arXiv subjects

I. M. Hook

Publications and source records attributed to I. M. Hook.

At least 127 records · Page 7Linked to original sources

arXiv2004

A survey for low luminosity quasars at redshift z~5

We present the results of a multi-colour (VIZ) survey for low luminosity (M_B<-23.5) quasars with z~5 using the 12K CCD mosaic camera on CFHT. The survey covers 1.8deg^2 to a limiting magnitude of m_z=22.5(Vega), about two magnitudes fainter than the SDSS quasar survey. 20 candidates were selected by their VIZ colours and spectra for 15 of these were obtained with GMOS on the Gemini North telescope. A single quasar with z=4.99 was recovered, the remaining candidates are all M stars. The detection of only a single quasar in the redshift range accessible to the survey (4.8<5.2) is indicative of a possible turn over in the luminosity function at faint quasar magnitudes, and a departure from the form observed at higher luminosities (in agreement with quasar lensing observations by Richards etal (2003)). However, the derived space densitys, of quasars more luminous than M_B(Vega)<-23.5, of 2.96x10^-7 Mpc^-3 is consistent at the 65% confidence level with extrapolation of the quasar luminosity function as derived by Fan etal (2001a) at m_i<19.6(Vega).

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arXiv2003

The Gemini Deep Deep Survey: II. Metals in Star-Forming Galaxies at Redshift 1.3<z<2

The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample of K<20.6 galaxies in the redshift range 0.8 1.3. The selected objects have colors typical of irregular and Sbc galaxies. Strong [OII] emission indicates high star formation activity in the HII regions (SFR~13-106 M_sun/yr). The high S/N composite spectrum shows strong ISM MgII and FeII absorption, together with weak MnII and MgI lines. The FeII column density, derived using the curve of growth analysis, is logN_FeII = 15.54^{+0.23}_{-0.13}. This is considerably larger than typical values found in damped Ly-alpha systems (DLAs) along QSO sight lines, where only 10 out of 87 (~11%) have logN_FeII > 15.2. High FeII column densities are observed in the z=2.72 Lyman break galaxy cB58 (logN_FeII ~ 15.25) and in gamma-ray burst host galaxies (logN_FeII ~ 14.8-15.9). Given our measured FeII column density and assuming a moderate iron dust depletion (delta_Fe ~ 1 dex), we derive an optical dust extinction A_V ~ 0.6. If the HI column density is log N(HI)<21.7 (as in 98% of DLAs), then the mean metallicity is Z/Z_sun > 0.2. The high completeness of the GDDS sample implies that these results are typical of star-forming galaxies in the 1<z<2 redshift range, an epoch which has heretofore been particularly challenging for observational programs.

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arXiv2002

The Parkes quarter-Jansky flat-spectrum sample 2. New Optical Spectra and Redshift Measurements

We present optical spectra and redshift measurements for 178 flat-spectrum objects from the Parkes quarter-Jansky flat-spectrum sample. These spectra were obtained in order to compile a complete sample of quasars for use in a study of quasar evolution. We present a composite optical spectrum made from the subset of 109 quasars that have flux densities in the range 0.25Jy < S(2.7GHz)< 0.5Jy, and make a comparison with a composite for radio-quiet QSOs from the Large Bright Quasar Survey. Our large sample of radio-loud quasars allows us to strengthen previous reports that the Ly-alpha and CIV emission lines have larger equivalent width in radio-loud quasars than radio-quiet QSOs to greater than the 3-sigma level. However we see no significant difference in the equivalent widths of CIII] or MgII. We also show that the flux decrements across the Lyman-alpha line (D_A) measured from these spectra show the same trend with redshift as for optically selected QSOs.

