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Register a new userAge Gradients Throughout the Galaxy with O-Miras
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We assemble the largest sample of oxygen rich Miras to date and highlight their importance for age-dating the components of the Galaxy. Using data from the Catalina Rapid Transient Survey and the All Sky Automated Survey for Supernovae, we extract a clean sample of $sim 2,400$ O-Miras, stretching from the Galactic Bulge to the distant halo. Given that the period of O-Miras correlates with age, this offers a new way of determining age gradients throughout the Galaxy. We use our sample to show (i) disk O-Miras have periods increasing on moving outwards from ~ 3 to 15 kpc, so the outer disk O-Miras are younger than the inner disk, (ii) the transition from younger disk to halo O-Miras occurs at r ~ 15 kpc and is marked by a plummeting in period, (iii) there exists a population of young O-Miras likely kicked from the disk to heights of order of |Z| ~ 10 kpc, (iv) great circle counts of old Miras show strong evidence for distant debris agglomeration associated with the Magellanic Clouds, (v) seven stars in our samples are located at distances between 200 and 500 kpc surpassing all previously established records, and, finally, (vi) O-Miras may be present in the Fornax, Sculptor, Sextans and Leo II Galactic dwarf spheroidals, as well as the distant globular cluster Pal 4. We spotlight the importance of O-Mira in the Era of Gaia as universal chronometers of the Galactic populations.
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Radial age gradients hold the cumulative record of the multitude of physical processes driving the build-up of stellar populations and the ensuing star formation (SF) quenching process in galaxy bulges, therefore potentially sensitive discriminators between competing theoretical concepts on bulge formation and evolution. Based on spectral modeling of integral field spectroscopy data from the CALIFA survey, we derive mass- and light-weighted stellar age gradients ($ abla$(t,B)L,M) within the photometrically determined bulge radius (RB) of a representative sample of local face-on late-type galaxies that span 2.6 dex in stellar mass. Our analysis documents a trend for decreasing $ abla$(t,B)L,M with increasing M,T, with high-mass bulges predominantly showing negative age gradients and vice versa. The inversion from positive to negative $ abla$(t,B)L,M occurs at logM,T ~ 10, which roughly coincides with the transition from lower-mass bulges whose gas excitation is powered by SF to bulges classified as Composite, LINER or Seyfert. We discuss two limiting cases for the origin of radial age gradients in massive LTG bulges. The first assumes that the stellar age in the bulge is initially spatially uniform, thus the observed age gradients arise from an inside-out SF quenching (ioSFQ) front that is radially expanding with a mean velocity vq. In this case, the age gradients translate into a slow ioSFQ that lasts until z~2, suggesting mild negative feedback by SF or an AGN. If negative age gradients in massive bulges are not due to ioSFQ but primarily due to their inside-out formation process, then the standard hypothesis of quasi-monolithic bulge formation has to be discarded in favor of a scenario that involves gradual buildup of stellar mass over 2-3 Gyr through, e.g., inside-out SF and inward migration of SF clumps from the disk. In this case, rapid AGN-driven ioSFQ cannot be ruled out.
We explore the origin of stellar metallicity gradients in simulated and observed dwarf galaxies. We use FIRE-2 cosmological baryonic zoom-in simulations of 26 isolated galaxies as well as existing observational data for 10 Local Group dwarf galaxies. Our simulated galaxies have stellar masses between $10^{5.5}$ and $10^{8.6} msun$. Whilst gas-phase metallicty gradients are generally weak in our simulated galaxies, we find that stellar metallicity gradients are common, with central regions tending to be more metal-rich than the outer parts. The strength of the gradient is correlated with galaxy-wide median stellar age, such that galaxies with younger stellar populations have flatter gradients. Stellar metallicty gradients are set by two competing processes: (1) the steady puffing of old, metal-poor stars by feedback-driven potential fluctuations, and (2) the accretion of extended, metal-rich gas at late times, which fuels late-time metal-rich star formation. If recent star formation dominates, then extended, metal-rich star formation washes out pre-existing gradients from the puffing process. We use published results from ten Local Group dwarf galaxies to show that a similar relationship between age and stellar metallicity-gradient strength exists among real dwarfs. This suggests that observed stellar metallicity gradients may be driven largely by the baryon/feedback cycle rather than by external environmental effects.
