arXiv subjects

Daniel Stern

Publications and source records attributed to Daniel Stern.

At least 523 records · Page 29Linked to original sources

arXiv2001

Serendipitously Detected Galaxies in the Hubble Deep Field

We present a catalog of 74 galaxies detected serendipitously during a campaign of spectroscopic observations of the Hubble Deep Field North (HDF) and its environs. Among the identified objects are five candidate Ly-alpha emitters at z > 5, a galaxy cluster at z = 0.85, and a Chandra source with a heretofore undetermined redshift of z = 2.011. We report redshifts for 25 galaxies in the central HDF, 13 of which had no prior published spectroscopic redshift. Of the remaining 49 galaxies, 30 are located in the single-orbit HDF Flanking Fields. We discuss the redshift distribution of the serendipitous sample, which contains galaxies in the range 0.10 < z < 5.77 with a median redshift of z = 0.85, and we present strong evidence for redshift clustering. By comparing our spectroscopic redshifts to optical/IR photometric studies of the HDF, we find that photometric redshifts are in most cases capable of producing reasonable predictions of galaxy redshifts. Finally, we estimate the line-of-sight velocity dispersion and the corresponding mass and expected X-ray luminosity of the galaxy cluster, we present strong arguments for interpreting the Chandra source as an obscured AGN, and we discuss in detail the spectrum of one of the candidate z > 5 Ly-alpha emitters.

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arXiv2001

The Farthest Known Supernova: Support for an Accelerating Universe and a Glimpse of the Epoch of Deceleration

We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of ~1.7, the farthest SN observed to date. SN 1997ff, was discovered in a repeat observation by the HST of the HDF-), and serendipitously monitored with NICMOS on HST throughout the GTO campaign. The SN type can be determined from the host galaxy type:an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse SNe. The class- ification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photo- metric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z=1.7+/-0.1, is in excellent agreement with the redshift estimate of z=1.65+/-0.15 derived from the U_300,B_450,V_606,I_814,J_110,J_125,H_160, H_165,K_s photometry of the galaxy. Optical and near-infrared spectra of the host provide a very tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe~Ia and a powerful test of the current accelerating Universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, Omega_M~1/3, Omega_Lambda~2/3. It is inconsistent with grey dust or simple luminosity evolution, candidate astro- physical effects which could mimic past evidence for an accelerating Universe from SNe Ia at z~0.5.We consider several sources of possible systematic error including lensing, SN misclassification, selection bias, and calibration errors. Currently, none of these effects appears likely to challenge our conclusions.

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arXiv2001

The Large Area Lyman Alpha Survey

The Lyman-$α$ line is expected to be strong in the presence of active star formation and the absence of dust, making it a good tool for finding chemically primitive galaxies in the early universe. We report on a new survey for high redshift Lyman-$α$ sources, the Large Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on the new $8192^2$ pixel CCD Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching known high redshift Lyman-$α$ galaxies have an observed surface density corresponding to $11000 \pm 700$ per square degree per unit redshift at $z=4.5$. Early spectroscopic followup from the Keck telescope suggests that $\sim 1/3$ of these are actually at $z\approx 4.5$, and has confirmed five $z > 4$ Lyman-$α$ emitters so far. Combining our photometric survey with spectroscopic results, we estimate a net density of $\sim 4000 $ Lyman-$α$ emitters per square degree per unit redshift at $z\approx 4.5$. The star formation rate density (estimated both from UV continuum and from line emission) is comparable to that of the Lyman break galaxy population within present uncertainties. The most extreme Lyman-$α$ emitters in our sample have rest frame equivalent widths $> 100Å$, consistent with the expectations for the first burst of star formation in a primitive, dust-free galaxy.