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arXiv2002

The Type Ia Supernova 1999aw: a Probable 1999aa-like Event In a Low-Luminosity Host Galaxy

SN 1999aw was discovered during the first campaign of the Nearby Galaxies Supernova Search (NGSS) project. This luminous, slow-declining (Delta m_{15} (B) = 0.81 \pm 0.03) Type Ia supernova was noteworthy in at least two respects. First, it occurred in an extremely low luminosity host galaxy that was not visible in the template images, nor in initial subsequent deep imaging. Secondly, the photometric and spectral properties of this supernova indicate that it very likely was similar to the subclass of Type Ia supernovae whose prototype is SN 1999aa. This paper presents the BVRI and J_{s} HK_{s} lightcurves of SN 1999aw (through 100 days past maximum light), as well as several epochs of optical spectra. From these data we calculate the bolometric light curve, and give estimates of the luminosity at maximum light and the initial ^{56}Ni mass. In addition, we present deep BVI images obtained recently with the Baade 6.5-meter telescope at Las Campanas Observatory which reveal the remarkably low-luminosity host galaxy.

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arXiv2002

Discovery of Radio-loud Quasars with Redshifts above 4 from the PMN sample

We present results of a new, large survey for high-redshift radio-loud quasars, which targets quasars with z>4. The survey is based on the PMN and NVSS radio surveys, optically identified using digitised UKST B, R and I plates. Six new z>4 flat-spectrum QSOs have been discovered, and one previously known z>4 QSO rediscovered, based on their red optical colours. The QSOs discovered in this survey are bright in both radio and optical bands; in particular PMN J1451-1512 (z=4.763, I=17.3, R=19.1) and PMN J0324-2918 (z=4.630, R=18.7) are very luminous. PMN J1451-1512 at z=4.763 is also now the most distant radio-selected quasar. In addition, 9 new quasars with 3.5 4 QSOs with S>= 72mJy and R< 21mag.

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arXiv2002

The distant Type Ia supernova rate

We present a measurement of the rate of distant Type Ia supernovae derived using 4 large subsets of data from the Supernova Cosmology Project. Within this fiducial sample, which surveyed about 12 square degrees, thirty-eight supernovae were detected at redshifts 0.25--0.85. In a spatially-flat cosmological model consistent with the results obtained by the Supernova Cosmology Project, we derive a rest-frame Type Ia supernova rate at a mean redshift $z\simeq0.55$ of $1.53 {^{+0.28}_{-0.25}} {^{+0.32}_{-0.31}} 10^{-4} h^3 {\rm Mpc}^{-3} {\rm yr}^{-1}$ or $0.58 {^{+0.10}_{-0.09}} {^{+0.10}_{-0.09}} h^2 {\rm SNu}$ (1 SNu = 1 supernova per century per $10^{10}$\Lbsun), where the first uncertainty is statistical and the second includes systematic effects. The dependence of the rate on the assumed cosmological parameters is studied and the redshift dependence of the rate per unit comoving volume is contrasted with local estimates in the context of possible cosmic star formation histories and progenitor models.

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arXiv2000

PMN J0525-3343: soft X-ray spectral flattening in a blazar at z=4.4

We report optical, radio and X-ray observations of a new distant blazar, PMN J0525-3343, at a redshift of 4.4. The X-ray spectrum measured from ASCA and BeppoSAX flattens below a few keV, in a manner similar to the spectra of two other z>4 blazars, GB 1428+4217 (z=4.72) reported by Boller et al and RXJ 1028.6-0844 (z=4.28) by Yuan et al. The spectrum is well fitted by a power-law continuum which is either absorbed or breaks at a few keV. An intrinsic column density corresponding to 2 x 10^23 H-atoms cm-2 at solar abundance is required by the absorption model. This is however a million times greater than the neutral hydrogen, or dust, column density implied by the optical spectrum, which covers the rest-frame UV emission of the blazar nucleus. We discuss the problems raised and suggest that, unless there is intrinsic flattening in the spectral distribution of the particles/seed photons producing X-rays via inverse Compton scattering, the most plausible solution is a warm absorber close to the active nucleus.

