arXiv subjects

Simon L. Morris

Publications and source records attributed to Simon L. Morris.

At least 37 records · Page 2Linked to original sources

arXiv2012

Large Scale Structure in Absorption: Gas within and around Galaxy Voids

We investigate the properties of the HI Ly-a absorption systems (Ly-a forest) within and around galaxy voids at z<0.1. We find a significant excess (>99% c.l.) of Ly-a systems at the edges of galaxy voids with respect to a random distribution, on ~5 h^{-1} Mpc scales. We find no significant difference in the number of systems inside voids with respect to the random expectation. We report differences between both column density (N_{HI}) and Doppler parameter (b_{HI}) distributions of Ly-a systems found inside and at the edge of galaxy voids at the >98% and >90% c.l. respectively. Low density environments (voids) have smaller values for both N_{HI} and b_{HI} than higher density ones (edges of voids). These trends are theoretically expected and also found in GIMIC, a state-of-the-art hydrodynamical simulation. Our findings are consistent with a scenario of at least three types of Ly-alpha: (1) containing embedded galaxies and so directly correlated with galaxies (referred as `halo-like'), (2) correlated with galaxies only because they lie in the same over-dense LSS, and (3) associated with under-dense LSS with a very low auto-correlation amplitude (~ random) that are not correlated with luminous galaxies. We argue the latter arise in structures still growing linearly from the primordial density fluctuations inside galaxy voids that have not formed galaxies because of their low densities. We estimate that these under-dense LSS absorbers account for 25-30% +- 6% of the current Ly-a population (N_{HI} > 10^{12.5} cm^{-2}) while the other two types account for the remaining 70-75% +- 12%. Assuming that only N_{HI} > 10^{14} cm^{-2} systems have embedded galaxies nearby, we have estimated the contribution of the `halo-like' Ly-a to be ~12-15% +- 4% and consequently ~55-60% +- 13% of the Ly-a systems to be associated with the over-dense LSS.

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arXiv2010

The association between gas and galaxies III: The Cross-correlation of Galaxies and Ly-alpha Absorbers at z ~ 1

We have measured the 2D 2-point correlation function, Xi_AG, between low column density Ly-alpha absorbers and galaxies at a redshift z ~ 1. We measured Ly-alpha absorbers between redshifts z=0.68 to 1.51 over a total redshift path length of Delta z=1.08 from HST STIS E230M absorption spectra towards the quasars HE 1122-1648 (z=2.4) and PKS 1127-145 (z=1.187). The column density of the Ly-alpha absorbers ranged from 13.2< log(N_HI (cm^-2))<17.4, with a median column density of log(N_HI)=14.0 . A total of 193 galaxy redshifts within the surrounding 6.8' x 5.7' field of view of both quasars were identified in a R magnitude limited survey (21.5<R_Vega<24.5) using the FORS2 spectrograph at the VLT, of which 95 were higher than the minimum redshift z=0.68 to be used in the correlation function. A 3 sigma upper-limit of Xi_AG=2.8 was found when 145 Ly-alpha absorber-galaxy pairs were binned in redshift space, in a bin of size Delta sigma=1.0, Delta pi=2.0 h^-1 Mpc along the projected separation and line of sight distances respectively. The upper-limit in the cross-correlation was found to be 5.4 sigma lower than the central peak in the galaxy auto-correlation within the same redshift range, Xi_GG, which was in our data equal to 10.7+/-1.4. Thus we have shown for the first time that the clustering between low column density absorbers and galaxies at a redshift of 1 is weaker than that between galaxies at the same redshift.

