arXiv subjects

Rogier A. Windhorst

Publications and source records attributed to Rogier A. Windhorst.

At least 217 records · Page 12Linked to original sources

arXiv2007

The Nearby and Extremely Metal-Poor Galaxy CGCG 269-049

We present Hubble Space Telescope (HST) and Spitzer Space Telescope images and photometry of the extremely metal-poor (Z = 0.03 Z_sol) blue dwarf galaxy CGCG 269-049. The HST images reveal a large population of red giant and asymptotic giant branch stars, ruling out the possibility that the galaxy has recently formed. From the magnitude of the tip of the red giant branch, we measure a distance to CGCG 269-049 of only 4.9 +/- 0.4 Mpc. The spectral energy distribution of the galaxy between ~3.6 - 70 microns is also best fitted by emission from predominantly ~10 Gyr old stars, with a component of thermal dust emission having a temperature of 52 +/- 10 K. The HST and Spitzer photometry indicate that more than 60% of CGCG 269-049's stellar mass consists of stars ~10 Gyr old, similar to other local blue dwarf galaxies. Our HST H-alpha image shows no evidence of a supernova-driven outflow that could be removing metals from the galaxy, nor do we find evidence that such outflows occurred in the past. Taken together with CGCG 269-049's large ratio of neutral hydrogen mass to stellar mass (~10), these results are consistent with recent simulations in which the metal deficiency of local dwarf galaxies results mainly from inefficient star formation, rather than youth or the escape of supernova ejecta.

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arXiv2007

Star Clusters in the Nearby Late-Type Galaxy NGC 1311

Ultraviolet, optical and near infrared images of the nearby (D ~ 5.5 Mpc) SBm galaxy NGC 1311, obtained with the Hubble Space Telescope, reveal a small population of 13 candidate star clusters. We identify candidate star clusters based on a combination of their luminosity, extent and spectral energy distribution. The masses of the cluster candidates range from ~1000 up to ~100000 Solar masses, and show a strong positive trend of larger mass with increasing with cluster age. Such a trend follows from the fading and dissolution of old, low-mass clusters, and the lack of any young super star clusters of the sort often formed in strong starbursts. The cluster age distribution is consistent with a bursting mode of cluster formation, with active episodes of age ~10 Myr, ~100 Myr and ~1 Gyr. The ranges of age and mass we probe are consistent with those of the star clusters found in quiescent Local Group dwarf galaxies.

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arXiv2006

Ultracompact Blue Dwarf Galaxies: Hubble Space Telescope Imaging and Stellar Population Analysis

We present deep Hubble Space Telescope Advanced Camera for Surveys / High Resolution Channel U, narrow-V, and I images of nine "ultracompact" blue dwarf galaxies (UCBDs) selected from the Sloan Digital Sky Survey. We define UCBDs as local (z < 0.01) star-forming galaxies having angular diameters < 6" and physical sizes < 1 kpc. They are also among the most metal-poor galaxies known, and are found to reside within voids. Both the HST images and the objects' optical spectra reveal that they are composites of young (~1 Myr) populations that dominate their light, and older (~10 Gyr) populations that dominate their stellar masses, which we estimate to be ~10^7 - 10^8 Msol. An intermediate-age population is also indicated in most cases. The objects are not as dynamically disturbed as the prototype UCBD, POX 186, but the structure of several of them suggests that their current starbursts have been triggered by the collisions/mergers of smaller clumps of stars. In one case, HS 0822+3542, the ACS/HRC images resolve the object into two small (~100 pc) components which appear to have recently collided, supporting this interpretation. In six of the objects much of their star formation is concentrated in Young Massive Star clusters. The evidence that the galaxies consist mainly of ~10 Gyr old stars establishes that they are not protogalaxies; their low metallicities are more likely to be the result of the escape of supernova ejecta, as opposed to youth. These results are consistent with recent galaxy formation simulations which predict that cosmic re-ionization at z ~ 6 significantly limited the subsequent star formation of dwarf galaxies in voids due to the photo-evaporation of baryons from their cold dark matter halos (Abridged).

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arXiv2006

The James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large (6.6m), cold (50K), infrared-optimized space observatory that will be launched early in the next decade. The observatory will have four instruments: a near-infrared camera, a near-infrared multi-object spectrograph, and a tunable filter imager will cover the wavelength range, 0.6 to 5.0 microns, while the mid-infrared instrument will do both imaging and spectroscopy from 5.0 to 29 microns. The JWST science goals are divided into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the early universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present day. The Birth of Stars and Protoplanetary Systems theme seeks to unravel the birth and early evolution of stars, from infall on to dust-enshrouded protostars to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. To enable these observations, JWST consists of a telescope, an instrument package, a spacecraft and a sunshield. The telescope consists of 18 beryllium segments, some of which are deployed. The segments will be brought into optical alignment on-orbit through a process of periodic wavefront sensing and control. The JWST operations plan is based on that used for previous space observatories, and the majority of JWST observing time will be allocated to the international astronomical community through annual peer-reviewed proposal opportunities.