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arXiv1999

The unusual afterglow of GRB 980326: evidence for the gamma-ray burst/supernova connection

Cosmic gamma-ray bursts (GRBs) have been firmly established as one of the most powerful phenomena in the Universe, releasing electromagnetic energy approaching the rest-mass energy of a neutron star in a few seconds. The two currently popular models for GRB progenitors are the coalescence of two compact objects (such as neutron stars or black holes) or collapse of a massive star. An unavoidable consequence of the latter model is that a bright supernovae should accompany the GRB. The emission from this supernova competes with the much brighter afterglow produced by the relativistic shock that gives rise to the GRB itself. Here we present evidence for an unusual light curve for GRB 980326 based on new optical observations. The transient brightened ~3 weeks after the burst to a flux sixty times larger than that extrapolated from the rapid decay seen at early time. Furthermore, the spectrum changed dramatically and became extremely red. We argue that the new source is the underlying supernova. If our hypothesis is true then this would be the first evidence for a supernova connection with GRBs at cosmological distances. We suggest that GRBs with long durations are associated with death of massive stars.

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arXiv1999

VLBI imaging of extremely high redshift quasars at 5 GHz

We present very long baseline interferometry (VLBI) images of ten very high redshift (z>3) quasars at 5 GHz. The sources 0004+139, 0830+101, 0906+041, 0938+119 and 1500+045 were observed in September 1992 using a global VLBI array, while 0046+063, 0243+181, 1338+381, 1428+423 and 1557+032 were observed in October 1996 with the European VLBI Network and Hartebeesthoek, South Africa. Most of the sources are resolved and show asymmetric structure. The sample includes 1428+423, the most distant radio loud quasar known to date (z=4.72). It is barely resolved with an angular resolution of about 2.0*1.4 mas.

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arXiv1998

Cosmology from Type Ia Supernovae

This presentation reports on first evidence for a low-mass-density/positive-cosmological-constant universe that will expand forever, based on observations of a set of 40 high-redshift supernovae. The experimental strategy, data sets, and analysis techniques are described. More extensive analyses of these results with some additional methods and data are presented in the more recent LBNL report #41801 (Perlmutter et al., 1998; accepted for publication in Ap.J.), astro-ph/9812133 . This Lawrence Berkeley National Laboratory reprint is a reduction of a poster presentation from the Cosmology Display Session #85 on 9 January 1998 at the American Astronomical Society meeting in Washington D.C. It is also available on the World Wide Web at http://supernova.LBL.gov/ This work has also been referenced in the literature by the pre-meeting abstract citation: Perlmutter et al., B.A.A.S., volume 29, page 1351 (1997).

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arXiv1998

Measurements of Omega and Lambda from 42 High-Redshift Supernovae

We report measurements of the mass density, Omega_M, and cosmological-constant energy density, Omega_Lambda, of the universe based on the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these SNe, at redshifts between 0.18 and 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of interest (Omega_M <~ 1.5). For a flat (Omega_M + Omega_Lambda = 1) cosmology we find Omega_M = 0.28{+0.09,-0.08} (1 sigma statistical) {+0.05,-0.04} (identified systematics). The data are strongly inconsistent with a Lambda = 0 flat cosmology, the simplest inflationary universe model. An open, Lambda = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(Lambda > 0) = 99%, including the identified systematic uncertainties. The best-fit age of the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h) Gyr for a flat cosmology. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calan/Tololo sample and our high-redshift sample. The conclusions are robust whether or not a width-luminosity relation is used to standardize the SN peak magnitudes.

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arXiv1998

A Search for High-redshift Quasars Among GB/FIRST Flat-Spectrum Radio Sources

We present the method and first results of a survey for high-redshift (z>3) radio-loud quasars, which is based on optical identifications of 2902 flat-spectrum radio sources with S(5GHz)>= 25mJy. The radio sample was defined over a 1600 sq degree region using the 5GHz Green Bank survey and the 1.4GHz VLA FIRST survey. 560 sources were identified to a limit of E=19.5 on APM scans of POSS-I plates and 337 of these optical counterparts are unresolved. From these a complete sample of 73 optically red (O-E >= 1.2) sources was defined for spectroscopic follow up. We have obtained spectra for 36 of these and an additional 14 had redshifts in the literature, thus 70% of the spectroscopic sample is completed. Six objects in the sample were found to be radio-loud quasars with z>3 of which two were previously known. The efficiency of the spectroscopic phase of the survey is therefore about 1 in 9. The six z>3 quasars were found in an effective area of 1100 square degrees, implying a surface density of one flat-spectrum z>3 radio-loud quasar per 190 square degrees to limits of E=19.5 and S(5GHz)>= 25mJy. This survey also produced the first known radio-loud BAL quasar, 1556+3517 with z=1.48, which has been reported in an earlier paper (Becker et al 1997). In addition we have obtained spectra of 22 GB/FIRST sources which are not part of the complete sample. We give positions, E (red) magnitudes, O-E colours, radio fluxes, radio spectral index and redshifts where possible for objects for which we have obtained spectra. We give spectra and finding charts for the z>3 quasars. (Abstract abridged)

