Searcharxiv⌕ Search

arXiv subjects

J. Kurk

Publications and source records attributed to J. Kurk.

36 records · Page 2Linked to original sources
arXiv2009

Bright Lyman-alpha Emitters at z~9: constraints on the luminosity function from HiZELS

New results are presented, as part of the Hi-z Emission Line Survey (HiZELS), from the largest area survey to date (1.4 sq.deg) for Lyman-alpha emitters (LAEs) at z~9. The survey, which is primarily targeting H-alpha emitters at z<3, uses the Wide Field CAMera on the United Kingdom Infrared Telescope and a custom narrow-band filter in the J band and reaches a Lyman-alpha luminosity limit of ~10^43.8 erg/s over a co-moving volume of 1.12x10^6 Mpc^3 at z=8.96+-0.06. Only 2 candidates were found out of 1517 line emitters and those were rejected as LAEs after follow-up observations. The limit on the space density of bright LAEs is improved by 3 orders of magnitude, consistent with suppression of the bright end of the Lyman-alpha luminosity function beyond z~6. Combined with upper limits from smaller but deeper surveys, this rules out some of the most extreme models for high-redshift LAEs. The potential contamination of future narrow-band Lyman-alpha surveys at z>7 by Galactic brown dwarf stars is also examined, leading to the conclusion that such contamination may well be significant for searches at 7.7<z<8.0, 9.1<z<9.5 and 11.7 < z < 12.2.

astro-ph.CO↗
arXiv2009

Witnessing the assembly at z=1.6 of a galaxy cluster (GMASS ultradeep spectroscopy of galaxies at z~2 - V)

We present a study of a galaxy overdensity at z=1.6, Cl 0332-2742, uncovered in the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). We conservatively estimate the overdensity in redshift space for the spike, containing 42 galaxies at z=1.6, to be 8.3+/-1.5 and find a velocity dispersion of 500 km/s. A map of the surface density of galaxies at z=1.6 in the GMASS field shows that its structure is irregular. The differences in the physical properties of Cl 0332-2742 member and field galaxies agree with the latest hierarchical galaxy formation models: for overdensity members, the star formation rate (SFR), and specific SFR, is approximately 50% lower than for the field galaxies; overdensity galaxies are twice the age, on average, of field galaxies; and there is a higher proportion of both massive (M>10^{10.7} M_sol), and early-type galaxies, inside Cl 0332-2742 than in the field. Among the 42 members, 7 have spectra consistent with being passively evolving, massive galaxies. In a z-J colour-magnitude diagram, the photometric data of these early-type galaxies are in close agreement with a theoretical red sequence of a galaxy cluster at redshift z=1.6, which formed most of its stars in a short burst of star formation at z~3. We conclude that the redshift spike at z=1.6 in the GMASS field represents a sheet-like structure in the cosmic web and the area with the highest surface density within this structure, containing already seven passively evolving galaxies, will evolve into a cluster of galaxies at a later time.

astro-ph.CO↗
arXiv2009

HiZELS: a high redshift survey of H-alpha emitters. II: the nature of star-forming galaxies at z=0.84

New results from a large survey of H-alpha emission-line galaxies at z=0.84 using WFCAM/UKIRT and a custom narrow-band filter in the J band are presented as part of the HiZELS survey. Reaching an effective flux limit of 1e-16 erg/s/cm^2 in a comoving volume of 1.8e5 Mpc^3, this represents the largest and deepest survey of its kind ever done at z~1. There are 1517 potential line emitters detected across 1.4 sq.deg of the COSMOS and UKIDSS UDS fields, of which 743 are selected as H-alpha emitters. These are used to calculate the H-alpha luminosity function, which is well-fitted by a Schechter function with phi*=10^(-1.92+-0.10) Mpc^-3, L*=10^(42.26+-0.05)erg/s, and alpha=-1.65+-0.15. The integrated star formation rate density (SFRD) at z=0.845 is 0.15+-0.01 M_sun/yr/Mpc^3. The results robustly confirm a strong evolution of SFRD from the present day out to z~1 and then flattening to z~2, using a single star-formation indicator. Out to z~1, both the characteristic luminosity and space density of the H-alpha emitters increase significantly; at higher redshifts, L* continues to increase, but phi* decreases. The z=0.84 H-alpha emitters are mostly disk galaxies (82+-3%), while 28+-4% of the sample show signs of merger activity and contribute ~20% to the total SFRD. Irregulars and mergers dominate the H-alpha luminosity function above L*, while disks are dominant at fainter luminosities. These results demonstrate that it is the evolution of 'normal' disk galaxies that drives the strong increase in the SFRD from the current epoch to z~1, although the continued strong evolution of L* beyond z=1 suggests an increasing importance of merger activity at higher redshifts.

