Arjun Dey - Academia.edu (original) (raw)
Papers by Arjun Dey
Astrophysical Journal, 2004
We present Keck/LRIS optical spectra of 17 Lyα-emitting galaxies and one Lyman break galaxy at z ... more We present Keck/LRIS optical spectra of 17 Lyα-emitting galaxies and one Lyman break galaxy at z ≈ 4.5 discovered in the Large Area Lyman Alpha (LALA) survey. The survey has identified a sample of ∼ 350 candidate Lyα-emitting galaxies at z ≈ 4.5 in a search volume of 1.5 × 10 6 comoving Mpc 3 . We targeted 25 candidates for spectroscopy; hence, the 18 confirmations presented herein suggest a selection reliability of 72%. The large equivalent widths (median W rest λ ≈ 80Å) but narrow physical widths (∆v < 500 km s −1 ) of the Lyα emission lines, along with the lack of accompanying high-ionization state emission lines, suggest that these galaxies are young systems powered by star formation rather than by AGN activity. Theoretical models of galaxy formation in the primordial Universe suggest that a small fraction of Lyα-emitting galaxies at z ≈ 4.5 may still be nascent, metal-free objects. Indeed, we find with 90% confidence that 3 to 5 of the confirmed sources show W rest λ > 240Å, exceeding the maximum Lyα equivalent width predicted for normal stellar populations. Nonetheless, we find no evidence for He II λ1640 emission in either individual or composite spectra, indicating that though these galaxies are young, they are not truly primitive, Population III objects.
Astrophysical Journal Supplement Series, 2007
We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1<z<5.2. Usi... more We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1<z<5.2. Using IRAC (3.6-8.0um), IRS (16um) and MIPS (24-160um) imaging, we decompose the rest-frame optical to infrared spectral energy distributions into stellar, AGN, and dust components and determine the contribution of host galaxy stellar emission at rest-frame H-band. Stellar masses derived from rest-frame near-IR data, where AGN and young star contributions are minimized, are significantly more reliable than those derived from rest-frame optical and UV data. We find that the fraction of emitted light at rest-frame H-band from stars is >60% for ~75% the high redshift radio galaxies. As expected from unified models of AGN, the stellar fraction of the rest-frame H-band luminosity has no correlation with redshift, radio luminosity, or rest-frame mid-IR (5um) luminosity. Additionally, while the stellar H-band luminosity does not vary with stellar fraction, the total H-band luminosity anti-correlates with the stellar fraction as would be expected if the underlying hosts of these radio galaxies comprise a homogeneous population. The resultant stellar luminosities imply stellar masses of 10^{11-11.5}Msun even at the highest redshifts. Powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGN, despite the stellar contribution to their mid-IR SEDs at shorter-wavelengths. The mid-IR luminosities alone classify most HzRGs as LIRGs or ULIRGs with even higher total-IR luminosities. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly-accreting AGN. We find a weak correlation of stellar mass with radio luminosity.
The high redshift radio galaxy 4C41.17 consists of a powerful radio source in which previous work... more The high redshift radio galaxy 4C41.17 consists of a powerful radio source in which previous work has shown that there is strong evidence for jet-induced star formation along the radio axis. We argue that nuclear photoionization is not responsible for the excitation of the emission line clouds and we construct a jet-cloud interaction model to explain the major features revealed by the data. The interaction of a high-powered jet with a dense cloud in the halo of 4C41.17 produces shock-excited emission-line nebulosity through ~1000 km/s shocks and induces star formation. The CIII to CIV line ratio and the CIV luminosity emanating from the shock, imply that the pre-shock density in the line-emitting cloud is high enough (~1-10 cm^-3) that shock initiated star formation could proceed on a timescale of order a few x 10^6 yrs, well within the estimated dynamical age of the radio source. Broad (FWHM ~ 100 - 1400 km/s) emission lines are attributed to the disturbance of the gas cloud by a partial bow--shock and narrow emission lines (FWHM ~ 500 - 650 km/s) (in particular CIV) arise in precursor emission in relatively low metallicity gas. The implied baryonic mass ~ 8 \times 10^{10} solar masses of the cloud is high and implies that Milky Way size condensations existed in the environments of forming radio galaxies at a redshift of 3.8. Our interpretation of the data provides a physical basis for the alignment of the radio, emission-line and UV continuum images in some of the highest redshift radio galaxies and the analysis presented here may form a basis for the calculation of densities and cloud masses in other high redshift radio galaxies.
