arXiv subjects

M. Dickinson

Publications and source records attributed to M. Dickinson.

At least 181 records · Page 10Linked to original sources

arXiv2006

Spitzer 70-micron Source Counts in GOODS-North

We present ultra-deep Spitzer 70-micron observations of GOODS-North. For the first time, the turn-over in the 70-micron Euclidean-normalized differential source counts is observed. We derive source counts down to a flux density of 1.2mJy. From the measured source counts and fluctuation analysis, we estimate a power-law approximation of the faint 70-micron source counts of dN/dS proportional to S^{-1.6}, consistent with that observed for the faint 24-micron sources. An extrapolation of the 70-micron source counts to zero flux density implies a total extragalactic background light (EBL) of 7.4+/-1.9 nW m^{-2} sr^{-1}. The source counts above 1.2mJy account for about 60% of the estimated EBL. From fluctuation analysis, we derive a photometric confusion level of sigma_c = 0.30+/-0.15 mJy (q=5) for the Spitzer 70-micron band.

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arXiv2006

Far-Ultraviolet Imaging of the Hubble Deep Field North: Star Formation in Normal Galaxies at z<1

We present far-ultraviolet (FUV) imaging of the Hubble Deep Field North (HDF-N) taken with the Solar Blind Channel of the Advanced Camera for Surveys (ACS/SBC) and the FUV MAMA detector of the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope. The full WFPC2 deep field has been observed at 1600 Angstroms. We detect 134 galaxies and one star down to a limit of FUV_{AB} ~ 29. All sources have counterparts in the WFPC2 image. Redshifts (spectroscopic or photometric) for the detected sources are in the range 0<z<1. We find that the FUV galaxy number counts are higher than those reported by GALEX, which we attribute at least in part to cosmic variance in the small HDF-N field of view. Six of the 13 Chandra sources at z<0.85 in the HDF-N are detected in the FUV, and those are consistent with starbursts rather than AGN. Cross-correlating with Spitzer sources in the field, we find that the FUV detections show general agreement with the expected L_IR/L_UV vs. Beta relationship. We infer star formation rates (SFRs), corrected for extinction using the UV slope, and find a median value of 0.3 Msun/yr for FUV-detected galaxies, with 75% of detected sources have SFR<1 Msun/yr. Examining the morphological distribution of sources, we find that about half of all FUV-detected sources are identied as spiral galaxies. Half of morphologically-selected spheroids at z<0.85 are detected in the FUV, suggesting that such sources have significant ongoing star-formation in the epoch since z=1.

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arXiv2006

Evidence for TP-AGB stars in high redshift galaxies, and their effect on deriving stellar population parameters

We explore the effects of stellar population models on estimating star formation histories, ages and masses of high redshift galaxies. The focus is on the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy among different evolutionary population synthesis. In particular, besides the models usually adopted in the literature, we use models (by Maraston 2005), in which the contribution of the TP-AGB phase is observationally calibrated and is the dominant source of near-IR energy for stellar populations in the age range 0.2 to 2 Gyr. We use a sample of high-z galaxies in the HUDF, with spectroscopic redshifts, and Spitzer IRAC and MIPS photometry from GOODS. We find that the TP-AGB phase plays a key role in the interpretation of Spitzer data for high-z galaxies, when the rest-frame near-IR is sampled. When fitting without dust reddening, the models with the empirically-calibrated TP-AGB phase always reproduce better the observed spectral energy distributions (SEDs). Allowing for dust reddening improves the fits with literature models in some cases. In both cases, the results from Maraston models imply younger ages by factors up to 6 and lower stellar masses (by ~60 % on average). The observed strengths of the MgUV spectral feature compare better to the predicted ones in the case of the Maraston models, implying a better overall consistency of SED fitting. Finally, we find that photometric redshifts improve significantly using these models on the SEDs extending over the IRAC bands. This work provides the first direct evidence of TP-AGB stars in the primeval Universe.