astro-ph.CO↗
arXiv2009

The SINS survey: modeling the dynamics of z~2 galaxies and the high-z Tully-Fisher relation

We present the modeling of SINFONI integral field dynamics of 18 star forming galaxies at z ~ 2 from Halpha line emission. The galaxies are selected from the larger sample of the SINS survey, based on the prominence of ordered rotational motions with respect to more complex merger induced dynamics. The quality of the data allows us to carefully select systems with kinematics dominated by rotation, and to model the gas dynamics across the whole galaxy using suitable exponential disk models. We obtain a good correlation between the dynamical mass and the stellar mass, finding that large gas fractions Mgas~M*) are required to explain the difference between the two quantities. We use the derived stellar mass and maximum rotational velocity Vmax from the modeling to construct for the first time the stellar mass Tully-Fisher relation at z ~ 2.2. The relation obtained shows a slope similar to what is observed at lower redshift, but we detect an evolution of the zero point. We find that at z ~ 2.2 there is an offset in log(M*) for a given rotational velocity of 0.41+-0.11 with respect to the local Universe. This result is consistent with the predictions of the latest N-body/hydrodynamical simulations of disk formation and evolution, which invoke gas accretion onto the forming disk in filaments and cooling flows. This scenario is in agreement with other dynamical evidence from SINS, where gas accretion from the halo is required to reproduce the observed properties of a large fraction of the z ~ 2 galaxies.

astro-ph.CO↗
arXiv2008

HiZELS: a high redshift survey of H-alpha emitters. I: the cosmic star-formation rate and clustering at z=2.23

We present results from a near-infrared narrow-band survey of emission-line galaxies at z=2.23, using the Wide Field Camera on the United Kingdom Infrared Telescope. The H2S1 narrow-band filter (lambda_c=2.121um) we employ selects the H-alpha emission line redshifted to z=2.23, and is thus suitable for selecting 'typical' star forming galaxies and active galactic nuclei at this epoch. The pilot study was undertaken in the well studied Cosmological Evolution Survey field (COSMOS) and is already the largest near-infrared narrow-band survey at this depth, with a line flux limit of F(H-alpha)~10^-16 erg/s/cm^2 over 0.60 square degrees, probing ~220x10^3 Mpc^3 (co-moving) down to a limiting star formation rate of 30 M_sun/yr (3sigma). In this paper we present the results from our pilot survey and evaluate the H-alpha luminosity function and estimate the clustering properties of H-alpha emitters at z=2.23 from 55 detected galaxies. The integrated luminosity function is used to estimate the volume averaged star formation rate at z=2.23: SFRD = 0.17^{+0.16}_{-0.09} M_sun/yr/Mpc^3 for L(H-alpha) > 10^42 erg/s. For the first time, we use the H-alpha star-formation tracer to reliably constrain SFRD out to z=2.23 demonstrating the rapid increase in SRFD out to this redshift as well as confirming the flattening in SFRD between z~1-2. In addition to the luminosity distribution, we analyse the clustering properties of these galaxies. Using the 2-point angular correlation function, we estimate a real space correlation length of r_0= 4.2^{+0.4}_{-0.2} h^-1 Mpc. In comparison to models of clustering which take into account bias evolution, we estimate that these galaxies are hosted by dark matter halos of mass M_halo ~ 10^12 M_sun consistent with the progenitors of the Milky Way.