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arXiv2005

Constraining the Distribution of L- & T-Dwarfs in the Galaxy

We estimate the thin disk scale height of the Galactic population of L- & T-dwarfs based on star counts from 15 deep parallel fields from the Hubble Space Telescope. From these observations, we have identified 28 candidate L- & T- dwarfs based on their (i'-z') color and morphology. By comparing these star counts to a simple Galactic model, we estimate the scale height to be 350+-50 pc that is consistent with the increase in vertical scale with decreasing stellar mass and is independent of reddening, color-magnitude limits, and other Galactic parameters. With this refined measure, we predict that less than 10^9 M_{sol} of the Milky Way can be in the form L- & T- dwarfs, and confirm that high-latitude, z~6 galaxy surveys which use the i'-band dropout technique are 97-100% free of L- & T- dwarf interlopers.

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arXiv2005

Hubble Space Telescope Imaging of the Ultracompact Blue Dwarf Galaxy HS 0822+3542: An Assembling Galaxy in a Local Void?

We present deep U, narrow-V, and I-band images of the ultracompact blue dwarf galaxy HS 0822+3542, obtained with the Advanced Camera for Surveys / High Resolution Channel of the Hubble Space Telescope. This object is extremely metal-poor (12 + log(O/H) = 7.45) and resides in a nearby void. The images resolve it into two physically separate components that were previously described as star clusters in a single galaxy. The primary component is only \~100 pc in maximum extent, and consists of starburst region surrounded by a ring-like structure of relatively redder stars. The secondary component is ~50 pc in size and lies at a projected distance of ~80 pc away from the primary, and is also actively star-forming. We estimate masses ~10^7 M(sol) and ~10^6 M(sol) for the two components, based on their luminosities, with an associated dynamical timescale for the system of a few Myr. This timescale and the structure of the components suggests that a collision between them triggered their starbursts. The spectral energy distributions of both components can be fitted by the combination of recent (few Myr old) starburst and an evolved (several Gyr old) underlying stellar population, similar to larger blue compact dwarf galaxies. This indicates that despite its metal deficiency the object is not forming its first generation of stars. However, the small sizes and masses of the two components suggests that HS 0822+3542 represents a dwarf galaxy in the process of assembling from clumps of stars intermediate in size between globular clusters and objects previously classified as galaxies. Its relatively high ratio of neutral gas mass to stellar mass (~1) and high specific star formation rate, log(SFR/M(sol) = -9.2, suggests that it is still converting much of its gas to stars.

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arXiv2005

UBVR and HST mid-UV and near-IR surface photometry and radial color gradients of late-type, irregular, and peculiar galaxies

We introduce a dataset of 142 mostly late-type spiral, irregular, and peculiar (interacting or merging) nearby galaxies observed in UBVR at the Vatican Advanced Technology Telescope (VATT), and present an analysis of their radial color gradients. We confirm that nearby elliptical and early- to mid-type spiral galaxies show either no or only small color gradients, becoming slightly bluer with radius. In contrast, we find that late-type spiral, irregular, peculiar, and merging galaxies become on average redder with increasing distance from the center. The scatter in radial color gradient trends increases toward later Hubble type. As a preliminary analysis of a larger data-set obtained with the Hubble Space Telescope (HST), we also analyze the color gradients of six nearby galaxies observed with NICMOS in the near-IR (H) and with WFPC2 in the mid-UV (F300W) and red (F814W). We discuss the possible implication of these results on galaxy formation and compare our nearby galaxy color gradients to those at high redshift. We present examples of images and UBVR radial surface brightness and color profiles, and of the tables of measurements; the full atlas and tables are published in the electronic edition only.