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arXiv2006

The Great Observatories Origins Deep Survey VLT/FORS2 Spectroscopy in the GOODS-South Field: Part II

We present the second campaign of the ESO/GOODS program of spectroscopy of faint galaxies in the GOODS-South field. Objects were selected as candidates for VLT/FORS2 observations primarily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams and comparing the results with public data. 807 spectra of 652 individua targets have been obtained in service mode with the FORS2 spectrograph at the ESO/VLT, providing 501 redshift determinations. The typical redshift uncertainty is estimated to be sigma_z ~ 0.001. Galaxies have been selected adopting three different color criteria and using the photometric redshifts.The resulting redshift distribution typically spans two redshift domains: from z=0.5 to 2 and z=3.5 to 6.2. In particular, 94 B435-,V606-,i775 "dropout" Lyman break galaxies have been observed, yielding redshifts for 65 objects in the interval 3.4<z<6.2. Three sources have been serendipitously discovered in the redshift interval 4.8<z<5.5. Together with the previous release, 930 sources have now been observed and 724 redshift determinations have been carried out. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/. Large scale structures are clearly detected at z=0.666, 0.734, 1.096, 1.221, 1.300, and 1.614. A sample of 34 sources with tilted [OII]3727 emission has been identified, 32 of them in the redshift range 0.9<z<1.5.

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arXiv2006

GMASS, Unveiling Distant Massive Galaxies with Spitzer

One of the main questions of galaxy formation is how and when galaxies assembled their mass. Observations suggest that most mass assembly and star formation occured at 1 < z < 3. However, in the same redshift range, fully assembled old and massive galaxies co-exist with star-bursting systems still in their assembling stage. In order to explore galaxy evolution in this critical redshift range, we started a project called GMASS (Galaxy Mass Assembly ultra-deep Spectroscopic Survey) based on an 145 hours ESO Large Program and aimed at doing ultradeep spectroscopy with the VLT of a sample of high redshift galaxies slected with IRAC/Spitzer m(4.5mu) < 23, z(phot) > 1.4) from the GOODS-South/HUDF region. The selection at 4.5 micron is crucial to select high redshift galaxies based on the stellar mass or objects affected by strong dust extinction. The spectroscopic deep integration times (15 - 50 hours) are needed to identify unambiguously the redshift and nature of these galaxies, which redshifts are mostly in the "desert" where spectroscopy is traditionally difficult due to the absence of strong emission lines at optical wavelengths. The survey is almost complete and some preliminary results are presented here.

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arXiv2006

The GOODS-North Radio Galaxies: On the Origin of the Radio Emission

We report on a preliminary study concerning the origin of radio emission within radio galaxies at L(1.4GHz)>1E24 W/Hz in the GOODS-N field. In the local universe, Condon et al. (2002) and Yun et al. (2001) have shown that in galaxies with radio luminosities greater than 1E23 W/Hz the majority of the radio emission originates from a `monster' i.e., an AGN. Using the Chandra 2Msec X-ray image centered on the GOODS-N field and a reprocessed VLA HDF A-array data plus newly acquired VLA B-array data (rms=5.3microJy), we find that radio galaxies (with spectroscopic redshifts; all have z>1) with L(1.4GHz)>1E24 W/Hz typically have an X-ray detection rate of 72% (60% emit hard X-rays suggesting an AGN origin for the radio emission) in contrast to 25% for radio galaxies with L < 1E23 W/Hz. The ACS images of these L(1.4 GHz) > 1E24 W/Hz galaxies typically show compact rather than extended galaxy morphology which is generally found for the less luminous radio emitting galaxies but a few appear to be ongoing galaxy mergers. We also present SED fitting for these luminous radio galaxies including Spitzer IRAC & MIPS 24um photometry and 60% show distinct power-law SED indicative of an AGN. Initial results tell us that the X-ray emitting radio galaxy population are generally not submm sources but the few (~10%) that are SCUBA sources appear to be the small AGN population found by Pope et al. and others.

