arXiv subjects

Kristine Spekkens

Publications and source records attributed to Kristine Spekkens.

133 records · Page 8Linked to original sources

arXiv2010

Disk Galaxy Scaling Relations in the SFI++: Intrinsic Scatter and Applications

We study the scaling relations between the luminosities, sizes, and rotation velocities of disk galaxies in the SFI++, with a focus on the size-luminosity (RL) and size-rotation velocity (RV) relations. Using isophotal radii instead of disk scale-lengths as a size indicator, we find relations that are significantly tighter than previously reported: the correlation coefficients of the template RL and RV relations are r=0.97 and r=0.85, which rival that of the more widely studied LV (Tully-Fisher) relation. The scatter in the SFI++ RL relation is 2.5-4 times smaller than previously reported for various samples, which we attribute to the reliability of isophotal radii relative to disk scale-lengths. After carefully accounting for all measurement errors, our scaling relation error budgets are consistent with a constant intrinsic scatter in the LV and RV relations for velocity widths logW>2.4, with evidence for increasing intrinsic scatter below this threshold. The scatter in the RL relation is consistent with constant intrinsic scatter that is biased by incompleteness at the low-luminosity end. Possible applications of the unprecedentedly tight SFI++ RV and RL relations are investigated. Just like the Tully-Fisher relation, the RV relation can be used as a distance indicator: we derive distances to galaxies with primary Cepheid distances that are accurate to 25%, and reverse the problem to measure a Hubble constant H_0=72+/-7 km/s/Mpc. Combining the small intrinsic scatter of our RL relation (0.034+/-0.001 log(kpc/h)) with a simple model for disk galaxy formation, we find an upper limit on the range of disk spin parameters that is a factor of ~7 smaller than that of the halo spin parameters predicted by cosmological simulations. This likely implies that the halos hosting Sc galaxies have a much narrower distribution of spin parameters than previously thought.

astro-ph.CO↗

arXiv2009

Aperture Synthesis Observations of the Nearby Spiral NGC 6503: Modeling the Thin and Thick HI Disks

We present sensitive aperture synthesis observations of the nearby, late-type spiral galaxy NGC 6503, and produce HI maps of considerably higher quality than previous observations by van Moorsel & Wells (1985). We find that the velocity field, while remarkably regular, contains clear evidence for irregularities. The HI is distributed over an area much larger than the optical image of the galaxy, with spiral features in the outer parts and localized holes within the HI distribution. The absence of absorption towards the nearby quasar 1748+700 yields an upper limit of 5 10^{17} cm^{-2} for the column density of cold HI gas along a line of sight which should intersect the disk at a radius of 29 kpc. This suggests that the radial extent of the HI disk is not much larger than that which we trace in HI emission (23 kpc). The observed HI distribution is inconsistent with models of a single thin or thick disk. Instead, the data require a model containing a thin disk plus a thicker low column-density HI layer that rotates more slowly than the thin disk and that extends only to approximately the optical radius. This suggests that the presence of extra-planar gas in this galaxy is largely the result of star formation in the disk rather than cold gas accretion. Improved techniques for interferometric imaging including multi-scale Clean that were used in this work are also described.

astro-ph.GA↗

arXiv2008

The Arecibo Legacy Fast ALFA Survey VII: A Neutral Hydrogen Cloud Complex in the Virgo Cluster

We present observations of an HI cloud complex most likely located in the Virgo galaxy cluster, first reported by Kent et al. (2007). The complex consists of five clouds, detected in the data set of the ALFALFA extragalactic HI survey at Arecibo. The clouds have radial velocities between cz~480 and 610 km/s. At the Virgo cluster distance, they are spread over a projected span of 170 kpc and have HI masses ranging from 0.48 to 1.7 x 10^8 Msun. The overall HI mass of the complex is 5.1 x 10^8 Msun. The clouds' velocity widths vary between 50 and 250 km/s. New results of follow-up aperture synthesis observations conducted with the Very Large Array are also presented, which yield a higher resolution view of two of the clouds in the complex. These two resolved clouds show no evidence of symmetry in the gas distribution or of any ordered motions. The possibility that the complex is a group of primordial objects, embedded in their own dark matter halo is thought to be unlikely. Scenarios in which the clouds have been removed from the disk of a galaxy traveling at high speed through the intracluster medium are considered. The most likely among those is thought to be one where the clouds were separated from NGC 4445 at a time >0.5 Gyr ago. The orbital velocity of the clouds and the putative parent galaxy would now be seen at a relatively large angle with respect to the line of sight.

astro-ph↗

arXiv2007

Optically Unseen HI Detections towards the Virgo Cluster detected in the Arecibo Legacy Fast ALFA Survey

