arXiv subjects

Pieter van Dokkum

Publications and source records attributed to Pieter van Dokkum.

At least 289 records · Page 16Linked to original sources

arXiv2007

Spitzer Constraints on the Stellar Populations of Lyman-Alpha Emitting Galaxies at z = 3.1

We investigate the stellar populations of a sample of 162 Lyman-alpha emitting galaxies (LAEs) at z = 3.1 in the Extended Chandra Deep Field South, using deep Spitzer IRAC data available from the GOODS and SIMPLE surveys to derive reliable stellar population estimates. We divide the LAEs according to their rest-frame near-IR luminosities into IRAC-detected and IRAC-undetected samples. About 70% of the LAEs are undetected in 3.6 micron down to [3.6] = 25.2 AB. Stacking analysis reveals that the average stellar population of the IRAC-undetected sample has an age of ~ 200 Myr and a mass of ~ 3x10^8 solar masses, consistent with the expectation that LAEs are mostly young and low-mass galaxies. On the other hand, the IRAC-detected LAEs are on average significantly older and more massive, with an average age > 1 Gyr and mass ~ 10^10 solar masses. Comparing the IRAC colors and magnitudes of the LAEs to z ~ 3 Lyman break galaxies (LBGs) shows that the IRAC-detected LAEs lie at the faint blue end of the LBG color-magnitude distribution, suggesting that IRAC-detected LAEs may be the low mass extension of the LBG population. We also present tentative evidence for a small fraction (~ 5%) of obscured AGN within the LAE sample. Our results suggest that LAEs posses a wide range of ages and masses. Additionally, the presence of evolved stellar populations inside LAEs suggests that the Lyman-alpha luminous phase of galaxies may either be a long-lasting or recurring phenomenon.

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arXiv2007

The Warm Spitzer Mission: Prospects for Studies of the Distant Universe

IRAC excels at detecting distant objects. Due to a combination of the shapes of the spectral energy distributions of galaxies and the low background achieved from space, IRAC reaches greater depth in comparable exposure time at 3.6 and 4.5 micron than any ground- or space-based facility currently can at 2.2 micron. Furthermore, the longer wavelengths probed by IRAC enable studies of the rest-frame optical and near-infrared light of galaxies and AGN to much higher redshift than is possible from the ground. This white paper explores the merits of different survey strategies for studying the distant universe during the warm mission. A three-tiered approach serves a wide range of science goals and uses the spacecraft effectively: 1) an ultra-deep survey of ~0.04 square degrees to a depth of ~250 hrs (in conjunction with an HST/WFC3 program), to study the Universe at 7 6 and characterize the relation between the build-up of dark matter halos and their constituent galaxies at 2<z<6, and 3) a 500 square degree survey to the SWIRE depth of 120 s, to systematically study large scale structure at 1<z<2 and characterize high redshift AGN. One or more of these programs could conceivably be implemented by the SSC, following the example of the Hubble Deep Field campaigns. As priorities in this field continuously shift it is also crucial that a fraction of the exposure time remains unassigned, thus enabling science that will reflect the frontiers of 2010 and beyond rather than those of 2007.

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arXiv2007

Galaxy Evolution and Environment

The properties of galaxies are strongly correlated with their environment, with red galaxies dominating galaxy clusters and blue galaxies dominating the general field. However, not all field galaxies are young: studies of the colors, line strengths, and M/L ratios of massive early-type galaxies at 0<z<1.3 show that the most massive galaxies do not seem to care about their surroundings, and have very similar ages irrespective of their environment. There is good evidence that the growth of these galaxies does continue longer in the field than in clusters, via (nearly) dissipationless mergers of already old galaxies. These results are consistent with predictions of recent galaxy formation models, which incorporate AGN feedback to suppress star formation in the most massive halos. Systematic studies of the relation of galaxies with their environment beyond z=1 are difficult, and still somewhat contradictory. Intriguingly both the DEEP2 and VVDS surveys find that the color-density relation disappears at z~1.3, unfortunately just at the point where both surveys become highly incomplete. On the other hand, clustering studies at z~2.5 have shown that red galaxies cluster more strongly than blue galaxies, implying that the color-density relation was already in place at that redshift.