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arXiv2006

Mid infrared properties of distant infrared luminous galaxies

We present evidence that the mid infrared (MIR) is a good tracer of the total infrared luminosity, L(IR), and star formation rate (SFR), of galaxies up to z 1.3. We use deep MIR images from the Infrared Space Observatory (ISO) and the Spitzer Space Telescope in the Northern field of the Great Observatories Origins Deep Survey (GOODS-N) together with VLA radio data to compute three independant estimates of L(IR). The L(IR,MIR) derived from the observed 15 and/or 24 um flux densities using a library of template SEDs, and L(IR,radio), derived from the radio (1.4 and/or 8.5 GHz) using the radio-far infrared correlation, agree with a 1-sigma dispersion of 40 %. We use the k-correction as a tool to probe different parts of the MIR spectral energy distribution (SED) of galaxies as a function of their redshift and find that on average distant galaxies present MIR SEDs very similar to local ones. However, in the redshift range z= 0.4-1.2, L(IR,24um) is in better agreement with L(IR,radio) than L(IR,15 um) by 20 %, suggesting that the warm dust continuum is a better tracer of the SFR than the broad emission features due to polycyclic aromatic hydrocarbons (PAHs). We find marginal evidence for an evolution with redshift of the MIR SEDs: two thirds of the distant galaxies exhibit rest-frame MIR colors (L(12 um)/L(7 um) and L(10 um)/L(15 um) luminosity ratios) below the median value measured for local galaxies. Possible explanations are examined but these results are not sufficient to constrain the physics of the emitting regions. We compare three commonly used SED libraries which reproduce the color-luminosity correlations of local galaxies with our data and discuss possible refinements to the relative intensities of PAHs, warm dust continuum and silicate absorption.

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arXiv2005

Spitzer Observations of Massive Red Galaxies at High Redshift

We investigate the properties of massive galaxies at z=1-3.5 using HST observations, ground-based near-IR imaging, and Spitzer Space Telescope observations at 3-24 micron. We identify 153 distant red galaxies (DRGs) with J-K > 2.3 mag (Vega) in the southern GOODS field. This sample is approximately complete in stellar mass for passively evolving galaxies above 10^11 solar masses and z < 3. The galaxies identified by this selection are roughly split between objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation, and galaxies whose light is dominated by heavily reddened starbursts. Very few of the galaxies (< 10%) have no indication of current star formation. Using SFR estimates that include the reradiated IR emission, the DRGs at z=1.5-3 with stellar masses > 10^11 solar masses have specific SFRs (SFRs per unit stellar mass) ranging from 0.2 to 10 Gyr^-1, with a mean value of ~2.4 Gyr^-1. The DRGs with stellar masses > 10^11 solar masses and 1.5 < z < 3 have integrated specific SFRs greater the global value over all galaxies. In contrast, we find that galaxies at z = 0.3-0.75 with these stellar masses have integrated specific SFRs less than the global value, and more than an order of magnitude lower than that for massive DRGs at z = 1.5-3. At z < 1, lower-mass galaxies dominate the overall cosmic mass assembly. This suggests that the bulk of star formation in massive galaxies occurs at early cosmic epochs and is largely complete by z~1.5. [Abridged]

