arXiv subjects

J. G. Hoessel

Publications and source records attributed to J. G. Hoessel.

15 recordsLinked to original sources

arXiv2008

The Red Sequence Luminosity Function in Massive Intermediate Redshift Galaxy Clusters

We measure the rest-frame B-band luminosity function of red-sequence galaxies (RSLF) of five intermediate-redshift (0.5 < z < 0.9), high-mass (sigma > 950 km/s) clusters. Cluster galaxies are identified through photometric redshifts based on imaging in seven bands (five broad, and two narrow) using the WIYN 3.5m telescope. The luminosity functions are well-fit down to M_B^*+3 for all of the clusters out to a radius of R_200. For comparison, the luminosity functions for a sample of 59 low redshift clusters selected from the SDSS are measured as well. There is a brightening trend (M_B^* increases by 0.7 mags by z=0.75) with redshift comparable to what is seen in the field for similarly defined galaxies, although there is a hint that the cluster red-sequence brightening is more rapid in the past (z>0.5), and relatively shallow at more recent times. Contrary to other claims, we find little evidence for evolution of the faint end slope. Previous indications of evolution may be due to limitations in measurement technique, bias in the sample selection, and cluster to cluster variation. As seen in both the low and high redshift sample, a significant amount of variation in luminosity functions parameters alpha and M^* exists between individual clusters.

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arXiv2005

Luminous Compact Blue Galaxies in Intermediate Redshift Galaxy Clusters: A Significant But Extreme Butcher-Oemler Population

We identify a population of Luminous Compact Blue Galaxies (LCBGs) in two galaxy clusters: MS0451.6-0305 (z=0.54) and Cl1604+4304 (z=0.9). LCBGs are identified via photometric characteristics and photometric redshifts derived from broad and narrow band images taken with the WIYN telescope and the Hubble Space Telescope. We analyze their surface densities and clustering properties to find they compose a statistically significant portion (42% and 53%) of the Butcher-Oemler galaxies in both clusters, and their spatial distributions are best characterized by a shell model. The enhancement of the projected space-density of LCBGs with M_B<-18.5 in the clusters relative to the field is 3-10 times higher than the BO population as a whole, but 2 times lower than the red population, except in the core where LCBGs are absent. Assuming some fading, a natural descendant would be small, low-luminosity galaxies found preferentially in today's clusters, such as dEs.

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arXiv2004

Cepheids and Long Period Variables in NGC 4395

Repeated imaging observations of NGC 4395 were made with the WIYN 3.5 m and the KPNO 2.1 m telescopes. From the photometry of the resolved brighter stars in this galaxy eleven Cepheids with periods ranging between 12 and 90 days have been identified. The true distance modulus has been derived from the apparent distance moduli in g, r and i. The distance modulus is 28.02 +/- 0.18 based on the LMC P-L relation by Sandage et al. 2003; this corresponds to a distance of 4.0 +/- 0.3 Mpc. Using the P-L relation from Madore & Freedman 1991, the distance modulus is 28.15 +/- 0.18; which corresponds to a distance of 4.3 +/- 0.4 Mpc. The reddening is calculated to be E(g-r) = 0.06 +/- 0.08 and E(r-i) = 0.10 +/- 0.08, again from the distance moduli mu_g, mu_r and mu_i. In addition, 37 other variables have been detected, the majority of which have definite periods. They are probably all red long period variables.

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arXiv2004

The Fate of Luminous Compact Blue Galaxies: An Environmental Approach

Luminous Compact Blue Galaxies (LCBGs) are a heterogeneous class which dominate an intermediate phase of galaxy evolution. These sources account for the majority of the star formation between 0.3<z<1, yet the identity of their present-day counterparts is an open question. An environmental dependence to their evolution may provide the answer. We have undertaken the first census of LCBGs in intermediate-redshift clusters using broad and narrow-band images from the WIYN Long Term Variability Study. Several key clusters are in the southern sky. The Southern African Large Telescope's (SALT) Prime Focus Imaging Spectrograph (PFIS), with a field of view matching our WIYN data, will allow us to determine several fundamental characteristics of cluster LCBGs, including (1) star formation rates (SFR) and metallicities from low dispersion Mulit-Object Spectroscopy (MOS); (2) dynamical masses from line-widths measured via high dispersion MOS; and (3) cluster velocity dispersions of LCBGs relative to other cluster emission-line galaxies via Fabry-Perot imaging or MOS. We tentatively connect these distant galaxies with their local, evolved counterparts, and discuss how PFIS observations will enable us to make the physical distinction between LCBGs in clusters and the field.