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arXiv2004

A Redshift z=5.4 Lyman alpha Emitting Galaxy with Linear Morphology in the GRAPES/UDF Field

We have discovered an extended Lyman alpha plume associated with a compact source at redshift 5.4 in slitless spectroscopic data from the Grism ACS Program for Extragalactic Science (GRAPES) project. The spatial extent of the emission is about 6 x 1.5 kpc (1 x 0.25 arcsec). Combining our grism data and the broadband images from the Hubble UltraDeep Field (UDF) images, we find a Lyman alpha line flux of 2e-17 erg/cm2/s and surface brightness 7e-17 erg/cm2/s/arcsec2. The UDF images show diffuse continuum emission associated with the Lyman alpha plume (hereafter UDF 5225), with three embedded knots. The morphology of UDF 5225 is highly suggestive of a galaxy in assembly. It is moreover possible that the prominent Lyman alpha emission from this object is due to an active nucleus, and that we are seeing the simultaneous growth through accretion of a galaxy and its central black hole. Followup observations at higher spectral resolution could test this hypothesis.

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arXiv2004

Candidates of z ~ 5.5--7 Galaxies in the HST Ultra Deep Field

We report results from our $z\simeq 5.5$--7 galaxy search in the HST Ultra Deep Field (UDF). Using the 400-orbit of ACS data, we found 108 plausible $5.5\leq z\leq 6.5$ (or $z\simeq 6$ for short) candidates to $m_{AB}(z_{850})=30.0$ mag. The contamination to the sample, either due to image artifacts or known types of astronomical objects, is likely negligible. The inferred surface densities of $z\simeq 6$ galaxies are consistent with our earlier predictions from $m_{AB}(z_{850})=26.5$ to 28.5 mag. After correcting for detection incompleteness, the counts of $z\simeq 6$ candidates to $m_{AB}(z_{850})=29.2$ mag suggests that the faint-end slope of the galaxy luminosity function (LF) at this redshift is likely between $α=-1.8$ and -1.9, which is sufficient to account for the entire Lyman photon budget necessary to complete the reionization of the universe at $z\simeq 6$. We also searched for $z\simeq 6.5$--7 candidates using the UDF NICMOS data, and have found four candidates to $J_{110}=27.2$ mag. However, the infrared colors of three candidates cannot be easily explained by galaxies in this redshift range. We tentatively derive an upper limit to the cumulative surface density of galaxies at $z\simeq 7$ of 0.36 per arcmin$^2$ to $J_{110}=26.6$ mag, which suggest a noticeable drop in the LF amplitude from $z\simeq 6$ to $z\simeq 7$.

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arXiv2004

Cosmological simulations of the high-redshift radio universe

Using self-consistent cosmological simulations of disc galaxy formation, we analyse the 1.4 GHz radio flux from high-redshift progenitors of present-day normal spirals within the context of present-day and planned next-generation observational facilities. We demonstrate that while current radio facilities such as the Very Large Array (VLA) are unlikely to trace these progenitors beyond redshifts z<0.2, future facilities such as the Square Kilometer Array (SKA) will readily probe their characteristics to redshifts z<2, and are likely to provide detections beyond z~3. We also demonstrate that the progenitors of present-day cD galaxies can emit in excess of 10 uJy of flux at redshifts z>1, and may be a non-negligible contributor to the micro-Jansky source counts derived from current deep VLA cm-wave surveys.

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arXiv2003

The Major Sources of the Cosmic Reionizing Background at z ~ 6

In this paper, we address which sources contributed most of the reionizing photons. Our argument assumes that the reionization ended around z ~ 6 and that it was a relatively quick process, i.e., that there was a non-negligible fraction of neutral hydrogen in the Universe at somewhat earlier epochs. Starting from our earlier estimate of the luminosity function (LF) of galaxies at z ~ 6, we quantitatively show that the major sources of reionization are most likely galaxies with L < L*. Our approach allows us to put stronger constraints to the LF of galaxies at z ~ 6. To have the Universe completely ionized at this redshift, the faint-end slope of the LF should be steeper than $α=-1.6$, which is the value measured at lower redshifts (z ~ 3), unless either the normalization (Phi*) of the LF or the clumping factor of the ionized hydrogen has been significantly underestimated. If Phi* is actually lower than what we assumed by a factor of two, a steep slope close to $α=-2.0$ is required. Our LF predicts a total of 50 -- 80 z ~ 6 galaxies in the HST Ultra Deep Field (UDF) to a depth of AB=28.4 mag, which can be used to constraint both Phi* and $α$. We conclude that the least luminous galaxies existing at this redshift should reach as low as some critical luminosity in order to accumulate the entire reionizing photon budget. On the other hand, the existence of significant amounts of neutral hydrogen at slightly earlier epochs, e.g. z ~ 7, requires that the least luminous galaxies should not be fainter than another critical value (i.e., the LF should cut-off at this point).