astro-ph↗
arXiv2008

GMASS Ultradeep Spectroscopy of Galaxies at z~2. III: The emergence of the color bimodality at z~2

The aim of this work is to study the evolution of the rest frame color distribution of galaxies with the redshift, in particular in the critical interval 1.4 10.1, and we study their morphological and spectro-photometric properties. We show that the contribution to this sample of early-type galaxies, defined as galaxies with a spheroidal morphology and no star formation, decreases from 60-70% at z<0.5 down to ~50% at redshift z=2. At z>2 we still find red galaxies in the mass complete sample, even if the bimodality is not seen any more. About 25% of these red galaxies at z>2 are passively evolving, with the bulk of their stars formed at redshift z>`3.

astro-ph↗
arXiv2008

The Nascent Red Sequence at z~2

We present new constraints on the evolution of the early-type galaxy color-magnitude relation (CMR) based on deep near-infrared imaging of a galaxy protocluster at z=2.16 obtained using NICMOS on-board the Hubble Space Telescope. This field contains a spectroscopically confirmed space-overdensity of Lyman-alpha and H-alpha emitting galaxies which surrounds the powerful radio galaxy MRC 1138-262. Using these NICMOS data we identify a significant surface-overdensity (= 6.2x) of red J-H galaxies in the color-magnitude diagram (when compared with deep NICMOS imaging from the HDF-N and UDF). The optical-NIR colors of these prospective red-sequence galaxies indicate the presence of on-going dust-obscured star-formation or recently formed (<~ 1.5 Gyr)stellar populations in a majority of the red galaxies. We measure the slope and intrinsic scatter of the CMR for three different red galaxy samples selected by a wide color cut, and using photometric redshifts both with and without restrictions on rest-frame optical morphology. In all three cases both the rest-frame $U-B$ slope and intrinsic color scatter are considerably higher than corresponding values for lower redshift galaxy clusters. These results suggest that while some relatively quiescent galaxies do exist in this protocluster both the majority of the galaxy population and hence the color-magnitude relation are still in the process of forming, as expected.

astro-ph↗
arXiv2008

GMASS Ultradeep Spectroscopy of Galaxies at redshift z~2. I. The stellar metallicity

Context: Galaxy metallicities have been measured to redshift z~2 by gas-phase oxygen abundances of the interstellar medium using the R23 and N2 methods. Galaxy stellar metallicities provide crucial data for chemical evolution models but have not been assessed reliably much outside the local Universe. Aims: We determine the iron-abundance, stellar metallicity of star-forming galaxies (SFGs) at redshift z~2, observed as part of the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). Methods: We compute the equivalent width of a rest-frame mid-ultraviolet, photospheric absorption-line index, the 1978 index found to vary monotonically with stellar metallicity by Rix and collaborators. We normalise and combine 75 SFG spectra from the GMASS survey to produce a spectrum corresponding to a total integration time 1652.5 hours (and a signal-to-noise ratio ~100 for our 1.5 angstrom binning) of FORS2 spectroscopic observations at the Very Large Telescope. Results: We measure an iron-abundance, stellar metallicity of log (Z/Zsolar) = -0.574+/-0.159 for our spectrum representative of a galaxy of stellar mass 9.4 x 10^9 Msolar assuming a Chabrier IMF. We find that the R04 model SFG spectrum for log (Z/Zsolar) = -0.699 solar metallicity provides the best description of our GMASS coadded spectrum. For similar galaxy stellar mass, our stellar metallicity is ~0.25 dex lower than the oxygen-abundance, gas-phase metallicity quantified by Erb and collaborators for UV-selected star-forming galaxies at z=2. Conclusions: We conclude that we are witnessing the establishment of a light-element overabundance in galaxies as they are being formed at redshift z~2. Our measurements are reminiscent of the alpha-element enhancement seen in low-redshift, galactic bulges and early-type galaxies. (Abridged)

astro-ph↗
arXiv2008

GMASS Ultradeep Spectroscopy of Galaxies at 1.4<z<2. II. Superdense passive galaxies: how did they form and evolve ?

