arXiv subjects

B. Mobasher

Publications and source records attributed to B. Mobasher.

At least 127 records · Page 7Linked to original sources

arXiv2005

The HST Cosmos Project: Contribution from the Subaru Telescope

The Cosmic Evolution Survey (COSMOS) is a Hubble Space Telescope (HST) treasury project.The COSMOS aims to perform a 2 square degree imaging survey of an equatorial field in $I$(F814W) band, using the Advanced Camera for Surveys (ACS). Such a wide field survey, combined with ground-based photometric and spectroscopic data, is essential to understand the interplay between large scale structure, evolution and formation of galaxies and dark matter. In 2004, we have obtained high-quality, broad band images of the COSMOS field ($B, V, r^\prime, i^\prime,$ and $ z^\prime$) using Suprime-Cam on the Subaru Telescope, and we have started our new optical multi-band program, COSMOS-21 in 2005. Here, we present a brief summary of the current status of the COSMOS project together with contributions from the Subaru Telescope. Our future Subaru program, COSMOS-21, is also discussed briefly.

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arXiv2004

The Phoenix Deep Survey: the clustering and the environment of Extremely Red Objects

In this paper we explore the clustering properties and the environment of the Extremely Red Objects (EROs; I-K>4mag) detected in a ~180arcmin^2 deep (Ks~20mag) Ks-band survey of a region within the Phoenix Deep Survey, an on-going multiwavelength program aiming to investigate the nature and the evolution of faint radio sources. Using our complete sample of 289 EROs brighter than Ks=20mag we estimate a statistically significant (~3.7sigma) angular correlation function signal with amplitude Aw=8.7^{+2.1}_{-1.7}x10^{-3} consistent with earlier work based on smaller samples. This amplitude suggests a clustering length in the range ro=12-17h^{-1}Mpc, implying that EROs trace regions of enhanced density. Using a novel method we further explore the association of EROs with galaxy overdensities by smoothing the K-band galaxy distribution using the matched filter algorithm of Postman et al. (1996) and then cross-correlating the resulting density maps with the ERO positions. Our analysis provides direct evidence that EROs are associated with overdensities at redshifts z>1. We also exploit the deep radio 1.4GHz data (limiting flux 60microJy) available to explore the association of EROs and faint radio sources and whether the two populations trace similar large scale structures. Cross-correlation of the two samples (after excluding 17EROs with radio counterparts) gives a 2sigma signal only for the sub-sample of high-z radio sources (z>0.6). Although the statistics are poor this suggests that it is the high-z radio sub-sample that traces similar structures with EROs.

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arXiv2004

Bar Evolution Over the Last Eight Billion Years: A Constant Fraction of Strong Bars in GEMS

One third of present-day spirals host optically visible strong bars that drive their dynamical evolution. However, the fundamental question of how bars evolve over cosmological times has yet to be addressed, and even the frequency of bars at intermediate redshifts remains controversial. We investigate the frequency of bars out to z~1.0 drawing on a sample of 1590 galaxies from the GEMS survey, which provides morphologies from HST ACS two-color images, and highly accurate redshifts from the COMBO-17 survey. We identify spiral galaxies using the Sersic index, concentration parameter, and rest-frame color. We characterize bars and disks by fitting ellipses to F606W and F850LP images, taking advantage of the two bands to minimize bandpass shifting. We exclude highly inclined (i>60 deg) galaxies to ensure reliable morphological classifications, and apply completeness cuts of M_v <= -19.3 and -20.6. More than 40% of the bars that we detect have semi major axes a<0.5" and would be easily missed in earlier surveys without the small PSF of ACS. The bars that we can reliably detect are fairly strong (with ellipticities e>=0.4) and have a in the range ~1.2-13 kpc. We find that the optical fraction of such strong bars remains at ~(30% +- 6%) from the present-day out to look-back times of 2-6 Gyr (z~0.2-0.7) and 6-8 Gyr (z~0.7-1.0); it certainly shows no sign of a drastic decline at z>0.7. Our findings of a large and similar bar fraction at these three epochs favor scenarios in which cold gravitationally unstable disks are already in place by z~1, and where on average bars have a long lifetime (well above 2 Gyr). The distributions of structural bar properties in the two slices are, however, not statistically identical and therefore allow for the possibility that the bar strengths and sizes may evolve over time.