![Research paper thumbnail of Spectroscopic Confirmation of Three Redshift [CLC][ITAL]z[/ITAL][/CLC] ≈ 5.7 L[CLC]y[/CLC]α Emitters from the Large-Area Lyman Alpha Survey](https://attachments.academia-assets.com/48900613/thumbnails/1.jpg)
](Astronomical Journal, 2003
Narrow-band searches for Lyman-α emission are an efficient way of identifying starforming galaxie... more Narrow-band searches for Lyman-α emission are an efficient way of identifying starforming galaxies at high redshifts. We present Keck telescope spectra confirming redshifts z ≈ 5.7 for three objects discovered in the Large Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory. All three spectra show strong, narrow emission lines with the asymmetric profile that is characteristically produced in high redshift Lyman-α emitters by preferential HI absorption in the blue wing of the line. These objects are undetected in deep B W , V , R, and λ ≈ 6600Å narrow-band images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyman-α forest absorption at redshift z ≈ 5.7. All three objects show large equivalent widths ( 150Å in the rest-frame), suggesting at least one of the following: a top-heavy initial mass function, very low stellar metallicity, or the presence of an active nucleus. We consider the case for an active nucleus to be weak in all three objects due to the limited width of the Lyman-α emission line (< 500 km s −1 ) and the absence of any other indicator of quasar activity. The three confirmed high redshift objects were among four spectroscopically observed targets drawn from the sample of 18 candidates presented by . Thus, these spectra support the Lyman-α emitter population statistics from our earlier photometric study, which imply little evolution in number density from z = 5.7 to z = 4.5 and provide strong evidence that the reionization redshift z r > 5.7.
Astrophysical Journal, 1997
The weak radio source LBDS 53W091 is associated with a very faint (R approx. 24.5) red (R-K appro... more The weak radio source LBDS 53W091 is associated with a very faint (R approx. 24.5) red (R-K approx. 5.8) galaxy. Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption-line redshift, z=1.552 +/- 0.002. The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6V star, and a single-burst old stellar population that matches the IR colors, the optical energy distribution and the spectral discontinuities has a minimum age of 3.5 Gyr. We present detailed population synthesis analyses of the observed spectrum in order to estimate the time since the last major epoch of star formation. We discuss the discrepancies in these estimates resulting from using different models, subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these techniques. The models most consistent with the data tend to yield ages at z=1.55 of >3.5Gyr, similar to that inferred for the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of Omega_0, only certain cosmological expansion times are consistent with the age of LBDS 53W091; in particular, for Omega_0=1, only models with H_0 < 45 km/s/Mpc are permitted. For H_0=50 km/s/Mpc and Omega_0=0.2, we derive a formation redshift, z_f >= 5.
Astrophysical Journal, 2003
We report deep Keck narrow-band Lya images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.0... more We report deep Keck narrow-band Lya images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100-200 kpc scale emission line nebulae, centered on these galaxies, which exhibit a wealth of morphological structure, including extended low surface brightness emission in the outer regions, radially directed filaments, cone-shaped structures and (indirect) evidence for extended Lya absorption. We discuss these features within a general scenario where the nebular gas cools gravitationally in large Cold Dark Matter (CDM) halos, forming stars and multiple stellar systems. Merging of these ``building'' blocks triggers large scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds super-massive black holes which produce the powerful radio jets and lobes. The radio sources, starburst superwinds and AGN radiation then disrupt the accretion process limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lya nebulae.
Astrophysical Journal, 1999
Astrophysical Journal, 2004
Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ul... more Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared continuum of normal galaxies is dominated by the composite stellar black body curve and peaks at approximately 1.6 microns, the ultraviolet to mid-infrared continuum of active galaxies is dominated by a power law. Consequently, with sufficient wavelength baseline, one can easily distinguish AGN from stellar populations. Mirroring the tendency of AGN to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGN tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. Based on this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGN, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1s. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGN and find evidence for a large population of optically obscured active galaxies.