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arXiv2007

The rest-frame optical luminosity functions of galaxies at 2<z<3.5

We present the rest-frame optical (B, V, and R band) luminosity functions (LFs) of galaxies at 2 2 are consistent with those at z~0. The characteristic magnitudes are significantly brighter than the local values (e.g., ~1.2 mag in the R band), while the measured values for Phi_star are typically ~5 times smaller. The B-band luminosity density at z~2.3 is similar to the local value, and in the R band it is ~2 times smaller than the local value. We present the LF of Distant Red Galaxies (DRGs), which we compare to that of non-DRGs. While DRGs and non-DRGs are characterized by similar LFs at the bright end, the faint-end slope of the non-DRG LF is much steeper than that of DRGs. The contribution of DRGs to the global densities down to the faintest probed luminosities is 14%-25% in number and 22%-33% in luminosity. From the derived rest-frame U-V colors and stellar population synthesis models, we estimate the mass-to-light ratios (M/L) of the different subsamples. The M/L ratios of DRGs are ~5 times higher (in the R and V bands) than those of non-DRGs. The global stellar mass density at 2<z<3.5 appears to be dominated by DRGs, whose contribution is of order ~60%-80% of the global value. Qualitatively similar results are obtained when the population is split by rest-frame U-V color instead of observed J-K color.

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arXiv2007

A Keck Spectroscopic Survey of MS1054-03 (z=0.83): Forming the Red Sequence

Using a magnitude-limited, spectroscopic survey of the X-ray luminous galaxy cluster MS1054-03, we isolate 153 cluster galaxies and measure MS1054's redshift and velocity dispersion to be z=0.8307 and sigma_z=1156 km/s. The absorption-line, post-starburst (``E+A''), and emission-line galaxies respectively make up 63%, 15%, and 23% of the cluster population. With photometry from HST/ACS, we find that the absorption-line members define an exceptionally tight red sequence over a span of ~3.5 magnitudes in i775: their intrinsic scatter in (V-i) color is only 0.048, corresponding to a (U-B)_z scatter of 0.041. Their color scatter is comparable to that of the ellipticals (sigma_Vi=0.055), but measurably smaller than that of the combined E+S0 sample (sigma_Vi=0.072). The color scatter of MS1054's absorption-line population is approximately twice that of the ellipticals in Coma; this difference is consistent with passive evolution where most of the absorption-line members (>75%) formed by z~2, and all of them by z~1.2. For red members, we find a trend (>95% confidence) of weakening Hdelta absorption with redder colors that we conclude is due to age: in MS1054, the color scatter on the red sequence is driven by differences in mean stellar age of up to ~1.5 Gyr. We also generate composite spectra and estimate that the average S0 in MS1054 is ~0.5-1 Gyr younger than the average elliptical; this difference in mean stellar age is mainly due to a number of S0s that are blue (18%) and/or are post-starburst systems (21%).

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arXiv2006

Clustering of K-selected Galaxies at 2<z<3.5: Evidence for a Color-Density Relation

We study the clustering properties of K-selected galaxies at 2 2.3 distant red galaxies (DRGs) have r_0 ~ 11 h^-1 Mpc. Contrary to findings for optically-selected galaxies, K-selected galaxies that are faint in the R-band cluster more strongly than brighter galaxies. These results suggest that a color-density relation was in place at z>2. Our results indicate that K-bright blue galaxies and K-bright red galaxies are fundamentally different, as they have different clustering properties. Using a simple model of one galaxy per halo, we infer halo masses ~ 5 x 10^12 M_sun for K<21 galaxies and ~ 2 x 10^13 M_sun for DRGs. A comparison of the observed space density of DRGs to the density of their host halos suggests large halo occupation numbers; however, this result is at odds with the lack of a strong small-scale excess in the angular correlation function. We find that the z=0 descendants of the galaxies considered here reside primarily in groups and clusters. [abridged]

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arXiv2006

The Multiwavelength Survey by Yale-Chile (MUSYC): Deep Near-Infrared Imaging and the Selection of Distant Galaxies