We report the discovery by the Arecibo Legacy Fast ALFA (ALFALFA) survey of eight HI features not coincident with stellar counterparts in the Virgo Cluster region. All of the HI clouds have cz < 3000 km/s and, if at the Virgo distance, HI masses between 1.9 x 10**7 and 1.1 x 10**9 solar masses. Four of the eight objects were reported or hinted at by previous studies and "rediscovered" by ALFALFA. While some clouds appear to be associated with optical galaxies in their vicinity, others show no clear association with a stellar counterpart. Two of them are embedded in relatively dense regions of the cluster and are associated with M49 and M86; they were previously known. The others are mostly located in peripheral regions of the cluster. Especially notable are a concentration of objects towards the so-called M cloud, 3 to 5 degrees to the NW of M87, and a complex of several clouds projected roughly halfway between M87 and M49. The object referred to as VIRGOHI21 and proposed to be a "dark galaxy" is also detected and shown to be a tidal feature associated with NGC 4254.

astro-ph↗

arXiv2007

Modeling Non-Circular Motions in Disk Galaxies: Application to NGC 2976

We present a new procedure to fit non-axisymmetric flow patterns to 2-D velocity maps of spiral galaxies. We concentrate on flows caused by bar-like or oval distortions to the total potential that may arise either from a non-axially symmetric halo or a bar in the luminous disk. We apply our method to high-quality CO and Halpha data for the nearby, low-mass spiral NGC 2976 previously obtained by Simon et al., and find that a bar-like model fits the data at least as well as their model with large radial flows. We find supporting evidence for the existence of a bar in the baryonic disk. Our model suggests that the azimuthally averaged central attraction in the inner part of this galaxy is larger than estimated by these authors. It is likely that the disk is also more massive, which will limit the increase to the allowed dark halo density. Allowance for bar-like distortions in other galaxies may either increase or decrease the estimated central attraction.

astro-ph↗

arXiv2005

The Arecibo Legacy Fast ALFA Survey: II. Results of Precursor Observations

In preparation for the full Arecibo Legacy Fast ALFA extragalactic HI survey, precursor observations were carried out in Aug--Sep 2004 with the 7-beam Arecibo L-band feed array (ALFA) receiver system and the WAPP spectral processors. While these observations were geared mainly at testing and debugging survey strategy, hardware and software, approximately 36 hours of telescope time yielded science--quality data. From those observations, an initial list of 730 tentative detections of varying degree of reliability was extracted. Ninety--eight high signal-to-noise candidates were deemed to be bona fide HI line detections. To test our ability to discriminate cosmic signals from RFI and noise, 165 candidates ranging in reliability likelihood were re--observed with the single beam L--band wide system at Arecibo in Jan--Feb 2005. Of those, 41% were confirmed as real. We present the results of both the ALFA and single beam observations for the sample of 166 confirmed HI sources, as well as our assessment of their optical counterparts. Of the 166 sources, 62 coincide with previously known HI sources, while optical redshifts were available for an additional 18 galaxies; thus, 52% of the redshifts reported here were previously unknown. Of the 166 HI detections, 115 are identified with previously cataloged galaxies, of either known or unknown redshift, leaving 51 objects identified for the first time. Because of the higher sensitivity of the Arecibo system, fewer than 10% of the 166 HI sources would have been detected by a HIPASS--like survey of the same region. Three of the objects have HI masses less than 10^7 solar masses. The full ALFALFA survey which commenced in February 2005 should detect more than 100 times as many objects of similarly low HI mass over the next 5 years.

astro-ph↗

arXiv2005

The Cusp/Core Problem in Galactic Halos: Long-Slit Spectra for a Large Dwarf Galaxy Sample

We derive inner dark matter halo density profiles for a sample of 165 low-mass galaxies using rotation curves obtained from high-quality, long-slit optical spectra assuming minimal disks and spherical symmetry. For $ρ(r) \sim r^{-α}$ near the galaxy center we measure median inner slopes ranging from $α_m = 0.22 \pm 0.08$ to $0.28 \pm 0.06$ for various subsamples of the data. This is similar to values found by other authors, and in stark contrast to the intrinsic cusps ($α_{int}\sim1$) predicted by simulations of halo assembly in cold dark matter (CDM) cosmologies. To elucidate the relationship between $α_m$ and $α_{int}$ in our data, we simulate long-slit observations of model galaxies with halo shapes broadly consistent with the CDM paradigm. Simulations with $α_{int}=1/2$ and 1 recover both the observed distribution of $α_m$ and correlations between $α_m$ and primary observational parameters such as distance and disk inclination, whereas those with $α_{int}=5/4$ are marginally consistent with the data. Conversely, the hypothesis that low-mass galaxies have $α_{int}=3/2$ is rejected. While the simulations do not imply that the data favor intrinsic cusps over cores, they demonstrate that the discrepancy between $α_m$ and $α_{int}\sim1$ for our sample does not necessarily imply a genuine conflict between our results and CDM predictions: rather, the apparent cusp/core problem may be reconciled by considering the impact of observing and data processing techniques on rotation curves derived from long-slit spectra.

astro-ph↗