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G. Morrison

Publications and source records attributed to G. Morrison.

35 records · Page 2Linked to original sources
arXiv2008

An AzTEC 1.1 mm Survey of the GOODS-N Field I: Maps, Catalogue, and Source Statistics

We have conducted a deep and uniform 1.1 mm survey of the GOODS-N field with AzTEC on the James Clerk Maxwell Telescope (JCMT). Here we present the first results from this survey including maps, the source catalogue, and 1.1 mm number-counts. The results presented here were obtained from a 245 sq-arcmin region with near uniform coverage to a depth of 0.96-1.16 mJy/beam. Our robust catalogue contains 28 source candidates detected with S/N >= 3.75, only 1-2 of which are expected to be spurious detections. Of these source candidates, 8 are also detected by SCUBA at 850 um in regions where there is good overlap between the two surveys. The major advantage of our survey over that with SCUBA is the uniformity of coverage. We calculate number counts using two different techniques: the first using a frequentist parameter estimation, and the second using a Bayesian method. The two sets of results are in good agreement. We find that the 1.1 mm differential number counts are well described in the 2-6 mJy range by the functional form dN/dS = N' (S'/S) exp(-S/S') with fitted parameters S' = 1.25 +/-0.38 mJy and dN/dS = 300 +/- 90 per mJy per sq-deg at 3 mJy.

astro-ph↗
arXiv2008

The role of internal chain dynamics on the rupture kinetics of adhesive contacts

We study the forced rupture of adhesive contacts between monomers that are not covalently linked in a Rouse chain. When the applied force ($f$) to the chain end is less than the critical force for rupture ($f_c$), the {\it reversible} rupture process is coupled to the internal Rouse modes. If $f/f_{c}$$>$1 the rupture is {\it irreversible}. In both limits, the non-exponential distribution of contact lifetimes, which depends sensitively on the location of the contact, follows the double-exponential (Gumbel) distribution. When two contacts are well separated along the chain, the rate limiting step in the {\it sequential} rupture kinetics is the disruption of the contact that is in the chain interior. If the two contacts are close to each other, they cooperate to sustain the stress, which results in an ``all-or-none'' transition.

cond-mat.soft↗
arXiv2007

Interferometric detections of GOODS 850-5 at 1 mm and 1.4 GHz

We have obtained a position (at sub-arcsecond accuracy) of the submillimeter bright source GOODS 850-5 (also known as GN10) in the GOODS North field using the IRAM Plateau de Bure interferometer at 1.25 mm wavelengths (MM J123633+6214.1, flux density: S(1.25 mm)=5.0+-1.0 mJy). This source has no optical counterpart in deep ACS imaging down to a limiting magnitude of i(775)=28.4 mag and its position is coincident with the position found in recent sub-millimeter mapping obtained at the SMA (Wang et al. 2007). Using deep VLA imaging at 20 cm, we find a radio source (S(20 cm)=32.7+-4.3 microJy) at the same position that is significantly brighter than reported in Wang et al. The source is detected by Spitzer in IRAC as well as at 24 microns. We apply different photometric redshift estimators using measurements of the dusty, mid/far-infrared part of the SED and derive a redshift z~4. Given our detection in the millimeter and radio we consider a significantly higher redshift (e.g., z~6 Wang et al. 2007) unlikely. MM J123633+6214.1 alias GOODS 850-5 nevertheless constitutes a bright representative of the high-redshift tail of the submillimeter galaxy population that may contribute a significant fraction to the (sub)millimeter background.

astro-ph↗
arXiv2007

Vigorous star formation with low efficiency in massive disk galaxies at z=1.5

We present the first detection of molecular gas cooling CO emission lines from ordinary massive galaxies at z=1.5. Two sources were observed with the IRAM Plateau de Bure Interferometer, selected to lie in the mass-star formation rate correlation at their redshift, thus being representative of massive high-z galaxies. Both sources were detected with high confidence, yielding L'_CO~10^{10}K km/s pc^2. For one of the sources we find evidence for velocity shear, implying CO sizes of ~10 kpc. With an infrared luminosity of L_FIR~10^{12}L_sun, these disk-like galaxies are borderline ULIRGs but with star formation efficiency similar to that of local spirals, and an order of magnitude lower than that in submm galaxies. This suggests a CO to total gas conversion factor similar to local spirals, gas consumption timescales approaching 1 Gyr or longer and molecular gas masses reaching ~10^11 M_sun, comparable to or larger than the estimated stellar masses. These results support a major role of 'in situ' gas consumption over cosmological timescales and with relatively low star formation efficiency, analogous to that of local spiral disks, for the formation of today's most massive galaxies and their central black holes. Given the high space density of similar galaxies, ~10^{-4}/Mpc^3, this implies a widespread presence of gas rich galaxies in the early Universe, many of which might be within reach of detailed investigations of current and planned facilities.