We present deep near-infrared JHK imaging of four 10'x10' fields. The observations were carried out as part of the Multiwavelength Survey by Yale-Chile (MUSYC) with ISPI on the CTIO 4m telescope. The typical point source limiting depths are J~22.5, H~21.5, and K~21 (5sigma; Vega). The effective seeing in the final images is ~1.0". We combine these data with MUSYC UBVRIz imaging to create K-selected catalogs that are unique for their uniform size, depth, filter coverage, and image quality. We investigate the rest-frame optical colors and photometric redshifts of galaxies that are selected using common color selection techniques, including distant red galaxies (DRGs), star-forming and passive BzKs, and the rest-frame UV-selected BM, BX, and Lyman break galaxies (LBGs). These techniques are effective at isolating large samples of high redshift galaxies, but none provide complete or uniform samples across the targeted redshift ranges. The DRG and BM/BX/LBG criteria identify populations of red and blue galaxies, respectively, as they were designed to do. The star-forming BzKs have a very wide redshift distribution, a wide range of colors, and may include galaxies with very low specific star formation rates. In comparison, the passive BzKs are fewer in number, have a different distribution of K magnitudes, and have a somewhat different redshift distribution. By combining these color selection criteria, it appears possible to define a reasonably complete sample of galaxies to our flux limit over specific redshift ranges. However, the redshift dependence of both the completeness and sampled range of rest-frame colors poses an ultimate limit to the usefulness of these techniques.

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arXiv2006

Spectroscopic Identification of Massive Galaxies at z~2.3 with Strongly Suppressed Star Formation

We present first results of a spectroscopic survey targeting K-selected galaxies at z=2.0-2.7 using the GNIRS instrument on Gemini-South. We obtained near-infrared spectra with a wavelength coverage of 1.0-2.5 micron for 26 K-bright galaxies (K<19.7) selected from the MUSYC survey using photometric redshifts. We successfully derived spectroscopic redshifts for all 26 galaxies using rest-frame optical emission lines or the redshifted Balmer/4000 Angstrom break. Twenty galaxies have spectroscopic redshifts in the range 2.0<z<2.7, for which bright emission lines like Halpha and [OIII] fall in atmospheric windows. Surprisingly, we detected no emission lines for nine of these 20 galaxies. The median 2 sigma upper limit on the rest-frame equivalent width of Halpha for these nine galaxies is ~10 Angstrom. The stellar continuum emission of these same nine galaxies is best fitted by evolved stellar population models. The best-fit star formation rate (SFR) is zero for five out of nine galaxies, and consistent with zero within 1 sigma for the remaining four. Thus, both the Halpha measurements and the independent stellar continuum modeling imply that 45% of our K-selected galaxies are not forming stars intensely. This high fraction of galaxies without detected line emission and low SFRs may imply that the suppression of star formation in massive galaxies occurs at higher redshift than is predicted by current CDM galaxy formation models. However, obscured star formation may have been missed, and deep mid-infrared imaging is needed to clarify this situation.

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arXiv2006

Direct Measurements of the Stellar Continua and Balmer/4000 Angstrom Breaks of Red z>2 Galaxies: Redshifts and Improved Constraints on Stellar Populations

We use near-infrared (NIR) spectroscopy obtained with GNIRS on Gemini, NIRSPEC on KECK, and ISAAC on the VLT to study the rest-frame optical continua of three `Distant Red Galaxies' (having Js - Ks > 2.3) at z>2. All three galaxy spectra show the Balmer/4000 Angstrom break in the rest-frame optical. The spectra allow us to determine spectroscopic redshifts from the continuum with an estimated accuracy dz/(1+z) ~ 0.001-0.04. These redshifts agree well with the emission line redshifts for the 2 galaxies with Halpha emission. This technique is particularly important for galaxies that are faint in the rest-frame UV, as they are underrepresented in high redshift samples selected in optical surveys and are too faint for optical spectroscopy. Furthermore, we use the break, continuum shape, and equivalent width of Halpha together with evolutionary synthesis models to constrain the age, star formation timescale, dust content, stellar mass and star formation rate of the galaxies. Inclusion of the NIR spectra in the stellar population fits greatly reduces the range of possible solutions for stellar population properties. We find that the stellar populations differ greatly among the three galaxies, ranging from a young dusty starburst with a small break and strong emission lines to an evolved galaxy with a strong break and no detected line emission. The dusty starburst galaxy has an age of 0.3 Gyr and a stellar mass of 1*10^11 Msun. The spectra of the two most evolved galaxies imply ages of 1.3-1.4 Gyr and stellar masses of 4*10^11 Msun. The large range of properties seen in these galaxies strengthens our previous much more uncertain results from broadband photometry. Larger samples are required to determine the relative frequency of dusty starbursts and (nearly) passively evolving galaxies at z~2.5.

