Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We present an analysis of the most comprehensive ...NIR variability data set of Sgr A* to date: eight 24 hr epochs of continuous monitoring of Sgr A* at 4.5 m with the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18 m data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12 m data from the NIRC2 camera at the Keck Observatory, in total 94,929 measurements. A new approximate Bayesian computation method for fitting the first-order structure function extracts information beyond current fast Fourier transformation (FFT) methods of power spectral density (PSD) estimation. With a combined fit of the data of all three observatories, the characteristic coherence timescale of Sgr A* is minutes (90% credible interval). The PSD has no detectable features on timescales down to 8.5 minutes (95% credible level), which is the ISCO orbital frequency for a dimensionless spin parameter a = 0.92. One light curve measured simultaneously at 2.12 and 4.5 m during a low flux-density phase gave a spectral index s = 1.6 0.1 ( ). This value implies that the Sgr A* NIR color becomes bluer during higher flux-density phases. The probability densities of flux densities of the combined data sets are best fit by log-normal distributions. Based on these distributions, the Sgr A* spectral energy distribution is consistent with synchrotron radiation from a non-thermal electron population from below 20 GHz through the NIR.
ABSTRACT
The deep, wide-area (∼800–900 arcmin
2
) near-infrared/WFC3/IR +
Spitzer
/IRAC observations over the CANDELS fields have been a remarkable resource for constraining the bright end of ...high-redshift UV luminosity functions. However, the lack of
Hubble Space Telescope
(
HST
) 1.05
μ
m observations over the CANDELS fields has made it difficult to identify
z
∼ 9–10 sources robustly, since such data are needed to confirm the presence of an abrupt Lyman break at 1.2
μ
m. Here, we report on the successful identification of many such
z
∼ 9–10 sources from a new
HST
program (z9-CANDELS) that targets the highest-probability
z
∼ 9–10 galaxy candidates with observations at 1.05
μ
m, to search for a robust Lyman-break at 1.2
μ
m. The potential
z
∼ 9–10 candidates were preselected from the full
HST
,
Spitzer
/IRAC S-CANDELS observations, and the deepest-available ground-based optical+near-infrared observations (CFHTLS-DEEP+HUGS+UltraVISTA+ZFOURGE). We identified 15 credible
z
∼ 9–10 galaxies over the CANDELS fields. Nine of these galaxies lie at
z
∼ 9 and five are new identifications. Our targeted follow-up strategy has proven to be very efficient in making use of scarce
HST
time to secure a reliable sample of
z
∼ 9–10 galaxies. Through extensive simulations, we replicate the selection process for our sample (both the preselection and follow-up) and use it to improve current estimates for the volume density of bright
z
∼ 9 and
z
∼ 10 galaxies. The volume densities we find are 5
and
lower, respectively, than those found at
z
∼ 8. When compared with the best-fit evolution (i.e.,
) in the UV luminosity densities from
z
∼ 8 to
z
∼ 4 integrated to
(−20 mag), these luminosity densities are
and
lower, respectively, than the extrapolated trends. Our new results are broadly consistent with the “accelerated evolution” scenario at
z
> 8, consistent with that seen in many models.
Abstract
Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We have analyzed a comprehensive ...submillimeter (including new observations simultaneous with NIR monitoring), NIR, and 2–8 keV data set. Submillimeter variations tend to lag those in the NIR by ∼30 minutes. An approximate Bayesian computation fit to the X-ray first-order structure function shows significantly less power at short timescales in the X-rays than in the NIR. Less X-ray variability at short timescales, combined with the observed NIR–X-ray correlations, means the variability can be described as the result of two strictly correlated stochastic processes, the X-ray process being the low-pass-filtered version of the NIR process. The NIR–X-ray linkage suggests a simple radiative model: a compact, self-absorbed synchrotron sphere with high-frequency cutoff close to NIR frequencies plus a synchrotron self-Compton scattering component at higher frequencies. This model, with parameters fit to the submillimeter, NIR, and X-ray structure functions, reproduces the observed flux densities at all wavelengths, the statistical properties of all light curves, and the time lags between bands. The fit also gives reasonable values for physical parameters such as magnetic flux density
B
≈ 13 G, source size
L
≈ 2.2
R
S
, and high-energy electron density
n
e
≈ 4 × 10
7
cm
−3
. An animation illustrates typical light curves, and we make public the parameter chain of our Bayesian analysis, the model implementation, and the visualization code.
