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P. G. van Dokkum

Publications and source records attributed to P. G. van Dokkum.

At least 55 records · Page 3Linked to original sources
arXiv2010

Very blue UV-continuum slopes of low luminosity z~7 galaxies from WFC3/IR: Evidence for extremely low metallicities?

We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z~7. We determine the UV-continuum slope beta in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in beta. For luminous L*(z=3) galaxies, we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to the beta~-2 derived at similar luminosities at z~5-6. However, for the lower luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is substantially bluer than is found for similar luminosity galaxies at z~4, just 800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected beta becomes >~-2.7. To produce these very blue beta's (i.e., beta~-3), extremely low metallicities and mechanisms to reduce the red nebular emission are likely required. For example, a large escape fraction (i.e., f_{esc}>~0.3) could minimize the contribution from this red nebular emission. If this is correct and the escape fraction in faint z~7 galaxies is >~0.3, it may help to explain how galaxies reionize the universe.

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arXiv2009

Ultradeep Infrared Array Camera Observations of sub-L* z~7 and z~8 Galaxies in the Hubble Ultra Deep Field: the Contribution of Low-Luminosity Galaxies to the Stellar Mass Density and Reionization

We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z=7 z_{850}-dropout galaxies and 5 z=8 Y_{105}-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6 micron and 4.5 micron. None of the galaxies are detected to [3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection for the z_{850}-dropouts and an upper limit for the Y_{105}-dropouts. We construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For the z_{850}-dropouts, we find z=6.9^{+0.1}_{-0.1}, (U-V)_{rest}=0.4, reddening A_V=0, stellar mass M*=1.2^{+0.3}_{-0.6} x 10^9 M_sun (Salpeter IMF). The best-fit ages ~300Myr, M/L_V=0.2, and SSFR=1.7Gyr^{-1} are similar to values reported for luminous z=7 galaxies, indicating the galaxies are smaller but not younger. The sub-L* galaxies observed here contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7<z<11. In contrast, the z=8.3^{+0.1}_{-0.2} Y_{105}-dropouts have stellar masses that are uncertain by 1.5 dex due to the near-complete reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the stellar mass density to M_{UV,AB} < -18 declines from rho*(z=7)=3.7^{+1.0}_{-1.8} x 10^6 M_sun Mpc^{-3} to rho*(z=8) < 8 x 10^5 M_sun Mpc^{-3}, following (1+z)^{-6} over 3<z<8. Lower masses at z=8 would signify more dramatic evolution, which can be established with deeper IRAC observations, long before the arrival of the James Webb Space Telescope.

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arXiv2009

z~7 Galaxies in the HUDF: First Epoch WFC3/IR Results

We present a sample of 16 robust z~7 z-drop galaxies detected by the newly installed WFC3/IR camera on the Hubble Space Telescope. Our analysis is based on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep Field with 60 orbits of Y, J, and H observations. These remarkable data cover 4.7 arcmin^2 and are the deepest NIR images ever taken, reaching to ~29 mag AB (5 sigma). The 16 z~6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z-Y) vs. (Y-J) colors. They have magnitudes J = 26.0-29.0 (AB), an average apparent half-light radius of ~0.16 arcsec (<~1 kpc), and show very blue colors (some even beta<~-2.5), in particular at low luminosities. The WFC3/IR data confirms previous NICMOS detections indicating that the dropout selection at z~7 is very reliable. Our data allow a first determination of the faint end slope of the z~7 luminosity function, reaching down to M_UV ~ -18, a full magnitude fainter than previous measurements. When fixing phi*=1.4e-3/Mpc^3/mag to the value previously measured at z~6, we find a best-fit value of alpha=-1.77+-0.20, with a characteristic luminosity of M_*=-19.91+-0.09. This steep slope is similar to what is seen at z~2-6 and indicates that low luminosity galaxies could potentially provide adequate flux to reionize the universe. The remarkable depth and resolution of these new images provide insights into the coming power of JWST.