We combine ultradeep optical spectroscopy from the GMASS project ("Galaxy Mass Assembly ultradeep Spectroscopic Survey") with GOODS multi-band photometry and HST imaging to study a sample of passive galaxiesat 1.39 2. These galaxies have morphologies that are predominantly compact and spheroidal.However, their sizes (R_e <~ 1 kpc) are much smaller than those of spheroids in the present--day Universe. Their stellar mass surface densities are consequently higher by ~1 dex if compared to spheroids at z~0 with the same mass. Their rest-frame B-band surface brightness scales with the effective radius, but the offset with respect to the surface brightness of the local Kormendy relation is too large to be explained by simple passive evolution. At z~1, a larger fraction of passive galaxies follows the z~0 size -- mass relation. Superdense relics with R_e~1 kpc are extremely rare at z~0 with respect to z>1, and absent if R_e<1 kpc. Because of the similar sizes and mass densities, we suggest that the superdense passive galaxies at 1 2. The results are compared with theoretical models and the main implications discussed in the framework of massive galaxy formation and evolution.

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↗
arXiv2007

The First Appearance of the Red Sequence of Galaxies in Proto-Clusters at 2<~z<~3

We explore the evolved galaxy population in the proto-clusters around four high-z radio galaxies at 2<~z<~3 based on wide-field near-infrared imaging. Three of the four fields are known proto-clusters as demonstrated by overdensities of line emitting galaxies at the same redshifts as the radio galaxies. We imaged the fields of three targets (PKS1138-262, USS0943-242 and MRC0316-257) to a depth of Ks~22 (5sigma) over a 4'x7' area centered on the radio galaxies with a new wide-field NIR camera, MOIRCS, on the Subaru Telescope. Another target (USS1558-003) was observed with SOFI on the NTT to a depth of Ks=20.5 over a 5'x5' area. We apply colour cuts in J-Ks and/or JHKs in order to exclusively search for galaxies located at high redshifts: z>2. To the 5sigma limiting magnitudes, we see a significant excess of NIR selected galaxies by a factor of two to three compared to those found in the field of GOODS-South. The spatial distribution of these NIR selected galaxies is not uniform and traces structures similar to those of emission line galaxies, although the samples of NIR selected galaxies and emitters show little overlap. We focus on the NIR colour-magnitude sequence of the evolved population and find that the bright-end (M_{stars}>10^{11}Msun) of the red sequence is well populated by z~2 but much less so in the z~3 proto-clusters. This may imply that the bright-end of the colour-magnitude sequence first appeared between z=3 and 2, an era coinciding with the appearance of submm galaxies and the peak of the cosmic star formation rate. Our observations show that during the same epoch, massive galaxies are forming in high density environments by vigorous star formation and assembly.

astro-ph↗
arXiv2005

MAMBO 1.2 Millimeter observations of BzK-selected star-forming Galaxies at z~2

We present MAMBO 1.2 mm observations of five BzK-pre-selected vigorous starburst galaxies at z~2. Two of these were detected at more than 99.5% confidence levels, with 1.2 mm fluxes around 1.5 mJy. These millimeter fluxes imply vigorous activity with star-formation rates (SFRs) approx. 500-1500 Msun/yr, confirmed also by detections at 24 microns with the MIPS camera on board of the Spitzer satellite. The two detected galaxies are the ones in the sample with the highest SFRs estimated from the rest-frame UV, and their far-IR- and UV-derived SFRs agree reasonably well. This is different from local ULIRGs and high-z submm/mm selected galaxies for which the UV is reported to underestimate SFRs by factors of 10-100, but similar to the average BzK-ULIRG galaxy at z~2. The two galaxies detected at 1.2 mm are brighter in K than the typical NIR-counterparts of MAMBO and SCUBA sources, implying also a significantly different K-band to submm/mm flux ratio. This suggests a scenario in which z~2 galaxies, after their rapid (sub)mm brightest phase opaque to optical/UV light, evolve into a longer lasting phase of K-band bright and massive objects. Targeting the most UV active BzKs could yield substantial detection rates at submm/mm wavelengths.