astro-ph↗
arXiv2007

Multiwavelength study of massive galaxies at z~2. I. Star formation and galaxy growth

Examining a sample of massive galaxies at 1.4 10^{11}L_sun, show a mid-IR excess which is likely due to the presence of obscured active nuclei, as shown in a companion paper. There is a tight and roughly linear correlation between stellar mass and SFR for 24um-detected galaxies. For a given mass, the SFR at z=2 was larger by a factor of ~4 and ~30 relative to that in star forming galaxies at z=1 and z=0, respectively. Typical ultraluminous infrared galaxies (ULIRGs) at z=2 are relatively 'transparent' to ultraviolet light, and their activity is long lived (~400 Myr), unlike that in local ULIRGs and high redshift submillimeter-selected galaxies. ULIRGs are the common mode of star formation in massive galaxies at z=2, and the high duty cycle suggests that major mergers are not the dominant trigger for this activity.Current galaxy formation models underpredict the normalization of the mass-SFR correlation by about a factor of 4, and the space density of ULIRGs by an orderof magnitude, but give better agreement for z>1.4 quiescent galaxies.

astro-ph↗
arXiv2007

Multiwavelength study of massive galaxies at z~2. II. Widespread Compton thick AGN and the concurrent growth of black holes and bulges

Approximately 20-30% of 1.4 6.2 keV. The stacked X-ray spectrum rises steeply at >10 keV, suggesting that these sources host Compton-thick Active Galactic Nuclei (AGNs) with column densities N_H~10^{24} cm^-2 and an average, unobscured X-ray luminosity L_{2-8 keV}~(1-4)x10^43 erg/s. Their sky density(~3200 deg^-2) and space density (~2.6x10^-4 Mpc^-3) are twice those of X-ray detected AGNs at z~2, and much larger than those of previously-known Compton thick sources at similar redshifts. The mid-IR excess galaxies are part of the long sought-after population of distant heavily obscured AGNs predicted by synthesis models of the X-ray background. The fraction of mid-IR excess objects increases with galaxy mass, reaching ~50-60% for M~10^11 M_sun, an effect likely connected with downsizing in galaxy formation. The ratio of theinferred black hole growth rate from these Compton-thick sources to the global star formation rate at z=2 is similar to the mass ratio of black holes to stars inlocal spheroids, implying concurrent growth of both within the precursors oftoday's massive galaxies.

astro-ph↗
arXiv2006

The GOODS-North Radio Galaxies: On the Origin of the Radio Emission

We report on a preliminary study concerning the origin of radio emission within radio galaxies at L(1.4GHz)>1E24 W/Hz in the GOODS-N field. In the local universe, Condon et al. (2002) and Yun et al. (2001) have shown that in galaxies with radio luminosities greater than 1E23 W/Hz the majority of the radio emission originates from a `monster' i.e., an AGN. Using the Chandra 2Msec X-ray image centered on the GOODS-N field and a reprocessed VLA HDF A-array data plus newly acquired VLA B-array data (rms=5.3microJy), we find that radio galaxies (with spectroscopic redshifts; all have z>1) with L(1.4GHz)>1E24 W/Hz typically have an X-ray detection rate of 72% (60% emit hard X-rays suggesting an AGN origin for the radio emission) in contrast to 25% for radio galaxies with L < 1E23 W/Hz. The ACS images of these L(1.4 GHz) > 1E24 W/Hz galaxies typically show compact rather than extended galaxy morphology which is generally found for the less luminous radio emitting galaxies but a few appear to be ongoing galaxy mergers. We also present SED fitting for these luminous radio galaxies including Spitzer IRAC & MIPS 24um photometry and 60% show distinct power-law SED indicative of an AGN. Initial results tell us that the X-ray emitting radio galaxy population are generally not submm sources but the few (~10%) that are SCUBA sources appear to be the small AGN population found by Pope et al. and others.

