Abstract
We present a new 3D map of interstellar dust reddening, covering three quarters of the sky (declinations of δ ≳ −30°) out to a distance of several kiloparsecs. The map is based on ...high-quality stellar photometry of 800 million stars from Pan-STARRS 1 and 2MASS. We divide the sky into sightlines containing a few hundred stars each, and then infer stellar distances and types, along with the line-of-sight dust distribution. Our new map incorporates a more accurate average extinction law and an additional 1.5 yr of Pan-STARRS 1 data, tracing dust to greater extinctions and at higher angular resolutions than our previous map. Out of the plane of the Galaxy, our map agrees well with 2D reddening maps derived from far-infrared dust emission. After accounting for a 25 per cent difference in scale, we find a mean scatter of ∼10 per cent between our map and the Planck far-infrared emission-based dust map, out to a depth of 0.8 mag in E(gP1 − rP1), with the level of agreement varying over the sky. Our map can be downloaded at http://argonaut.skymaps.info, or from the Harvard Dataverse (Green 2017).
Earth is bombarded by meteors, occasionally by one large enough to cause a significant explosion and possible loss of life. It is not possible to detect all hazardous asteroids, and the efforts to ...detect them years before they strike are only advancing slowly. Similarly, ideas for mitigation of the danger from an impact by moving the asteroid are in their infancy. Although the odds of a deadly asteroid strike in the next century are low, the most likely impact is by a relatively small asteroid, and we suggest that the best mitigation strategy in the near term is simply to move people out of the way. With enough warning, a small asteroid impact should not cause loss of life, and even portable property might be preserved. We describe an early warning system that could provide a week’s notice of most sizeable asteroids or comets on track to hit the Earth. This may be all the mitigation needed or desired for small asteroids, and it can be implemented immediately for relatively low cost. This system, dubbed Asteroid Terrestrial-Impact Last Alert System (ATLAS), comprises two observatories separated by about 100 km that simultaneously scan the visible sky twice a night. Software automatically registers a comparison with the unchanging sky and identifies everything that has moved or changed. Communications between the observatories lock down the orbits of anything approaching the Earth, within one night if its arrival is less than a week. The sensitivity of the system permits detection of 140 m asteroids (100 Mton impact energy) three weeks before impact and 50 m asteroids a week before arrival. An ATLAS alarm, augmented by other observations, should result in a determination of impact location and time that is accurate to a few kilometers and a few seconds. In addition to detecting and warning of approaching asteroids, ATLAS will continuously monitor the changing universe around us: most of the variable stars in our Galaxy, many microlensing events from stellar alignments, luminous stars and novae in nearby galaxies, thousands of supernovae, nearly a million quasars and active galactic nuclei, tens of millions of galaxies, and a billion stars. With two views per day ATLAS will make the variable universe as familiar to us as the sunrise and sunset.
We provide a calculation of Pan-STARRS' ability to detect objects similar to the interstellar object 1I/2017 U1 (hereafter 'Oumuamua), including the most detectable approach vectors and the effect of ...object size on detection efficiency. Using our updated detection cross section, we infer an interstellar number density of such objects ( ). This translates to a mass density of that cannot be populated unless every star is contributing. We find that, given current models, such a number density cannot arise from the ejection of inner solar system material during planet formation. We note that a stellar system's Oort cloud will be released after a star's main-sequence life time and may provide enough material to obtain the observed density. The challenge is that Oort cloud bodies are icy and 'Oumuamua was observed to be dry, which necessitates a crust-generation mechanism.
We present an algorithm to identify the type of an SN spectrum and to determine its redshift and age. This algorithm, based on the correlation techniques of Tonry & Davis, is implemented in the ...Supernova Identification (SNID) code. It is used by members of ongoing high-redshift SN searches to distinguish between Type Ia and Ib/c SNe and to identify "peculiar" SNe Ia. We develop a diagnostic to quantify the quality of a correlation between the input and template spectra, which enables a formal evaluation of the associated redshift error. Furthermore, by comparing the correlation redshifts obtained using SNID with those determined from narrow lines in the SN host galaxy spectrum, we show that accurate redshifts (with a typical error sigma sub(z) unk 0.01) can be determined for SNe Ia without a spectrum of the host galaxy. Last, the age of an input spectrum is determined with a typical accuracy sigma sub(i) unk 3 days, shown here by using high-redshift SNe Ia with well-sampled light curves. The success of the correlation technique confirms the similarity of some SNe Ia at low and high redshifts. The SNID code, which will be made available to the community, can also be used for comparative studies of SN spectra, as well as comparisons between data and models.