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arXiv2004

Evolution and Impact of Bars over the Last Eight Billion Years: Early Results from GEMS

Bars drive the dynamical evolution of disk galaxies by redistributing mass and angular momentum, and they are ubiquitous in present-day spirals. Early studies of the Hubble Deep Field reported a dramatic decline in the rest-frame optical bar fraction f_opt to below 5% at redshifts z>0.7, implying that disks at these epochs are fundamentally different from present-day spirals. The GEMS bar project, based on ~8300 galaxies with HST-based morphologies and accurate redshifts over the range 0.2-1.1, aims at constraining the evolution and impact of bars over the last 8 Gyr. We present early results indicating that f_opt remains nearly constant at ~30% over the range z=0.2-1.1,corresponding to lookback times of ~2.5-8 Gyr. The bars detected at z>0.6 are primarily strong with ellipticities of 0.4-0.8. Remarkably, the bar fraction and range of bar sizes observed at z>0.6 appear to be comparable to the values measured in the local Universe for bars of corresponding strengths. Implications for bar evolution models are discussed.

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arXiv2004

The Phoenix Deep Survey: the radio properties of the hard X-ray selected sample

The radio properties of hard (2-8keV) X-ray selected sources are explored by combining a single 50ks XMM-Newton pointing with the ultra-deep and homogeneous Phoenix radio (1.4GHz) survey (Hopkins et al. 2003). A total of 43 sources are detected above the X-ray flux limit f_X(2-8keV)=7.7e-15cgs with 14 of them exhibiting radio emission above ~40muJy (3sigma). The X-ray/radio matched population lies in the borderline between radio loud and quiet AGNs and comprises sources with both soft and hard X-ray spectral properties suggesting both obscured and unobscured systems. The spectroscopically identified sub-sample (total of 6 X-ray/radio matches) comprises narrow emission line AGNs (4) with hard X-ray spectral properties and broad line sources (2) with soft X-ray spectra. We find evidence that the fraction of X-ray/radio matches increases from ~20% for sources with rest-frame column density N_H<1e22cm^-2 to \~50% for more absorbed systems. Poor statistics however, limit the significance of the above result to the ~2sigma level. Also, the X-ray/radio matched sources have flatter coadded X-ray spectrum (Gamma=1.78^{+0.05}_{-0.03}) compared to sources without radio emission (Gamma=2.00^{+0.03}_{-0.04}). A possible explanation for the higher fraction of absorbed sources with radio emission at the muJy level is the presence of circum-nuclear starburst activity that both feeds and obscures the central engine. For a small sub-sample of z~0.4 radio emitting AGNs with N_H>1e22cm^-2 their combined spectrum exhibits a soft X-ray component that may be associated with star-formation activity, although other possibilities cannot be excluded. We also find that radio emitting AGNs make up about 13-20% of the hard-band X-ray background depending on the adopted normalisation.

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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

Newcomers Meet the Intracluster Medium in the Coma Cluster

A main topic at this meeting is how galaxies are affected when they enter for the first time the cluster environment from the outskirts. Most of the times we are forced to infer the environmental effects indirectly, relying on systematic variations of galaxy properties with environment, but there aren't many examples of direct observations able to unveil ongoing transformations taking place, and the corresponding mechanism producing it. We present a case in which it is possible to identify the cluster environment, and in particular the intracluster medium and the recent infall history of galaxies onto the cluster, as the cause for a recent, abrupt change in the evolutionary history of galaxies.

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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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arXiv2003

Radio Properties of EROs in the Phoenix Deep Survey

Insensitive to dust obscuration, radio wavelengths are ideal to study star-forming galaxies free of dust induced biases. Using data from the Phoenix Deep Survey, we have identified a sample of star-forming extremely red objects (EROs). Stacking of the radio images of the radio-undetected star-forming EROs revealed a significant radio detection. Using the expected median redshift, we estimate an average star-formation rate of 61 M_sun/yr for these galaxies.

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arXiv2003

The Phoenix Deep Survey: Evolution of Star Forming Galaxies

The Phoenix Deep Survey (PDS) is a multiwavelength survey based on deep 1.4 GHz radio observations used to identify a large sample of star forming galaxies to z=1. Photometric redshifts are estimated for the optical counterparts to the radio-detected galaxies, and their uncertainties quantified by comparison with spectroscopic redshift measurements. The photometric redshift estimates and associated best-fitting spectral energy distributions are used in a stacking analysis exploring the mean radio properties of U-band selected galaxies. Average flux densities of a few microJy are measured.