ABSTRACT We analyze the spatial distribution of dusty young stellar objects (YSOs) identified in the Spitzer Survey of the Orion Molecular clouds, augmenting these data with Chandra X-ray ...observations to correct for incompleteness in dense clustered regions. We also devise a scheme to correct for spatially varying incompleteness when X-ray data are not available. The local surface densities of the YSOs range from 1 pc−2 to over 10,000 pc−2, with protostars tending to be in higher density regions. This range of densities is similar to other surveyed molecular clouds with clusters, but broader than clouds without clusters. By identifying clusters and groups as continuous regions with surface densities ≥10 pc−2, we find that 59% of the YSOs are in the largest cluster, the Orion Nebula Cluster (ONC), while 13% of the YSOs are found in a distributed population. A lower fraction of protostars in the distributed population is evidence that it is somewhat older than the groups and clusters. An examination of the structural properties of the clusters and groups shows that the peak surface densities of the clusters increase approximately linearly with the number of members. Furthermore, all clusters with more than 70 members exhibit asymmetric and/or highly elongated structures. The ONC becomes azimuthally symmetric in the inner 0.1 pc, suggesting that the cluster is only ∼2 Myr in age. We find that the star formation efficiency (SFE) of the Orion B cloud is unusually low, and that the SFEs of individual groups and clusters are an order of magnitude higher than those of the clouds. Finally, we discuss the relationship between the young low mass stars in the Orion clouds and the Orion OB 1 association, and we determine upper limits to the fraction of disks that may be affected by UV radiation from OB stars or dynamical interactions in dense, clustered regions.
Of several dozen galaxies observed spectroscopically that are candidates for having a redshift (z) in excess of seven, only five have had their redshifts confirmed via Lyman α emission, at z = 7.008, ...7.045, 7.109, 7.213 and 7.215 (refs 1-4). The small fraction of confirmed galaxies may indicate that the neutral fraction in the intergalactic medium rises quickly at z > 6.5, given that Lyman α is resonantly scattered by neutral gas. The small samples and limited depth of previous observations, however, makes these conclusions tentative. Here we report a deep near-infrared spectroscopic survey of 43 photometrically-selected galaxies with z > 6.5. We detect a near-infrared emission line from only a single galaxy, confirming that some process is making Lyman α difficult to detect. The detected emission line at a wavelength of 1.0343 micrometres is likely to be Lyman α emission, placing this galaxy at a redshift z = 7.51, an epoch 700 million years after the Big Bang. This galaxy's colours are consistent with significant metal content, implying that galaxies become enriched rapidly. We calculate a surprisingly high star-formation rate of about 330 solar masses per year, which is more than a factor of 100 greater than that seen in the Milky Way. Such a galaxy is unexpected in a survey of our size, suggesting that the early Universe may harbour a larger number of intense sites of star formation than expected.
We present a uniform mid-infrared imaging and photometric survey of 36 young, nearby, star-forming clusters and groups using Spitzer IRAC and MIPS. We have confidently identified and classified 2548 ...young stellar objects (YSOs) using recently established mid-infrared color-based methods. We have devised and applied a new algorithm for the isolation of local surface density enhancements from point source distributions, enabling us to extract the overdense cores of the observed star-forming regions for further analysis. We have compiled several basic structural measurements of these cluster cores from the data, such as mean surface densities of sources, cluster core radii, and aspect ratios, in order to characterize the ranges for these quantities. We find that a typical cluster core is 0.39 pc in radius, has 26 members with infrared excess in a ratio of Class II to Class I sources of 3.7, is embedded in a AK = 0.8 mag cloud clump, and has a surface density of 60 pc-2. We examine the nearest neighbor distances among the YSOs in several ways, demonstrating similarity in the spacings between Class II and Class I sources but large member clusters appear more dense than smaller clusters. We demonstrate that near-uniform source spacings in cluster cores are common, suggesting that simple Jeans fragmentation of parsec-scale cloud clumps may be the dominant process governing star formation in nearby clusters and groups. Finally, we compare our results to other similar surveys in the literature and discuss potential biases in the data to guide further interpretation.
Size-controlling rootstocks are critical to grow small, efficient trees that enable early and high yield in plantings of apple. Improved knowledge of rootstock-related size-control processes is ...important for breeding and cultural practices. The research objective was to determine hormone profiles within trees grafted on Malling rootstocks. Buds from ‘Gala’, ‘Fuji’, M.7, M.9, M.27, and MM.111 were grafted to rootstocks M.7, M.9, M.27, and MM.111 and then grown in a greenhouse and the field. After 2 years, heights of trees grafted to growth-controlling rootstocks decreased in the order M.9 > M.7 > MM.111 in the greenhouse, the reverse trend that was measured in the field. Although soil resources were not measured, ample availability of resources such as water and mineral nutrients in the greenhouse may have been responsible for the reversal of rootstock effects on tree height in greenhouse and field. Elevated abscisic acid (ABA) and ABA metabolites were associated with ‘Gala’, MM.111 and M.9 scion that were grafted on M.9 rootstocks. Abscisic acid and abscisic acid glucose ester were generally greater in root, rootstock stem below the graft union, scion above the graft, and xylem exudate of rootstock M.9 than MM.111. Reduced gibberellin (GA₁₉) was found in roots and xylem exudate of ‘Gala’ grafted to M.9 than MM.111 rootstocks. These results support the hypothesis that hormone signals from rootstocks control tree growth but it is likely that stage of development, time after planting, and environmental resources will also interact to influence growth effects of size-controlling rootstocks. It is proposed that gene expression associated with hormone metabolism can be developed to further understand the underpinnings of size-controlling rootstocks and assist the selection of rootstocks for size control and, possibly, for other hormone-related characteristics.