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arXiv2009

Galaxies at Redshift ~0.5 Around Three Closely Spaced Quasar Sightlines

We examine the relationship between galaxies and the intergalactic medium at z < 1 using a group of three closely spaced background QSOs with z_em ~1 observed with the Hubble Space Telescope. Using a new grouping algorithm, we identify groups of galaxies and absorbers across the three QSO sightlines that may be physically linked. There is an excess number of such groups compared to the number we expect from a random distribution of absorbers at a confidence level of 99.9%. The same search is performed with mock spectra generated using a hydrodynamic simulation, and we find the vast majority of such groups arise in dense regions of the simulation. We find that at z<0.5, groups in the simulation generally trace the large-scale filamentary structure as seen in the projected 2-d distribution of the HI column density in a ~30 h^-1 Mpc region. We discover a probable sub-damped Lyman-alpha system at z=0.557 showing strong, low-ionisation metal absorption lines. Previous analyses of absorption across the three sightlines attributed these metal lines to HI. We show that even when the new line identifications are taken into account, evidence remains for planar structures with scales of ~1 Mpc absorbing across the three sightlines. We identify a galaxy at z=0.2272 with associated metal absorption in two sightlines, each 200 kpc away. By constraining the star formation history of the galaxy, we show the gas causing this metal absorption may have been enriched and ejected by the galaxy during a burst of star formation 2 Gyr ago.

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arXiv2009

The colour of galaxies in distant groups

(Abridged) We present new optical and near-infrared imaging for a sample of 98 spectroscopically-selected galaxy groups at 0.25<z<0.55. We measure accurate colours for group members and the surrounding field population, statistically complete above a stellar mass limit of M=1E10 Msun. The overall colour distribution is bimodal in both the field and group samples; but at fixed luminosity the fraction of group galaxies populating the red peak is larger, by 20+/-7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in HST imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. We show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group sample, and shows little dependence on near infrared luminosity. Thus, the fraction of bright group members with no sign of star formation or AGN activity is 54+/-6 per cent. Our field sample, which includes galaxies in all environments, contains 35+/-3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions, that dense environments at z<0.5 are associated with a premature cessation of star formation in some galaxies; in particular we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the timescale of this quenching equally for all group members distorts the colour distribution in a way that is inconsistent with observations.

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arXiv2006

The stellar mass content of distant galaxy groups

We have obtained near-infrared imaging of 58 galaxy groups, in the redshift range 0.1<z<0.6, from the William Herschel Telescope and from the Spitzer IRAC data archive. The groups are selected from the CNOC2 redshift survey, with additional spectroscopy from the Baade telescope (Magellan). Our group samples are statistically complete to K(Vega}=17.7 (INGRID) and [3.6](AB)=19.9 (IRAC). From these data we construct near-infrared luminosity functions, for groups in bins of velocity dispersion, up to 800 km/s, and redshift. The total amount of near-infrared luminosity per group is compared with the dynamical mass, estimated from the velocity dispersion, to compute the mass-to-light ratio, M/Lk. We find that the M/Lk values in these groups are in good agreement with those of their statistical descendants at z=0, with no evidence for evolution beyond that expected for a passively evolving population. There is a trend of M/Lk with group mass, which increases from M/Lk~10 for groups with sigma<250 km/s to M/Lk~100 for sigma=425-800 km/s. This trend is weaker, but still present, if we estimate the total mass from weak lensing measurements. In terms of stellar mass, stars make up ~2 per cent of the mass in the smallest groups, and ~1 per cent in the most massive groups. We also use the near-infrared data to consider the correlations between stellar populations and stellar masses, for group and field galaxies at 0.1<z<0.6. We find that fewer group galaxies show strong [OII] emission compared with field galaxies of the same stellar mass and at the same redshift. We conclude that most of the stellar mass in these groups was already in place by z~0.4, with little environment-driven evolution to the present day.

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arXiv2006

The association between gas and galaxies I: CFHT spectroscopy and pair analysis

We investigate the relative distribution of the gaseous contents of the Universe (as traced by a sample of Lyman alpha (lya) absorbers), and the luminous baryonic matter (as traced by a redshift survey of galaxies in the same volume searched for lya absorbers), along 16 lines-of-sight (LOS) between redshifts 0 and 1. Our galaxy redshift survey was made with the Multi-Object Spectrograph (MOS) on Canada-France-Hawaii Telescope (CFHT) and, when combined with galaxies from the literature in the same LOS, gives us a galaxy sample of 636 objects. By combining this with an absorption line sample of 406 absorbing systems drawn from published works, we are able to study the relationship between gas and galaxies over the latter half of the age of the Universe. A correlation between absorbers and galaxies is detected out to separation of 1.5 Mpc. This correlation is weaker than the galaxy-galaxy correlation. There is also some evidence that the absorbing systems seen in CIV are more closely related to galaxies, although this correlation could be with column density rather than metallicity. The above results are all consistent with the absorbing gas and the galaxies co-existing in dark matter filaments and knots as predicted by current models, where the column density of the absorbing gas is correlated with the underlying matter density.