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arXiv2003

Deep HST Imaging of Sextans A. III. The Star Formation History

We present a measurement of the star formation history of Sextans A, based on WFPC2 photometry that is 50% complete to V=27.5 (M_V ~+ 1.9) and I=27.0. The star formation history and chemical enrichment history have been measured through modeling of the CMD. We find evidence for increased reddening in the youngest stellar populations and an intrinsic metallicity spread at all ages. Sextans A has been actively forming stars at a high rate for ~ 2.5 Gyr ago, with an increased rate beginning ~ 0.1 Gyr ago. We find a non-zero number of stars older than 2.5 Gyr, due to the limited depth of the photometry, a detailed star formation history at intermediate and older ages has considerable uncertainties. The mean metallicity was found to be [M/H] ~- 1.4 over the measured history of the galaxy, with most of the enrichment happening at ages of at least 10 Gyr. We also find that an rms metallicity spread of 0.15 dex at all ages allows the best fits to the observed CMD. We revisit our determination of the recent star formation history (age <= 0.7 Gyr) using BHeB stars and find good agreement for all but the last 25 Myr, a discrepancy resulting primarily from different distances used in the two analyses and the differential extinction in the youngest populations. This indicates that star formation histories determined solely from BHeB stars should be confined to CMD regions where no contamination from reddened MS stars is present.

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arXiv2002

Deep HST Imaging of Sextans A. II. Cepheids and Distance

We have identified 82 short-period variable stars in Sextans A from deep WFPC2 observations. All of the periodic variables appear to be short-period Cepheids, with periods as small as 0.8 days for fundamental-mode Cepheids and 0.5 days for first-overtone Cepheids. These objects have been used, along with measurements of the RGB tip and red clump, to measure a true distance modulus to Sextans A of (m-M)_0 = 25.61 +/- 0.07, corresponding to a distance of d = 1.32 +/- 0.04 Mpc. Comparing distances calculated by these techniques, we find that short-period Cepheids (P < 2 days) are accurate distance indicators for objects at or below the metallicity of the SMC. As these objects are quite numerous in low-metallicity star-forming galaxies, they have the potential for providing extremely precise distances throughout the Local Group. We have also compared the relative distances produced by other distance indicators. We conclude that calibrations of RR Lyraes, the RGB tip, and the red clump are self-consistent, but that there appears to be a small dependence of long-period Cepheid distances on metallicity. Finally, we present relative distances of Sextans A, Leo A, IC 1613, and the Magellanic Clouds.

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arXiv2000

Deep HST Imaging of IC 1613. I. Variable Stars and Distance

We present WFPC2 VI photometry of a field in the halo of IC 1613, finding 13 RR Lyraes and 11 Cepheids. Our photometry of the red giant branch tip and red clump is used to derive distances to IC 1613, which are consistent with each other and with distances based on the variable stars. We compare these values with similarly-measured distances for the Magellanic Clouds, and are able to measure metallicity dependencies of the RR Lyrae and Cepheid distances by requiring consistent relative distance measurements from the four techniques. For metallicities of [Fe/H] = -1.3 (RR Lyraes) and -1.0 (Cepheids), we find a relatively steep slope of 0.34 +/- 0.20 magnitudes per dex for the RR Lyraes and a shallow slope of -0.07 +/- 0.16 mag/dex for the Cepheids, both values within the range of theoretical and empirical results in the literature. We find that a dependence of the red clump absolute magnitude on age, in addition to metallicity, is required to produce self-consistent relative distances between IC 1613 and the Magellanic Clouds. Adopting such a red clump calibration and self-consistent calibrations for the other three distance indicators, we find that the distances to all three objects are in excellent agreement. Our best distance modulus to IC 1613 is mu_0 = 24.31 +/- 0.06, corresponding to a distance of 730 +/- 20 kpc. This distance produces an RR Lyrae absolute magnitude of MV = 0.61 +/- 0.08.

