The electromagnetic counterpart to the Galactic center supermassive black hole, Sgr A*, has been observed in the near-infrared for over 20 years and is known to be highly variable. We report new Keck Telescope observations showing that Sgr A* reached much brighter flux levels in 2019 than ever measured at near-infrared wavelengths. In the K band, Sgr A* reached flux levels of ∼ 6 mJy, twice the level of the previously observed peak flux from > 13, 000 measurements over 130 nights with the VLT and Keck Telescopes. We also observe a factor of 75 change in flux over a 2-hour time span with no obvious color changes between 1.6 µm and 2.1 µm. The distribution of flux variations observed this year is also significantly different than the historical distribution. Using the most comprehensive statistical model published, the probability of a single night exhibiting peak flux levels observed this year, given historical Keck observations, is less than 0.3%. The probability to observe the flux levels similar to all 4 nights of data in 2019 is less than 0.05%. This increase in brightness and variability may indicate a period of heightened activity from Sgr A* or a change in its accretion state. It may also indicate that the current model is not sufficient to model Sgr A* at high flux levels and should be updated. Potential physical origins of Sgr A*'s unprecedented brightness may be from changes in the accretion-flow as a result of the star S0-2's closest passage to the black hole in 2018 or from a delayed reaction to the approach of the dusty object G2 in 2014. Additional multi-wavelength observations will be necessary to both monitor Sgr A* for potential state changes and to constrain the physical processes responsible for its current variability.
We present a new Milky Way microlensing simulation code, dubbed PopSyCLE (Population Synthesis for Compact object Lensing Events). PopSyCLE is the first resolved microlensing simulation to include a compact object distribution derived from numerical supernovae explosion models and both astrometric and photometric microlensing effects. We demonstrate the capabilities of PopSyCLE by investigating the optimal way to find black holes (BHs) with microlensing. Candidate BHs have typically been selected from wide-field photometric microlensing surveys, such as OGLE, by selecting events with long Einstein crossing times (t E > 120 days). These events can be selected at closest approach and monitored astrometrically in order to constrain the mass of each lens; PopSyCLE predicts a BH detection rate of ∼40% for such a program. We find that the detection rate can be enhanced to ∼ 85% by selecting events with both t E > 120 days and a microlensing parallax of π E < 0.08. Unfortunately, such a selection criterion cannot be applied during the event as π E requires both pre-and post-peak photometry. However, historical microlensing events from photometric surveys can be revisited using this new selection criteria in order to statistically constrain the abundance of BHs in the Milky Way. The future WFIRST microlensing survey provides both precise photometry and astrometry and will yield individual masses of O(100 − 1000) black holes, which is at least an order of magnitude more than is possible with individual candidate follow-up with current facilities. The resulting sample of BH masses from WFIRST will begin to constrain the shape of the black hole present-day mass function, BH multiplicity, and BH kick velocity distributions.
As a young massive cluster in the Central Molecular Zone, the Arches cluster is a valuable probe of the stellar Initial Mass Function (IMF) in the extreme Galactic Center environment. We use multi-epoch Hubble Space Telescope observations to obtain high-precision proper motion and photometric measurements of the cluster, calculating cluster membership probabilities for stars down to ∼1.8 M between cluster radii of 0.25 pc -3.0 pc. We achieve a cluster sample with just ∼6% field contamination, a significant improvement over photometrically-selected samples which are severely compromised by the differential extinction across the field. Combining this sample with K-band spectroscopy of 5 cluster members, we forward model the Arches cluster to simultaneously constrain its IMF and other properties (such as age and total mass) while accounting for observational uncertainties, completeness, mass segregation, and stellar multiplicity. We find that the Arches IMF is best described by a 1-segment power law that is significantly top-heavy: α = 1.80 ± 0.05 (stat) ± 0.06 (sys), where dN/dm ∝ m −α , though we cannot discount a 2-segment power law model with a high-mass slope only slightly shallower than local star forming regions (α = 2.04 +0.14 −0.19 ± 0.04) but with a break at 5.8 +3.2 −1.2 ± 0.02 M . In either case, the Arches IMF is significantly different than the standard IMF. Comparing the Arches to other young massive clusters in the Milky Way, we find tentative evidence for a systematically top-heavy IMF at the Galactic Center.
We present a quantitative analysis of the low-resolution (∼4.5Å) spectra of 12 late-B and early-A blue supergiants (BSGs) in the metal-poor dwarf galaxy NGC 3109. A modified method of analysis is presented which does not require use of the Balmer jump as an independent T ef f indicator, as used in previous studies. We determine stellar effective temperatures, gravities, metallicities, reddening, and luminosities, and combine our sample with the early-B type BSGs analyzed by Evans et al. (2007) to derive the distance to NGC 3109 using the Flux-weighted Gravity-Luminosity Relation (FGLR). Using primarily Fe-group elements, we find an average metallicity of [Z] = -0.67 ± 0.13, and no evidence of a metallicity gradient in the galaxy. Our metallicities are higher than those found by Evans et al. (2007) based on the oxygen abundances of early-B supergiants ([Z] = −0.93 ± 0.07), suggesting a low α/Fe ratio for the galaxy. We adjust the position of NGC 3109 on the BSG-determined galaxy mass-metallicity relation accordingly and compare it to metallicity studies of HII regions in star-forming galaxies. We derive an FGLR distance modulus of 25.55 ± 0.09 (1.27 Mpc) that compares well with Cepheid and tip of the red giant branch (TRGB) distances. The FGLR itself is consistent with those found in other galaxies, demonstrating the reliability of this method as a measure of extragalactic distances.