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arXiv2004

The abundance of Lyman-(α) emitters in hierarchical models

We present predictions for the abundance of Ly-$α$ emitters in hierarchical structure formation models. We use the {\tt GALFORM} semi-analytical model to explore the impact on the predicted counts of varying assumptions about the escape fraction of Ly-$α$ photons, the redshift at which the universe reionised and the cosmological density parameter. A model with a fixed escape fraction gives a remarkably good match to the observed counts over a wide redshift interval. We present predictions for the expected counts in a typical observation with the Multi Unit Spectroscopic Explorer instrument proposed for the Very Large Telescope.

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arXiv2001

Low Redshift Lyman Alpha absorbers and their Connection with Galaxies

We review the ongoing debate about the relationship between low redshift Lyman Alpha absorbers and luminous galaxies. In particular, we discuss the difficulty of `assigning' a particular absorber to a particular galaxy, and consider methods of circumventing this problem. We also provide a status report on an ongoing project collecting more data to address this issue, and show some results for a close together pair of QSOs providing two adjacent lines of sight through the inter-galactic medium.

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arXiv2000

Active Galactic Nuclei in the CNOC2 Field Galaxy Redshift Survey

We present a sample of 47 confirmed and 14 candidate Active Galactic Nuclei (AGN) discovered in the Canadian Network for Observational Cosmology field galaxy redshift survey (CNOC2). The sample consists of 38 objects identified from broad emission lines, 8 from narrow [NeV] emission, and 15 candidates from FeII or MgII absorption lines, one of which has been confirmed as a broad-line AGN via infrared spectroscopy. Redshifts of these AGN range from z=0.27 to z=4.67, and the average absolute magnitude is M_B=-22.25. No color selection criteria were involved in selecting this spectroscopically discovered sample. The sample is also unbiased against objects with luminous host galaxies since the spectroscopy preferentially targeted extended objects. Simple color-color diagram selection criteria can recover 81+-6% of the CNOC2 AGN, but several of the most unusual objects would be missing from such a color-selected sample. In the subsample with broad emission lines, the average equivalent widths for MgII and CIII] agree with the predictions of previous studies of the Baldwin effect. However, the average equivalent widths for CIV and Ly-alpha are smaller than predicted by previous studies of the Baldwin effect at lower redshift. This may imply that the slopes of the CIV and Ly-alpha Baldwin effects evolve with redshift, steepening with cosmic time. The broad emission line AGN also show a higher incidence of associated MgII and CIV absorption than in most previous surveys. This may arise from strong absorption being anti-correlated with optical luminosity or becoming less frequent with cosmic time, or possibly because our selection method is not biased against objects with resolved spatial structure or reddened by dust associated with the absorbing gas. (Abridged)

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arXiv2000

H-alpha Photometry of Abell 2390

We present the results of a search for strong H-alpha emission line galaxies (rest frame equivalent widths greater than 50 A) in the z=0.23 cluster Abell 2390. The survey contains 1189 galaxies over 270 square arcminutes, and is 50% complete at M_r=-17.5+5 log h. The fraction of galaxies in which H-alpha is detected at the 2sigma level rises from 0.0 in the central regions (excluding the cD galaxy) to 12.5+/-8% at R200. For 165 of the galaxies in our catalogue, we compare the H-alpha equivalent widths with their [OII] equivalent widths, from the CNOC1 spectra. The fraction of strong H-alpha emission line galaxies is consistent with the fraction of strong [OII] emission galaxies in the CNOC1 sample: only 2+/-1% have no detectable [OII] emission and yet significant (>2sigma) H-alpha equivalent widths. Dust obscuration, non-thermal ionization, and aperture effects are all likely to contribute to this non-correspondence of emission lines. We identify six spectroscopically 'secure' k+a galaxies (W(OII)<5 A and W(Hdelta)>5 A); at least two of these show strong signs in H-alpha of star formation in regions that are covered by the slit from which the spectra were obtained. Thus, some fraction of galaxies classified k+a based on spectra shortward of 6000 A are likely to be undergoing significant star formation. These results are consistent with a 'strangulation' model for cluster galaxy evolution, in which star formation in cluster galaxies is gradually decreased, and is neither enhanced nor abruptly terminated by the cluster environment.

