In the last 30 days

• Open/close The Nature of the Extended H I Gas around NGC 4449: The Dr. Jekyll/Mr. Hyde of Irregular Galaxies

  Deidre A. Hunter et al 1998 ApJ 495 L47 Tag this article

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  We present interferometric H I 21 cm line observations of the extended gas around the irregular galaxy NGC 4449 covering 67' on the sky at a resolution of ~1'. The main star-forming body of NGC 4449 is relatively normal for a Magellanic irregular galaxy, but the galaxy is unusual in that it has two counterrotating gas systems and H I that extends to 6 times the Holmberg radius. Our new, detailed H I maps of this extended gas show that most of the extended H I is located in large, highly structured, extended clouds and very long streamers. We compare NGC 4449 with other systems in the context of possible models for the origin of these structures, the most likely of which involves an interaction with another galaxy. Thus, NGC 4449 no longer fits the standard picture of an irregular galaxy quietly evolving in isolation.

• Open/close Temporal Changes in Sunspot Umbral Magnetic Fields and Temperatures

  M. J. Penn and W. Livingston 2006 ApJ 649 L45 Tag this article

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  We have observed high-resolution intensity spectra near the Fe I 1564.8 nm line at a single umbral point corresponding to the darkest position in over 900 sunspots from 1998 through 2005. From these data we determine that the maximum sunspot magnetic fields have been decreasing at about 52 G yr ^-1. The same data set shows a concurrent increase in the normalized umbral intensity from 0.60 to 0.75 (corresponding to a blackbody temperature rise from 5137 to 5719 K) and a decrease of more than 50% in the molecular OH line strength. The magnetic field and intensity changes observed over time in the sunspot umbrae from different spots behave in the same way as the magnetic field and intensity changes observed spatially across single sunspots.

• Open/close The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy

  Michael H. Siegel et al 2007 ApJ 667 L57 Tag this article

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  As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ~ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised using both isochrone and main-sequence fitting to ( m - M) ₀ = 17.27 and E( B - V) = 0.15. Preliminary assessment finds the M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the globular cluster. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H] = -0.4 to -0.6. We also clearly show, for the first time, a prominent, ~2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (~0.1-0.8 Gyr old), more metal-rich ([Fe/H] ~ 0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.

• Open/close A Fundamental Relation between Supermassive Black Holes and Their Host Galaxies

  Laura Ferrarese and David Merritt 2000 ApJ 539 L9 Tag this article

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  The masses of supermassive black holes correlate almost perfectly with the velocity dispersions of their host bulges, M _bh σ ^α, where α = 4.8 ± 0.5. The relation is much tighter than the relation between M _bh and bulge luminosity, with a scatter no larger than expected on the basis of measurement error alone. Black hole masses recently estimated by Magorrian et al. lie systematically above the M _bh-σ relation defined by more accurate mass estimates, some by as much as 2 orders of magnitude. The tightness of the M _bh-σ relation implies a strong link between black hole formation and the properties of the stellar bulge.

• Open/close Star Formation and Dust Obscuration at z 2: Galaxies at the Dawn of Downsizing

  M. Pannella et al 2009 ApJ 698 L116 Tag this article

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  We present first results of a study aimed to constrain the star formation rate (SFR) and dust content of galaxies at z 2. We use a sample of BzK-selected star-forming galaxies, drawn from the Cosmic Evolution Survey, to perform a stacking analysis of their 1.4 GHz radio continuum as a function of different stellar population properties, after cleaning the sample from contamination by active galactic nuclei. Dust unbiased SFRs are derived from radio fluxes assuming the local radio-IR correlation. The main results of this work are: (1) specific star formation rate (SSFR)s are constant over about 1 dex in stellar mass and up to the highest stellar mass probed, (2) the dust attenuation is a strong function of galaxy stellar mass with more massive galaxies being more obscured than lower mass objects, (3) a single value of the UV extinction applied to all galaxies would lead to a gross underestimate of the SFR in massive galaxies, (4) correcting the observed UV luminosities for dust attenuation based on the Calzetti recipe provides results in very good agreement with the radio derived ones, (5) the mean SSFR of our sample steadily decreases by a factor of ~4 with decreasing redshift from z = 2.3 to 1.4 and a factor of ~40 down the local universe. These empirical SFRs would cause galaxies to dramatically overgrow in mass if maintained all the way to low redshifts; we suggest that this does not happen because star formation is progressively quenched, likely starting from the most massive galaxies.

• Open/close Time Dilation and Quasar Variability

  M. R. S. Hawkins 2001 ApJ 553 L97 Tag this article

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  The timescale of quasar variability is widely expected to show the effects of time dilation. In this Letter, we analyze the Fourier power spectra of a large sample of quasar light curves to look for such an effect. We find that the timescale of quasar variation does not increase with redshift, as required by time dilation. Possible explanations of this result all conflict with widely held consensus in the scientific community.

