Gregory Dobler

The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze

The Fermi Gamma-ray Space Telescope reveals a diffuse inverse Compton (IC) signal in the inner Galaxy with a similar spatial morphology to the microwave haze observed by WMAP, supporting the synchrotron interpretation of the microwave signal. Using spatial templates, we regress out π0 gammas, as well as IC and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess toward the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with IC from a hard population of electrons. The morphology and spectrum are consistent with it being the IC counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition, the implied electron spectrum is hard; electrons accelerated in supernova shocks in the disk which then diffuse a few kpc to the haze region would have a softer spectrum. We describe the full-sky Fermi maps used in this analysis and make them available for download.

Extended Anomalous Foreground Emission in the WMAP Three-Year Data

We study the spectral and morphological characteristics of the diffuse Galactic emission in theWMAPtemperature data using a template-based multilinear regression, and obtain the following results. (1) We confirm previous observationsof abumpinthedust-correlatedspectrum,consistentwiththeDraine&Lazarianspinningdustmodel.(2)We alsoconfirmthe‘‘haze’’signalintheinnerGalaxy,andarguethatitdoesnotfollowafree-freespectrumasfirstthought, but instead is synchrotron emissionfrom a hard electron cosmic-raypopulation. (3) In a departure from previous work, we allow the spectrum of H� -correlated emission (which is used to trace the free-free component) to float in the fit, andfind that it does not follow the expectedfree-free spectrum. Instead there is a bump near 50 GHz, modifying the spectrum at the 20% level, which we speculate is caused by spinning dust in the warm ionized medium. (4) The derived cross-correlation spectra are not sensitive to the map zero points, but are sensitive to the choice of CMB estimator. In cases where the CMB estimator is derived by minimizing variance of a linear combination of the WMAP bands, we show that a bias proportional to the cross-correlation of each template and the true CMB is always present. Thisbiascanbelargerthananyof theforegroundsignalsinsomebands.(5)Lastly,weconsiderthefrequencycoverage andsensitivityof thePlanckmission,andsuggestlinearcombinationcoefficientsfortheCMBtemplatethatwillreduce boththestatisticalandsystematicuncertaintyinthesynchrotronandhazespectrabymorethananorderof magnitude. Subject headingg diffuse radiation — dust, extinction — ISM: clouds — radiation mechanisms: nonthermal — radio continuum: ISM

Possible evidence for dark matter annihilations from the excess microwave emission around the center of the Galaxy seen by the Wilkinson Microwave Anisotropy Probe

The WMAP experiment has revealed an excess of microwave emission from the region around the center of our Galaxy. It has been suggested that this signal, known as the “WMAP Haze”, could be synchrotron emission from relativistic electrons and positrons generated in dark matter annihilations. In this letter, we revisit this possibility. We find that the angular distribution of the WMAP Haze matches the prediction for dark matter annihilations with a cusped density profile, ρ(r) ∝ r in the inner kiloparsecs. Comparing the intensity in different WMAP frequency bands, we find that a wide range of possible WIMP annihilation modes are consistent with the spectrum of the haze for a WIMP with a mass in the 100 GeV to multi-TeV range. Most interestingly, we find that to generate the observed intensity of the haze, the dark matter annihilation cross section is required to be approximately equal to the value needed for a thermal relic, σv ∼ 3×10 cm/s. No boost factors are required. If dark matter annihilations are in fact responsible for the WMAP Haze, and the slope of the halo profile continues into the inner Galaxy, GLAST is expected to detect gamma rays from the dark matter annihilations in the Galactic Center if the WIMP mass is less than several hundred GeV.