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arXiv2003

The relation between galactic properties and cluster structure

A satisfactory understanding of the origin of the dependence of galaxy properties on their environment has remained, so far, out of reach. In the light of numerous observational results and substantial theoretical progress obtained for clusters of galaxies in the last years, a primary goal is to understand how the star formation activity depends on cluster substructure, i.e. on the merging/accretion history of a cluster. In this contribution we present a case in which it is possible to identify the cluster environment, and in particular the intracluster medium and the recent infall history of galaxies onto the cluster, as the cause for an abrupt change in the star formation histories of a subset of galaxies in the Coma cluster.

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arXiv2003

Faint 1.4 GHz sources in the 2dF Galaxy Redshift Survey

The Australia Telescope Compact Array (ATCA) has been used to survey at 1.4GHz, a small region (< 3 sq deg) overlapping with the 2dF Galaxy Redshift Survey (Colless et al. 2001, MNRAS, 328, 1039). We surveyed with a varying radio sensitivity, ranging from 1mJy - 20microJy (1 sigma). There are 365 2dFGRS sources with z > 0.001 lying within the surveyed region, of which 316 have reliable spectral classification. Following Sadler et al. 2002 (MNRAS, 329, 227), we visually classified 176 as AGN or early-type galaxies, and 140 as star-forming galaxies. We derived radio flux density measurement or upperlimits for each of the 365 2dFGRS sources. The fraction of radio detected 2dFGRS star-forming galaxies increases from ~ 50% at ~ 0.7mJy up to ~ 60% at ~ 0.2mJy. The mean redshift for the fraction of radio detected star-forming galaxies increases with increasing radio detection sensitivity, while the mean redshift is fairly constant for the AGN/early-type fraction. We found very similar radio detection rates of 2dFGRS galaxies for both the AGN/early-type and star-forming components. The radio detection rate increases approximately linearly with respect to the rate of increase in radio detection sensitivity. We derived the radio luminosity function for our sample and it was found to be consistent with that of Sadler et al. 2002. We have also compared the total flux densities of NVSS sources common to our survey, and we discuss strategies for a large-scale radio survey of the 2dFGRS sample.

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arXiv2003

A comparison of the galaxy populations in the Coma and distant clusters: the evolution of k+a galaxies and the role of the intracluster medium

The spectroscopic properties of galaxies in the Coma cluster are compared with those of galaxies in rich clusters at $z \sim 0.5$, to investigate the evolution of the star formation history in clusters. Luminous galaxies with $M_V \leq -20$ and post-starburst/post-starforming (k+a) spectra which constitute a significant fraction of galaxies in distant cluster samples are absent in Coma, where spectacular cases of k+a spectra are found instead at $M_V>-18.5$ and represent a significant proportion of the cluster dwarf galaxy population. A simple inspection of their positions on the sky indicates that this type of galaxy does not show a preferential location within the cluster, but the bluest and strongest-lined group of k+a's lies in projection towards the central 1.4 Mpc of Coma and have radial velocities significantly higher than the cluster mean. We find a striking correlation between the positions of these young and strong post-starburst galaxies and substructure in the hot intracluster medium (ICM) identified from {\it XMM-Newton} data, with these galaxies lying close to the edges of two infalling substructures. This result strongly suggests that the interaction with the dense ICM could be responsible for the quenching of the star formation (thus creating the k+a spectrum), and possibly, for any previous starburst. The evolution with redshift of the luminosity distribution of k+a galaxies can be explained by a ``downsizing effect'', with the maximum luminosity/mass of actively star-forming galaxies infalling onto clusters decreasing at lower redshift. We discuss the possible physical origin of this downsizing effect and the implications of our results for current scenarios of environmental effects on the star formation in galaxies.

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arXiv2003

Extremely red galaxies in the Phoenix Deep Survey

The Phoenix Deep Survey (PDS) is a multiwavelength survey based on deep 1.4 GHz radio observations used to identify a large sample of star forming galaxies to z=1. Here we present an exploration of the evolutionary constraints on the star-forming population imposed by the 1.4 GHz source counts, followed by an analysis of the average properties of extremely red galaxies in the PDS, by using the "stacking" technique.