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arXiv2005

Star Formation in Distant Red Galaxies: Spitzer observations in the Hubble Deep Field South

We present Spitzer 24micron imaging of 1.5 < z < 2.5 Distant Red Galaxies (DRGs) in the 10arcmin by 10arcmin Extended Hubble Deep Field South of the Multiwavelength Survey by Yale-Chile. We detect 65% of the DRGs with K_AB < 23.2 mag at S_24micron > 40uJy, and conclude that the bulk of the DRG population are dusty active galaxies. A mid-infrared (MIR) color analysis with IRAC data suggests that the MIR fluxes are not dominated by buried AGN, and we interpret the high detection rate as evidence for a high average star formation rate of = 130+/-30 M/yr. From this, we infer that DRGs are important contributors to the cosmic star formation rate density at z ~ 2, at a level of \~0.02 M/yr/Mpc^3 to our completeness limit of K_AB = 22.9 mag

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arXiv2005

Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS1054-03 (z=0.83)

We present follow-up spectroscopy of the galaxy cluster MS1054-03 (z=0.83) confirming that at least six of the nine merging galaxy pairs identified by van Dokkum et al. (1999) are indeed bound systems: they have projected separations of R_s<10 kpc and relative line-of sight velocities of dv<165 km/s. For the remaining three pairs, we were unable to obtain redshifts of both constituent galaxies. To identify a more objective sample of merging systems, we select bound red galaxy pairs (R_s<=30 kpc, dv<=300 km/s) from our sample of 121 confirmed cluster members: galaxies in bound red pairs make up 15.7+/-3.6% of the cluster population. The (B-K_s) color-magnitude diagram shows that the pair galaxies are as red as the E/S0 members and have a homogeneous stellar population. The red pair galaxies span a large range in luminosity and internal velocity dispersion to include some of the brightest, most massive members (L>L*, sigma>200 km/s); these bound galaxy pairs must evolve into E/S0 members by z~0.7. These results combined with MS1054's high merger fraction and reservoir of likely future mergers indicates that most, if not all, of its early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.

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arXiv2004

Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6

Using wide-field HST/WFPC2 imaging and extensive Keck/LRIS spectroscopy, we present a detailed study of the galaxy populations in MS2053--04, a massive, X-ray luminous cluster at z=0.5866. Analysis of 149 confirmed cluster members shows that MS2053 is composed of two structures that are gravitationally bound to each other; their respective velocity dispersions are 865 km/s (113 members) and 282 km/s (36 members). MS2053's total dynamical mass is 1.2x10^15 Msun. MS2053 is a classic Butcher-Oemler cluster with a high fraction of blue members (24%) and an even higher fraction of star-forming members (44%), as determined from their [OII] emission. The number fraction of blue/star-forming galaxies is much higher in the infalling structure than in the main cluster. This result is the most direct evidence to date that the Butcher-Oemler effect is linked to galaxy infall. In terms of their colors, luminosities, estimated internal velocity dispersions, and [OII] equivalent widths, the infalling galaxies are indistinguishable from the field population. MS2053's deficit of S0 galaxies combined with its overabundance of blue spirals implies that many of these late-types will evolve into S0 members. The properties of the blue cluster members in both the main cluster and infalling structure indicate they will evolve into low mass, L 3.