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arXiv2005

Evidence for a Massive Post-Starburst Galaxy at z ~ 6.5

We present results from a search for high-redshift J--band ``dropout'' galaxies in the portion of the GOODS southern field that is covered by extremely deep imaging from the Hubble Ultradeep Field (HUDF).Using observations at optical, near-infrared and mid-infrared wavelengths from the Hubble and Spitzer Space Telescopes and the ESO-VLT, we search for very massive galaxies at high redshifts and find one particularly remarkable candidate. Its spectral energy distribution is consistent with a galaxy at z ~ 6.5 and a stellar mass of 6x10e11 M(sun) (for a Salpeter IMF). We interpret a prominent photometric break between the near-infrared and Spitzer bandpasses as the 3646A Balmer discontinuity. The best-fitting models have low reddening and ages of several hundred Myr, placing the formation of the bulk of the stars at z > 9. Alternative models of dusty galaxies at z ~ 2.5 are possible but provide significantly poorer fits. The object is detected with Spitzer at 24 micron. This emission originats from an obscured active nucleus or star formation. We present optical and near-infrared spectroscopy which has, thus far, failed to detect any spectral features. This helps limit the solution in which the galaxy is a starburst or active galaxy at z ~ 2.5. If the high-redshift interpretation is correct, this object would be an example of a galaxy that formed by a process strongly resembling traditional models of monolithic collapse, in a way which a very large mass of stars formed within a remarkably short period of time, at very high redshift.

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arXiv2005

Direct evidence for an early reionization of the Universe?

We examine the possible reionization of the intergalactic medium (IGM) by the source UDF033238.7-274839.8 (hereafter HUDF-JD2), which was discovered in deep {\it HST}/VLT/{\it Spitzer} images obtained as part of the Great Observatory Origins Deep Survey and {\it Hubble} Ultra-Deep Field projects. Mobasher et al (2005) have identified HUDF-JD2 as a massive ($\sim6\times10^{11}M_\odot$) post-starburst galaxy at redshift z$\gtrsim6.5$. We find that HUDF-JD2 may be capable of reionizing its surrounding region of the Universe, starting the process at a redshift as high as z$\approx 15 \pm5$.

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arXiv2005

The population of BzK selected ULIRGs at z~2

We investigate the multi-wavelength emission of BzK selected star forming galaxies at z~2 in the Great Observatories Origins Deep Survey (GOODS) North region. Most (82%) of the sources are individually detected at 24um in the Spitzer MIPS imaging, and one fourth (26%) in the VLA radio data. Significant detections of the individually undetected objects are obtained through stacking in the radio, submm and X-ray domains. The typical star forming galaxy with stellar mass ~10^{11}Mo at z=2 is an Ultra-luminous Infrared Galaxy (ULIRG), with L_IR ~ 1-2x10^{12}Lo and star formation rate SFR 200-300Mo/yr, implying a comoving density of ULIRGs at z=2 at least 3 orders of magnitude above the local one. SFRs derived from the reddening corrected UV luminosities agree well, on average, with the longer wavelength estimates. The high 24um detection rate suggests a relatively large duty cycle for the BzK star forming phase, consistently with the available independent measurements of the space density of passively evolving galaxies at z>1.4. If the IMF at z=2 is similar to the local one, and in particular is not a top-heavy IMF, this suggests that a substantial fraction of the high mass tail (>10^{11}Mo) of the galaxy stellar mass function was completed by z~1.4.

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arXiv2005

Evolution in the Color-Magnitude Relation of Early-Type Galaxies in Clusters of Galaxies at z~=1

We present a study of the color evolution of elliptical and S0 galaxies in six clusters of galaxies inside the redshift range 0.78 < z < 1.27. For each cluster, we used imaging from the Hubble Space Telescope to determine morphological types by both an automated technique and from visual inspection. We performed simulations to determine the accuracy of the automated classifications and found a success rate of ~75% at m(L*) or brighter magnitudes for most of our HST imaging data with the fraction of late--type galaxies identified as early--type galaxies to be ~10% at m(L*) to ~20% at m(L*)+2. From ground based optical and near-infrared imaging, we measured the zero-point and scatter in the color--magnitude relation of the early-type populations, which when combined with Stanford et al. (1998), yields a sample of cluster early--type galaxies that span a lookback time of 9 gigayears from the present. We see the colors of the early--type cluster members become bluer with increasing redshift. We fit a set of models to the change in the color as a function of redshift with the best fitting values ranging from a formation redshift of 3^+2_-1 to 5_-3. The large scatter in resulting formation epochs, which depends on the details of the models used, implies that we can conclude that the oldest stars in the elliptical galaxies appear to have formed at redshifts of z>3. We find possible evolution in the scatter of the colors, with some high redshift clusters showing scatter as small as the Coma cluster but others showing much larger scatter. Those clusters with a small scatter imply either a formation redshift of at least z ~ 3 or a smaller spread in the range of formation redshifts at lower redshifts, assuming a Gaussian distribution of star-formation around the mean epoch.