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arXiv2003

Photometric Redshifts for Galaxies in the GOODS Southern Field

We use extensive multi-wavelength photometric data from the Great Observatories Origins Deep Survey (GOODS) to estimate photometric redshifts for a sample of 434 galaxies with spectroscopic redshifts in the Chandra Deep Field South. Using the Bayesian method, which incorporates redshift/magnitude priors, we estimate photometric redshifts for galaxies in the range 18 < R (AB) < 25.5, giving an rms scatter of 0.11. The outlier fraction is < 10%, with the outlier-clipped rms being 0.047. We examine the accuracy of photometric redshifts for several, special sub--classes of objects. The results for extremely red objects are more accurate than those for the sample as a whole, with rms of 0.051 and very few outliers (3%). Photometric redshifts for active galaxies, identified from their X-ray emission, have a dispersion of 0.104, with 10% outlier fraction, similar to that for normal galaxies. Employing a redshift/magnitude prior in this process seems to be crucial in improving the agreement between photometric and spectroscopic redshifts.

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arXiv2003

Lower Mass Black Holes in the GOODS? Off-nuclear X-ray Sources

We have identified a sample of 10 highly reliable off-nuclear (> 2 kpc) X-ray sources at z=0.03-0.25 in late-type host galaxies within the two GOODS fields (i.e., the two Chandra Deep Fields). The combination of the superb spatial resolution and great depth of the HST ACS and Chandra ACIS coverage in the GOODS fields is critical to the identification of these sources. We extend the study of this enigmatic population up to higher redshifts and larger look-back times than has been possible before, finding that the fraction of optically luminous galaxies exhibiting luminous off-nuclear sources at z~0.1 is larger than at the current time (~36% versus ~8%). The X-ray luminosities of the GOODS off-nuclear X-ray sources are comparable to or slightly greater than the integrated point-source X-ray luminosities of nearby galaxies, providing a possible constraint on the X-ray Luminosity Function at higher star-formation rates. X-ray variability demonstrates that several of these sources are likely single accreting black holes; for the majority, however, the number of X-ray counts is too low for X-ray variability studies.

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arXiv2003

The Phoenix Deep Survey: X-ray properties of faint radio sources

In this paper we use a 50ks XMM-Newton pointing overlapping with the Phoenix Deep Survey, a homogeneous radio survey reaching muJy sensitivities, to explore the X-ray properties and the evolution of star-forming galaxies. UV, optical and NIR photometry is available and is used to estimate photometric redshifts and spectral types for radio sources brighter than R=21.5mag (total of 82). Sources with R<21.5mag and spiral galaxy SEDs (34) are grouped into two redshift bins with a median of z=0.240 and 0.455 respectively. Stacking analysis for both the 0.5-2 and 2-8keV bands is performed on these subsamples. A high confidence level signal (>3.5sigma) is detected in the 0.5-2keV band corresponding to a mean flux of ~3e-16cgs for both subsamples. This flux translates to mean luminosities of ~5e40 and 1.5e41cgs for the z=0.240 and 0.455 subsamples respectively. Only a marginally significant signal (2.6sigma) is detected in the 2-8keV band for the z=0.455 subsample. We argue that the stacked signal above is dominated by star-formation. The mean L_X/L_B ratio and the mean L_X of the two subsamples are found to be higher than optically selected spirals and similar to starbursts. We also find that the mean L_X and L_1.4 of the faint radio sources studied here are consistent with the L_X-L_1.4 correlation of local star-forming galaxies. Moreover, the X-ray emissivity of sub-mJy sources to z~0.3 is found to be elevated compared to local HII galaxies. The observed increase is consistent with L_X evolution of the form (1+z)^3. Assuming that our sample is indeed dominated by starbursts this is direct evidence for evolution of such systems at X-ray wavelengths. Using an empirical L_X to SFR conversion we estimate a global SFR density at z~0.3 of \~0.029M_o/yr/Mpc in agreement with previous studies.

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arXiv2003

Dependence of Dust Obscuration on Star Formation Rates in Galaxies

Many investigations of star formation rates (SFRs) in galaxies have explored details of dust obscuration, with a number of recent analyses suggesting that obscuration appears to increase in systems with high rates of star formation. To date these analyses have been primarily based on nearby (z < 0.03) or UV selected samples. Using 1.4 GHz imaging and optical spectroscopic data from the Phoenix Deep Survey, the SFR-dependent obscuration is explored. The use of a radio selected sample shows that previous studies exploring SFR-dependent obscurations have been biased against obscured galaxies. The observed relation between obscuration and SFR is found to be unsuitable to be used as an obscuration measure for individual galaxies. Nevertheless, it is shown to be successful as a first order correction for large samples of galaxies where no other measure of obscuration is available, out to intermediate redshifts (z ~ 0.8).

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