Most stars are born in rich young stellar clusters (YSCs) embedded in giant molecular clouds. The most massive stars live out their short lives there, profoundly influencing their natal environments by ionizing HII regions, inflating wind-blown bubbles, and soon exploding as supernovae. Thousands of lower-mass pre-main sequence stars accompany the massive stars, and the expanding HII regions paradoxically trigger new star formation as they destroy their natal clouds. While this schematic picture is established, our understanding of the complex astrophysical processes involved in clustered star formation have only just begun to be elucidated. The technologies are challenging, requiring both high spatial resolution and wide fields at wavelengths that penetrate obscuring molecular material and remove contaminating Galactic field stars. We outline several important projects for the coming decade: the IMFs and structures of YSCs; triggered star formation around YSC; the fate of OB winds; the stellar populations of Infrared Dark Clouds; the most massive star clusters in the Galaxy; tracing star formation throughout the Galactic Disk; the Galactic Center region and YSCs in the Magellanic Clouds. Programmatic recommendations include: developing a 30m-class adaptive optics infrared telescope; support for high-resolution and wide field X-ray telescopes; large-aperture sub-millimeter and far-infrared telescopes; multi-object infrared spectrographs; and both numerical and analytical theory.
The pace and pattern of star formation leading to rich young stellar clusters is quite uncertain. In this context, we analyze the spatial distribution of ages within 19 young (median t<3 Myr on the Siess et al. (2000) timescale), morphologically simple, isolated, and relatively rich stellar clusters. Our analysis is based on young stellar object samples from the MYStIX and SFiNCs surveys, and a new estimator of pre-main sequence (PMS) stellar ages, AgeJX, derived from X-ray and near-infrared photometric data. Median cluster ages are computed within four annular subregions of the clusters. We confirm and extend the earlier result of Getman et al. (2014): 80% percent of the clusters show age trends where stars in cluster cores are younger than in outer regions. Our cluster stacking analyses establish the existence of an age gradient to high statistical significance in several ways. Time scales vary with the choice of PMS evolutionary model; the inferred median age gradient across the studied clusters ranges from 0.75 Myr/pc to 1.5 Myr/pc. The empirical finding reported in the present study -- late or continuing formation of stars in the cores of star clusters with older stars dispersed in the outer regions -- has a strong foundation with other observational studies and with the astrophysical models like the global hierarchical collapse model of Vazquez-Semadeni et al. (2017).
Measurement and astrophysical interpretation of characteristic gamma-ray lines from nucleosynthesis was one of the prominent science goals of the INTEGRAL mission and in particular its spectrometer SPI. Emission from 26Al and from 60Fe decay lines originates from accumulated ejecta of nucleosynthesis sources, and appears diffuse in nature. 26Al and 60Fe are believed to originate mostly from massive star clusters. Gamma-ray observations open an interesting window to trace the fate and flow of nucleosynthesis ejecta, after they have left the immediate sources and their birth sites, and on their path to mix with ambient interstellar gas. The INTEGRAL 26Al emission image confirms earlier findings of clumpiness and an extent along the entire plane of the Galaxy, supporting its origin from massive-star groups. INTEGRAL spectroscopy resolved the line and found Doppler broadenings and systematic shifts from large-scale galactic rotation. But an excess velocity of ~200 km/s suggests that 26Al decays preferentially within large superbubbles that extend in forward directions between spiral arms. The detection of 26Al line emission from nearby Orion and the Eridanus superbubble supports this interpretation. Positrons from beta+ decays of 26Al and other nucleosynthesis ejecta have been found to not explain the morphology of positron annihilation gamma-rays at 511 keV that have been measured by INTEGRAL. The 60Fe signal measured by INTEGRAL is diffuse but too weak for an imaging interpretation, an origin from point-like/concentrated sources is excluded. The 60Fe/26Al ratio is constrained to a range 0.2-0.4. Beyond improving precision of these results, diffuse nucleosynthesis contributions from novae (through 22Na radioactivity) and from past neutron star mergers in our Galaxy (from r-process radioactivity) are exciting new prospects for the remaining mission extensions.
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