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arXiv2001

Large Area Lyman Alpha survey: finding young galaxies at z=4.5

Strong Lyman-alpha emission is a signpost of young stars and the absence of dust and thus indicates young galaxies. To find such a population of young galaxies at z=4.5 we started the Large Area Lyman Alpha survey (LALA). This survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on a large format (36x36 square-arcminutes) camera on the 4 meter telescope at KPNO. The volume density and star-formation contribution of the Lyman-alpha emitters at z=4.5 is comparable to that of Lyman break galaxies. With many candidates and a few spectroscopic confirmations in hand we discuss what the properties of Ly-alpha emitters imply for galaxy and star formation in the early universe.

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arXiv2000

First Results from the SPICES Survey

We present first results from SPICES, the Spectroscopic, Photometric, Infrared-Chosen Extragalactic Survey. SPICES is comprised of four ~30 square arcminute high Galactic latitude fields with deep BRIzJK imaging reaching depths of ~25th magnitude (AB) in the optical and ~23rd magnitude (AB) in the near-infrared. To date we have 626 spectroscopic redshifts for infrared-selected SPICES sources with K<20 (Vega). The project is poised to address galaxy formation and evolution to redshift z~2. We discuss initial results from the survey, including the surface density of extremely red objects and the fraction of infrared sources at z>1. One of the SPICES fields has been the target of a deep 190 ksec Chandra exposure; we discuss initial results from analysis of that data set. Finally, we briefly discuss a successful campaign to identify high-redshift sources in the SPICES fields.

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arXiv2000

Spectroscopy of Ultra Steep Spectrum Radio Sources

We present optical spectroscopy of 62 objects selected from several samples of ultra steep spectrum (USS) radio sources. 46 of these are from our primary catalog, consisting of 669 sources with radio spectral indices alpha < -1.30 (S_nu ~ nu^alpha); this first spectroscopic sub-sample was selected on the basis of their faint optical and near-IR identifications. Most are identified as narrow-lined radio galaxies with redshifts ranging from z=0.25 to z=5.19. Ten objects are at z>3, nearly doubling the number of such sources known to date. Four of the USS radio sources are identified with quasars, of which at least three have very red spectral energy distributions. The source TN J0936-2242 is identified with an extremely red object (ERO, R-K>5); both it and a close companion are at z=1.479. The spectrum of the ERO closely resembles that of previously discovered radio galaxies at z~1.5. Five sources show continuum emission, but fail to show any clear emission or absorption features, despite integrations of ~1 h with the Keck telescope. We suggest that these objects could be (i) radio galaxies with faint emission lines in the ``redshift desert'' at 1.5 <~ z <~ 2.3, (ii) radio galaxies with an obscured AGN, which are dominated by a stellar continuum observed with insufficient S/N, or (iii) pulsars. Three radio sources identified with faint objects in the K-band images remain undetected in 50-90 min spectroscopic integrations with the Keck telescope, and are possible z>7 candidates.

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arXiv2000

Evidence Against a Redshift z>6 for the Galaxy STIS 123627+621755

The identification of galaxies at extreme distances provides our most direct information on the earliest phases of galaxy formation. The distance implied by redshifts z > 5 makes this a challenging endeavor for even the most luminous sources; interpretation of low signal-to-noise ratio observations of faint galaxies is difficult and occasional misidentifications will occur. Here we report on STIS 123627+621755, a galaxy with a suggested spectroscopic redshift of z = 6.68 and the most distant spectroscopically-identified object claimed. For the suggested redshift and reported spectral energy distribution, the galaxy should be essentially invisible shortward of 9300 Angstroms because the intervening intergalactic medium absorbs essentially all light energetic enough to ionize neutral hydrogen (lambda < (1 + z) x 912 Angstroms; the redshifted Lyman limit). The galaxy should be relatively bright in the near-infrared with f(nu) ~ 1 microJy. Here we report a detection of the galaxy at 6700 Angstroms, below the Lyman limit for z = 6.68. We also report a non-detection at 1.2 microns, implying the flux has dropped by a factor of three or more between rest 1216 Angstroms and rest 1560 Angstroms for z = 6.68. Our observations are inconsistent with the suggested extreme distance of STIS 123627+621755 and conservatively require z < 6.