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arXiv2003

Evidence for a Major Merger Origin of High Redshift Sub-millimeter Galaxies

Sub-mm detected galaxies located at redshifts $z > 1$ host a major fraction of the bolometric luminosity at high redshifts due to thermal emission from heated dust grains, yet the nature of these objects remains a mystery. The major problem in understanding their origin is whether the dust heating mechanism is predominantly caused by star formation or active galactic nuclei, and what triggered this activity. We address this issue by examining the structures of a sample of 11 sub-mm galaxies imaged with STIS on the Hubble Space Telescope. We argue that ~61pm21% of these sub-mm sources are undergoing an active major merger using the CAS quantitative morphological system. We rule out at ~5sigma confidence that these sub-mm galaxies are normal Hubble types at high redshift. This merger fraction appears to be higher than for Lyman-break galaxies undergoing mergers at similar redshifts. Using reasonable constraints on the stellar masses of Lyman-Break galaxies and these sub-mm sources, we further argue that at redshifts z~2-3 systems with high stellar masses are more likely than lower mass galaxies to be involved in major mergers.

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arXiv2003

Searching for z ~ 6 Objects with the HST Advanced Camera for Surveys: Preliminary Analysis of a Deep Parallel Field

Recent results suggest that z ~ 6 marks the end of the reionization era. A large sample of objects at z ~ 6, therefore, will be of enormous importance, as it will enable us to observationally determine the exact epoch of the reionization and the sources that are responsible for it. With the HST Advanced Camera for Surveys (ACS) coming on line, we now have an unique opportunity to discover a significant number of objects at z ~ 6. The pure parallel mode implemented for the Wide Field Camera (WFC) has greatly enhanced this ability. We present our preliminary analysis of a deep ACS/WFC parallel field at |b|=74.4^o. We find 30 plausible z ~ 6 candidates, all of which have S/N > 7 in the F850LP-band. The major source of contamination could be faint Galactic cool dwarfs, and we estimated that they would contribute at most 4 objects to our candidate list. We derived the cumulative number density of galaxies at 6.0 <= z <= 6.5 as 2.3 arcmin^{-2} to a limit of 28.0 mag in the F850LP-band, which is slightly higher than our prediction. If this is not due to an underestimated contamination rate, it could possibly imply that the faint-end slope of the z ~ 6 luminosity function is steeper than alpha=-1.6. At the very least, our result suggests that galaxies with L<L^* do exist in significant number at z ~ 6 and they could be the major sources that contributed to the reionizing photons.

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arXiv2003

The Hubble Space Telescope WFPC2 B-Band Parallel Survey: A Study of Galaxy Morphology for 18 < B < 27 mag

[Abridged] We present the results of the HST B-Band Parallel Survey (BBPS). It covers 0.0370 sq. deg. and consists of 31 shallow (4-6 orbit), randomly selected high latitude HST WFPC2 parallel fields with images taken in both the B (F450W) and I (F814W) filters. The goal is to morphologically classify the galaxies in a homogeneous manner and study galaxy properties as a function of type and B-band magnitude for 18<b_J<23.5 mag. The B-band selected galaxies are classified using an artificial neural network (ANN) galaxy classifier on the higher S/N I-band images. These provide reliable types for I<24 mag (or b_J<26 mag), since these I-band classifications are less subject to the uncertain redshifted rest-frame UV morphology. The ANN classification depends on the shape of the surface brightness profile, but not on color. These results are combined with deeper studies in the HDF-N and the deep WFPC2 field surrounding the radio galaxy 53W002, for which galaxies have been classified to b_J<27 mag. The galaxy counts for the combined B-band selected samples show adequate statistics for a range 19<b_J<27 mag, and are in good agreement with other studies in the flux range where they overlap, while showing improved statistics at the bright end. The galaxies are divided into 3 morphological classes: E/S0, Sabc and Sd/Irr, and the B-band counts are presented for each class, as well as the total counts. The faint end of the counts is dominated by the Sd/Irr galaxies. These type dependent counts are compared to models based on local luminosity functions which include the effects of the Cosmological Constant. The BBPS sample, along with the two deeper fields, is used to delineate the general trends of effective radius and (B-I) color as a function of both morphological type and apparent magnitude 18<b_J<27 mag.