We characterize the spatial density of the Pan-STARRS1 (PS1) sample of Rrab stars to study the properties of the old Galactic stellar halo. This sample, containing 44,403 sources, spans ...galactocentric radii of 0.55 kpc ≤ Rgc ≤ 141 kpc with a distance precision of 3% and thus is able to trace the halo out to larger distances than most previous studies. After excising stars that are attributed to dense regions such as stellar streams, the Galactic disk and bulge, and halo globular clusters, the sample contains ∼11,000 sources within 20 kpc ≤ Rgc ≤ 131 kpc. We then apply forward modeling using Galactic halo profile models with a sample selection function. Specifically, we use ellipsoidal stellar density models (l, b, Rgc) with a constant and a radius-dependent halo flattening q(Rgc). Assuming constant flattening q, the distribution of the sources is reasonably well fit by a single power law with n = 4.40 − 0.04 + 0.05 and q = 0.918 − 0.014 + 0.016 and comparably well fit by an Einasto profile with n = 9.53 − 0.28 + 0.27 , an effective radius reff = 1.07 0.10 kpc, and a halo flattening of q = 0.923 0.007. If we allow for a radius-dependent flattening q(Rgc), we find evidence for a distinct flattening of q ∼ 0.8 of the inner halo at ∼25 kpc. Additionally, we find that the south Galactic hemisphere is more flattened than the north Galactic hemisphere. The results of our work are largely consistent with many earlier results (e.g., Watkins et al.; Iorio et al.). We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after masking all known overdensities.
We present (g 475 - z 850) color and z 850-band surface brightness fluctuations (SBFs) measurements for 43 early-type galaxies in the Fornax cluster imaged with the Hubble Space Telescope Advanced ...Camera for Surveys. These are combined with our earlier measurements for Virgo cluster galaxies to derive a revised, nonlinear calibration of the z 850-band SBF absolute magnitude as a function of (g 475 - z 850) color, valid for the AB color range 0.8 < (g 475 - z 850) < 1.6. In all, we tabulate recalibrated SBF distances for 134 galaxies in Virgo, Fornax, the Virgo W' group, and NGC 4697 in the Virgo Southern Extension. The calibration procedure yields a highly precise relative distance modulus for Fornax with respect to Virgo of (m - M) FV = 0.42 ± 0.03 mag, or a distance ratio dF /dV = 1.214 ± 0.017. The resulting Fornax distance modulus is (m - M)For = 31.51 ± 0.03 ± 0.15 mag, corresponding to dF = 20.0 ± 0.3 ± 1.4 Mpc, where the second set of error bars reflects the total systematic uncertainty from our assumed Virgo distance of 16.5 Mpc. The rms distance scatter for the early-type Fornax cluster galaxies is s d = 0.49+0.11 -0.15 Mpc, or a total line-of-sight depth of 2.0+0.4 -0.6 Mpc, consistent with its compact appearance on the sky. This translates to a depth scatter smaller than the intrinsic, or 'cosmic,' scatter scos in the SBF calibration, unlike the case for the larger Virgo cluster. As a result, we are able to place the first tight constraints on the value of scos. We find scos = 0.06 ± 0.01 mag, with a firm upper limit of scos < 0.08 mag, for the subsample of galaxies with (g 475 - z 850)>1.02, but it is about twice as large for bluer galaxies. We also present an alternative SBF calibration based on the 'fluctuation count' parameter , a proxy for galaxy mass. This gives a consistent relative distance but with larger intrinsic scatter, and we adopt the result from the calibration on (g 475 - z 850) because of its basis in stellar population properties alone. Finally, we find no evidence for systematic trends of the galaxy distances with position or velocity (e.g., no current infall); the Fornax cluster appears both compact and well virialized.
