Myr at z form ∼ 4. This evolution cannot be solely attributed to an increased number of quiescent galaxies at later times; several Heavy Metal galaxies have extreme chemical properties not found in massive galaxies at z ∼ 0.0–0.7. Thus, the chemical properties of individual galaxies must evolve over time. Minor mergers also cannot fully account for this evolution as they cannot increase Fe/H, particularly in galaxy centers. Consequently, the buildup of massive quiescent galaxies since z ∼ 2.1 may require further mechanisms, such as major mergers and/or central star formation.
We present the Bayesian Extinction And Stellar Tool (BEAST), a probabilistic approach to modeling the dust extinguished photometric spectral energy distribution of an individual star while accounting ...for observational uncertainties common to large resolved star surveys. Given a set of photometric measurements and an observational uncertainty model, the BEAST infers the physical properties of the stellar source using stellar evolution and atmosphere models and constrains the line of sight extinction using a newly developed mixture model that encompasses the full range of dust extinction curves seen in the Local Group. The BEAST is specifically formulated for use with large multi-band surveys of resolved stellar populations. Our approach accounts for measurement uncertainties and any covariance between them due to stellar crowding (both systematic biases and uncertainties in the bias) and absolute flux calibration, thereby incorporating the full information content of the measurement. We illustrate the accuracy and precision possible with the BEAST using data from the Panchromatic Hubble Andromeda Treasury. While the BEAST has been developed for this survey, it can be easily applied to similar existing and planned resolved star surveys.
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