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arXiv2000

High-Redshift, Radio-Loud Quasars

I discuss two programs to study radio-loud quasars at high (z>4) redshift. Quasars are the most luminous, non-transient objects known and are observed to the earliest cosmic epochs. At lower redshifts, radio-loud quasars are associated with host galaxies having deVaucoleurs profiles. By association, identifying and studying a sample of high-redshift, radio-loud quasars provides important clues to the early Universe and potentially probes early-type galaxy formation. The first aspect of this proceeding discusses an extensive search for high-redshift, radio-loud quasars culled from matching the FIRST radio catalog with the 2nd generation Palomar Sky Survey. The second aspect of this proceeding concerns studying the radio properties of optically-selected z>4 quasars. At the time of this conference, only seven z>4 quasars were in the literature. The first program discussed herein uncovers three new such sources, while the second program identifies an additional five. We use the new samples to study the evolution of the radio-loud fraction, finding no evolution in this quantity between redshift z>2 and z>4 for -26>M(B)>-28.

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arXiv2000

The FIRST Sample of Ultraluminous Infrared Galaxies at High Redshift I. Sample and Near-IR Morphologies

We present a new sample of distant ultraluminous infrared galaxies. The sample was selected from a positional cross--correlation of the IRAS Faint Source Catalog with the FIRST database. Objects from this set were selected for spectroscopy by virtue of following the well-known star-forming galaxy correlation between 1.4 GHz and 60 micron flux, and by being optically faint on the POSS. Optical identification and spectroscopy were obtained for 108 targets at the Lick Observatory 3m telescope. Most objects show spectra typical of starburst galaxies, and do not show the high ionization lines of active galactic nuclei. The redshift distribution covers 0.1 < z < 0.9, with 13 objects at z > 0.5 and an average redshift of 0.31. K-band images were obtained at the IRTF, Lick, and Keck observatories in sub-arcsec seeing of all optically identified targets. About 2/3 of the objects appear to be interacting galaxies, while the other 1/3 appear to be normal. Nearly all the identified objects have far-IR luminosities greater than 10^11 L_sun, and ~25% have L_FIR > 10^12 L_sun.

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arXiv2000

Stellar Populations and Galaxy Morphology at High Redshift

In this article we investigate the morphology and stellar populations of high-redshift galaxies through multi-waveband HST imaging and ground-based spatially-resolved spectroscopy. We study the redshift evolution of galaxy morphology in the Hubble Deep Field, using the deep IDT-NICMOS near-infrared HST imaging coupled with spectroscopic and photometric redshifts. Using the multi-waveband data to compare the appearance of galaxies at the same rest-frame wavelengths reveals that "morphological k-corrections" (the change in appearance when viewing high-z objects at shorter rest-frame wavelengths) are only important in a minority of cases, and that galaxies were intrinsically more peculiar at high redshift. One example of significant morphological k-corrections is spiral galaxies, which often show more pronounced barred structure in the near-infrared than in the optical. Therefore, the apparent decline in the fraction of barred spirals at faint magnitudes in the optical HDF may be due to band-shifting effects at the higher redshifts, rather than intrinsic evolution. Using such features as the age-sensitive Balmer+4000Ang break, the spatially- resolved colours of distant galaxies in optical/near-infrared imaging can also be used to study their component stellar populations. We supplement this with deep Keck/LRIS spectroscopy of two extended sources: a chain galaxy at z=2.8 (HDF4-555.1, the "Hot Dog" - the brightest U-drop Lyman-break galaxy in the HDF) and a pair of z=4.04 gravitationally lensed arcs behind the cluster Abell 2390. The absence of measurable rotation across the z=2.8 chain galaxy implies that it is unikely to be a disk viewed edge on. With the resolution enhancement from lensing, we detect stellar populations of different ages in the z=4 arcs.