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arXiv2000

A search for the submillimetre counterparts to Lyman break galaxies

We have carried out targetted sub-mm observations as part of a programme to explore the connection between the rest-frame UV and far-IR properties of star-forming galaxies at high redshift, which is currently poorly understood. On the one hand the Lyman break technique is very effective at selecting z~3 galaxies. On the other hand `blank field' imaging in the sub-mm seems to turn up sources routinely, amongst which some are star forming galaxies at similar redshifts. Already much work has been done searching for optical identifications of objects detected using the SCUBA instrument. Here we have taken the opposite approach, performing sub-mm photometry for a sample of Lyman break galaxies whose UV properties imply high star formation rates. The total signal from our Lyman break sample is undetected in the sub-mm, at an RMS level of 0.5 mJy, which implies that the population of Lyman break galaxies does not constitute a large part of the recently detected blank-field sub-mm sources. However, our one detection suggests that with reasonable SCUBA integrations we might expect to detect those few LBGs that are far-IR brightest.

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arXiv2000

Spectroscopic Gravitational Lens Candidates in the CNOC2 Field Galaxy Redshift Survey

We present five candidate gravitational lenses discovered spectroscopically in the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2), along with one found in followup observations. Each has a secure redshift based on several features, plus a discrepant emission line which does not match any known or plausible feature and is visible in multiple direct spectral images. We identify these lines as Lyman-alpha or [OII] emission from galaxies lensed by, or projected onto, the CNOC2 target galaxies. Einstein radii estimated from the candidate deflector galaxy luminosities indicate that for two candidates the lines are probably [OII] from projected z<1 galaxies (consistent with the detection of H-beta as well as [OII] in one of them), but that in the remaining four cases the lines could be Ly-alpha from lensed z>3 galaxies. We estimate that only 1.9+-0.7 [OII]-emitting galaxies are expected to project onto target galaxies in the original CNOC2 sample, consistent with three or four of the six candidates being true gravitational lenses.

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arXiv2000

The Origin of Star Formation Gradients in Rich Galaxy Clusters

We examine the origin of clustercentric gradients in the star formation rates and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formation rates decline on a timescale of a few Gyrs, the typical gas consumption timescale of disk galaxies in the field. Such behaviour might be expected if tides and ram pressure strip off the gaseous envelopes that normally fuel star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster formation in a Lambda-CDM universe, we reproduce the systematic differences observed in the color distribution of cluster and field galaxies, as well as the strong suppression of star formation in cluster galaxies and its dependence on clustercentric radius. The simulations also indicate that a significant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explains naturally why star formation in the outskirts of clusters (and as far out as two virial radii) is systematically suppressed relative to the field. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the galaxy is accreted into the main body of the cluster. We conclude that the star formation rates of cluster galaxies depend primarily on the time elapsed since their accretion onto massive virialized systems, and that the cessation of star formation may have taken place gradually over a few Gyrs.

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arXiv1999

Near-IR adaptive optics imaging of nuclear spiral structure in the Seyfert galaxy, NGC3227

We present high spatial resolution, near-IR images in J, H, K of the nucleus of NGC3227, obtained with the Adaptive Optics bonnette on CFHT. The ~0.15 arcsec (17pc) resolution allows structures to be probed in the core region. Dust obscuration becomes significantly less pronounced at longer wavelengths, revealing the true geometry of the core region. We are able to identify two main features in our maps: (i) a spiraling association of knots with a counterpart in an HST F606W image; (ii) a smaller scale annulus, orthogonal to the spiral of knots. These features may provide a means to transport material inwards to fuel the active nucleus.

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arXiv1999

Adaptive optics near-IR imaging of NGC2992 - unveiling core structures related to radio figure-8 loops

We present near-IR adaptive optics, VLA radio and HST optical imaging of the nearby Seyfert galaxy NGC2992. Spiral structure and an extension to the West are traced down to the core region at the limiting resolution of our near-IR images. A faint, diffuse loop of near-IR and radio emission is also observed to the north, embedded within the prominent 2 arcsec radio loop previously observed to the northwest. Near-IR color maps, and CO narrowband imaging, are then used to identify which regions may not be purely reddened stellar populations. Our new data provide evidence that the VLA radio-loop morphology in the shape of a figure-8 represents two components superimposed: 1) outflow bubbles out of the plane of the disk, coincident with the extended emission line region (EELR); 2) star formation along the spiral arm within the galaxy disk and through the dust lane. The near-IR continuum emission associated with the outflowing radio bubbles suggest that the radio loops are driven by the active nucleus.

