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<jats:title>Abstract</jats:title> <jats:p>Dense cores are the final place where turbulence is dissipated. It has been proposed from theoretical arguments that the nonthermal velocity dispersion should be narrower both for molecular ions (compared to neutrals) and for transitions with higher critical densities. To test these hypotheses, we compare the velocity dispersion of <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{N}}}_{2}{{\rm{H}}}^{+}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn1.gif" xlink:type="simple" /> </jats:inline-formula> (1–0) (<jats:inline-formula> <jats:tex-math> <?CDATA ${n}_{\mathrm{crit}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn2.gif" xlink:type="simple" /> </jats:inline-formula>  = 6 × 10<jats:sup>4</jats:sup> <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathrm{cm}}^{-3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn3.gif" xlink:type="simple" /> </jats:inline-formula>) and <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathrm{NH}}_{3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn4.gif" xlink:type="simple" /> </jats:inline-formula> (<jats:inline-formula> <jats:tex-math> <?CDATA ${n}_{\mathrm{crit}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn5.gif" xlink:type="simple" /> </jats:inline-formula>  = 2 × 10<jats:sup>3</jats:sup> <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathrm{cm}}^{-3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn6.gif" xlink:type="simple" /> </jats:inline-formula>), in the dense core Barnard 5. We analyze well-resolved and high signal-to-noise observations of <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathrm{NH}}_{3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn7.gif" xlink:type="simple" /> </jats:inline-formula> (1,1) and (2,2) obtained with combining Robert C. Byrd Green Bank Telescope (GBT) and Very Large Array (VLA) data, and <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{N}}}_{2}{{\rm{H}}}^{+}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn8.gif" xlink:type="simple" /> </jats:inline-formula> (1–0) obtained with GBT Argus, which present a similar morphology. Surprisingly, the nonthermal velocity dispersion of the ion is systematically higher than that of the neutral by 20%. The derived sonic Mach number, <jats:inline-formula> <jats:tex-math> <?CDATA ${{ \mathcal M }}_{s}={\sigma }_{\mathrm{NT}}/{c}_{s}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn9.gif" xlink:type="simple" /> </jats:inline-formula>, has peak values <jats:inline-formula> <jats:tex-math> <?CDATA ${{ \mathcal M }}_{s,{{\rm{N}}}_{2}{{\rm{H}}}^{+}}=0.59$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn10.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA ${{ \mathcal M }}_{s,{\mathrm{NH}}_{3}}=0.48$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn11.gif" xlink:type="simple" /> </jats:inline-formula> for <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{N}}}_{2}{{\rm{H}}}^{+}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn12.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA ${\mathrm{NH}}_{3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabebddieqn13.gif" xlink:type="simple" /> </jats:inline-formula>, respectively. This observed difference may indicate that the magnetic field even deep within the dense core is still oscillating, as it is in the turbulent region outside the core. The ions should be more strongly dynamically coupled to this oscillating field than the neutrals, thus accounting for their broader line width. If corroborated by further observations, this finding would shed additional light on the transition to quiescence in dense cores.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a new study of the merger dynamics of A1775 by analyzing the high-quality Chandra and XMM-Newton archival data. We confirm/identify an arc-shaped edge (i.e., the head) at ∼48 kpc west of the X-ray peak, a split cold gas tail that extends eastward to ∼163 kpc, and a plume of spiral-like X-ray excess (within about 81–324 kpc northeast of the cluster core) that connects to the end of the tail. The head, across which the projected gas temperature rises outward from <jats:inline-formula> <jats:tex-math> <?CDATA ${3.39}_{-0.18}^{+0.28}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf09eieqn1.gif" xlink:type="simple" /> </jats:inline-formula> to <jats:inline-formula> <jats:tex-math> <?CDATA ${5.30}_{-0.43}^{+0.54}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf09eieqn2.gif" xlink:type="simple" /> </jats:inline-formula> keV, is found to be a cold front with a Mach number of <jats:inline-formula> <jats:tex-math> <?CDATA ${ \mathcal M }\sim 0.79$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf09eieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. Along the surfaces of the cold front and tail, typical Kelvin–Helmholtz instability features (noses and wings, etc.) are found and are used to constrain the upper limit of the magnetic field (∼11.2 <jats:italic>μ</jats:italic>G) and the viscosity suppression factor (∼0.01). Combining optical and radio evidence, we propose a two-body merger (instead of systematic motion in a large-scale gas environment) scenario and have carried out idealized hydrodynamic simulations to verify it. We find that the observed X-ray emission and temperature distributions can be best reproduced with a merger mass ratio of 5 after the first pericentric passage. The NAT radio galaxy is thus more likely to be a single galaxy falling into the cluster center at a relative velocity of 2800 km s<jats:sup>−1</jats:sup>, a speed constrained by its radio morphology. The infalling subcluster is expected to have a relatively low gas content, because only a gas-poor subcluster can cause central-only disturbances as observed in such an off-axis merger.