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arXiv1999

The Extragalactic Distance Scale Key Project XVIII. The Discovery of Cepheids and a New Distance to NGC 4535 Using the Hubble Space Telescope

We report on the discovery of Cepheids in the Virgo spiral galaxy NGC 4535, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. NGC 4535 is one of 18 galaxies observed as a part of The HST Key Project on the Extragalactic Distance Scale which aims to measure the Hubble constant to 10% accuracy. NGC 4535 was observed over 13 epochs using the F555W filter, and over 9 epochs using the F814W filter. The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent programs, DoPHOT and DAOPHOT II/ALLFRAME. Period-luminosity relations in the V and I bands were constructed using 39 high-quality Cepheids present in our set of 50 variable candidates. We obtain a distance modulus of 31.02+/-0.26 mag, corresponding to a distance of 16.0+/-1.9 Mpc. Our distance estimate is based on values of mu = 18.50 +/- 0.10 mag and E(V-I) = 0.13 mag for the distance modulus and reddening of the LMC, respectively.

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arXiv1998

A Cepheid Distance to the Fornax Cluster

The Hubble Space Telescope is being used to measure accurate Cepheid distances to nearby galaxies with the ultimate aim of determining the Hubble constant, H_0. For the first time, it has become feasible to use Cepheid variables to derive a distance to a galaxy in the southern hemisphere cluster of Fornax. Based on the discovery of 37 Cepheids in the Fornax galaxy NGC 1365, a distance to this galaxy of 18.6 +/- 0.6 Mpc (statistical error only) is obtained. This distance leads to a value of H_0 = 70 +/- 7 (random) +/- 18 (systematic) km/sec/Mpc in good agreement with estimates of the Hubble constant further afield.

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arXiv1998

The HST Key Project on the Extragalactic Distance Scale XIV. The Cepheids in NGC 1365

We report the detection of Cepheid variable stars in the barred spiral galaxy NGC 1365, located in the Fornax cluster, using the Hubble Space Telescope Wide Field and Planetary Camera 2. Twelve V (F555W) and four I (F814W) epochs of observation were obtained. The two photometry packages, ALLFRAME and DoPHOT, were separately used to obtain profile-fitting photometry of all the stars in the HST field. The search for Cepheid variable stars resulted in a sample of 52 variables, with periods between 14 and 60 days, in common with both datasets. ALLFRAME photometry and light curves of the Cepheids are presented. A subset of 34 Cepheids were selected on the basis of period, light curve shape, similar ALLFRAME and DoPHOT periods, color, and relative crowding, to fit the Cepheid period-luminosity relations in V and I for both ALLFRAME and DoPHOT. The measured distance modulus to NGC 1365 from the ALLFRAME photometry is 31.31 +/- 0.20 (random) +/- 0.18 (systematic) mag, corresponding to a distance of 18.3 +/- 1.7 (random) +/- 1.6 (systematic) Mpc. The reddening is measured to be E(V-I) = 0.16 +/- 0.08 mag. These values are in excellent agreement with those obtained using the DoPHOT photometry, namely a distance modulus of 31.26 +/- 0.10 mag, and a reddening of 0.15 +/- 0.10 mag (internal errors only).

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arXiv1998

A WFPC2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

The stellar population of the Pegasus dwarf irregular galaxy is investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with WFPC2. These and ground-based data are combined to produce color-magnitude diagrams which show the complex nature of the stellar population in this small galaxy. A young (< 0.5 Gyr) main sequence stellar component is present and clustered in two centrally-located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of A_V = 0.47 mag. The mean color of the well-populated red giant branch is relatively blue, consistent with a moderate metallicity young, or older, metal-poor stellar population. The red giant branch also has significant width in color, implying a range of stellar ages and/or metallicities. A small number of extended asymptotic giant branch stars are found beyond the red giant branch tip. Near the faint limits of our data is a populous red clump superimposed on the red giant branch. Efforts to fit self-consistent stellar population models based on the Geneva stellar evolution tracks yield a revised distance of 760 kpc. Quantitative fits to the stellar population are explored as a means to constrain the star formation history. The numbers of main sequence and core helium-burning blue loop stars require that the star formation rate was higher in the recent past, by a factor of 3-4 about 1 Gyr ago. Unique results cannot be obtained for the star formation history over longer time baselines without better information on stellar metallicities and deeper photometry. The youngest model consistent with the data contains stars with constant metallicity of Z = 0.001 which mainly formed 2-4 Gyr ago. Even at its peak of star forming activity, the Pegasus dwarf most likely remained relatively dim with M_V ~ -14.