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arXiv2004

Galaxies at z~7-8: z-dropouts in the Hubble Ultra Deep Field

We have detected likely z~7-8 galaxies in the 144"x144" NICMOS observations of the Hubble Ultra Deep Field. Objects are required to be >3 sigma detections in both NICMOS bands, J and H. The selection criteria for this sample are (z-J)>0.8, (z-J) > 0.66 (J-H)+0.8, (J-H)<1.2, and no detection at <8500 Angstroms. The 5 selected sources have total magnitudes H~27. Four of the five sources are quite blue compared to typical lower-redshift dropout galaxies and are clustered within a 1 arcmin**2 region. Because all 5 sources are near the limit of the NICMOS data, we have carefully evaluated their reality. Each of the candidates is visible in different splits of the data and a median stack. We analyzed several noise images and estimate the number of spurious sources to be 1+/-1. A search using an independent reduction of this same data set clearly revealed 3 of the 5 candidates and weakly detected a 4th candidate, suggesting the contamination could be higher. For comparison with predictions from lower redshift samples we take a conservative approach and adopt four z~7-8 galaxies as our sample. With the same detection criteria on simulated datasets, assuming no-evolution from z~3.8, we predict 10 sources at z~7-8, or 14 if we use a more realistic (1+z)^{-1} size scaling. We estimate that the rest-frame continuum UV (~1800 Angstroms) luminosity density at z~7.5 (integrated down to 0.3L*) is just 0.20_{-0.08}^{+0.12}x that found at z~3.8. Effectively this sets an upper limit on the luminosity density down to 0.3 L*. This result is consistent with significant evolution at the bright end of the luminosity function from z~7.5 to z~3.8. Even with the lower UV luminosity density at z~7.5, it appears that galaxies could still play an important role in reionization at these redshifts, though definitive measurements remain to be made. [abridged]

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arXiv2004

Stellar Populations and Kinematics of Red Galaxies at z>2: Implications for the Formation of Massive Galaxies

We recently identified a substantial population of galaxies at z>2 with red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion J-K>2.3. In this paper we present NIR spectroscopy with Keck/NIRSPEC of six DRGs at 2.4<z<3.2. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of H alpha are 20-30 Ang, smaller than measured for LBGs and nearby LIRGs, and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate, or that they are very dusty. Fitting both the photometry and the H alpha lines, we find continuum extinction A_V=1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 solar masses/yr, and stellar masses 1-5x10^11 solar masses for models with constant star formation rates. From [NII]/H alpha ratios we infer that the metallicities are high, 1-1.5 x Solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130-240 km/s, and by combining data for LBGs and DRGs we find significant correlations between linewidth and restframe U-V color, and between linewidth and stellar mass. The latter correlation has a similar slope and offset as the ``baryonic Tully-Fisher relation'' for nearby galaxies. The median dynamical mass is ~2x10^11 solar masses, supporting the high stellar masses inferred from the photometry. We find that the median M/L_V ~ 0.8, a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal rich, more massive, and have higher ages than z=3 LBGs of the same rest-frame V-band luminosity. Their high M/L ratios imply that they contribute significantly to the stellar mass density at z~2.5. [ABRIDGED]

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arXiv2004

Field E+A Galaxies at Intermediate Redshifts (0.3<z<1)

We select E+A candidates from a spectroscopic dataset of ~800 field galaxies and measure the E+A fraction at 0.3<z<1 to be 2.7+/-1.1%, a value lower than that in galaxy clusters at comparable redshifts (11+/-3%). HST/WFPC2 imaging for five of our six E+A's shows they have a heterogeneous parent population: these E+A's span a range in half-light radius (0.8-8 kpc) and estimated internal velocity dispersion (50-220 km/s), and they include luminous systems (-21.6<M_Bz-5logh<-19.2). Despite their diversity in some aspects, the E+A's share several common characteristics that indicate the E+A phase is an important link in the evolution of star-forming galaxies into passive systems: the E+A's are uniformly redder than the blue, star-forming galaxies that make up the majority of the field, they are more likely to be bulge-dominated than the average field galaxy, and they tend to be morphologically irregular. We find E+A's make up ~9% of the absorption line systems in this redshift range, and estimate that ~25% of passive galaxies in the local field had an E+A phase at z<1.