Astrophysical Journal, 2006
We study the distribution of Eddington luminosity ratios, L_bol/L_edd, of active galactic nuclei ... more We study the distribution of Eddington luminosity ratios, L_bol/L_edd, of active galactic nuclei (AGNs) discovered in the AGN and Galaxy Evolution Survey (AGES). We combine H-beta, MgII, and CIV line widths with continuum luminosities to estimate black hole (BH) masses in 407 AGNs, covering the redshift range z~0.3-4 and the bolometric luminosity range L_bol~10^45-10^47 erg/s. The sample consists of X-ray or mid-infrared (24 micron) point sources with optical magnitude R<=21.5 mag and optical emission line spectra characteristic of AGNs. For the range of luminosity and redshift probed by AGES, the distribution of estimated Eddington ratios is well described as log-normal with a peak at L_bol/L_edd ~ 1/4 and a dispersion of 0.3 dex. Since additional sources of scatter are minimal, this dispersion must account for contributions from the scatter between estimated and true BH mass and the scatter between estimated and true bolometric luminosity. Therefore, we conclude that: (1) neither of these sources of error can contribute more than \~0.3 dex rms; and (2) the true Eddington ratios of optically luminous AGNs are even more sharply peaked. Because the mass estimation errors must be smaller than ~0.3 dex, we can also investigate the distribution of Eddington ratios at fixed BH mass. We show for the first time that the distribution of Eddington ratios at fixed BH mass is peaked, and that the dearth of AGNs at a factor ~10 below Eddington is real and not an artifact of sample selection. These results provide strong evidence that supermassive BHs gain most of their mass while radiating close to the Eddington limit, and they suggest that the fueling rates in luminous AGNs are ultimately determined by BH self-regulation of the accretion flow rather than galactic scale dynamical disturbances.
Astrophysical Journal, 2005
We report the discovery of a galaxy cluster at z = 1.41. ISCS J143809+341419 was found in the Spi... more We report the discovery of a galaxy cluster at z = 1.41. ISCS J143809+341419 was found in the Spitzer/IRAC Shallow Survey of the Bootes field in the NOAO Deep Wide-Field Survey carried out by IRAC. The cluster candidate was initially identified as a high density region of objects with photometric redshifts in the range 1.3 < z < 1.5. Optical spectroscopy of a limited number of objects in the region shows that 5 galaxies within a ~120 arcsec diameter region lie at z = 1.41 +/- 0.01. Most of these member galaxies have broad--band colors consistent with the expected spectral energy distribution of a passively--evolving elliptical galaxy formed at high redshift. The redshift of ISCS J143809+341419 is the highest currently known for a spectroscopically-confirmed cluster of galaxies.
Astrophysical Journal, 2000
Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as si... more Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as signposts to high-density environments in the early universe, which subsequently evolve into the cores of the richest clusters seen at the present day. These regions are characterised by over-densities of young galaxies, perhaps including a population of dusty, interaction-driven starbursts -the progenitors of massive cluster ellipticals. By searching for this population at submillimeter (submm) wavelengths we can therefore test both galaxy-and structure-formation models. We have undertaken such a search in the field of a z = 3.8 radio galaxy, 4C 41.17, with the SCUBA submm camera. Our extremely deep 450-and 850-µm maps reveal an order-of-magnitude over-density of luminous submm galaxies compared to typical fields (the likelihood of finding such an over-density in a random field is < 2 × 10 −3 ). The SCUBA galaxies have bolometric luminosities, > 10 13 L ⊙ , which imply star-formation rates (SFRs) consistent with those required to form a massive galaxy in only a few 10 8 years. We also note that this field exhibits an over-density of extremely red objects (EROs), some of which may be associated with the submm sources, and Lyman-break galaxies. We propose that the over-densities of both submm and ERO sources in this field represent young dusty, starburst galaxies forming within a proto-cluster centered on the radio galaxy at z = 3.8, which is also traced by a less-obscured population of Lyman-break galaxies.
![Research paper thumbnail of A [ITAL]z[/ITAL] = 5.34 Galaxy Pair in the Hubble Deep Field](https://attachments.academia-assets.com/48900437/thumbnails/1.jpg)
](Astronomical Journal, 1998
We present spectrograms of the faint V -drop (V 606 = 28.1, I 814 = 25.6) galaxy pair HDF 3-951.1... more We present spectrograms of the faint V -drop (V 606 = 28.1, I 814 = 25.6) galaxy pair HDF 3-951.1 and HDF 3-951.2 obtained at the Keck II Telescope. Fernández-Soto, Lanzetta, & Yahil (1998) derive a photometric redshift of z ph = 5.28 +0.34 −0.41 (2σ) for these galaxies; our integrated spectrograms show a large and abrupt discontinuity near 7710 ± 5Å. This break is almost certainly due to the Lyα forest as its amplitude (1 − f short ν /f long ν > 0.87, 95% confidence limit) exceeds any discontinuities observed in stellar or galaxian rest-frame optical spectra. The resulting absorption-break redshift is z = 5.34 ± 0.01.