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arXiv2002

UV-Optical Pixel Maps of Face-On Spiral Galaxies -- Clues for Dynamics and Star Formation Histories

UV and optical images of the face-on spiral galaxies NGC 6753 and NGC 6782 reveal regions of strong on-going star formation that are associated with structures traced by the old stellar populations. We make NUV--(NUV-I) pixel color-magnitude diagrams (pCMDs) that reveal plumes of pixels with strongly varying NUV surface brightness and nearly constant I surface brightness. The plumes correspond to sharply bounded radial ranges, with (NUV-I) at a given NUV surface brightness being bluer at larger radii. The plumes are parallel to the reddening vector and simple model mixtures of young and old populations, thus neither reddening nor the fraction of the young population can produce the observed separation between the plumes. The images, radial surface-brightness, and color plots indicate that the separate plumes are caused by sharp declines in the surface densities of the old populations at radii corresponding to disk resonances. The maximum surface brightness of the NUV light remains nearly constant with radius, while the maximum I surface brightness declines sharply with radius. An MUV image of NGC 6782 shows emission from the nuclear ring. The distribution of points in an (MUV-NUV) vs. (NUV-I) pixel color-color diagram is broadly consistent with the simple mixture model, but shows a residual trend that the bluest pixels in (MUV-NUV) are the reddest pixels in (NUV-I). This may be due to a combination of red continuum from late-type supergiants and [SIII] emission lines associated with HII regions in active star-forming regions. We have shown that pixel mapping is a powerful tool for studying the distribution and strength of on-going star formation in galaxies. Deep, multi-color imaging can extend this to studies of extinction, and the ages and metallicities of composite stellar populations in nearby galaxies.

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arXiv2002

The Infrared Counterparts of the Optically Unidentified CDF-S 1Ms Sources

The Chandra Deep Field South (CDF-S) 1Ms exposure produced a catalog of 346 X-ray sources, of which 59 were not visible on the VLT/FORS1 and the ESO-MPI/WFI deep R-band images to a limit of R_{vega}=26.1--26.7 mag. Using the first release of the ESO VLT/ISAAC JHK_s data on the CDF-S, we identified six of the twelve such objects that were within the coverage of these IR observations. The VLT/FORS1 I-band data further confirms that five of these six objects are undetected in the optical. The photometric properties of these six counterparts are compared against those of the optically brighter counterparts of Chandra sources in the same field. We found that the location of these optically brighter Chandra sources in the near-IR color space was bifurcated, with the color of one branch being consistent with that of E/S0 galaxies at 0 <= z <= 1.5 and the other branch being consistent with that of unreddened AGN/QSOs at 0 <= z <= 3.5. The six counterparts that we identified seemed to lie on the E/S0 branch and its extension, suggesting that these X-ray source hosts are mostly luminous E/S0 galaxies (M_V ~ -20 mag in AB system) at 1 <= z <= 2.5. On the other hand, some of them can also be explained by AGN/QSOs over a wide redshift range (0 <= z <= 5) if a range of internal extinction (A_V=0--1 mag) is allowed. However, the later interpretation requires fine-tuning extinction together with redshift for these objects individually. If they are indeed AGN/QSOs, the most luminous of them is just barely qualified for being a QSO. Finally, we point out that neither high-redshift (z > 5) star-forming galaxies nor irregular galaxies at lower redshift can be a viable explanation to the nature of these six counterparts.

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arXiv2002

15 colour photometry of Landolt SA95 standard star field

In this paper, we present a set of photometric observations in 15 colors of stars in the Landolt covered by the system range from 300 nm to 1000 nm. Visual magnitudes of the stars being studied in the field are from 10th to 20th mag. The observational methodology and the data reduction procedures are described. The relationships between the BATC intermediate-band system and the Landolt UBVRI broad band system are obtained. A catalogue of the photometry has been produced which contains the SEDs of 3613 stars. The electronic form of this catalogue can be accessed at the CDS via anonymous ftp to cdsarc.u-strasbg.fr.

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arXiv2002

A Simple Prediction on the Surface Density of Galaxies at z ~ 6

Systematic surveys are being proposed to discover a significant number of galaxies at z ~ 6, which is now suggested as the epoch when the reionization era of the Universe ends. To plan such surveys, we need a reasonable expectation of the surface density of high redshift galaxies at different flux limits. Here we present a simple prediction of the surface density of 5.5 < z < 6.5 galaxies in the optical regime, extrapolating from what is already known about galaxies at z ~ 3. This prediction is consistent with the results of nearly all known searches for objects at z ~ 6, giving confidence that we may use it to plan optimal combination of survey depth and sky coverage in searching for such objects. We suggest that the most efficient strategy with existing ground-based facilities is to do medium-depth (m_{AB}= 24.0 -- 24.5 mag), wide-field (a couple of square degrees) survey using a wide-field camera at a 4m-class telescope. As the predicted surface density at this brightness level is very sensitive to the value of L*, the result of such a survey can be easily used to constrain the luminosity evolution from z ~ 3 to 6.

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