SELECTING QUASARS BY THEIR INTRINSIC VARIABILITY

We present a new and simple technique for selecting extensive, complete, and pure quasar samples, based on their intrinsic variability. We parameterize the single-band variability by a power-law model for the light-curve structure function, with amplitude A and power-law index γ. We show that quasars can be efficiently separated from other non-variable and variable sources by the location of the individual sources in the A-γ plane. We use ~60 epochs of imaging data, taken over ~5 years, from the SDSS stripe 82 (S82) survey, where extensive spectroscopy provides a reference sample of quasars, to demonstrate the power of variability as a quasar classifier in multi-epoch surveys. For UV-excess selected objects, variability performs just as well as the standard SDSS color selection, identifying quasars with a completeness of 90% and a purity of 95%. In the redshift range 2.5 < z < 3, where color selection is known to be problematic, variability can select quasars with a completeness of 90% and a purity of 96%. This is a factor of 5-10 times more pure than existing color selection of quasars in this redshift range. Selecting objects from a broad griz color box without u-band information, variability selection in S82 can afford completeness and purity of 92%, despite a factor of 30 more contaminants than quasars in the color-selected feeder sample. This confirms that the fraction of quasars hidden in the stellar locus of color space is small. To test variability selection in the context of Pan-STARRS 1 (PS1) we created mock PS1 data by down-sampling the S82 data to just six epochs over 3 years. Even with this much sparser time sampling, variability is an encouragingly efficient classifier. For instance, a 92% pure and 44% complete quasar candidate sample is attainable from the above griz-selected catalog. Finally, we show that the presented A-γ technique, besides selecting clean and pure samples of quasars (which are stochastically varying objects), is also efficient at selecting (periodic) variable objects such as RR Lyrae.

Prospects for polarized foreground removal

In this report we discuss the impact of polarized foregrounds on a future CMBPol satellite mission. We review our current knowledge of Galactic polarized emission at microwave frequencies, including synchrotron and thermal dust emission. We use existing data and our understanding of the physical behavior of the sources of foreground emission to generate sky templates, and start to assess how well primordial gravitational wave signals can be separated from foreground contaminants for a CMBPol mission. At the estimated foreground minimum of ∼100 GHz, the polarized foregrounds are expected to be lower than a primordial polarization signal with tensor‐to‐scalar ratio r = 0.01, in a small patch (∼1%) of the sky known to have low Galactic emission. Over 75% of the sky we expect the foreground amplitude to exceed the primordial signal by about a factor of eight at the foreground minimum and on scales of two degrees. Only on the largest scales does the polarized foreground amplitude exceed the primordial signal by a larger factor of about 20. The prospects for detecting an r = 0.01 signal including degree‐scale measurements appear promising, with 5σ_r∼0.003 forecast from multiple methods. A mission that observes a range of scales offers better prospects from the foregrounds perspective than one targeting only the lowest few multipoles. We begin to explore how optimizing the composition of frequency channels in the focal plane can maximize our ability to perform component separation, with a range of typically 40 ≲ ν ≲ 300 GHz preferred for ten channels. Foreground cleaning methods are already in place to tackle a CMBPol mission data set, and further investigation of the optimization and detectability of the primordial signal will be useful for mission design.

CONSTRAINING SPINNING DUST PARAMETERS WITH THE WMAP FIVE-YEAR DATA

We characterize spinning dust emission in the warm ionized medium (WIM) by comparing templates of Galactic dust and Hα with the five-year maps from the Wilkinson Microwave Anisotropy Probe (WMAP). The Hα-correlated microwave emission deviates from the thermal bremsstrahlung (free-free) spectrum expected for ionized gas, exhibiting an additional broad bump peaked at ~40 GHz which provides ~20% of the peak intensity. We confirm that the bump is consistent with a modified Draine and Lazarian spinning dust model, though the peak frequency of the emission is somewhat lower than the 50 GHz previously claimed. This frequency shift results from systematic errors in the large-scale modes of the three-year WMAP data which have been corrected in the five-year data release. We show that the bump is not the result of errors in the Hα template by analyzing regions of high free-free intensity, where the WMAP K-band map may be used as the free-free template. We rule out a pure free-free spectrum for the Hα-correlated emission at high confidence: ~27σ for the nearly full-sky fit, even after marginalizing over the cosmic microwave background cross-correlation bias. We also extend the previous analysis by searching the parameter space of the Draine and Lazarian model but letting the amplitude float. The best fit for reasonable values of the characteristic electric dipole moment and density requires an amplitude factor of ~0.3. This suggests that small polycyclic aromatic hydrocarbons in the WIM are depleted by a factor of ~3.