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arXiv2005

Evidence for Primordial Clustering Around the QSO SDSS J1030+0524 at z=6.28

We present tentative evidence for primordial clustering, manifested as an excess of color-selected objects in the field of the QSO SDSS J1030+0524 at redshift z=6.28. We have selected objects red in i_{775}-z_{850} on the basis of Hubble Space Telescope Advanced Camera for Surveys imaging of a field centered on the QSO. Compared to data at comparable depth obtained by the GOODS survey, we find an excess of objects with (i_{775}-z_{850}) \geq 1.5 in the QSO field. The significance of the detection is estimated to be ~97% on the basis of the counts alone and increases to 99.4% if one takes into account the color distribution. If confirmed this would represent the highest redshift example of galaxy clustering and would have implications on models for the growth of structure. Bias-driven clustering of first luminous objects forming in the highest peaks of the primordial density field is expected in most models of early structure formation. The redshift of one of the candidates has been found to be z=5.970 by our spectroscopy with Keck I/LRIS, confirming the validity of our color selection.

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arXiv2004

Optical and Infrared Photometry of the Type Ia Supernovae 1991T, 1991bg, 1999ek, 2001bt, 2001cn, 2001cz, and 2002bo

We present optical and/or infrared photometry of the Type Ia supernovae SN 1991T, SN 1991bg, SN 1999ek, SN 2001bt, SN 2001cn, SN 2001cz, and SN 2002bo. All but one of these supernovae have decline rate parameters Delta m_15(B) close to the median value of 1.1 for the whole class of Type Ia supernovae. The addition of these supernovae to the relationship between the near-infrared absolute magnitudes and Delta m_15(B) strengthens the previous relationships we have found, in that the maximum light absolute magnitudes are essentially independent of the decline rate parameter. (SN 1991bg, the prototype of the subclass of fast declining Type Ia supernovae, is a special case.) The dispersion in the Hubble diagram in JHK is only ~0.15 mag. The near-infrared properties of Type Ia supernovae continue to be excellent measures of the luminosity distances to the supernova host galaxies, due to the need for only small corrections from the epoch of observation to maximum light, low dispersion in absolute magnitudes at maximum light, and the minimal reddening effects in the near-infrared.

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arXiv2004

The B-Band Luminosity Function of Red and Blue Galaxies up to z=3.5

We have explored the redshift evolution of the luminosity function of red and blue galaxies up to $z=3.5$. This was possible joining a deep I band composite galaxy sample, which includes the spectroscopic K20 sample and the HDFs samples, with the deep $H_{AB}=26$ and $K_{AB}=25$ samples derived from the deep NIR images of the Hubble Deep Fields North and South, respectively. About 30% of the sample has spectroscopic redshifts and the remaining fraction well-calibrated photometric redshifts. This allowed to select and measure galaxies in the rest-frame blue magnitude up to $z\sim 3$ and to derive the redshift evolution of the B-band luminosity function of galaxies separated by their rest-frame $U-V$ color or specific (i.e. per unit mass) star-formation rate. The class separation was derived from passive evolutionary tracks or from their observed bimodal distributions. Both distributions appear bimodal at least up to $z\sim 2$ and the locus of red/early galaxies is clearly identified up to these high redshifts. Both luminosity and density evolutions are needed to describe the cosmological behaviour of the red/early and blue/late populations. The density evolution is greater for the early population with a decrease by one order of magnitude at $z\sim 2-3$ with respect to the value at $z\sim 0.4$. The luminosity densities of the early and late type galaxies with $M_B<-20.6$ appear to have a bifurcation at $z>1$. Indeed while star-forming galaxies slightly increase or keep constant their luminosity density, "early" galaxies decrease in their luminosity density by a factor $\sim 5-6$ from $z\sim 0.4$ to $z\sim 2.5-3$. A comparison with one of the latest versions of the hierarchical CDM models shows a broad agreement with the observed number and luminosity density evolutions of both populations.