• Open/close The Relation between Black Hole Mass, Bulge Mass, and Near-Infrared Luminosity

  Alessandro Marconi and Leslie K. Hunt 2003 ApJ 589 L21 Tag this article

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  We present new accurate near-infrared (NIR) spheroid (bulge) structural parameters obtained by a two-dimensional image analysis of all galaxies with a direct black hole (BH) mass determination. As expected, NIR bulge luminosities L _bul and BH masses are tightly correlated, and if we consider only those galaxies with a secure BH mass measurement and an accurate L _bul (27 objects), the spread of M _BH- L _bul is similar to M _BH-σ _e , where σ _e is the effective stellar velocity dispersion. We find an intrinsic rms scatter of 0.3 dex in log M _BH. By combining the bulge effective radii R _e measured in our analysis with σ _e , we find a tight linear correlation (rms 0.25 dex) between M _BH and the virial bulge mass ( R _e σ ), with M _BH/ M _bul ~ 0.002. A partial correlation analysis shows that M _BH depends on both σ _e and R _e and that both variables are necessary to drive the correlations between M _BH and other bulge properties.

• Open/close Star Formation in AEGIS Field Galaxies since z = 1.1: The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-forming Galaxies

  K. G. Noeske et al 2007 ApJ 660 L43 Tag this article

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  We analyze star formation (SF) as a function of stellar mass ( M _*) and redshift z in the All-Wavelength Extended Groth Strip International Survey. For 2905 field galaxies, complete to 10 ¹⁰(10 ^10.8) M at z < 0.7(1), with Keck spectroscopic redshifts out to z = 1.1, we compile SF rates (SFRs) from emission lines, GALEX, and Spitzer MIPS 24 μm photometry, optical-NIR M _* measurements, and HST morphologies. Galaxies with reliable signs of SF form a distinct "main sequence" (MS), with a limited range of SFRs at a given M _* and z (1 σ ±0.3 dex), and log(SFR) approximately proportional to log M _*. The range of log(SFR) remains constant to z > 1, while the MS as a whole moves to higher SFR as z increases. The range of the SFR along the MS constrains the amplitude of episodic variations of SF and the effect of mergers on the SFR. Typical galaxies spend ~67%(95%) of their lifetime since z = 1 within a factor of 2(4) of their average SFR at a given M _* and z. The dominant mode of the evolution of SF since z ~ 1 is apparently a gradual decline of the average SFR in most individual galaxies, not a decreasing frequency of starburst episodes, or a decreasing factor by which SFRs are enhanced in starbursts. LIRGs at z ~ 1 seem to mostly reflect the high SFR typical for massive galaxies at that epoch. The smooth MS may reflect that the same set of few physical processes governs SF prior to additional quenching processes. A gradual process like gas exhaustion may play a dominant role.

• Open/close Smoothed Particle Hydrodynamics Simulations of Apsidal and Nodal Superhumps

  Matt A. Wood et al 2000 ApJ 535 L39 Tag this article

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  In recent years, a handful of systems have been observed to show "negative" (nodal) superhumps, with periods slightly shorter than the orbital period. It has been suggested that these modes are a consequence of the slow retrograde precession of the line of nodes in a disk tilted with respect to the orbital plane. Our simulations confirm and refine this model: they suggest a roughly axisymmetric, retrogradely precessing, tilted disk that is driven at a period slightly less than half the orbital period as the tidal field of the orbiting secondary encounters, in turn, the two halves of the disk above and below the midplane. Each of these passings leads to viscous dissipation on one face of an optically thick disk—observers on opposite sides of the disk would each observe one brightening per orbit, but 180° out of phase with each other.

• Open/close Fermi Observations of GRB 090902B: A Distinct Spectral Component in the Prompt and Delayed Emission

  A. A. Abdo et al 2009 ApJ 706 L138 Tag this article

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  We report on the observation of the bright, long gamma-ray burst (GRB), GRB 090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on-board the Fermi observatory. This was one of the brightest GRBs to have been observed by the LAT, which detected several hundred photons during the prompt phase. With a redshift of z = 1.822, this burst is among the most luminous detected by Fermi. Time-resolved spectral analysis reveals a significant power-law component in the LAT data that is distinct from the usual Band model emission that is seen in the sub-MeV energy range. This power-law component appears to extrapolate from the GeV range to the lowest energies and is more intense than the Band component, both below ~50 keV and above 100 MeV. The Band component undergoes substantial spectral evolution over the entire course of the burst, while the photon index of the power-law component remains constant for most of the prompt phase, then hardens significantly toward the end. After the prompt phase, power-law emission persists in the LAT data as late as 1 ks post-trigger, with its flux declining as t ^–1.5. The LAT detected a photon with the highest energy so far measured from a GRB, 33.4 ^+2.7 _–3.5 GeV. This event arrived 82 s after the GBM trigger and ~50 s after the prompt phase emission had ended in the GBM band. We discuss the implications of these results for models of GRB emission and for constraints on models of the extragalactic background light.