arXiv subjects

G. R. Meurer

Publications and source records attributed to G. R. Meurer.

51 records · Page 3Linked to original sources

arXiv2003

Advanced Camera for Surveys Photometry of the Cluster RDCS1252.9-2927: The Color-Magnitude Relation at z=1.24

We investigate the color-magnitude (CM) relation of galaxies in the distant X-ray selected cluster RDCS1252.9-2927 at z=1.24 using images obtained with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope in the F775W and F850LP bandpasses. We select galaxies based on morphological classifications extending about 3.5 mag down the galaxy luminosity function, augmented by spectroscopic membership information. At the core of the cluster is an extensive early-type galaxy population surrounding a central pair of galaxies that show signs of dynamical interaction. The early-type population defines a tight sequence in the CM diagram, with an intrinsic scatter in observed (i-z) of $0.029\pm0.007$ mag based on 52 galaxies, or $0.024\pm0.008$ mag for $\sim30$ ellipticals. Simulations using the latest stellar population models indicate an age scatter for the ellipticals of about 34%, with a mean age $\taul\gta2.6$ Gyr (corresponding to $z_L\gta2.7$), and the last star formation occurring at $z_{end}\gta1.5$. Transforming to rest-frame $(U-B)$, we conclude that the slope and scatter in the CM relation for morphologically selected early-type galaxies show little or no evidence for evolution out to $z\approx1.2$. Thus, elliptical galaxies were already well established in X-ray luminous clusters when the universe was a third of its present age.

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arXiv2003

Star Formation at z~6: i-dropouts in the ACS GTO fields

Using an i-z dropout criterion, we determine the space density of z~6 galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of 23 objects are found over 46 arcmin^2, or ~0.5 objects/arcmin^2 down to z~27.3 (6 sigma; all AB mag) (including one probable z~6 AGN). Combining deep ISAAC data for our RDCS1252-2927 field (J~25.7 and Ks~25.0 (5 sigma)) and NICMOS data for the HDF North (JH~27.3 (5 sigma)), we verify that these dropouts have flat spectral slopes. i-dropouts in our sample range in luminosity from ~1.5 L* (z~25.6) to ~0.3 L* (z~27.3) with the exception of one very bright candidate at z~24.2. The half-light radii vary from 0.09" to 0.29", or 0.5 kpc to 1.7 kpc. We derive the z~6 rest-frame UV luminosity density using three different procedures, each utilizing simulations based on a CDF South V dropout sample. First, we compare our findings with a no-evolution projection of this V-dropout sample. We find 23+/-25% more i-dropouts than we predict. Adopting previous results to z~5, this works out to a 20+/-29% drop in the luminosity density from z~3 to z~6. Second, we use these same V-dropout simulations to derive a selection function for our i-dropout sample and compute the UV-luminosity density (7.2+/-2.5 x 10^25 ergs/s/Hz/Mpc^3 down to z~27). We find a 39+/-21% drop over the same redshift range. This is our preferred value and suggests a star formation rate of 0.0090+/-0.0031 M_sol/yr/Mpc^3 to z~27, or ~0.036+/- 0.012 M_sol/yr/Mpc^3 extrapolating the LF to the faint limit. Third, we follow a very similar procedure, but assume no incompleteness, finding a luminosity density which is ~2-3X lower. This final estimate constitutes a lower limit. All three estimates are within the canonical range of luminosity densities necessary for reionization of the universe at this epoch. (abridged)

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arXiv2003

HST/ACS Coronagraphic Imaging of the Circumstellar Disk around HD1415659A

Multicolor coronagraphic images of the circumstellar disk around HD141569A have been obtained with the Hubble Space Telescope (HST) Advanced Camera for Surveys. B, V, and I images show that the disk's previously-described multiple-ring structure is actually a continuous distribution of dust with a tightly-wound spiral structure. Two, more open spiral arms extend from the disk, one of which appears to reach the nearby binary star HD141569BC. Diffuse dust is seen up to 1200AU from HD141569A. Although planets may exist in the inner region of the disk, tidal interaction with HD141569BC seems more likely to be the cause of these phenomena. The disk appears redder than the star (B-V = 0.21 and V-I = 0.25), and its color is spatially uniform. A scattering asymmetry factor of g = 0.25-0.35 is derived. The azimuthal density distribution is asymmetric, varying by a factor of ~3 at some radii.