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arXiv2009

The Dead Sequence: A Clear Bimodality in Galaxy Colors from z=0 to z=2.5

We select 25,000 galaxies from the NEWFIRM Medium Band Survey (NMBS) to study the rest-frame U-V color distribution of galaxies at 0 < z < 2.5. The five unique NIR filters of the NMBS enable the precise measurement of photometric redshifts and rest-frame colors for 9,900 galaxies at 1 < z < 2.5. The rest-frame U-V color distribution at all z<~2.5 is bimodal, with a red peak, a blue peak, and a population of galaxies in between (the green valley). Model fits to the optical-NIR SEDs and the distribution of MIPS-detected galaxies indicate that the colors of galaxies in the green valley are determined largely by the amount of reddening by dust. This result does not support the simplest interpretation of green valley objects as a transition from blue star-forming to red quiescent galaxies. We show that correcting the rest-frame colors for dust reddening allows a remarkably clean separation between the red and blue sequences up to z~2.5. Our study confirms that dusty starburst galaxies can contribute a significant fraction to red sequence samples selected on the basis of a single rest-frame color (i.e. U-V), so extra care must be taken if samples of truly "red and dead" galaxies are desired. Interestingly, of galaxies detected at 24 microns, 14% remain on the red sequence after applying the reddening correction.

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arXiv2009

The size-star formation relation of massive galaxies at 1.5<z<2.5

We study the relation between size and star formation activity in a complete sample of 225 massive (M > 5 x 10^10 Msun) galaxies at 1.5<z<2.5, selected from the FIREWORKS UV-IR catalog of the CDFS. Based on stellar population synthesis model fits to the observed restframe UV-NIR SEDs, and independent MIPS 24 micron observations, 65% of galaxies are actively forming stars, while 35% are quiescent. Using sizes derived from 2D surface brightness profile fits to high resolution (FWHM_{PSF}~0.45 arcsec) groundbased ISAAC data, we confirm and improve the significance of the relation between star formation activity and compactness found in previous studies, using a large, complete mass-limited sample. At z~2, massive quiescent galaxies are significantly smaller than massive star forming galaxies, and a median factor of 0.34+/-0.02 smaller than galaxies of similar mass in the local universe. 13% of the quiescent galaxies are unresolved in the ISAAC data, corresponding to sizes <1 kpc, more than 5 times smaller than galaxies of similar mass locally. The quiescent galaxies span a Kormendy relation which, compared to the relation for local early types, is shifted to smaller sizes and brighter surface brightnesses and is incompatible with passive evolution. The progenitors of the quiescent galaxies, were likely dominated by highly concentrated, intense nuclear star bursts at z~3-4, in contrast to star forming galaxies at z~2 which are extended and dominated by distributed star formation.

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arXiv2008

Hubble Space Telescope Images of Red Mergers: How Dry are They?

Mergers between red galaxies are observed to be common in the nearby Universe, and are thought to be the dominant mechanism by which massive galaxies grow their mass at late times. These ``dry'' mergers can be readily identified in very deep ground based images, thanks to their extended low surface brightness tidal features. However, ground-based images lack the required resolution to determine the morphologies of the merging galaxies, and to measure the amount of dust and associated gas. We present HST/ACS and WFPC2 observations of a sample of 31 bulge-dominated red-sequence galaxies at z~0.1, comprised of ongoing mergers, merger remnants, and undisturbed galaxies. Nearly all galaxies have early-type morphologies and most are well-fit by r^1/4 law surface brightness profiles. We find that only 10% of the galaxies show evidence for the presence of dust. The amount of cold gas (or its upper limit) is calculated from the mean color-excess, assuming a simple relation between gas mass and dust mass. The gas mass is low for all galaxies, and we find that Mgas/Mstellar < 3x10^-4. We infer that red mergers in the nearby Universe mostly involve early-type galaxies containing little cold gas and dust. This may imply that the progenitors were mostly devoid of gas and/or that feedback mechanisms are very effective in preventing the gas to cool. The lack of gas in these objects may also imply a relatively large fraction of binary black holes in the centers of massive ellipticals.