Low resolution (∼ 4.5Å) ESO VLT/FORS spectra of blue supergiant stars are analyzed to determine stellar metallicities (based on elements such as iron, titanium, magnesium) in the extended disk of the spiral galaxy NGC 3621. Mildly subsolar metallicity (-0.30 dex) is found for the outer objects beyond 7 kpc independent of galactocentric radius and compatible with the absence of a metallicity gradient confirming the results of a recent investigation of interstellar medium H II region gas oxygen abundances. The stellar metallicities are slightly higher than those from the H II regions when based on measurements of the weak forbidden auroral oxygen line at 4363Å but lower than the ones obtained with the R 23 strong line method. It is shown that the present level of metallicity in the extended disk cannot be the result of chemical evolution over the age of the disk with the present rate of in situ star formation. Additional mechanisms must be involved. In addition to metallicity, stellar effective temperatures, gravities, interstellar reddening, and bolometric magnitudes are determined. After application of individual reddening corrections for each target the flux-weighted gravityluminosity relationship of blue supergiant stars is used to obtain a distance modulus of 29.07 ± 0.09 mag (distance D = 6.52 ± 0.28 Mpc). This new distance is discussed in relation to Cepheid and tip of the red giant branch distances.
We present the first direct determination of a stellar metallicity in the spiral galaxy NGC 4258 (D = 7.6 Mpc) based on the quantitative analysis of a low-resolution (∼ 5Å) Keck LRIS spectrum of a blue supergiant star located in its disk. A determination of stellar metallicity in this galaxy is important for the absolute calibration of the Cepheid Period-Luminosity relation as an anchor for the extragalactic distance scale and for a better characterization of its dependence as a function of abundance. We find a value 0.2 dex lower than solar metallicity at a galactocentric distance of 8.7 kpc, in agreement with recent H II region studies using the weak forbidden auroral oxygen line at 4363Å. We determine the effective stellar temperature, gravity, luminosity and line-of-sight extinction of the blue supergiant being studied. We show that it fits well on the flux-weighted gravity-luminosity relation (FGLR), strengthening the potential of this method as a new extragalactic distance indicator.
At a projected distance of ∼26 pc from Sgr A * , the Arches cluster provides insight into star formation in the extreme Galactic center (GC) environment. Despite its importance, many key properties, such as the cluster's internal structure and orbital history, are not well known. We present an astrometric and photometric study of the outer region of the Arches cluster (R > 6 25) using Hubble Space Telescope WFC3IR. Using proper motions, we calculate membership probabilities for stars down to F153M = 20 mag (∼2.5 M e ) over a 120″ × 120″ field of view, an area 144 times larger than previous astrometric studies of the cluster. We construct the radial profile of the Arches to a radius of 75″ (∼3 pc at 8 kpc), which can be well described by a single power law. From this profile we place a 3σ lower limit of 2.8 pc on the observed tidal radius, which is larger than the predicted tidal radius (1-2.5 pc). Evidence of mass segregation is observed throughout the cluster, and no tidal tail structures are apparent along the orbital path. The absence of breaks in the profile suggests that the Arches has not likely experienced its closest approach to the GC between ∼0.2 and 1 Myr ago. If accurate, this constraint indicates that the cluster is on a prograde orbit and is located in front of the sky plane that intersects Sgr A * . However, further simulations of clusters in the GC potential are required to interpret the observed profile with more confidence.
The Quintuplet star cluster is one of only three known young (< 10 Myr) massive (M > 10 4 M ) clusters within ∼ 100 pc of the Galactic Center. In order to explore star cluster formation and evolution in this extreme environment, we analyze the Quintuplet's dynamical structure. Using the HST WFC3-IR instrument, we take astrometric and photometric observations of the Quintuplet covering a 120 × 120 field-of-view, which is 19 times larger than those of previous proper motion studies of the Quintuplet. We generate a catalog of the Quintuplet region with multi-band, near-infrared photometry, proper motions, and cluster membership probabilities for 10, 543 stars. We present the radial density profile of 715 candidate Quintuplet cluster members with M 4.7 M out to 3.2 pc from the cluster center. A 3σ lower limit of 3 pc is placed on the tidal radius, indicating the lack of a tidal truncation within this radius range. Only weak evidence for mass segregation is found, in contrast to the strong mass segregation found in the Arches cluster, a second and slightly younger massive cluster near the Galactic Center. It is possible that tidal stripping hampers a mass segregation signature, though we find no evidence of spatial asymmetry. Assuming that the Arches and Quintuplet formed with comparable extent, our measurement of the Quintuplet's comparatively large core radius of 0.62 +0.10 −0.10 pc provides strong empirical evidence that young massive clusters in the Galactic Center dissolve on a several Myr timescale.
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