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>From our position embedded within the Milky Way’s interstellar medium, we have limited ability to detect gas at low relative velocities in the extended Galactic halo because those spectral lines are blended with much stronger signals from dense foreground gas. As a result, the content of the Milky Way’s circumgalactic medium (CGM) is poorly constrained at ∣<jats:italic>v</jats:italic> <jats:sub>LSR</jats:sub>∣ ≲150 km s<jats:sup>−1</jats:sup>. To overcome this complication, the QuaStar survey applies a spectral differencing technique using paired quasar−star sight lines to measure the obscured content of the Milky Way’s CGM for the first time. We present measurements of the C <jats:sc>iv</jats:sc> doublet (<jats:italic>λ</jats:italic> <jats:italic>λ</jats:italic>1548, 1550), a rest-frame UV metal line transition, detected in Hubble Space Telescope/Cosmic Origins Spectrograph spectra of 30 halo-star/quasar pairs evenly distributed across the sky at Galactic latitudes ∣<jats:italic>b</jats:italic>∣ &gt; 30°. The 30 halo stars have well-constrained distances (<jats:italic>d</jats:italic> ≈ 5–14 kpc) and are paired with quasars separated by &lt;2.°8. We argue that the difference in absorption between the quasar and stellar sight lines originates primarily in the Milky Way’s extended CGM beyond ∼10 kpc. For the Milky Way’s extended, low-velocity CGM (∣<jats:italic>v</jats:italic>∣ &lt;150 km s<jats:sup>−1</jats:sup>), we place an upper limit on the mean C <jats:sc>iv</jats:sc> column density of <jats:inline-formula> <jats:tex-math> <?CDATA ${\rm{\Delta }}\mathrm{log}{N}_{\mathrm{LVCGM}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>LVCGM</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabeb6bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> &lt; 13.39 and find a covering fraction of <jats:inline-formula> <jats:tex-math> <?CDATA ${f}_{{\rm{C}}{\rm\small{IV}},\mathrm{LVCGM}}(\mathrm{log}N\gt 13.65)\,=$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em" /> <mml:mi mathsize="small" mathvariant="normal">IV</mml:mi> <mml:mo>,</mml:mo> <mml:mi>LVCGM</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:mi>log</mml:mi> <mml:mi>N</mml:mi> <mml:mo>&gt;</mml:mo> <mml:mn>13.65</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mspace width="0.25em" /> <mml:mo>=</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabeb6bieqn2.gif" xlink:type="simple" /> </jats:inline-formula> 20% [6/30], a value significantly lower than the covering fraction for star-forming galaxies at low redshift. Our results suggest either that the bulk of Milky Way’s C <jats:sc>iv</jats:sc> -traced CGM lies at low Galactic latitudes or that the Milky Way’s CGM is lacking in warm, ionized material compared to low-redshift (<jats:italic>z</jats:italic> &lt; 0.1) star-forming galaxy halos.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>To date, at least three comets—2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd)—have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H<jats:sub>2</jats:sub>O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H<jats:sub>2</jats:sub>O of unity, and one model reaches a CO/H<jats:sub>2</jats:sub>O ratio &gt;10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H<jats:sub>2</jats:sub>O ice. In light of this, a population of CO-ice-enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own solar system and in extrasolar systems, than previously expected.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present initial results from the Cosmic Origins Spectrograph (COS) and Gemini Mapping the Circumgalactic Medium (CGMCGM ≡ CGM<jats:sup>2</jats:sup>) survey. The CGM<jats:sup>2</jats:sup> survey consists of 1689 galaxies, all with high-quality Gemini-GMOS spectra, within 1 Mpc of 22 <jats:italic>z</jats:italic> ≲ 1 quasars, all with a signal-to-noise ratio of ∼10 Hubble Space Telescope/COS G130M+G160M spectra. For 572 of these galaxies with stellar masses 10<jats:sup>7</jats:sup> <jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub> &lt; <jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub> &lt; 10<jats:sup>11</jats:sup> <jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub> and <jats:italic>z</jats:italic> ≲ 0.5, we show that the H <jats:sc>i</jats:sc> covering fraction above a threshold of <jats:italic>N</jats:italic> <jats:sub>HI</jats:sub> &gt; 10<jats:sup>14</jats:sup>cm<jats:sup>−2</jats:sup> is ≳0.5 within 1.5 virial radii (<jats:italic>R</jats:italic> <jats:sub>vir</jats:sub> ∼ <jats:italic>R</jats:italic> <jats:sub>200m</jats:sub>). We examine the H <jats:sc>i</jats:sc> kinematics and find that the majority of absorption lies within ±250 km s<jats:sup>−1</jats:sup> of the galaxy systemic velocity. We examine H <jats:sc>i</jats:sc> covering fractions over a range of impact parameters to infer a characteristic size of the CGM, <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, as a function