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arXiv2000

First Results from the Large Area Lyman Alpha Survey

We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching known high redshift Lyman alpha galaxies (i.e., observed equivalent width above 80 Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in an 80 Angstrom filter) have an observed surface density corresponding to 11000 +- 700 per square degree per unit redshift at z=4.5. Spatial variation in this surface density is apparent on comparison between counts in 6561 and 6730 Angstrom filters. Early spectroscopic followup results from the Keck telescope included three sources meeting our criteria for good Lyman alpha candidates. Of these, one is confirmed as a z=4.52 source, while another remains consistent with either z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies per square degree per unit redshift.

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arXiv2000

A Serendipitous Search for High-Redshift Lyman alpha Emission: Two Primeval Galaxy Candidates at z~3

In the course of our ongoing search for serendipitous high-redshift Lyman alpha (LyA) Emissionin deep archival Keck spectra, we discovered two very high equivalent width (W_{obs} ~ 450A, 2-sigma) LyA emission line candidates at z ~3 in a moderate dispersion (R~1200) spectrogram. Both lines have low velocity dispersions (sigma_v ~ 60 km/s) and deconvolved radii r ~ 1 kpc (h = 0.5). We argue that the lines are LyA, and are powered by stellar ionization. The surface density of robust, high equivalent width LyA candidates is estimated to be ~3 \pm 2 per arcmin^2 per unit redshift at z ~ 3, consistent with the estimate of Cowie etal (1998). The LyA emission line source characteristics are consistent with the galaxies undergoing their first burst of star formation, ie, with being primeval. Source sizes and velocity dispersions are comparable to the theoretical primeval galaxy model of Lin and Murray (1992) based on the inside-out, self-similar collapse of an isothermal sphere. In this model, star formation among field galaxies is a protracted process. Galaxies are thought to be able to display high equivalent widths for only the first few x 10 Myr. This time is short in relation to the difference in look back times between z=3 and z=4, and implies that a substantial fraction of strong line-emitting galaxies at z=3 were formed at redshifts z < 4. We discuss the significance of high-equivalent width LyA-emitting galaxies in terms of the emerging picture of the environment, and the specific characteristics of primeval galaxy formation at high redshift.

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arXiv2000

Discovery of a Color-Selected Quasar at z=5.50

We present observations of RD J030117+002025, a quasar at z=5.50 discovered from deep, multi-color, ground-based observations covering 74 square arcmin. This is the most distant quasar or AGN currently known. The object was targeted as an R-band dropout, with R(AB)>26.3 (3-sigma limit in a 3 arcsec diameter region), I(AB)=23.8, and z(AB)=23.4. The Keck/LRIS spectrum shows broad Lyman-alpha/NV emission and sharp absorption decrements from the highly-redshifted hydrogen forests. The fractional continuum depression due to the Lyman-alpha forest is D(A)=0.90. RD J030117+002025 is the least luminous, high-redshift quasar known (M(B)~-22.7).

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arXiv2000

One-Line Redshifts and Searches for High-Redshift Lyman-Alpha Emission

We report the serendipitous discovery of two objects close in projection with fairly strong emission lines at long wavelength (~9190 A). One (A) seems not to be hosted by any galaxy brighter than V(555)=27.5, or I(814)=26.7 (Vega-based 3-sigma limits in 1.0 arcsec diameter apertures), while the other line is associated with a faint (I(814)~24.4) red galaxy (B) offset by 2.7 arcsec and 7 A spectrally. Both lines are broad (FWHM 700 km/s), extended spatially, and have high equivalent widths (W(A,obs)>1225 A, 95% confidence limit; W(B,obs)~150 A). No secondary spectral features are detected for galaxy A. Blue continuum and the marginal detection of a second weak line in the spectrum of galaxy B is consistent with [OII] (the strong line) and MgII (the weak line) at z=1.466. By association, galaxy A is likely at z=1.464, implying a rest-frame equivalent width of the [OII] emission line in excess of 600 A and a projected separation of 30 kpc for the galaxy pair. Conventional wisdom states that isolated emission lines with rest-frame equivalent widths larger than ~200 A are almost exclusively Lyman-alpha. This moderate-redshift discovery therefore compromises recent claims of high-redshift Lyman-alpha emitters for which other criteria (i.e., line profile, associated continuum decrements) are not reported. We discuss observational tests to distinguish Lyman-alpha emitters at high redshift from foreground systems.