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arXiv1998

WFPC2 Observations of Leo A: A Predominantly Young Galaxy within the Local Group

The unprecedented detail of the WFPC2 colour-magnitude diagrams of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops and the tip of the red giant branch, we obtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occured in this galaxy. Leo A provides a nearby laboratory for studying young metal poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.

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arXiv1998

A Study of Nine High-Redshift Clusters of Galaxies : III. HST Morphology of Clusters 0023+0423 and 1604+4304

We present a detailed morphological analysis of the galaxy populations in the first two clusters to be completed in an extensive observational study of nine high-redshift clusters of galaxies (Oke, Postman & Lubin 1998). These two clusters, CL0023+0423 and CL1604+4304, are at redshifts of z = 0.84 and z = 0.90, respectively. The morphological studies are based on high-angular resolution imagery taken with WFPC2 aboard the Hubble Space Telescope. These data are combined with deep, ground-based BVRI photometry and spectra taken with the Keck 10-meter telescopes. Based on these observations, CL0023+0423 has a galaxy population which is more similar to groups of galaxies and the field. This system is almost completely dominated by spiral galaxies. CL1604+4304, however, has a morphological composition which is more typical of a normal, present-day cluster; early-type galaxies comprise approximately 76% of all galaxies brighter than M_V = -19.0 + 5 log h in the central 0.5 h^{-1} Mpc. The ratio of S0 galaxies to ellipticals in this cluster is 1.7 +/- 0.9, consistent with local cluster populations. The morphological results support the conclusions of the dynamical analysis presented in the second paper of this series (Postman, Lubin & Oke 1998). CL0023+0423 consists of two galaxy groups which are separated by approximately 2900 km/s in radial velocity. On the other hand, CL1604+4304 has a morphological composition, velocity dispersion, and implied mass which are consistent with an Abell richness class 2 or 3 cluster.

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arXiv1997

Hubble Space Telescope Observations of the Draco Dwarf Spheroidal

We present an F606W-F814W color-magnitude diagram for the Draco dwarf spheroidal galaxy based on Hubble Space Telescope WFPC2 images. The luminosity function is well-sampled to 3 magnitudes below the turn-off. We see no evidence for multiple turnoffs and conclude that, at least over the field of the view of the WFPC2, star formation was primarily single-epoch. If the observed number of blue stragglers is due to extended star formation, then roughly 6% (upper limit) of the stars could be half as old as the bulk of the galaxy. The color difference between the red giant branch and the turnoff is consistent with an old population and is very similar to that observed in the old, metal-poor Galactic globular clusters M68 and M92. Despite its red horizontal branch, Draco appears to be older than M68 and M92 by 1.6 +/- 2.5 Gyrs, lending support to the argument that the ``second parameter'' which governs horizontal branch morphology must be something other than age. Draco's observed luminosity function is very similar to that of M68, and the derived initial mass function is consistent with that of the solar neighborhood.

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arXiv1995

The Palomar Distant Cluster Survey: I. The Cluster Catalog

We present an optical/near IR selected catalog of 79 clusters distributed over an area of 5.1~square~degrees. The catalog was constructed from images obtained with the 4-Shooter CCD mosaic camera on the Hale 5m telescope operating in ``scan" mode. The survey, hereafter known as the Palomar Distant Cluster Survey (PDCS), was conducted in two broad band filters that closely resemble~$V$ and~$I$. The $4σ$ limiting magnitudes for our 300 s exposures are~$\sim$~23.8~($V$) and~22.5~($I$). A matched filter algorithm was developed and employed to identify the cluster candidates by using positional and photometric data simultaneously. The clusters cover the range $0.2 \simless z \simless 1.2$, based on the redshift estimates derived in the cluster detection procedure. An accurate selection function is generated from extensive simulations. We find that the cumulative surface density of clusters with richness class R $\ge$ 1 is about a factor of 5 higher than the extrapolated density of R $\ge$ 1 Abell clusters. The PDCS results are consistent with a constant comoving density of clusters to $z \simless 0.6$, albeit at the above high density level. Constraints on cluster abundances at $z > 0.6$ will be possible with the acquisition of spectroscopic redshifts for a large subset of these cluster candidates. We also present a supplemental catalog of 28 clusters that do not satisfy all our selection criteria but which include some of the most distant systems detected in the survey. Finding charts for all 107 cluster candidates are provided.

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