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arXiv2007

The UV colours of high-redshift early-type galaxies: evidence for recent star formation and stellar mass assembly over the last 8 billion years

We combine deep UBVRIzJK photometry from the Multiwavelength Survey by Yale-Chile (MUSYC) with redshifts from the COMBO-17 survey to perform a large-scale study of the rest-frame ultraviolet (UV) properties of 674 high-redshift (0.5 -20.5) counterparts potentially forming 30-60 percent of their mass in the same redshift range. (abridged)

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arXiv2007

Ly-alpha Emission-Line Galaxies at z = 3.1 in the Extended Chandra Deep Field South

We describe the results of an extremely deep, 0.28 deg^2 survey for z = 3.1 Ly-alpha emission-line galaxies in the Extended Chandra Deep Field South. By using a narrow-band 5000 Anstrom filter and complementary broadband photometry from the MUSYC survey, we identify a statistically complete sample of 162 galaxies with monochromatic fluxes brighter than 1.5 x 10^-17 ergs cm^-2 s^-1 and observers frame equivalent widths greater than 80 Angstroms. We show that the equivalent width distribution of these objects follows an exponential with a rest-frame scale length of w_0 = 76 +/- 10 Angstroms. In addition, we show that in the emission line, the luminosity function of Ly-alpha galaxies has a faint-end power-law slope of alpha = -1.49 +/- 0.4, a bright-end cutoff of log L^* = 42.64 +/- 0.2, and a space density above our detection thresholds of 1.46 +/- 0.12 x 10^-3 h70^3 galaxies Mpc^-3. Finally, by comparing the emission-line and continuum properties of the LAEs, we show that the star-formation rates derived from Ly-alpha are ~3 times lower than those inferred from the rest-frame UV continuum. We use this offset to deduce the existence of a small amount of internal extinction within the host galaxies. This extinction, coupled with the lack of extremely-high equivalent width emitters, argues that these galaxies are not primordial Pop III objects, though they are young and relatively chemically unevolved.

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arXiv2007

Low Star Formation Rates for z=1 Early-Type Galaxies in the Very Deep GOODS-MIPS Imaging: Implications for their Optical/Near-Infrared Spectral Energy Distributions

We measure the obscured star formation in z~1 early-type galaxies. This constrains the influence of star formation on their optical/near-IR colors, which, we found, are redder than predicted by the model by Bruzual & Charlot (2003). From deep ACS imaging we construct a sample of 95 morphologically selected early-type galaxies in the HDF-N and CDF-S with spectroscopic redshifts in the range 0.85 2 sigma detections (~25 muJy} is 17(-4,+9)%. Of the 15 galaxies with significant detections at least six have an AGN. Stacking the MIPS images of the galaxies without significant detections and adding the detected galaxies without AGN we find an upper limit on the mean star formation rate (SFR) of 5.2+/-3.0 Msol yr^-1, and on the mean specific SFR of 4.6+/-2.2 * 10^-11 yr^-1. Under the assumption that the average SFR will decline at the same rate as the cosmic average, the in situ growth in stellar mass of the early-type galaxy population is less than 14+/-7% between z=1 and the present. We show that the typically low IR luminosity and SFR imply that the effect of obscured star formation (or AGN) on their rest-frame optical/near-IR SEDs is negligible for ~90% of the galaxies in our sample. Hence, their optical/near-IR colors are most likely dominated by evolved stellar populations. This implies that the colors predicted by the Bruzual & Charlot (2003) model for stellar populations with ages similar to those of z~1 early-type galaxies (~1-3 Gyr) are most likely too blue, and that stellar masses of evolved, high-redshift galaxies can be overestimated by up to a factor of ~2.

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arXiv2006

Spectroscopy of z~6 i-dropout Galaxies: Frequency of Ly-alpha Emission and the Sizes of Ly-alpha Emitting Galaxies

We report on deep spectroscopy using LRIS on Keck I and FORS2 on the VLT of a sample of 22 candidate z~6 Lyman Break galaxies (LBGs) selected by the i-z> 1.3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5.5 - 6.1, confirming the efficiency of the i-z color selection technique. Six of the confirmed galaxies show Ly-alpha emission. Assuming that the 14 objects without redshifts are z~6 LBGs, but lack detectable Ly-alpha emission lines, we infer that the fraction of Ly-alpha emitting LBGs with Ly-alpha equivalent widths greater than 20 Angstroms among z~6 LBGs is ~30%, similar to that found at z~3. Every Ly-alpha emitting object in our sample is compact with r <= 0.14". Furthermore, all the Ly-alpha emitting objects in our sample are more compact than average relative to the observed size-magnitude relation of a large i-dropout sample (332 candidate z~6 objects). We can reject the hypothesis that the Ly-alpha emitting population is a subset of the rest of the z~6 LBG population at >97% confidence. We speculate the small sizes of Ly-alpha emitting LBGs are due to these objects being less massive than other LBGs at z~6.