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arXiv1998

The ASCA spectrum of the z=4.72 blazar, GB 1428+4217

The X-ray luminous quasar GB 1428+4217 at redshift 4.72 has been observed with ASCA. The observed 0.5-10 keV flux is 3.2E-12 erg/s/cm2. We report here on the intrinsic 4-57 keV X-ray spectrum, which is very flat (photon index of 1.29). We find no evidence for flux variability within the ASCA dataset or between it and ROSAT data. We show that the overall spectral energy distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars and present models which fit the available data. The Doppler beaming factor is likely to be at least 8. We speculate on the number density of such high redshift blazars, which must contain rapidly-formed massive black holes.

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arXiv1998

The Redshift Cutoff and the Quasar Epoch

We have obtained complete redshift information for a sample of 442 radio-loud quasars with flux densities S(11cm) >= 0.25Jy. These come from a completely-identified sample of 878 flat-spectrum radio sources, so there is no optical magnitude limit. With these quasars, therefore, we can map out the entire quasar epoch, unhindered by optical selection effects and any intervening dust. The rapid decline in true space density above redshifts of about 2.5 is confirmed. It agrees closely with that obtained from optically-selected samples, indicating that these are also not significantly affected by dust. The evolution of the quasar space density is similar to that deduced for the global star formation rate, implying that the quasar epoch may also be that of star formation. This sample of radio-selected quasars can now be used in a variety of other studies, including unbiased searches for damped Ly-alpha absorbers, studies of the phenomenon of ``red quasars'', and searches for high-redshift molecular absorption systems.

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arXiv1998

The Evolution of Radio-loud Quasars at High Redshift

We present results from two surveys for radio-loud quasars. These are (1) a northern survey reaching S(5GHz)=0.2Jy and (2) the Parkes QSO survey reaching S(2.7GHz)=0.25Jy, which together cover most of the sky. Both surveys use digitised plate material to identify the majority of the radio sources and CCD images to identify the optically fainter sources. The northern survey was targeted specifically at z > 3 QSOs and has produced more than 20, one of which, at z=4.72, is the most distant radio source known. This sample is identified to the POSS-I plate limit (20 mag) over an area of 3.7sr, or 12 000 sq deg. With the addition of CCD identifications, an effective sub area of 2400 sq degrees is 97% identified. In the South the Parkes QSO survey now has complete redshift information for all 442 flat-spectrum quasars. These cover the redshift range 0 < z < 4.5 and show a clear drop-off in space density at redshifts above 3. The form of this decline is remarkably similar in form to that seen in optically-selected samples of quasars. Since radio emission is unaffected by dust this implies that dust has a minimal effect on the observed drop-off seen in optical samples. Analysis of our radio-selected samples suggests that the decline in space density at high redshift is more pronounced for less powerful radio sources. We are planning a new survey that will be sensitive to bright quasars with redshifts up to 6.