astro-ph↗
arXiv2006

Chandra and Spitzer unveil heavily obscured quasars in the SWIRE/Chandra Survey

Using the large multi-wavelength data set in the chandra/SWIRE Survey (0.6 square degrees in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGN, estimate a lower limit to their surface density and characterize their multi-wavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain 1) 5 X-ray sources with hard X-ray spectra and column densities > 10^24 cm-2, and 2) 120 infrared sources with red and AGN-dominated infrared spectral energy distributions (SEDs). We estimate a surface density of at least 25 Compton-thick AGN per square degree detected in the infrared in the chandra/SWIRE field of which ~40% show distinct AGN signatures in their optical/near-infrared SEDs, the remainings being dominated by the host-galaxy emission. Only ~33% of all Compton-thick AGN are detected in the X-rays at our depth (F(0.3-8 keV)>10^-15 erg/cm2/s. We report the discovery of two sources in our sample of Compton-thick AGN, SWIRE_J104409.95+585224.8 (z=2.54) and SWIRE_J104406.30+583954.1 (z=2.43), which are the most luminous Compton-thick AGN at high-z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities and black-hole masses.

astro-ph↗
arXiv2006

Mid infrared properties of distant infrared luminous galaxies

We present evidence that the mid infrared (MIR) is a good tracer of the total infrared luminosity, L(IR), and star formation rate (SFR), of galaxies up to z 1.3. We use deep MIR images from the Infrared Space Observatory (ISO) and the Spitzer Space Telescope in the Northern field of the Great Observatories Origins Deep Survey (GOODS-N) together with VLA radio data to compute three independant estimates of L(IR). The L(IR,MIR) derived from the observed 15 and/or 24 um flux densities using a library of template SEDs, and L(IR,radio), derived from the radio (1.4 and/or 8.5 GHz) using the radio-far infrared correlation, agree with a 1-sigma dispersion of 40 %. We use the k-correction as a tool to probe different parts of the MIR spectral energy distribution (SED) of galaxies as a function of their redshift and find that on average distant galaxies present MIR SEDs very similar to local ones. However, in the redshift range z= 0.4-1.2, L(IR,24um) is in better agreement with L(IR,radio) than L(IR,15 um) by 20 %, suggesting that the warm dust continuum is a better tracer of the SFR than the broad emission features due to polycyclic aromatic hydrocarbons (PAHs). We find marginal evidence for an evolution with redshift of the MIR SEDs: two thirds of the distant galaxies exhibit rest-frame MIR colors (L(12 um)/L(7 um) and L(10 um)/L(15 um) luminosity ratios) below the median value measured for local galaxies. Possible explanations are examined but these results are not sufficient to constrain the physics of the emitting regions. We compare three commonly used SED libraries which reproduce the color-luminosity correlations of local galaxies with our data and discuss possible refinements to the relative intensities of PAHs, warm dust continuum and silicate absorption.

astro-ph↗
arXiv2005

The population of BzK selected ULIRGs at z~2

We investigate the multi-wavelength emission of BzK selected star forming galaxies at z~2 in the Great Observatories Origins Deep Survey (GOODS) North region. Most (82%) of the sources are individually detected at 24um in the Spitzer MIPS imaging, and one fourth (26%) in the VLA radio data. Significant detections of the individually undetected objects are obtained through stacking in the radio, submm and X-ray domains. The typical star forming galaxy with stellar mass ~10^{11}Mo at z=2 is an Ultra-luminous Infrared Galaxy (ULIRG), with L_IR ~ 1-2x10^{12}Lo and star formation rate SFR 200-300Mo/yr, implying a comoving density of ULIRGs at z=2 at least 3 orders of magnitude above the local one. SFRs derived from the reddening corrected UV luminosities agree well, on average, with the longer wavelength estimates. The high 24um detection rate suggests a relatively large duty cycle for the BzK star forming phase, consistently with the available independent measurements of the space density of passively evolving galaxies at z>1.4. If the IMF at z=2 is similar to the local one, and in particular is not a top-heavy IMF, this suggests that a substantial fraction of the high mass tail (>10^{11}Mo) of the galaxy stellar mass function was completed by z~1.4.

astro-ph↗
arXiv2005

The shape of a flexible polymer in a cylindrical pore

We calculate the mean end-to-end distance ($R$) of a self-avoiding polymer encapsulated in an infinitely long cylinder with radius $D$. A self-consistent perturbation theory is used to calculate $R$ as a function of $D$ for impenetrable hard walls and soft walls. In both cases, $R$ obeys the predicted scaling behavior in the limit of large and small $D$. The crossover from the three dimensional behavior ($D\to\infty$) to the fully stretched one dimensional case ($D\to 0$) is non-monotonic. The minimum value of $R$ is found at $D\sim 0.46 R_F$, where $R_F$ is the Flory radius of $R$ at $D \to \infty$. The results for soft walls map onto the hard wall case with a larger cylinder radius.

cond-mat.soft↗
arXiv2005

The 24 micron Luminosity Function of spectroscopic SWIRE sources from the Lockman validation field