![Research paper thumbnail of An X-Ray–Selected Galaxy Cluster at [CLC][ITAL]z[/ITAL][/CLC] = 1.26](https://attachments.academia-assets.com/48900418/thumbnails/1.jpg)
](Astronomical Journal, 1999
We report the discovery of an X-ray luminous galaxy cluster at z = 1.26. RXJ0848.9+4452 was selec... more We report the discovery of an X-ray luminous galaxy cluster at z = 1.26. RXJ0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey, on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray 1 Based in part on observations obtained at the W.M. Keck Observatory 2 Based in part on observations obtained at the Kitt Peak National Observatory -2emission, with a significant excess of red objects with J − K colors typical of elliptical galaxies at z > 1. Spectroscopic observations at the Keck II telescope have secured 6 galaxy redshifts in the range 1.257 < z < 1.267 (< z >= 1.261), within a 35 arcsec radius around the peak X-ray emission. This system lies only 4.2 arcmin away (5.0 h −1 50 comoving Mpc, q 0 = 0.5) from the galaxy cluster ClG J0848+4453, which was identified by at z = 1.273 in a near-IR field galaxy survey, and is also known to be X-ray luminous. Assuming that the X-ray emission is entirely due to hot intra-cluster gas, both these systems have similar rest frame luminosities L X ≈ 1 × 10 44 ergs s −1 (0.5-2.0 keV band). In combination with our spectrophotometric data for the entire 30 arcmin 2 field, this suggests the presence of a superstructure, consisting of two collapsed, possibly virialized clusters, the first detected to date at z > 1.
Astrophysical Journal, 2008
We have traced the past 7 Gyr of red galaxy stellar mass growth within dark matter halos. We have... more We have traced the past 7 Gyr of red galaxy stellar mass growth within dark matter halos. We have determined the halo occupation distribution, which describes how galaxies reside within dark matter halos, using the observed luminosity function and clustering of 40,696 0.2<z<1.0 red galaxies in Bootes. Half of 10^{11.9} Msun/h halos host a red central galaxy, and this fraction increases with increasing halo mass. We do not observe any evolution of the relationship between red galaxy stellar mass and host halo mass, although we expect both galaxy stellar masses and halo masses to evolve over cosmic time. We find that the stellar mass contained within the red population has doubled since z=1, with the stellar mass within red satellite galaxies tripling over this redshift range. In cluster mass halos most of the stellar mass resides within satellite galaxies and the intra-cluster light, with a minority of the stellar mass residing within central galaxies. The stellar masses of the most luminous red central galaxies are proportional to halo mass to the power of a third. We thus conclude that halo mergers do not always lead to rapid growth of central galaxies. While very massive halos often double in mass over the past 7 Gyr, the stellar masses of their central galaxies typically grow by only 30%.
Nature, 1996
ONE of the most direct methods of constraining the epoch at which the first galaxies formed-and t... more ONE of the most direct methods of constraining the epoch at which the first galaxies formed-and thereby to constrain the age of the Universe-is to identify and date the oldest galaxies at high redshift. But most distant galaxies have been identified on the basis of their abnormal brightness in some spectral region1-4 such selection criteria are biased towards objects with
Astrophysical Journal, 2000
Early spectroscopic followup results from the Keck telescope included three sources meeting our c... more 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.
We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81)... more We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81). We detect the broad line in the nuclear spectrum and in the spatially extended galaxian component, both near the nucleus and in the spectrum of an off-nuclear knot located 31 kpc south east of the nucleus of the galaxy. These data provide strong support for the simplest form of the unification hypothesis, that radio galaxies are quasars whose optical radiation is directed in the plane of the sky rather than into our line of sight. These data also strongly support the scattering model for the alignment of the UV continuum emission with the radio axis. In 3C265, if the axis of the anisotropically emitted UV continuum radiation is identified with the major axis of the radio source, then the observed rest frame UV continuum emission implies that the opening angle of the radiation cone is large (half angle approximately 45 degrees). We also derive a mass estimate of 8x10^{10} Msun for the central region of 3C265 from its rotation curve. The implied mass-to-light ratio is low (M/L is roughly 2), and suggests that a significant fraction of the rest frame UV continuum emission from this galaxy is dominated by reprocessed radiation from the buried AGN. Finally, we detect the CaII\lambda3933 K line in absorption in the integrated spectrum of 3C265. This provides direct spectroscopic evidence for the existence of stars in a high redshift radio galaxy.