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arXiv2003

Discovery of Two Distant Type Ia Supernovae in the Hubble Deep Field North with the Advanced Camera for Surveys

We present observations of the first two supernovae discovered with the recently installed Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The supernovae were found in Wide Field Camera images of the Hubble Deep Field North taken with the F775W, F850LP, and G800L optical elements as part of the ACS guaranteed time observation program. Spectra extracted from the ACS G800L grism exposures confirm that the objects are Type Ia supernovae (SNe Ia) at redshifts z=0.47 and z=0.95. Follow-up HST observations have been conducted with ACS in F775W and F850LP and with NICMOS in the near-infrared F110W bandpass, yielding a total of 9 flux measurements in the 3 bandpasses over a period of 50 days in the observed frame. We discuss many of the important issues in doing accurate photometry with the ACS. We analyze the multi-band light curves using two different fitting methods to calibrate the supernovae luminosities and place them on the SNe Ia Hubble diagram. The resulting distances are consistent with the redshift-distance relation of the accelerating universe model, although evolving intergalactic grey dust remains as a less likely possibility. The relative ease with which these SNe Ia were found, confirmed, and monitored demonstrates the potential ACS holds for revolutionizing the field of high-redshift SNe Ia, and therefore of testing the accelerating universe cosmology and constraining the "epoch of deceleration".

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arXiv2002

An Automatic Image Reduction Pipeline for the Advanced Camera for Surveys

We have written an automatic image processing pipeline for the Advanced Camera for Surveys (ACS) Guaranteed Time Observation (GTO) program. The pipeline, known as Apsis, supports the different cameras available on the ACS instrument and is written in Python with a flexible object-oriented design that simplifies the incorporation of new pipeline modules. The processing steps include empirical determination of image offsets and rotation, cosmic ray rejection, image combination using the drizzle routine called via the STScI Pyraf package, object detection and photometry using SExtractor, and photometric redshift estimation in the event of multiple bandpasses. The products are encapsulated in XML markup for automated ingestion into the ACS Team archive.

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arXiv2002

Calibration of Geometric Distortion in the ACS Detectors

The off-axis location of the Advanced Camera for Surveys (ACS) is the chief (but not sole) cause of strong geometric distortion in all detectors: the Wide Field Camera (WFC), High Resolution Camera (HRC), and Solar Blind Camera (SBC). Dithered observations of rich star cluster fields are used to calibrate the distortion. We describe the observations obtained, the algorithms used to perform the calibrations and the accuracy achieved.

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arXiv2002

Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intra-cluster media in the distant and younger Universe.

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arXiv2001

FUSE Observations of Outflowing OVI in the Dwarf Starburst Galaxy NGC1705

We report FUSE far-UV spectroscopy of the prototypical dwarf starburst galaxy NGC 1705. These data allow us for the first time to probe the coronal-phase gas (T = 10E5 to 10E6 K) that may dominate the radiative cooling of the supernova-heated ISM and thereby determine the dynamical evolution of starburst-driven outflows. We detect a broad (100 km/s) and blueshifted (by 80 km/s) OVI absorption-line arising in the previously-known galactic outflow. The properties of the OVI absorption are inconsistent with the standard superbubble model in which this gas arises in a conductive interface inside the outer shell. We show that the superbubble in NGC 1705 is blowing out of the galaxy ISM. During blow-out, coronal-phase gas can be created by hydrodynamical mixing as hot gas rushes out through fissures in the fragmenting shell of cool gas. As the coronal gas cools radiatively, it can naturally produce the observed OVI column density and outflow speed. The OVI data show that the cooling rate in the coronal-phase gas is less than about 10% of the supernova heating rate. Since the X-ray luminosity from hotter gas is even smaller, we conclude that radiative losses are insignificant. The outflow should be able to vent its metals and kinetic energy out of the galaxy. This process has potentially important implications for the evolution of dwarf galaxies and the IGM.