A spectroscopic follow-up of SWIRE sources from the Lockman Validation Field has allowed the determination of the SWIRE 24 micron Luminosity Function (LF). The spectroscopic sample was chosen above a 24 micron flux limit at 260 microJy and an r-band optical limit of r<21. A spectroscopic completeness of 82.5% was achieved. We found the median redshift for the sample to be z_med=0.29. Markov-Chain Monte-Carlo (MCMC) and Maximum Likelihood Estimator (MLE) techniques were employed to fit a parametric LF. Our result of the local LF (LLF) is consistent with the local 25 micron determination from Shupe et al. 1998. We split the sample at a redshift of z_split=0.36 and find strong evidence for galaxy evolution.

astro-ph↗
arXiv2004

Spectral energy distributions and luminosities of galaxies and AGN in the SPITZER SWIRE Legacy Survey

We discuss optical associations, spectral energy distributions and photometric redshifts for SWIRE sources in the ELAIS-N1 area and the Lockman Validation Field. The band-merged IRAC (3.6, 4.5, 5.8 and 8.0 mu) and MIPS (24, 70, 160 mu) data have been associated with optical UgriZ data from the INT Wide Field Survey in ELAIS-N1, and with our own optical Ugri data in Lockman-VF. The spectral energy distributions of selected ELAIS sources in N1 detected by SWIRE, most with spectroscopic redshifts, are modelled in terms of a simple set of galaxy and quasar templates in the optical and near infrared, and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and AGN dust torus) in the mid infrared. The optical data, together with the IRAC 3.6 and 4.5 mu data, have been used to determine photometric redshifts. For galaxies with known spectroscopic redshifts there is a notable improvement in the photometric redshift when the IRAC data are used, with a reduction in the rms scatter from 10% in (1+z) to 7%. The photometric redshifts are used to derive the 3.6 and 24 mu redshift distribution and to compare this with the predictions of models. For those sources with a clear mid infrared excess, relative to the galaxy starlight model used for the optical and near infrared, the mid and far infrared data are modelled in terms of the same dust emission templates. The proportions found of each template type are: cirrus 31%, M82 29%, Arp 220 10%, AGN dust tori 29%. The distribution of the different infrared sed types in the L_{ir}/L_{opt} versus L_{ir} plane, where L_{ir} and L_{opt} are the infrared and optical bolometric luminosities, is discussed.

astro-ph↗
arXiv2001

Afterglow upper limits for four short duration, hard spectrum gamma-ray bursts

We present interplanetary network localization, spectral, and time history information for four short-duration, hard spectrum gamma-ray bursts, GRB000607, 001025B, 001204, and 010119. All of these events were followed up with sensitive radio and optical observations (the first and only such bursts to be followed up in the radio to date), but no detections were made, demonstrating that the short bursts do not have anomalously intense afterglows. We discuss the upper limits, and show that the lack of observable counterparts is consistent both with the hypothesis that the afterglow behavior of the short bursts is like that of the long duration bursts, many of which similarly have no detectable afterglows, as well as with the hypothesis that the short bursts have no detectable afterglows at all. Small number statistics do not allow a clear choice between these alternatives, but given the present detection rates of various missions, we show that progress can be expected in the near future.

astro-ph↗
arXiv2001

The Unusually Long Duration Gamma-ray Burst GRB 000911

Of all the well localized gamma-ray bursts, GRB 000911 has the longest duration (T_90 ~ 500 s), and ranks in the top 1% of BATSE bursts for fluence. Here, we report the discovery of the afterglow of this unique burst. In order to simultaneously fit our radio and optical observations, we are required to invoke a model involving an hard electron distribution, p ~ 1.5 and a jet-break time less than 1.5 day. A spectrum of the host galaxy taken 111 days after the burst reveals a single emission line, interpreted as [OII] at a redshift z = 1.0585, and a continuum break which we interpret as the Balmer limit at this redshift. Despite the long T_90, the afterglow of GRB 000911 is not unusual in any other way when compared to the set of afterglows studied to date. We conclude that the duration of the GRB plays little part in determining the physics of the afterglow.

astro-ph↗
arXiv2001

Multi-Color Observations of the GRB000926 Afterglow

We present multi-color light-curves of the optical afterglow of GRB 000926. Beginning ~1.5 days after the burst, the light-curves of this GRB steepen measurably. The existence of such achromatic breaks are usually taken to be an important observational signature that the ejecta are not expanding isotropically, but rather have a collimated jet-like geometry. If we interpret the data in this context, we derive an opening angle of 5 degrees, which reduces the energy release compared to an isotropic model by a factor of ~275, to 1.7 x 10^51 erg. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11 - 0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host.

astro-ph↗