Astrophysical Journal, 1996
We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81)... more We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81). We detect the broad line in the nuclear spectrum and in the spatially extended galaxian component, both near the nucleus and in the spectrum of an o -nuclear knot located 31 kpc south east of the nucleus of the galaxy. These data provide strong support for the simplest form of the uni cation hypothesis, that radio galaxies are quasars whose optical radiation is directed in the plane of the sky rather than into our line of sight. These data also strongly support the scattering model for the alignment of the UV continuum emission with the radio axis. In 3C265, if the axis of the anisotropically emitted UV continuum radiation is identi ed with the major axis of the radio source, then the observed rest frame UV continuum emission implies that the opening angle of the radiation cone is large (half angle 45 ).
Astrophysical Journal, 2004
We present Keck/LRIS optical spectra of 17 Lyα-emitting galaxies and one Lyman break galaxy at z ... more We present Keck/LRIS optical spectra of 17 Lyα-emitting galaxies and one Lyman break galaxy at z ≈ 4.5 discovered in the Large Area Lyman Alpha (LALA) survey. The survey has identified a sample of ∼ 350 candidate Lyα-emitting galaxies at z ≈ 4.5 in a search volume of 1.5 × 10 6 comoving Mpc 3 . We targeted 25 candidates for spectroscopy; hence, the 18 confirmations presented herein suggest a selection reliability of 72%. The large equivalent widths (median W rest λ ≈ 80Å) but narrow physical widths (∆v < 500 km s −1 ) of the Lyα emission lines, along with the lack of accompanying high-ionization state emission lines, suggest that these galaxies are young systems powered by star formation rather than by AGN activity. Theoretical models of galaxy formation in the primordial Universe suggest that a small fraction of Lyα-emitting galaxies at z ≈ 4.5 may still be nascent, metal-free objects. Indeed, we find with 90% confidence that 3 to 5 of the confirmed sources show W rest λ > 240Å, exceeding the maximum Lyα equivalent width predicted for normal stellar populations. Nonetheless, we find no evidence for He II λ1640 emission in either individual or composite spectra, indicating that though these galaxies are young, they are not truly primitive, Population III objects.
Astrophysical Journal Supplement Series, 2007
We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1<z<5.2. Usi... more We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1<z<5.2. Using IRAC (3.6-8.0um), IRS (16um) and MIPS (24-160um) imaging, we decompose the rest-frame optical to infrared spectral energy distributions into stellar, AGN, and dust components and determine the contribution of host galaxy stellar emission at rest-frame H-band. Stellar masses derived from rest-frame near-IR data, where AGN and young star contributions are minimized, are significantly more reliable than those derived from rest-frame optical and UV data. We find that the fraction of emitted light at rest-frame H-band from stars is >60% for ~75% the high redshift radio galaxies. As expected from unified models of AGN, the stellar fraction of the rest-frame H-band luminosity has no correlation with redshift, radio luminosity, or rest-frame mid-IR (5um) luminosity. Additionally, while the stellar H-band luminosity does not vary with stellar fraction, the total H-band luminosity anti-correlates with the stellar fraction as would be expected if the underlying hosts of these radio galaxies comprise a homogeneous population. The resultant stellar luminosities imply stellar masses of 10^{11-11.5}Msun even at the highest redshifts. Powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGN, despite the stellar contribution to their mid-IR SEDs at shorter-wavelengths. The mid-IR luminosities alone classify most HzRGs as LIRGs or ULIRGs with even higher total-IR luminosities. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly-accreting AGN. We find a weak correlation of stellar mass with radio luminosity.
The high redshift radio galaxy 4C41.17 consists of a powerful radio source in which previous work... more The high redshift radio galaxy 4C41.17 consists of a powerful radio source in which previous work has shown that there is strong evidence for jet-induced star formation along the radio axis. We argue that nuclear photoionization is not responsible for the excitation of the emission line clouds and we construct a jet-cloud interaction model to explain the major features revealed by the data. The interaction of a high-powered jet with a dense cloud in the halo of 4C41.17 produces shock-excited emission-line nebulosity through ~1000 km/s shocks and induces star formation. The CIII to CIV line ratio and the CIV luminosity emanating from the shock, imply that the pre-shock density in the line-emitting cloud is high enough (~1-10 cm^-3) that shock initiated star formation could proceed on a timescale of order a few x 10^6 yrs, well within the estimated dynamical age of the radio source. Broad (FWHM ~ 100 - 1400 km/s) emission lines are attributed to the disturbance of the gas cloud by a partial bow--shock and narrow emission lines (FWHM ~ 500 - 650 km/s) (in particular CIV) arise in precursor emission in relatively low metallicity gas. The implied baryonic mass ~ 8 \times 10^{10} solar masses of the cloud is high and implies that Milky Way size condensations existed in the environments of forming radio galaxies at a redshift of 3.8. Our interpretation of the data provides a physical basis for the alignment of the radio, emission-line and UV continuum images in some of the highest redshift radio galaxies and the analysis presented here may form a basis for the calculation of densities and cloud masses in other high redshift radio galaxies.