RR Lyrae stars may be the best practical tracers of Galactic halo (sub-)structure and kinematics. The PanSTARRS1 (PS1) 3 π survey offers multi-band, multi-epoch, precise photometry across much of the ...sky, but a robust identification of RR Lyrae stars in this data set poses a challenge, given PS1's sparse, asynchronous multi-band light curves ( 12 epochs in each of five bands, taken over a 4.5 year period). We present a novel template fitting technique that uses well-defined and physically motivated multi-band light curves of RR Lyrae stars, and demonstrate that we get accurate period estimates, precise to 2 s in > 80 % of cases. We augment these light-curve fits with other features from photometric time-series and provide them to progressively more detailed machine-learned classification models. From these models, we are able to select the widest (three-fourths of the sky) and deepest (reaching 120 kpc) sample of RR Lyrae stars to date. The PS1 sample of ∼45,000 RRab stars is pure (90%) and complete (80% at 80 kpc) at high galactic latitudes. It also provides distances that are precise to 3%, measured with newly derived period-luminosity relations for optical/near-infrared PS1 bands. With the addition of proper motions from Gaia and radial velocity measurements from multi-object spectroscopic surveys, we expect the PS1 sample of RR Lyrae stars to become the premier source for studying the structure, kinematics, and the gravitational potential of the Galactic halo. The techniques presented in this study should translate well to other sparse, multi-band data sets, such as those produced by the Dark Energy Survey and the upcoming Large Synoptic Survey Telescope Galactic plane sub-survey.
The ACS Virgo Cluster Survey consists of HST ACS imaging for 100 early-type galaxies in the Virgo Cluster, observed in the F475W (-SDSS g) and F850LP (-SDSS z) filters. We derive distances for 84 of ...these galaxies using the method of surface brightness fluctuations (SBFs), present the SBF distance catalog, and use this database to examine the three-dimensional distribution of early-type galaxies in the Virgo Cluster. The SBF distance moduli have a mean (random) measurement error of 0.07 mag (0.5 Mpc), or roughly 3 times better than previous SBF measurements for Virgo Cluster galaxies. Five galaxies lie at a distance of d - 23 Mpc and are members of the W' cloud. The remaining 79 galaxies have a narrow distribution around our adopted distance of < d > = 16.5 c0.1 (random mean error) c1.1 Mpc (systematic). The rms distance scatter of this sample is s(d) = 0.6 c0.1 Mpc, with little or no dependence on morphological type or luminosity class (i.e., 0.7 c0.1 and 0.5 c0.1 Mpc for the giants and dwarfs, respectively). The back-to-front depth of the cluster measured from our sample of early-type galaxies is 2.4 c 0.4 Mpc (i.e., c2 s of the intrinsic distance distribution). The M87 (cluster A) and M49 (cluster B) subclusters are found to lie at distances of 16.7 c 0.2 and 16.4 c 0.2 Mpc, respectively. There may be a third subcluster associated with M86. A weak correlation between velocity and line-of-sight distance may be a faint echo of the cluster velocity distribution not having yet completely virialized. In three dimensions, Virgo's early-type galaxies appear to define a slightly triaxial distribution, with axis ratios of (1: 0.7: 0.5). The principal axis of the best-fit ellipsoid is inclined 620-40 from the line of sight, while the galaxies belonging to the W' cloud lie on an axis inclined by 610-15.
We present a detailed analysis of the morphology, isophotal parameters, and surface brightness profiles for 100 early-type members of the Virgo Cluster, from dwarfs (M sub(B) = -15.1 mag) to giants ...(M sub(B) = -21.8 mag), imaged in the g and z passbands using the Advanced Camera for Surveys on board the Hubble Space Telescope, Dust and complex morphological structures are common. Dust is detected in 42% of galaxies brighter than B sub(T) = 12.15 mag, while kiloparsec-scale stellar disk, bars, and nuclear stellar disks are seen in 60% of galaxies with intermediate luminosity. Isophotal parameters are derived typically within 8 kpc from the center for the brightest galaxies, and 1.5 kpc for the faintest systems, with a resolution of 7 pc. For most galaxies, the surface brightness profiles are well described by a Sersic model with index n that increases steadily with the galaxy luminosity; only for 8 of the 10 brightest galaxies are the inner profiles (typically within 100 pc of the center) lower than expected based on an extrapolation of the outer Sersic model, and are better described by a single power-law function. Contrary to previous claims, we find no evidence in support of a strong bimodal behavior of the logarithmic slope of the inner surface brightness profile, 7; in particular the 7 distribution for galaxies that do not show evidence of multiple morphological components is unimodal across the entire magnitude range spanned by the ACSVCS galaxies. Although the brightest galaxies have shallow inner profiles, the shallowest profiles are found in faint dwarf systems. The widely adopted separation of early-type galaxies between "core" and "power-law" types is questioned based on the present study.
PDF