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arXiv1999

X-ray Mass Estimates at $z\sim0.3$ for the CNOC Cluster Sample

Results are presented from the analysis of ROSAT HRI and PSPC observations of the CNOC subsample of the EMSS high redshift galaxy clusters. X-ray surface brightness profiles of 14 clusters with $0.17 < z < 0.55$ are constructed and fit to isothermal $β$ models. Where possible, we use both the HRI and PSPC data to constrain the fit. Under the assumptions of isothermality, hydrostatic equilibrium, and spherical symmetry, we derive total X-ray masses within a range of radii from 141 to $526 h^{-1}_{100}$ kpc. These masses are compared with both the dynamical masses obtained from galaxy velocities and the projected masses from published gravitational lensing studies. We find no systematic bias between X-ray and dynamical methods across the sample, with an average $M_{Dyn}/M_X=1.04\pm0.07$, although individual clusters exhibit mass discrepancies up to a factor of 2. We estimate that the systematic effects due to cooling flows, non-equilibrium systems and temperature gradients affect the average mass ratio by no more than $15-20%$. Weak gravitational lensing masses appear to be systematically higher than X-ray results by factors of $\sim50%$, while strong lensing estimates show larger discrepancies (factors of $\sim2.5$). However, these comparisons are complicated by the need to extrapolate the X-ray data to larger or smaller radii. X-ray derived cluster gas masses are calculated, from which we obtain a cluster baryon fraction of $\sim5%h^{-3/2}_{100}$, yielding $Ω_0 \sim0.3h^{-1/2}_{100}$.

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arXiv1998

The core structure of AGN: perils of adaptive optics artifacts

As part of a project to map nearby Seyfert galaxies in the near-IR with adaptive optics, we present high spatial resolution, near-IR images in J,H, and K of the nuclei of NGC 3227 and NGC 2992, obtained with the Adaptive Optics Bonnette (AOB) on CFHT. The ~0.15" resolution allows us to probe structures in the core region at unprecedented scales. With NGC 3227, we are able to identify an inwards spiraling starburst in all three near-IR bands. NGC 2992 shows evidence for emission along a radio loop. Compared with HST optical images, dust obscuration becomes significantly less pronounced at longer wavelengths, revealing the true geometry of the core regions. The observed structures may help to elucidate fueling mechanisms for the central engine, as well as providing insight into the unification paradigms. The results are tempered with the discovery of AOB-related artifacts in the central arcsec of observed AGN galaxies which take on the appearance of spiral/disk structures.

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arXiv1998

The Dependence of Cluster Galaxy Star Formation Rates on the Global Environment

A comparison of star formation properties as a function of environment is made from the spectra of identically selected cluster and field galaxies in the CNOC 1 redshift survey of over 2000 galaxies in the fields of fifteen X-ray luminous clusters at 0.18<z<0.55. The ratio of bulge luminosity to total galaxy luminosity (B/T) is computed for galaxies in this sample, and this measure of morphology is compared with the galaxy star formation rate as determined from the [OII]3727 emission line. The mean star formation rate of cluster galaxies brighter than M_r= -17.5 + 5 log h is found to vary from 0.17 +- 0.02 h^{-2} M_sun/yr at R200 (1.5-2 Mpc/h) to zero in the cluster center, and is always less than the mean star formation rate of field galaxies, which is 0.39 +- 0.01 h^{-2}M_sun/yr. It is demonstrated that this significant difference is not due exclusively to the difference in morphological type, as parameterized by the B/T value, by correcting for the B/T-radius relation. The distribution of [OII] equivalent widths among cluster galaxies is skewed toward lower values relative to the distribution for field galaxies of comparable physical size, B/T and redshift, with a statistical significance of more than 99%. The cluster environment affects not only the morphological mix of the galaxy population, but also suppresses the star formation rate within those galaxies, relative to morphologically similar galaxies in the field.

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