Astronomical Journal, 2003
Narrow-band searches for Lyman-α emission are an efficient way of identifying starforming galaxie... more Narrow-band searches for Lyman-α emission are an efficient way of identifying starforming galaxies at high redshifts. We present Keck telescope spectra confirming redshifts z ≈ 5.7 for three objects discovered in the Large Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory. All three spectra show strong, narrow emission lines with the asymmetric profile that is characteristically produced in high redshift Lyman-α emitters by preferential HI absorption in the blue wing of the line. These objects are undetected in deep B W , V , R, and λ ≈ 6600Å narrow-band images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyman-α forest absorption at redshift z ≈ 5.7. All three objects show large equivalent widths ( 150Å in the rest-frame), suggesting at least one of the following: a top-heavy initial mass function, very low stellar metallicity, or the presence of an active nucleus. We consider the case for an active nucleus to be weak in all three objects due to the limited width of the Lyman-α emission line (< 500 km s −1 ) and the absence of any other indicator of quasar activity. The three confirmed high redshift objects were among four spectroscopically observed targets drawn from the sample of 18 candidates presented by . Thus, these spectra support the Lyman-α emitter population statistics from our earlier photometric study, which imply little evolution in number density from z = 5.7 to z = 4.5 and provide strong evidence that the reionization redshift z r > 5.7.
Astrophysical Journal, 1997
The weak radio source LBDS 53W091 is associated with a very faint (R approx. 24.5) red (R-K appro... more The weak radio source LBDS 53W091 is associated with a very faint (R approx. 24.5) red (R-K approx. 5.8) galaxy. Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption-line redshift, z=1.552 +/- 0.002. The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6V star, and a single-burst old stellar population that matches the IR colors, the optical energy distribution and the spectral discontinuities has a minimum age of 3.5 Gyr. We present detailed population synthesis analyses of the observed spectrum in order to estimate the time since the last major epoch of star formation. We discuss the discrepancies in these estimates resulting from using different models, subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these techniques. The models most consistent with the data tend to yield ages at z=1.55 of >3.5Gyr, similar to that inferred for the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of Omega_0, only certain cosmological expansion times are consistent with the age of LBDS 53W091; in particular, for Omega_0=1, only models with H_0 < 45 km/s/Mpc are permitted. For H_0=50 km/s/Mpc and Omega_0=0.2, we derive a formation redshift, z_f >= 5.
Astrophysical Journal, 2003
We report deep Keck narrow-band Lya images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.0... more We report deep Keck narrow-band Lya images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100-200 kpc scale emission line nebulae, centered on these galaxies, which exhibit a wealth of morphological structure, including extended low surface brightness emission in the outer regions, radially directed filaments, cone-shaped structures and (indirect) evidence for extended Lya absorption. We discuss these features within a general scenario where the nebular gas cools gravitationally in large Cold Dark Matter (CDM) halos, forming stars and multiple stellar systems. Merging of these ``building'' blocks triggers large scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds super-massive black holes which produce the powerful radio jets and lobes. The radio sources, starburst superwinds and AGN radiation then disrupt the accretion process limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lya nebulae.
Astrophysical Journal, 1999
Astrophysical Journal, 2004
Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ul... more Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared continuum of normal galaxies is dominated by the composite stellar black body curve and peaks at approximately 1.6 microns, the ultraviolet to mid-infrared continuum of active galaxies is dominated by a power law. Consequently, with sufficient wavelength baseline, one can easily distinguish AGN from stellar populations. Mirroring the tendency of AGN to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGN tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. Based on this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGN, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1s. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGN and find evidence for a large population of optically obscured active galaxies.