astro-ph↗
arXiv2002

BRIGHT lights, BIG city: Massive galaxies, giant Ly-a nebulae, and proto--clusters

High redshift radio galaxies are great cosmological tools for pinpointing the most massive objects in the early Universe: massive forming galaxies, active super--massive black holes and proto--clusters. We report on deep narrow--band imaging and spectroscopic observations of several z > 2 radio galaxy fields to investigate the nature of giant Ly-alpha nebulae centered on the galaxies and to search for over--dense regions around them. We discuss the possible implications for our understanding of the formation and evolution of massive galaxies and galaxy clusters.

astro-ph↗
arXiv2001

Deep VLT spectroscopy of high redshift radio galaxy MRC 2104-242: Evidence for a metallicity gradient

In this contribution we will present deep VLT spectroscopy observations of the giant emission line halo around the z=2.49 radio galaxy MRC 2104-242. The morphology of the halo is dominated by two spatially resolved regions. Lya is extended by >12" along the radio axis, C IV and He II are extended by ~8". The overall spectrum is typical for that of high redshift radio galaxies. Interestingly, N V is present in the spectrum of the region associated with the center of the galaxy hosting the radio source, the northern region, while absent in the southern region. Using a simple photoionization model, the difference in N V can be explained due to a metallicity gradient within the halo. This is consistent with a scenario in which the halo is formed by a massive cooling flow or originates from the debris of the merging of two or more galaxies. However, also other mechanisms such as jet-cloud interactions or starburst-winds could be important.

astro-ph↗
arXiv2001

Deep VLT spectroscopy of the z=2.49 Radio Galaxy MRC 2104-242: Evidence for a metallicity gradient in its extended emission line region

We present spectroscopic observations of the rest-frame UV line emission around radio galaxy MRC 2104-242 at z=2.49, obtained with FORS1 on VLT Antu. The morphology of the halo is dominated by two spatially resolved regions. Lya is extended by >12 arcsec along the radio axis, CIV and HeII are extended by ~8 arcsec. The overall spectrum is typical for that of high redshift radio galaxies. The most striking spatial variation is that NV is present in the spectrum of the region associated with the center of the galaxy hosting the radio source, the northern region, while absent in the southern region. Assuming that the gas is photoionized by a hidden quasar, the difference in NV emission can be explained by a metallicity gradient within the halo. This is consistent with a scenario in which the gas is associated with a massive cooling flow or originates from the debris of the merging of two or more galaxies.

astro-ph↗
arXiv2000

The rise and fall of V4334 Sgr (Sakurai's Object)

CCD UBVRi photometry of the final helium flash object V4334 Sgr (Sakurai's Object), carried out during 1997 - 1999, is presented, and the light curve from its pre-discovery rise to the dust obscuration phase is constructed. The optical light curve can be divided into four sections, the rise to maximum, the maximum, the dust onset, and the massive dust shell phase. The color indices show a general increase with time, first because of the photospheric expansion and cooling, and later because of the dust forming events. The energy distributions for the years 1996 - 1999 show that an increasing part of the energy is radiated at infrared wavelengths. In 1996, the infrared excess is likely caused by free-free radiation in the stellar wind. Starting from 1997 or 1998 at the latest, carbon dust grains are responsible for the more and more dramatic decrease of optical radiation and the growing infrared excess. Its photometric behavior in 1998 - 1999 mimics the ``red declines'' of R CrB variables, the amplitude, however, is more extreme than any fading ever observed in an R CrB star. Evidence is given that a complete dust shell has formed around V4334 Sgr. It therefore shows similarities with dust-forming classical novae, although evolving ~20 times more slowly. Its luminosity increased by a factor 4 between 1996 and 1998. A comparison of time scales of the final helium flash objects FG Sge, V605 Aql and V4334 Sgr shows that the observed photometric and spectroscopic features are similar, while V4334 Sgr is the most rapidly evolving object to date.

astro-ph↗