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arXiv2001

How to Correct for Dust Absorption in Starbursts

We review new and published results to examine how well the bolometric flux of starbursts can be recovered from ultraviolet (UV) and optical observations. We show that the effective absorption of starbursts can be substantial, up to \~10 mag in the far UV, and ~5 mag in H-alpha, but apparently not as high as some claims in the literature (several tens to a thousand mag). The bolometric fluxes of an IUE sample of starbursts can be recovered to 0.14 dex accuracy using the UV flux and spectral slope. However, this relationship breaks down for Ultra Luminous Infrared Galaxies (ULIGs). The H-alpha flux combined with the Balmer decrement can be used to predict the bolometric flux to 0.5 dex accuracy for starbursts including most ULIGs. These results imply a foreground screen component to the dust distribution.

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arXiv2001

Starbursts, Dark Matter and Dwarf Galaxy Evolution

Optical and HI imaging of both dwarf irregular (dI) and Blue Compact Dwarf (BCD) galaxies reveal important clues on how dwarf galaxies evolve and their star formation is regulated. Both usually show evidence for stellar and gaseous disks. However, their total mass is dominated by dark matter. Gas rich dwarfs form with a range of disk structural properties. These have been arbitrarily separated them into two classes on the basis of central surface brightness. Dwarfs with mu_{0}(B) > ~22 mag arcsec^{-2} are usually classified as BCDs, while those fainter than limit are usually classified as dIs. Both classes experience bursts of star formation, but with an absolute intensity correlated with the disk surface brightness. Even in BCDs the bursts typically represent only a modest < ~1 mag enhancement to the B luminosity of the disk. While starbursts are observed to power significant galactic winds, the fractional ISM loss remains modest. Dark matter halos play an important role in determining dwarf galaxy morphology by setting the equilibrium surface brightness of the disk.

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arXiv2000

Hiden Star Formation: The Ultraviolet Perspective

Many recent estimates of the star formation rate density at high redshift rely on rest-frame ultraviolet (UV) data. These are highly sensitive to dust absorption. Applying a correlation between the far-infrared (FIR) to UV flux ratio and UV color found in a local starbursts to galaxy samples out to z ~ 3, one can account for most of the FIR background. However, the correlation is based on a sample that does not include the most extreme starbursts, Ultra Luminous Infrared Galaxies (ULIGs). Our new UV images of ULIGs show that their FIR fluxes are underpredicted by this correlation by factors ranging from 7 to 70. We discuss how ULIGs compare to the various types of high-z galaxies: sub-mm sources, Lyman Break Galaxies, and Extremely Red Objects.

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arXiv2000

Star Clusters and the Duration of Starbursts

The duration of starbursts is important for determining how they are regulated and the impact they have on their environment. Starbursts contain numerous prominent star clusters which typically comprise in total ~20% of the ultraviolet light, embedded in a diffuse glow of recently formed stars responsible for the dominant ~80%. Hubble Space Telescope images have been obtained for four starburst galaxies in order to determine their burst duration from the ages of their star clusters. Preliminary results for NGC3310 are reported here. The UBI colors of its clusters and diffuse light give consistent results. The clusters have a broad range of colors, consistent with a population of instantaneous bursts with ages ranging from 0 to a few 100 Myr. The diffuse light has a narrow range of color consistent with continuous star formation over timescales ranging from ~ 10 to 100 Myr. Hence NGC3310's starburst has lasted ~100 Myr, or about 10 times the crossing time. Other results noted in the literature also indicate bursts lasting several times longer than the crossing time. These results suggest that starbursts are not self-extinguishing flashes. Rather they are sustainable, perhaps self-regulated.