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arXiv1998

Discovery of radio-loud quasars with z=4.72 and z=4.01

We report the discovery of two radio-loud quasars with redshifts greater than four; GB1428+4217 with z=4.72 and GB1713+2148 with z=4.01. This doubles the number of published radio-selected quasars with z > 4, bringing the total to 4. GB1428+4217 is the third most distant quasar known and the highest redshift radio and X-ray source currently known. It has a radio flux density at 5GHz of 259 +/- 31 mJy and an optical magnitude of R approximately 20.9. The rest frame absolute UV magnitude, M(nu=1450A), is -26.7 similar to that of the archetypal radio-selected quasar 3C273 (z=0.158; M(nu=1450A)=-26.4). GB1428+4217 has a tentative detection in ROSAT PSPC observations, which has been confirmed by more recent ROSAT observations, described in a companion paper by Fabian et al. Both quasars were discovered during the CCD imaging phase of an investigation into the evolution of the space density of radio-loud quasars at high redshift. Combined with our earlier survey results these objects give a lower limit on the space density of quasars with radio power P(5GHz) > 5.8 x 10^{26} W/Hz/sr between z=4 and z=5 of 1.4 +/- 0.9 x 10^{-10} Mpc^{-3}. This can be compared to 2.9 +/- 0.2 x 10^{-10} Mpc^-3 at z=2 from Dunlop & Peacock (1990) for flat-spectrum sources of the same luminosity.

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arXiv1997

Discovery of a Supernova Explosion at Half the Age of the Universe and its Cosmological Implications

The ultimate fate of the universe, infinite expansion or a big crunch, can be determined by measuring the redshifts, apparent brightnesses, and intrinsic luminosities of very distant supernovae. Recent developments have provided tools that make such a program practicable: (1) Studies of relatively nearby Type Ia supernovae (SNe Ia) have shown that their intrinsic luminosities can be accurately determined; (2) New research techniques have made it possible to schedule the discovery and follow-up observations of distant supernovae, producing well over 50 very distant (z = 0.3 -- 0.7) SNe Ia to date. These distant supernovae provide a record of changes in the expansion rate over the past several billion years. By making precise measurements of supernovae at still greater distances, and thus extending this expansion history back far enough in time, we can distinguish the slowing caused by the gravitational attraction of the universe's mass density Omega_M from the effect of a possibly inflationary pressure caused by a cosmological constant Lambda. We report here the first such measurements, with our discovery of a Type Ia supernova (SN 1997ap) at z = 0.83. Measurements at the Keck II 10-m telescope make this the most distant spectroscopically confirmed supernova. Over two months of photometry of SN 1997ap with the Hubble Space Telescope and ground-based telescopes, when combined with previous measurements of nearer SNe Ia, suggests that we may live in a low mass-density universe. Further supernovae at comparable distances are currently scheduled for ground and space-based observations.

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arXiv1997

Measurements of the Cosmological Parameters Omega and Lambda from the First 7 Supernovae at z >= 0.35

We have developed a technique to systematically discover and study high-redshift supernovae that can be used to measure the cosmological parameters. We report here results based on the initial seven of >28 supernovae discovered to date in the high-redshift supernova search of the Supernova Cosmology Project. We find a dispersion in peak magnitudes of sigma_{M_B} = 0.27 this dispersion narrows to sigma_{M_B,corr} = 0.19 after "correcting" the magnitudes using the light-curve "width-luminosity" relation found for nearby (z <= 0.1) type Ia supernovae from the Calan/Tololo survey (Hamuy et al. 1996). Comparing lightcurve-width-corrected magnitudes as a function of redshift of our distant (z = 0.35-0.46) supernovae to those of nearby type Ia supernovae yields a global measurement of the mass density, Omega_M = 0.88^{+0.69}_{-0.60} for a Lambda = 0 cosmology. For a spatially flat universe (i.e., Omega_M +Omega_Lambda = 1), we find Omega_M = 0.94 ^{+0.34}_{-0.28} or, equivalently, a measurement of the cosmological constant, Omega_Lambda = 0.06 ^{+0.28}_{-0.34} (<0.51 at the 95% confidence level). For the more general Friedmann-Lemaitre cosmologies with independent Omega_M and Omega_Lambda, the results are presented as a confidence region on the Omega_M-Omega_Lambda plane. This region does not correspond to a unique value of the deceleration parameter q_0. We present analyses and checks for statistical and systematic errors, and also show that our results do not depend on the specifics of the width-luminosity correction. The results for Omega_Lambda-versus-Omega_M are inconsistent with Lambda-dominated, low density, flat cosmologies that have been proposed to reconcile the ages of globular cluster stars with higher Hubble constant values.

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