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arXiv2004

The Great Observatories Origins Deep Survey - VLT/FORS2 Spectroscopy in the GOODS-South Field

We present the first results of the ESO/GOODS program of spectroscopy of faint galaxies in the Chandra Deep Field South (CDF-S). 399 spectra of 303 unique targets have been obtained in service mode with the FORS2 spectrograph at the ESO/VLT, providing 234 redshift determinations (the median of the redshift distribution is at 1.04). The typical redshift uncertainty is estimated to be sig(z) ~ 0.001. Galaxies have been color selected in a way that the resulting redshift distribution typically spans from z=0.5 to 2. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/ Large scale structure is clearly detected at z ~ 0.67, 0.73, 1.10 and 1.61. Three Lyman-break galaxies have also been included as targets and are confirmed to have redshifts z=4.800, 4.882 and 5.828. In a few cases, we observe clear [OII]3727 rotation curves, even at the relatively low resolution (R = 860) of the present observations. Assuming that the observed velocity structure is due to dynamically-relaxed rotation, this is an indication of large galactic masses (few times 10^(11) solar masses) at z ~ 1.

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arXiv2004

The Hubble Higher-Z Supernova Search: Supernovae to z=1.6 and Constraints on Type Ia Progenitor Models

We present results from the Hubble Higher-z Supernova Search, the first space-based open field survey for supernovae (SNe). In cooperation with the Great Observatories Origins Deep Survey, we have used the Hubble Space Telescope with the Advanced Camera for Surveys to cover 300 square arcmin in the area of the Chandra Deep Field South and the Hubble Deep Field North on five separate search epochs (separated by 45 day intervals) to a limiting magnitude of z'=26. These deep observations have allowed us to discover 42 SNe in the redshift range 0.2 < z < 1.6. As these data span a large range in redshift, they are ideal for testing the validity of Type Ia supernova progenitor models with the distribution of expected ``delay times,'' from progenitor star formation to SN Ia explosion, and the SN rates these models predict. Through a Bayesian maximum likelihood test, we determine which delay-time models best reproduce the redshift distribution of SNe Ia discovered in this survey. We find that models that require a large fraction of ``prompt'' (less than 2 Gyr) SNe Ia poorly reproduce the observed redshift distribution and are rejected at 95% confidence. We find that Gaussian models best fit the observed data for mean delay times in the range of 3 to 4 Gyr.

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arXiv2004

Evolution of Disk Galaxies in the GOODS-South Field: Number Densities and Size Distribution

We examine the evolution of the sizes and number densities of disk galaxies using the high resolution images obtained by the Great Observatories Origins Deep Survey (GOODS) with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). The multiwavelength (B,V,i,z) images allow us to classify galaxies based on their rest-frame B-band morphologies out to redshift z = 1.25. In order to minimize the effect of selection biases, we confine our analysis to galaxies which occupy the region of magnitude-size plane where the survey is almost 90% complete at all redshifts. The observed size distribution is consistent with a log-normal distribution as seen for the disk galaxies in the local Universe and does not show any significant evolution over the redshift range 0.25 < z < 1.25. We find that the number densities of disk galaxies remains fairly constant over this redshift range, although a modest evolution by a factor of four may be possible within the 2-sigma uncertainties.

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