of galaxy mass. <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> is the impact parameter at which the probability of observing an absorber with <jats:italic>N</jats:italic> <jats:sub>HI</jats:sub> &gt;10<jats:sup>14</jats:sup> cm<jats:sup>−2</jats:sup> is &gt;50%. In this framework, the radial extent of the CGM of <jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub> &gt; 10<jats:sup>9.9</jats:sup> <jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub> galaxies is <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}={346}_{-53}^{+57}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> kpc or <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}\simeq 1.2{R}_{\mathrm{vir}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn4.gif" xlink:type="simple" /> </jats:inline-formula>. Intermediate-mass galaxies with 10<jats:sup>9.2</jats:sup> &lt; <jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub>/<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> &lt; 10<jats:sup>9.9</jats:sup> have an extent of <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}={353}_{-50}^{+64}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn5.gif" xlink:type="simple" /> </jats:inline-formula> kpc or <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}\simeq 2.4{R}_{\mathrm{vir}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn6.gif" xlink:type="simple" /> </jats:inline-formula>. Low-mass galaxies, <jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub> &lt; 10<jats:sup>9.2 </jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, show a smaller physical scale of <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}={177}_{-65}^{+70}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn7.gif" xlink:type="simple" /> </jats:inline-formula> kpc and extend to <jats:inline-formula> <jats:tex-math> <?CDATA ${R}_{\mathrm{CGM}}^{14}\simeq 1.6{R}_{\mathrm{vir}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabea14ieqn8.gif" xlink:type="simple" /> </jats:inline-formula>. Our analysis suggests that using <jats:italic>R</jats:italic> <jats:sub>vir</jats:sub> as a proxy for the characteristic radius of the CGM likely underestimates its extent.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on Mg and Si isotope data of 86 presolar silicate grains identified through NanoSIMS oxygen ion imaging in thin sections of carbonaceous and ordinary chondrites. The O, Mg, and Si isotope data of 106 presolar silicates (including grains studied previously by our group) suggest division of O isotope Group 1 grains into four subpopulations: (i) “normal,” (ii) <jats:sup>25</jats:sup>Mg-rich, (iii) <jats:sup>26</jats:sup>Mg-rich, and (iv) <jats:sup>25</jats:sup>Mg-poor. Normal Group 1 grains (∼60% of Group 1 grains) formed in the winds of low-mass asymptotic giant branch (AGB) stars, with Mg and Si defining linear arrays with slopes of ∼0.9 and 1.37, respectively, in three-isotope representations, most likely representing Galactic chemical evolution (GCE). The <jats:sup>25</jats:sup>Mg-rich grains (∼25%) show enrichments in <jats:sup>25</jats:sup>Mg of up to a factor 2.4 relative to solar composition and most likely formed in supernova (SN) ejecta or the winds of intermediate-mass AGB stars. The <jats:sup>26</jats:sup>Mg-rich and <jats:sup>25</jats:sup>Mg-poor Group 1 grains lie below the Mg GCE line and their isotopic compositions favor origins from supergiants or SNe. The O isotope Group 2 grains show a wide range of Mg-isotopic compositions, similar to Group 1 grains, with likely origins from massive AGB stars, super-AGB stars, supergiants, and SNe. The Mg- and Si-isotopic compositions of Group 4 grains are compatible with previously proposed SN origins. Our results suggest that &gt;30% of presolar silicates formed in the winds of supergiants and in SN ejecta, and that low-mass AGB stars appear to have contributed only some 50% to presolar silicates, less than previously thought.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using six years of spectroscopic monitoring of the luminous quasar HE 0435-4312 (<jats:italic>z</jats:italic> = 1.2231) with the Southern African Large Telescope, in combination with photometric data (CATALINA, OGLE, SALTICAM, and BMT), we determined a rest-frame time delay of <jats:inline-formula> <jats:tex-math> <?CDATA ${296}_{-14}^{+13}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe9b2ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> days between the Mg <jats:sc>ii</jats:sc> broad-line emission and the ionizing continuum using seven different time-delay inference methods. Time-delay artifact peaks and aliases were mitigated using the bootstrap method and prior weighting probability function, as well as by analyzing unevenly sampled mock light curves. The Mg <jats:sc>ii</jats:sc> emission is considerably variable with a fractional variability of ∼5.4%, which is comparable to the continuum variability (∼4.8%). Because of its high luminosity (<jats:italic>L</jats:italic> <jats:sub>3000</jats:sub> = 10<jats:sup>46.4</jats:sup> erg s<jats:sup>−1</jats:sup>), the source is beneficial for a further reduction of the scatter along the Mg <jats:sc>ii</jats:sc>-based radius–luminosity relation and its extended versions, especially when the highly accreting subsample that has an rms scatter of ∼0.2 dex is considered. This opens up the possibility of using the high-accretor Mg <jats:sc>ii</jats:sc>-based radius–luminosity relation for constraining cosmological parameters. With the current sample of 27 reverberation-mapped sources, the best-fit cosmological parameters (Ω<jats:sub>m</jats:sub>, Ω<jats:sub>Λ</jats:sub>) = (0.19; 0.62) are consistent with the standard cosmological model within the 1<jats:italic>σ</jats:italic> confidence level.