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arXiv2000

Radio Properties of z>4 Optically-Selected Quasars

We report on two programs to address differential evolution between the radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both programs entail studying the radio properties of optically-selected quasars. First, we have observed 32 optically-selected, high-redshift (z>4) quasars with the VLA at 6 cm (5 GHz). These sources comprise a statistically complete and well-understood sample. We detect four quasars above our 3-sigma limit of ~0.15 mJy, which is sufficiently sensitive to detect all radio-loud quasars at the probed redshift range. Second, we have correlated 134 z>4 quasars, comprising all such sources that we are aware of as of mid-1999, with FIRST and NVSS. These two recent 1.4 GHz VLA sky surveys reach 3-sigma limits of approximately 0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z>4 quasars, of which six were not previously known to be radio-loud. The depth of these surveys does not reach the radio-loud/radio-quiet demarcation luminosity density (L(1.4 GHz) = 10^32.5 h(50)^(-2) ergs/s/Hz) at the redshift range considered; this correlation therefore only provides a lower limit to the radio-loud fraction of quasars at high-redshift. The two programs together identify eight new radio-loud quasars at z>4, a significant increase over the seven currently in the published literature. We find no evidence for radio-loud fraction depending on optical luminosity for -25 > M_B > -28 at z~2, or for -26>M_B>-28 at z>4. Our results also show no evolution in the radio-loud fraction between z~2 and z>4 (-26>M_B>-28).

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arXiv1999

Search Techniques for Distant Galaxies

How and when do galaxies form? Studies of the microwave background radiation reveal that the Universe is spectacularly homogenous at redshift z~1000. Observations of the local Universe reveal that by z=0 much of the luminous matter has condensed into mature, gravitationally-bound structures. One of the primary challenges to astronomers today is to achieve a robust understanding of this process of galaxy formation and evolution. Locating and studying young galaxies at large look-back times is an essential aspect of this program. We review the systematic observational techniques used to identify galaxies at early cosmic epochs. In the past few years, the study of normal, star-forming galaxies at z>3 has become possible; indeed, successful methods have been developed to push the frontier past z=5. We are now directly observing individual galaxies within a Gyr of the Big Bang. We present a detailed review of the many search methods used for identifying distant galaxies, consider the biases inherent in different search strategies, and discuss early results of these studies. We conclude with goals for future studies at the start of the 21st century.

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arXiv1999

A Serendipitous Search for Hy-Redshift Ly$Alpha$ Emission: A Case Study of Two Sources at $z\simeq 3$

In the course of our on-going search for serendipitous high-redshift LyA emission in deep archival Keck spectra, we discovered two LyA emission line candidates in a moderate dispersion ($λ/{Δλ} +~ 1200$) spectrogram. Both lines have high equivalent width (EW_{obs}>= 450A), low velocity dispersions ($σ_v ~ 60 km/s), and deconvolved effective radii $r_e ~ 1.0 h_{50}^{-1}$ kpc. Their sizes and luminosities are suggestive of the primeval galaxy model of Lin & Murray (1992), based on the self-similar collapse of an isothermal sphere. We argue that the line emission is LyA, and it is stellar in origin. The sources are consistent with being primeval.

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arXiv1999

Ultra-Steep Spectrum Radio Galaxies at Hy Redshifts

Radio sources have traditionally provided convenient beacons for probing the early Universe. Hy Spinrad was among the first of the tenacious breed of observers who would attempt to obtain optical identifications and spectra of the faintest possible `radio galaxies' to investigate the formation and evolution of galaxies at hy redshift. Modern telescopes and instruments have made these tasks much simpler, although not easy, and here we summarize the current status of our hunts for hy redshift radio galaxies (HyZRGs) using radio spectral and near-IR selection.

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