Foreground Science Knowledge and Prospects

Detecting “B‐mode” (i.e., divergence free) polarization in the Cosmic Microwave Background (CMB) would open a new window on the very early Universe. However, the polarized microwave sky is dominated by polarized Galactic dust and synchrotron emissions, which may hinder our ability to test inflationary predictions. In this paper, we report on our knowledge of these “Galactic foregrounds,” as well as on how a CMB satellite mission aiming at detecting a primordial B‐mode signal (“CMBPol”) will contribute to improving it. We review the observational and analysis techniques used to constrain the structure of the Galactic magnetic field, whose presence is responsible for the polarization of Galactic emissions. Although our current understanding of the magnetized interstellar medium is somewhat limited, dramatic improvements in our knowledge of its properties are expected by the time CMBPol flies. Thanks to high resolution and high sensitivity instruments observing the whole sky at frequencies between 30 GHz and 850 GHz, CMBPol will not only improve this picture by observing the synchrotron emission from our galaxy, but also help constrain dust models. Polarized emission form interstellar dust indeed dominates over any other signal in CMBol’s highest frequency channels. Observations at these wavelengths, combined with ground‐based studies of starlight polarization, will therefore enable us to improve our understanding of dust properties and of the mechanism(s) responsible for the alignment of dust grains with the Galactic magnetic field. CMBPol will also shed new light on observations that are presently not well understood. Morphological studies of anomalous dust and synchrotron emissions will indeed constrain their natures and properties, while searching for fluctuations in the emission from heliospheric dust will test our understanding of the circumheliospheric interstellar medium. Finally, acquiring more information on the properties of extra‐Galactic sources will be necessary in order to maximaize the cosmological constrainsts extracted from CMBPol’s observations of CMB lensing.

Microlensing of central images in strong gravitational lens systems

We study microlensing of the faint images that form close to the centres of strong gravitational lens galaxies. These central images, which have finally begun to yield to observations, naturally appear in dense stellar fields and may be particularly sensitive to fine granularity in the mass distribution. The microlensing magnification maps for overfocused (i.e. demagnified) images differ strikingly from those for magnified images. In particular, the familiar ‘fold’ and ‘cusp’ features of maps for magnified images are only present for certain values of the fraction f of the surface mass density contained in stars. For central images, the dispersion in microlensing magnifications is generally larger than for normal (minimum and saddle) images, especially when the source is comparable to or larger than the stellar Einstein radius. The dispersion depends in a complicated way on f; this behaviour may hold the key to using microlensing as a probe of the relative densities of stars and dark matter in the cores of distant galaxies. Quantitatively, we predict that the central image C in PMN J1632−0033 has a magnification dispersion of 0.6 mag for Rsrc/RE 1, or 0.3 mag for Rsrc/RE = 10. For comparison, the dispersions are 0.5‐0.6 mag for image B and 0.05‐0.1 mag for image A, if Rsrc/RE 1; and just 0.1 mag for B and 0.008 mag for A if Rsrc/RE = 10. (The dispersions can be extrapolated to larger sources sizes as σ ∝ R −1 src .) Thus, central images are more susceptible than other lensed images to microlensing and hence good probes for measuring source sizes.

Planck intermediate results IX

Using precise full-sky observations from Planck, and applying several methods of component separation, we identify and characterise the emission from the Galactic “haze” at microwave wavelengths. The haze is a distinct component of diffuse Galactic emission, roughly centered on the Galactic centre, and extends to | b | ~ 35−50° in Galactic latitude and | l | ~ 15−20° in longitude. By combining the Planck data with observations from the Wilkinson Microwave Anisotropy Probe, we were able to determine the spectrum of this emission to high accuracy, unhindered by the strong systematic biases present in previous analyses. The derived spectrum is consistent with power-law emission with a spectral index of −2.56 ± 0.05, thus excluding free-free emission as the source and instead favouring hard-spectrum synchrotron radiation from an electron population with a spectrum (number density per energy) dN/dE ∝ E-2.1. At Galactic latitudes | b | < 30°, the microwave haze morphology is consistent with that of the Fermi gamma-ray “haze” or “bubbles”, while at b ~ −50° we have identified an edge in the microwave haze that is spatially coincident with the edge in the gamma-ray bubbles. Taken together, this indicates that we have a multi-wavelength view of a distinct component of our Galaxy. Given both the very hard spectrum and the extended nature of the emission, it is highly unlikely that the haze electrons result from supernova shocks in the Galactic disk. Instead, a new astrophysical mechanism for cosmic-ray acceleration in the inner Galaxy is implied.