Astrophysical Journal, 2006
We study the distribution of Eddington luminosity ratios, L_bol/L_edd, of active galactic nuclei ... more We study the distribution of Eddington luminosity ratios, L_bol/L_edd, of active galactic nuclei (AGNs) discovered in the AGN and Galaxy Evolution Survey (AGES). We combine H-beta, MgII, and CIV line widths with continuum luminosities to estimate black hole (BH) masses in 407 AGNs, covering the redshift range z~0.3-4 and the bolometric luminosity range L_bol~10^45-10^47 erg/s. The sample consists of X-ray or mid-infrared (24 micron) point sources with optical magnitude R<=21.5 mag and optical emission line spectra characteristic of AGNs. For the range of luminosity and redshift probed by AGES, the distribution of estimated Eddington ratios is well described as log-normal with a peak at L_bol/L_edd ~ 1/4 and a dispersion of 0.3 dex. Since additional sources of scatter are minimal, this dispersion must account for contributions from the scatter between estimated and true BH mass and the scatter between estimated and true bolometric luminosity. Therefore, we conclude that: (1) neither of these sources of error can contribute more than \~0.3 dex rms; and (2) the true Eddington ratios of optically luminous AGNs are even more sharply peaked. Because the mass estimation errors must be smaller than ~0.3 dex, we can also investigate the distribution of Eddington ratios at fixed BH mass. We show for the first time that the distribution of Eddington ratios at fixed BH mass is peaked, and that the dearth of AGNs at a factor ~10 below Eddington is real and not an artifact of sample selection. These results provide strong evidence that supermassive BHs gain most of their mass while radiating close to the Eddington limit, and they suggest that the fueling rates in luminous AGNs are ultimately determined by BH self-regulation of the accretion flow rather than galactic scale dynamical disturbances.
Astrophysical Journal, 2005
We report the discovery of a galaxy cluster at z = 1.41. ISCS J143809+341419 was found in the Spi... more We report the discovery of a galaxy cluster at z = 1.41. ISCS J143809+341419 was found in the Spitzer/IRAC Shallow Survey of the Bootes field in the NOAO Deep Wide-Field Survey carried out by IRAC. The cluster candidate was initially identified as a high density region of objects with photometric redshifts in the range 1.3 < z < 1.5. Optical spectroscopy of a limited number of objects in the region shows that 5 galaxies within a ~120 arcsec diameter region lie at z = 1.41 +/- 0.01. Most of these member galaxies have broad--band colors consistent with the expected spectral energy distribution of a passively--evolving elliptical galaxy formed at high redshift. The redshift of ISCS J143809+341419 is the highest currently known for a spectroscopically-confirmed cluster of galaxies.
Astrophysical Journal, 2000
Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as si... more Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as signposts to high-density environments in the early universe, which subsequently evolve into the cores of the richest clusters seen at the present day. These regions are characterised by over-densities of young galaxies, perhaps including a population of dusty, interaction-driven starbursts -the progenitors of massive cluster ellipticals. By searching for this population at submillimeter (submm) wavelengths we can therefore test both galaxy-and structure-formation models. We have undertaken such a search in the field of a z = 3.8 radio galaxy, 4C 41.17, with the SCUBA submm camera. Our extremely deep 450-and 850-µm maps reveal an order-of-magnitude over-density of luminous submm galaxies compared to typical fields (the likelihood of finding such an over-density in a random field is < 2 × 10 −3 ). The SCUBA galaxies have bolometric luminosities, > 10 13 L ⊙ , which imply star-formation rates (SFRs) consistent with those required to form a massive galaxy in only a few 10 8 years. We also note that this field exhibits an over-density of extremely red objects (EROs), some of which may be associated with the submm sources, and Lyman-break galaxies. We propose that the over-densities of both submm and ERO sources in this field represent young dusty, starburst galaxies forming within a proto-cluster centered on the radio galaxy at z = 3.8, which is also traced by a less-obscured population of Lyman-break galaxies.
Astronomical Journal, 1998
We present spectrograms of the faint V -drop (V 606 = 28.1, I 814 = 25.6) galaxy pair HDF 3-951.1... more We present spectrograms of the faint V -drop (V 606 = 28.1, I 814 = 25.6) galaxy pair HDF 3-951.1 and HDF 3-951.2 obtained at the Keck II Telescope. Fernández-Soto, Lanzetta, & Yahil (1998) derive a photometric redshift of z ph = 5.28 +0.34 −0.41 (2σ) for these galaxies; our integrated spectrograms show a large and abrupt discontinuity near 7710 ± 5Å. This break is almost certainly due to the Lyα forest as its amplitude (1 − f short ν /f long ν > 0.87, 95% confidence limit) exceeds any discontinuities observed in stellar or galaxian rest-frame optical spectra. The resulting absorption-break redshift is z = 5.34 ± 0.01.