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arXiv2006

Could GRB 060614 and its presumed host galaxy be a chance superposition?

The lack of an observed supernova associated with GRB 060614 appears to require a new paradigm for the formation of (a subset of) long-duration GRBs. This requirement is based on the presumed low redshift of the burst, which was inferred from the spatial coincidence of the afterglow with a z=0.125 galaxy. We explore the possibility that this low-redshift galaxy is a chance superposition along the line of sight to GRB 060614. We examine the galaxy distribution of the field of GRB 060614 and find that the probability of a chance association with a galaxy at least as bright as the putative host is only ~ 0.5 - 1.9%. However, for the current ensemble of ~180 Swift GRBs it is likely that several such coincidences have occurred, and given the "non-standard" nature of GRB 060614 it is not implausible that this is one such occurrence. Thus the conclusion that GRB 060614 requires a revision to the formation paradigm for long-duration GRBs should be approached with caution.

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arXiv2006

Line Strengths in Early-Type Cluster Galaxies at z=0.33: Implications for alpha/Fe, Nitrogen and the Histories of E/S0s

[Heavily Abbreviated] In this paper we analyze previously published spectra with high signal-to-noise ratios of E/S0 galaxies in the rich cluster CL1358+62 at z=0.33, and introduce techniques for fitting stellar population models to the data. Here we focus on the 19 E and S0 galaxies with an homogeneous set of eight blue Lick indices. We explore the galaxy properties using six-parameter stellar population models from the literature, and describe an approach for fitting the models differentially, such that the largest systematic errors are avoided. We find: (1) no differences between the stellar population parameters of Es and S0s, at fixed sigma; (2) the stars in the Es and S0s are uniformly old, consistent with previously published results using M/L ratios; (3) a significant correlation of [Z/H] with sigma, in a manner consistent with the observed B-V colors of the galaxies; (4) no significant correlation of [alpha/Fe] with sigma; and (5) a significant anti-correlation of [alpha/N] with [Z/H], which we interpret as the signature of secondary nitrogen. Neither [alpha/C], nor [alpha/Ca] shows significant variation. While the differences between our conclusions and the current view of stellar populations may point to serious deficiencies, our deduced correlation of mean metallicity with sigma does reproduce the B-V colors of the galaxies, as well as the slope of the local Mg-sigma relation. In matching the inferred population trends with published data on nearby galaxies, the line strength-line width relations match well, save for the narrow iron indices. Taken together, these results reduce early-type galaxies in clusters to a family with one-parameter, velocity dispersion, greatly simplifying scenarios for their formation and evolution.

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arXiv2006

SN 2006aj and the nature of low-luminosity gamma-ray bursts

We present SMARTS consortium optical/IR light curves of SN 2006aj associated with GRB 060218. We find that this event is broadly similar to two previously observed events SN 1998bw/GRB 980425 and SN 2003lw/GRB 031203. In particular, all of these events are greatly under-luminous in gamma-rays compared to typical long-duration GRBs. We find that the observation by Swift of even one such event implies a large enough true event rate to create difficulties in interpreting these events as typical GRBs observed off-axis. Thus these events appear to be intrinsically different from and much more common than high-luminosity GRBs, which have been observed in large numbers out to a redshift of at least 6.3. The existence of a range of intrinsic energies of GRBs may present challenges to using GRBs as standard candles.

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arXiv2005

The Evolution of Rest-Frame K-band Properties of Early-Type Galaxies from z=1 to the Present

We measure the evolution of the rest-frame K-band Fundamental Plane from z=1 to the present by using IRAC imaging of a sample of early-type galaxies in the Chandra Deep Field-South at z~1 with accurately measured dynamical masses. We find that $M/L_K$ evolves as $Δ\ln{(M/L_K)}=(-1.18\pm0.10)z$, which is slower than in the B-band ($Δ\ln{(M/L_B)}=(-1.46\pm0.09)z$). In the B-band the evolution has been demonstrated to be strongly mass dependent. In the K-band we find a weaker trend: galaxies more massive than $M=2\times10^{11}M_{\odot}$ evolve as $Δ\ln{(M/L_K)}=(-1.01\pm0.16)z$; less massive galaxies evolve as $Δ\ln{(M/L_K)}=(-1.27\pm0.11)z$. As expected from stellar population models the evolution in $M/L_K$ is slower than the evolution in $M/L_B$. However, when we make a quantitative comparison, we find that the single burst Bruzual-Charlot models do not fit the results well, unless large dust opacities are allowed at z=1. Models with a flat IMF fit better, Maraston models with a different treatment of AGB stars fit best. These results show that the interpretation of rest-frame near-IR photometry is severely hampered by model uncertainties and therefore that the determination of galaxy masses from rest-frame near-IR photometry may be harder than was thought before.

