These are the latest articles published in The Astrophysical Journal.

• Open/close Clustering of Star-forming Galaxies Detected in Mid-infrared with the Spitzer Wide-area Survey

  S. Starikova et al. 2012 ApJ 751 126 Tag this article

  Article References Full text PDF (1.53 MB) View as HTML

  We discuss the clustering properties of galaxies with signs of ongoing star formation detected by the Spitzer Space Telescope at the 24 μm band in the SWIRE Lockman Hole field. The sample of mid-IR-selected galaxies includes ~20,000 objects detected above a flux threshold of S _{24 μm} = 310 μJy. We adopt optical/near-IR color selection criteria to split the sample into the lower-redshift and higher-redshift galaxy populations. We measure the angular correlation function on scales of θ = 0 01-3 5 , from which, using the Limber inversion along with the redshift distribution established for similarly selected source populations in the GOODS fields (Rodighiero et al.), we obtain comoving correlation lengths of r ₀ = 4.98 ± 0.28 h ^–1 Mpc and r ₀ = 8.04 ± 0.69 h ^–1 Mpc for the low- z ( z _mean = 0.7) and high- z ( z _mean = 1.7) subsamples, respectively. Comparing these measurements with the correlation functions of dark matter (DM) halos identified in the Bolshoi cosmological simulation, we find that the high-redshift objects reside in progressively more massive halos reaching M _tot 3 × 10 ¹² h ^–1 M _☉, compared to M _tot 7 × 10 ¹¹ h ^–1 M _☉ for the low-redshift population. Approximate estimates of the IR luminosities based on the catalogs of 24 μm sources in the GOODS fields show that our high- z subsample represents a population of "distant ULIRGs" with L _IR > 10 ¹² L _☉, while the low- z subsample mainly consists of "LIRGs," L _IR ~ 10 ¹¹ L _☉. The comparison of number density of the 24 μm selected galaxies and of DM halos with derived minimum mass M _tot shows that only 20% of such halos may host star-forming galaxies.

• Open/close Observational Constraints on the Molecular Gas Content in Nearby Starburst Dwarf Galaxies

  Kristen B. W. McQuinn et al. 2012 ApJ 751 127 Tag this article

  Article References Full text PDF (1.48 MB) View as HTML

  Using star formation histories derived from optically resolved stellar populations in 19 nearby starburst dwarf galaxies observed with the Hubble Space Telescope, we measure the stellar mass surface densities of stars newly formed in the bursts. By assuming a star formation efficiency (SFE), we then calculate the inferred gas surface densities present at the onset of the starbursts. Assuming an SFE of 1%, as is often assumed in normal star-forming galaxies, and assuming that the gas was purely atomic, translates to very high H I surface densities (~10 ²-10 ³ M _☉ pc ^–2), which are much higher than have been observed in dwarf galaxies. This implies either higher values of SFE in these dwarf starburst galaxies or the presence of significant amounts of H ₂ in dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with observations of more massive starbursts associated with merging galaxies), still results in H I surface densities higher than observed in 10 galaxies. Thus, these observations appear to require that a significant fraction of the gas in these dwarf starbursts galaxies was in the molecular form at the onset of the bursts. Our results imply molecular gas column densities in the range 10 ¹⁹-10 ²¹ cm ^–2 for the sample. In the galaxies where CO observations have been made, these densities correspond to values of the CO–H ₂ conversion factor ( X _CO) in the range >(3-80) × 10 ²⁰ cm ^–2 (K km s ^–1) ^–1, or up to 40 × greater than Galactic X _CO values.