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arXiv2003

The Nature of E+A Galaxies in Intermediate Redshift Clusters

[Abridged] Combining HST/WFPC2 mosaics with extensive ground-based spectroscopy, we study the nature of E+A galaxies in three intermediate redshift clusters (z=0.33, 0.58, & 0.83). From a sample of ~500 confirmed cluster members, we isolate 46 E+A candidates to determine the E+A fraction and study their physical properties. We find E+A's comprise a non-negligible component (~7-13%) of the cluster population at these redshifts, and their diverse nature indicates a heterogeneous parent population. Cluster E+A's span the range in Hubble type and cover a wide range in luminosity, internal velocity dispersion, and half-light radius. From their velocity dispersions and half-light radii, we infer that the descendants of E+A's in our highest redshift cluster are massive early-type galaxies. We find a decrease in the characteristic E+A mass similar to the decrease in luminosity of rapidly star-forming field galaxies since z~1, i.e. galaxy ``down-sizing.'' In addition, we argue our statistics imply that >30% of the E-S0 members have undergone an E+A phase; the true fraction could be 100% if the effects of E+A down-sizing, an increasing E+A fraction with redshift, and the conversion of spirals into early-types are also considered. Thus, the E+A phase may indeed be an important stage in the transformation of star-forming galaxies into early-type cluster members.

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arXiv2001

Hydrogen 21-Centimeter Emission from a Galaxy at Cosmological Distance

We have detected the neutral atomic hydrogen (HI) emission line at a cosmologically significant distance (z=0.18) in the rich galaxy cluster Abell 2218 with the Westerbork Synthesis Radio Telescope. The HI emission originates in a spiral galaxy 2.0 megaparsecs from the cluster core. No other significant detections have been made in the cluster, suggesting that the mechanisms that remove neutral gas from cluster galaxies are efficient. We infer that fewer than three gas-rich galaxies were accreted by Abell 2218 over the past 10^9 years. This low accretion rate is qualitatively consistent with low-density cosmological models in which clusters are largely assembled at z>1.

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arXiv2001

A K-band Selected Photometric Redshift Catalog in the HDF-S: Sampling the Rest-Frame V-Band to z=3

We present the first results from the Faint Infra-Red Extragalactic Survey (FIRES) of the Hubble Deep Field South. Using a combination of deep near infrared data obtained with ISAAC at the VLT with the WFPC2 HST data, we construct a K-band selected sample of 136 galaxies with K(AB)<=23.5. We derive zphot's using a newly developed technique which models the observed spectral energy distribution with a linear combination of empirical galaxy templates. Testing our zphot technique against spectroscopic redshifts in the HDF-N yields dz/(1+z)~0.07 for z<6. We show that we can derive realistic error estimates in zphot by combining the systematic uncertainties derived from the HDF-N with errors in zphot which depend on the observed flux errors. The redshift histogram of galaxies in the HDF-S shows distinct structure with a sharp peak at z~0.5 and a broad enhancement at z~1-1.4. We find that 12% of our galaxies with K(vega)<21 lie at z>=2. While this is higher than the fraction predicted in Omega_M=1 hierarchical models of galaxy formation we find that published predictions using pure luminosity evolution models produce too many bright galaxies at redshifts greater than unity. Finally, we use our broad wavelength coverage to measure the rest-frame UBV luminosities Lrest for z<=3. There is a paucity of galaxies brighter than Lrest_V>=1.4e10 h^{-2}Lsun at z~1.5-2, however, at z>2 we find very luminous galaxies with Lrest_V>=5e10 h^{-2}Lsun (for Omega_M=0.3,Lambda=0.7, H_o=100 h km s^{-1}Mpc^{-1}). Local B-band luminosity functions predict 0.1 galaxies in the redshift range 2<=z<=3.5 and with Lrest_B>= 5e10 h^{-2}Lsun,B but we find 9. The discrepancy can be explained if L*_B increases by a factor of 2.4-3.2 with respect to locally determined values (abridged).

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