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arXiv1999

The Shape and Figure Rotation of NGC 2915's Dark Halo

NGC 2915 is a blue compact dwarf galaxy with a very extended HI disk showing a short central bar and extended spiral arms, both reaching far beyond the optical component. We use Tremaine & Weinberg (1984) method to measure the pattern speed of the bar from HI radio synthesis data. Our measurements yield a pattern speed of 0.21+/-0.06 km/s/arcsec (8.0+/-2.4 km/s/kpc for D=5.3 Mpc), in disagreement with the general view that corotation in barred disks lies just outside the end of the bar, but consistent with recent models of barred galaxies with dense dark matter halos. Our adopted bar semi-length puts corotation at more than 1.7 bar radii. The existence of the pattern is also problematic. Because NGC 2915 is isolated, interactions cannot account for the structure observed in the HI disk. We also demonstrate that the low observed disk surface density and the location of the pseudo-rings make it unlikely that swing amplification or bar-driven spiral arms could explain the bar and spiral pattern. Based on the similarity of the dark matter and HI surface density profiles, we discuss the possibility of dark matter distributed in a disk and following closely the HI distribution. The disk then becomes unstable and can naturally form a bar and spiral pattern. However, this explanation is hard to reconcile with some properties of NGC 2915. We also consider the effect of a massive and extended triaxial dark matter halo with a rotating figure. The existence of such halos is supported by CDM simulations showing strongly triaxial dark halos with slow figure rotation. The observed structure of the HI disk can then arise through forcing by the rotating triaxial figure. We associate the measured pattern speed in NGC 2915 with the figure rotation of its dark halo.

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arXiv1996

NGC 2915. II. A Dark Spiral Galaxy With A Blue Compact Dwarf Core

Abridged abstract: We present HI synthesis observations of the weak blue compact dwarf (BCD) galaxy NGC~2915. NGC~2915 has the HI properties of a late type spiral galaxy (Sd - Sm), including a double horn global profile, and HI spiral arms. The HI extends out to over five times the Holmberg radius. The optical counterpart corresponds to a central HI bar. The rotation curve is derived and fitted with a mass model consisting of a stellar disk, a neutral gas disk, and a dark matter (DM) halo. The DM halo dominates at nearly all radii. The total mass to blue light ratio is 76 within the last measured point, making NGC~2915 one of the darkest disk galaxies known. The core of the DM halo is unusually dense (rho_0 ~ 0.1, M_sun pc^{-3}) and compact (R_core approx 1 kpc). The neutral gas component, with mass M_g = 1.27x10^9 M_sun is probably more massive than the stellar component. Split and broad HI lines (line of sight velocity dispersion, sigma ~ 35 km/s) are seen in the central region. Pressure support is probably significant, and it is not clear whether the core is in equilibrium. Beyond the Holmberg radius, the average HI sigma is 8 km/s, which is normal for disk galaxies. NGC~2915 does not obey the Tully-Fisher (1977) relation, being underluminous by a factor of nine. A simple HI surface density threshold of about 10^{21} cm^{-2} adequately describes the location of current star formation. The HI properties are compared to "normal" disk galaxies and other BCDS.

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arXiv1995

Starbursts and Star Clusters in the Ultraviolet

Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies reveal them to be highly irregular, even after excluding compact sources (clusters and resolved stars). Most (7/9) are found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The brightest clusters, or super star clusters (SSC), are preferentially found at the very heart of starbursts. The size of the nearest SSCs are consistent with those of Galactic globular clusters. The luminosity function of SSCs is well represented by a power law with a slope alpha ~ -2. There is a strong correlation between the far infrared excess and the UV spectral slope. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust.

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