• Open/close Three-dimensional Evolution of Solar Wind during Solar Cycles 22–24

  P. K. Manoharan 2012 ApJ 751 128 Tag this article

  Article References Full text PDF (1.28 MB) View as HTML

  This paper presents an analysis of three-dimensional evolution of solar wind density turbulence and speed at various levels of solar activity between solar cycles 22 and 24. The solar wind data used in this study have been obtained from the interplanetary scintillation (IPS) measurements made at the Ooty Radio Telescope, operating at 327 MHz. Results show that (1) on average, there was a downward trend in density turbulence from the maximum of cycle 22 to the deep minimum phase of cycle 23; (2) the scattering diameter of the corona around the Sun shrunk steadily toward the Sun, starting from 2003 to the smallest size at the deepest minimum, and it corresponded to a reduction of ~50% in the density turbulence between the maximum and minimum phases of cycle 23; (3) the latitudinal distribution of the solar wind speed was significantly different between the minima of cycles 22 and 23. At the minimum phase of solar cycle 22, when the underlying solar magnetic field was simple and nearly dipole in nature, the high-speed streams were observed from the poles to ~30° latitudes in both hemispheres. In contrast, in the long-decay phase of cycle 23, the sources of the high-speed wind at both poles, in accordance with the weak polar fields, occupied narrow latitude belts from poles to ~60° latitudes. Moreover, in agreement with the large amplitude of the heliospheric current sheet, the low-speed wind prevailed in the low- and mid-latitude regions of the heliosphere. (4) At the transition phase between cycles 23 and 24, the high levels of density and density turbulence were observed close to the heliospheric equator and the low-speed solar wind extended from the equatorial-to-mid-latitude regions. The above results in comparison with Ulysses and other in situ measurements suggest that the source of the solar wind has changed globally, with the important implication that the supply of mass and energy from the Sun to the interplanetary space has been significantly reduced in the prolonged period of low solar activity. The IPS results are consistent with the onset and growth of the current solar cycle 24, starting from the middle of 2009. However, the width of the high-speed wind at the northern high latitudes has almost disappeared and indicates that the ascending phase of the current cycle has almost reached the maximum phase in the northern hemisphere of the Sun. However, in the southern part of the hemisphere, the solar activity has yet to develop and/or increase.

• Open/close The Kinematics and Chemistry of Red Horizontal Branch Stars in the Sagittarius Streams

  W. B. Shi et al. 2012 ApJ 751 130 Tag this article

  Article References Full text PDF (7.91 MB) HTML with Enhancements

  We have selected 556 red horizontal branch stars along the streams of the Sagittarius (Sgr) dwarf galaxy from Sloan Digital Sky Survey DR7 spectroscopic data using a theoretical model. The metallicity and α-element distributions are investigated for stars in the Sgr streams and for Galactic stars at the same locations. We find that the Sgr stars have two peaks in the metallicity distribution while the Galactic stars have a more prominent metal-poor peak. Meanwhile, [α/Fe] ratios of the Sgr stars are lower than those of the Galactic stars. Among the Sgr stars, we find a difference in the metallicity distribution between the leading and trailing arms of the Sgr tidal tails. The metallicity and [α/Fe] distribution of the leading arm is similar to that of the Galaxy. The trailing arm is composed mainly of a metal-rich component and [α/Fe] is obviously lower than that of the Galactic stars. The metallicity gradient is –(1.8 ± 0.3)× 10 ^–3 dex deg ^–1 in the first wrap of the trailing arm and –(1.5 ± 0.4)× 10 ^–3 dex deg ^–1 in the first wrap of the leading arm. No significant gradient exists along the second wraps of the leading or trailing arms. It seems that the Sgr dwarf galaxy initially lost the metal-poor component in the second wrap (older) arms due to the tidal force of our Galaxy and then the metal-rich component is disrupted in the first wrap (younger) arms. Finally, we found that the velocity dispersion of the trailing arm from 88° < Λ _☉ < 112° is σ = 9.808 ± 1.0 km s ^–1, which is consistent with previous work in the literature.

• Open/close Empirical Determination of the Energy Loss Rate of Accelerated Electrons in a Well-observed Solar Flare

  Gabriele Torre et al. 2012 ApJ 751 129 Tag this article

  Article References Full text PDF (1.35 MB) View as HTML

  We present electron images of an extended solar flare source, deduced from RHESSI hard X-ray imaging spectroscopy data. We apply the electron continuity equation to these maps in order to determine empirically the form of the energy loss rate for the bremsstrahlung-emitting electrons. We show that this form is consistent with an energy transport model involving Coulomb collisions in a target with a temperature of about 2 × 10 ⁷ K, with a continuous injection of fresh deka-keV electrons at a rate of approximately 10 ^–2 electrons s ^–1 per ambient electron.