Astronomical Journal, 1999
We report the discovery of an X-ray luminous galaxy cluster at z = 1.26. RXJ0848.9+4452 was selec... more We report the discovery of an X-ray luminous galaxy cluster at z = 1.26. RXJ0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey, on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray 1 Based in part on observations obtained at the W.M. Keck Observatory 2 Based in part on observations obtained at the Kitt Peak National Observatory -2emission, with a significant excess of red objects with J − K colors typical of elliptical galaxies at z > 1. Spectroscopic observations at the Keck II telescope have secured 6 galaxy redshifts in the range 1.257 < z < 1.267 (< z >= 1.261), within a 35 arcsec radius around the peak X-ray emission. This system lies only 4.2 arcmin away (5.0 h −1 50 comoving Mpc, q 0 = 0.5) from the galaxy cluster ClG J0848+4453, which was identified by at z = 1.273 in a near-IR field galaxy survey, and is also known to be X-ray luminous. Assuming that the X-ray emission is entirely due to hot intra-cluster gas, both these systems have similar rest frame luminosities L X ≈ 1 × 10 44 ergs s −1 (0.5-2.0 keV band). In combination with our spectrophotometric data for the entire 30 arcmin 2 field, this suggests the presence of a superstructure, consisting of two collapsed, possibly virialized clusters, the first detected to date at z > 1.
Astrophysical Journal, 2008
We have traced the past 7 Gyr of red galaxy stellar mass growth within dark matter halos. We have... more We have traced the past 7 Gyr of red galaxy stellar mass growth within dark matter halos. We have determined the halo occupation distribution, which describes how galaxies reside within dark matter halos, using the observed luminosity function and clustering of 40,696 0.2<z<1.0 red galaxies in Bootes. Half of 10^{11.9} Msun/h halos host a red central galaxy, and this fraction increases with increasing halo mass. We do not observe any evolution of the relationship between red galaxy stellar mass and host halo mass, although we expect both galaxy stellar masses and halo masses to evolve over cosmic time. We find that the stellar mass contained within the red population has doubled since z=1, with the stellar mass within red satellite galaxies tripling over this redshift range. In cluster mass halos most of the stellar mass resides within satellite galaxies and the intra-cluster light, with a minority of the stellar mass residing within central galaxies. The stellar masses of the most luminous red central galaxies are proportional to halo mass to the power of a third. We thus conclude that halo mergers do not always lead to rapid growth of central galaxies. While very massive halos often double in mass over the past 7 Gyr, the stellar masses of their central galaxies typically grow by only 30%.
Nature, 1996
ONE of the most direct methods of constraining the epoch at which the first galaxies formed-and t... more ONE of the most direct methods of constraining the epoch at which the first galaxies formed-and thereby to constrain the age of the Universe-is to identify and date the oldest galaxies at high redshift. But most distant galaxies have been identified on the basis of their abnormal brightness in some spectral region1-4 such selection criteria are biased towards objects with
Astrophysical Journal, 2000
Early spectroscopic followup results from the Keck telescope included three sources meeting our c... more 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.
We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81)... more We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81). We detect the broad line in the nuclear spectrum and in the spatially extended galaxian component, both near the nucleus and in the spectrum of an off-nuclear knot located 31 kpc south east of the nucleus of the galaxy. These data provide strong support for the simplest form of the unification hypothesis, that radio galaxies are quasars whose optical radiation is directed in the plane of the sky rather than into our line of sight. These data also strongly support the scattering model for the alignment of the UV continuum emission with the radio axis. In 3C265, if the axis of the anisotropically emitted UV continuum radiation is identified with the major axis of the radio source, then the observed rest frame UV continuum emission implies that the opening angle of the radiation cone is large (half angle approximately 45 degrees). We also derive a mass estimate of 8x10^{10} Msun for the central region of 3C265 from its rotation curve. The implied mass-to-light ratio is low (M/L is roughly 2), and suggests that a significant fraction of the rest frame UV continuum emission from this galaxy is dominated by reprocessed radiation from the buried AGN. Finally, we detect the CaII\lambda3933 K line in absorption in the integrated spectrum of 3C265. This provides direct spectroscopic evidence for the existence of stars in a high redshift radio galaxy.
Astrophysical Journal, 1996
We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81)... more We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81). We detect the broad line in the nuclear spectrum and in the spatially extended galaxian component, both near the nucleus and in the spectrum of an o -nuclear knot located 31 kpc south east of the nucleus of the galaxy. These data provide strong support for the simplest form of the uni cation hypothesis, that radio galaxies are quasars whose optical radiation is directed in the plane of the sky rather than into our line of sight. These data also strongly support the scattering model for the alignment of the UV continuum emission with the radio axis. In 3C265, if the axis of the anisotropically emitted UV continuum radiation is identi ed with the major axis of the radio source, then the observed rest frame UV continuum emission implies that the opening angle of the radiation cone is large (half angle 45 ).