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present multiband Hubble Space Telescope imaging that spans rest-frame near-ultraviolet through near-infrared wavelengths (<jats:inline-formula> <jats:tex-math> <?CDATA ${\lambda }_{\mathrm{rest}}=0.3$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>–1.1 <jats:italic>μ</jats:italic>m) for 12 compact starburst galaxies at <jats:italic>z</jats:italic> = 0.4–0.8. These massive galaxies (<jats:inline-formula> <jats:tex-math> <?CDATA ${{ \mathcal M }}_{* }\sim {10}^{11}\,{{ \mathcal M }}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>) are driving very fast outflows (<jats:inline-formula> <jats:tex-math> <?CDATA ${v}_{\max }=1000$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>–3000 km s<jats:sup>−1</jats:sup>), and their light profiles are dominated by an extremely compact starburst component (half-light radius ∼ 100 pc). Our goal is to constrain the physical mechanisms responsible for launching these fast outflows by measuring the physical conditions within the central kiloparsec. Based on our stellar population analysis, the central component typically contributes ≈25% of the total stellar mass, and the central escape velocities <jats:inline-formula> <jats:tex-math> <?CDATA ${v}_{\mathrm{esc},\mathrm{central}}\approx 900$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> are a factor of two smaller than the observed outflow velocities. This Requires physical mechanisms that can accelerate gas to speeds significantly beyond the central escape velocities, and it makes clear that these fast outflows are capable of traveling into the circumgalactic medium, and potentially beyond. We find central stellar densities <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{\Sigma }}}_{{\rm{e}},\mathrm{central}}\approx 3\times {10}^{11}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn5.gif" xlink:type="simple" /> </jats:inline-formula> <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn6.gif" xlink:type="simple" /> </jats:inline-formula> kpc<jats:sup>−2</jats:sup> comparable to theoretical estimates of the Eddington limit, and we estimate <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{\Sigma }}}_{1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn7.gif" xlink:type="simple" /> </jats:inline-formula> surface densities within the central kiloparsec comparable to those of compact massive galaxies at <jats:inline-formula> <jats:tex-math> <?CDATA $0.5\lt z\lt 3.0$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn8.gif" xlink:type="simple" /> </jats:inline-formula>. Relative to “red nuggets” and “blue nuggets” at <jats:inline-formula> <jats:tex-math> <?CDATA $z\sim 2$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn9.gif" xlink:type="simple" /> </jats:inline-formula>, we find significantly smaller <jats:italic>r</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> values at a given stellar mass, which we attribute to the dominance of a young stellar component in our sample and the better physical resolution for rest-frame optical observations at <jats:inline-formula> <jats:tex-math> <?CDATA $z\sim 0.6$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn10.gif" xlink:type="simple" /> </jats:inline-formula> versus <jats:inline-formula> <jats:tex-math> <?CDATA $z\sim 2$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe935ieqn11.gif" xlink:type="simple" /> </jats:inline-formula>. We compare to theoretical scenarios involving major mergers and violent disk instability, and we speculate that our galaxies are progenitors of power-law ellipticals in the local universe with prominent stellar cusps.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Modern astronomical surveys are observing spectral data for millions of stars. These spectra contain chemical information that can be used to trace the Galaxy’s formation and chemical enrichment history. However, extracting the information from spectra and making precise and accurate chemical abundance measurements is challenging. Here we present a data-driven method for isolating the chemical factors of variation in stellar spectra from those of other parameters (i.e., <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub>, log <jats:italic>g</jats:italic>, [Fe/H]). This enables us to build a spectral projection for each star with these parameters removed. We do this with no ab initio knowledge of elemental abundances themselves and hence bypass the uncertainties and systematics associated with modeling that rely on synthetic stellar spectra. To remove known nonchemical factors of variation, we develop and implement a neural network architecture that learns a disentangled spectral representation. We simulate our recovery of chemically identical stars using the disentangled spectra in a synthetic APOGEE-like data set. We show that this recovery declines as a function of the signal-to-noise ratio but that our neural network architecture outperforms simpler modeling choices. Our work demonstrates the feasibility of data-driven abundance-free chemical tagging.</jats:p>