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arXiv2005

Faint InfraRed Extragalactic Survey: Data and Source Catalogue of the MS1054-03 field

We present deep near-infrared Js, H, and Ks band imaging of a field around MS1054-03, a massive cluster at z=0.83. The observations were carried out with ISAAC at the ESO VLT as part of the Faint InfraRed Extragalactic Survey (FIRES). The total integration time amounts to 25.9h in Js, 24.4h in H, and 26.5h in Ks, divided nearly equally between four pointings covering 5.5'x5.3'. The 3-sigma total limiting AB magnitudes for point sources from the shallowest to deepest pointing are Js=26.0-26.2, H=25.5-25.8, and Ks=25.3-25.7. The effective spatial resolution of the coadded images has FWHM=0.48", 0.46", and 0.52" in Js, H, and Ks. We complemented the ISAAC data with deep optical imaging using existing HST WFPC2 mosaics in the F606W and F814W filters and new U, B and V band data from VLT FORS1. We constructed a Ks-band limited multicolour source catalogue to Ks(total,AB)=25 (about 5-sigma for point sources). The catalogue contains 1858 objects, of which 1663 have eight-band photometry. We describe the observations, data reduction, source detection and photometric measurements method. We present the number counts, colour distributions, and photometric redshifts z_ph of the catalogue sources. We find that our counts at the faint end 22 =0.078. The MS1054-03 field observations complement our HDF-South data set with nearly five times larger area at about 0.7 brighter magnitudes. [ABRIDGED]

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arXiv2005

The Multiwavelength Survey by Yale-Chile (MUSYC): Survey Design and Deep Public UBVRIz' Images and Catalogs of the Extended Hubble Deep Field South

We present UBVRIz' optical images of the 0.32 square degree Extended Hubble Deep Field South. This is one of four fields comprising the MUSYC survey, which is optimized for the study of galaxies at z=3, AGN demographics, and Galactic structure. We calculate corrected aperture photometry and its uncertainties and find through tests that these provide a significant improvement upon standard techniques. Our photometric catalog of 62968 objects is complete to a total magnitude of R_AB=25. We select z=3 Lyman break galaxy (LBG) candidates from their UVR colors and find a sky surface density of 1.4/sq. arcmin and an angular correlation function w(theta) = 2.3+-1.0 theta^{-0.8}. (Abridged)

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arXiv2005

Submillimeter observations of Distant Red Galaxies: uncovering the 1mJy 850um-population

We present a study of the submillimeter (submm) emission of Distant Red Galaxies (DRGs). The DRGs are selected by the criterion (J-K)>2.3, and are generally massive galaxies at redshifts higher than 2, with red rest-frame optical colors. Using a deep SCUBA submm image of a field centred on the cluster MS1054-03, we obtain a statistical detection of the DRGs at redshift z=2-3.5, with an average 850um flux density of 1.11+-0.28mJy. The detection implies an average star formation rate (SFR) of 127+-34 M_sun/yr (lensing corrected), assuming that the far-infrared (FIR) spectral energy distribution (SED) is well-described by a modified blackbody. The SFR derived from the submm agrees well with SFRs derived from SED fitting of optical-near-infrared data and average X-ray emission. Constant Star Formation models imply ages of 2Gyr, extinction A_V=2.4 mag, which is consistent with the FIR to rest-frame optical luminosity ratio of ~15. DRGs are older and have lower SFRs relative to optical luminosity than (ultra-)luminous infrared galaxies, although their FIR luminosities are similar. The DRGs at 2 4) at 1<z<2, which were also investigated, contribute 5.7 and 5.9 Jy deg^-2 respectively to the submm background. Simple estimates suggest that these populations contribute ~50% of the flux from sources with 0.5 < f_850 < 5 mJy, which is where the peak of energy is produced. We have therefore uncovered one of the most important populations of galaxies contributing to the sub-mm background.

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