We present an independent estimate of the interstellar extinction law for the Spitzer IRAC bands, as well as a first attempt at extending the law to the 24 mu m MIPS band. The source data for these ...measurements are observations of five nearby star-forming regions: the Orion A cloud, NGC 2068/2071, NGC 2024/2023, Serpens, and Ophiuchus. Color excess ratios E/Ek,- were measured for stars without infrared excess dust emission from clrcumstellar disks/envelopes. For four of these five regions, the extinction laws are similar at all wavelengths and differ systematically from a previous determination of the extinction law, which was dominated by the diffuse ISM, derived for the IRAC bands. This difference could be due to the difference in the dust properties of the dense molecular clouds observed here and those of the diffuse ISM. The extinction law at longer wavelengths toward the Ophiuchus region lies between that to the other four regions studied here and that for the ISM. In addition, we extended our extinction law determination to 24 mu m for Serpens and NGC 2068/2071 using Spitzer MIPS data. We compare these results against several ISO extinction law determinations, although in each case there are assumptions which make absolute comparison uncertain. However, our work confirms a relatively flatter extinction curve from 4 to 8 mu m than the previously assumed standard, as noted by all of these recent studies. The extinction law at 24 mu m is consistent with previous measurements and models, although there are relatively large uncertainties.
ABSTRACT We present Hubble WFC3/IR slitless grism spectra of a remarkably bright z 10 galaxy candidate, GN-z11, identified initially from CANDELS/GOODS-N imaging data. A significant spectroscopic ...continuum break is detected at . The new grism data, combined with the photometric data, rule out all plausible lower redshift solutions for this source. The only viable solution is that this continuum break is the Ly break redshifted to , just ∼400 Myr after the Big Bang. This observation extends the current spectroscopic frontier by 150 Myr to well before the Planck (instantaneous) cosmic reionization peak at z ∼ 8.8, demonstrating that galaxy build-up was well underway early in the reionization epoch at z > 10. GN-z11 is remarkably, and unexpectedly, luminous for a galaxy at such an early time: its UV luminosity is 3× larger than measured at z ∼ 6−8. The Spitzer IRAC detections up to 4.5 m of this galaxy are consistent with a stellar mass of ∼109 M . This spectroscopic redshift measurement suggests that James Webb Space Telescope (JWST) will be able to similarly and easily confirm such sources at z > 10 and characterize their physical properties through detailed spectroscopy. Furthermore, WFIRST, with its wide-field near-IR imaging, would find large numbers of similar galaxies and contribute greatly to JWST's spectroscopy, if it is launched early enough to overlap with JWST.
Emission from Saggitarius A* is highly variable at both X-ray and infrared (IR) wavelengths. Observations over the last ∼20 yr have revealed X-ray flares that rise above a quiescent thermal ...background about once per day, while faint X-ray flares from Sgr A* are undetectable below the constant thermal emission. In contrast, the IR emission of Sgr A* is observed to be continuously variable. Recently, simultaneous observations have indicated a rise in IR flux density around the same time as every distinct X-ray flare, while the opposite is not always true (peaks in the IR emission may not be coincident with an X-ray flare). Characterizing the behavior of these simultaneous X-ray/IR events and measuring any time lag between them can constrain models of Sgr A*'s accretion flow and the flare emission mechanism. Using 100+ hours of data from a coordinated campaign between the Spitzer Space Telescope and the Chandra X-ray Observatory, we present results of the longest simultaneous IR and X-ray observations of Sgr A* taken to date. The cross-correlation between the IR and X-ray light curves in this unprecedented data set, which includes four modest X-ray/IR flares, indicates that flaring in the X-ray may lead the IR by approximately 10-20 min with 68% confidence. However, the 99.7% confidence interval on the time-lag also includes zero, i.e., the flaring remains statistically consistent with simultaneity. Long-duration and simultaneous multi-wavelength observations of additional bright flares will improve our ability to constrain the flare timing characteristics and emission mechanisms, and must be a priority for Galactic Center observing campaigns.
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