Modulating Cepheid radial velocity curves - a systematic uncertainty for Baade-Wesselink distances
Richard Anderson (Geneva Observatory)
Cepheid distance determination using variations of the Baade-Wesselink technique requires measurements of the angular and linear radius variations due to pulsation.
I here report the discovery of significant (~ 1km/s) amplitude modulations in the line of sight velocity curves of four classical Cepheids.
Such modulations can lead to systematic errors on Baade-Wesselink distances in the order of 5 to 15% when the angular and linear radius variations were measured during different epochs.
Both short (< 5d) and long (> 30d) period Cepheids showing this phenomenon are presented, thus suggesting modulations to be common among Cepheids.
This highlights the necessity to employ contemporaneous observational data in Baade-Wesselink analyses in order to ensure the accuracy of the derived distances.
A new study of Cepheids p-factor : Application on κ Pav and RS Pup
Joanne Breitfelder (ESO - Chile)
Cepheids are known from more than a century as fundamental distance indicators, but their Period-Luminosity relationship still needs to be better calibrated. Accurate distance measurements are therefore required, and they are mostly computed by Baade-Wesselink (BW) methods. A measure of both the linear and angular diameters provides us with a quasi-geometrical distance. This elegant technique is although limited by the projection factor (p-factor) that converts the measured radial velocities into actual pulsating velocities, and whose value is matter of debate. We present here the results we obtained on two peculiar stars : the Type II Cepheid κ Pav and the long-period Cepheid RS Pup.
Quasars as Cosmic Distance Indicators - How does it work and how well does it work?
Kelly Denney (Ohio State University - Astronomy)
It has recently been realized that the potential exists for active galactic nuclei (AGN) or quasars, to be used as reliable cosmic distance indicators (Watson et al. 2011). This is possible because both the AGN continuum and broad emission line flux are variable, and the variations in the line flux trace those of the continuum with an observed time delay. This time delay corresponds to the light travel time between the continuum-emitting region, presumably the accretion disk, and broad line-emitting region. The time delay, i.e., the radius at which we see the emission-line response, is also dependent on the intrinsic luminosity of the AGN continuum. This is because photoionization physics regulates the production of line-emitting photons coming from the broad line region, which have been reprocessed from the ionizing continuum photons from the accretion disk. This leads to an empirically identified, though physically well-understood, "radius-luminosity" relationship, which, when cast in terms of the observables - the emission-line time delay and the observed continuum flux - is proportional to the luminosity distance to the AGN. Taking advantage of this simple physical relationship provides the only currently viable means by which AGN can be used as standardizable candles for studying cosmology. We show that while the current sample is small, covering only z<~0.3, and has a scatter of ~0.3 mag, programs are currently underway to increase the sample size and extend the redshift range covered by this sample to z>2. Furthermore, we identify multiple sources of scatter in the current sample that will be mitigated by future efforts. Our work suggests that AGN have the potential to be powerful cosmic distance indicators that will complement other methods over the redshift range 0 < z <~ 3-4.
Assessing the error budget in lensing-based distance scale measurements
Chris Fassnacht (University of California, Davis)
Distance scale measurements with gravitational lenses are
independent from, and thus highly complementary to, more traditional
techniques. Furthermore, since each gravitational lens provides an
independent measurement of cosmological parameters, it is possible to
achieve a roughly sqrt(N) improvement in the precision of the
measurement as sample sizes increase. This poster presents the
H0licow project, which is expanding the number of fully-analyzed good
lens systems from a sample of two to a sample of five. We are
addressing two of the major components of the error budget, the
modeling of the mass distribution of the lens and the line-of-sight
contribution, through deep spectroscopy and multiband imaging. In
addition, in looking to future lens samples that are orders of
magnitude larger than current samples, we are conducting a blind Time
Delay Challenge to assess whether different delay-measuring techniques
may introduce systematic biases at the sub-percent level.
A Geometric Distance to the Megamaser Galaxy NGC5765b
Feng Gao (NRAO/SHAO)
As part of the Megamaser Cosmology Project (MCP), here we present a preliminary distance measurement to the galaxy NGC5765b. Recent VLBI observations have confirmed that the water masers trace a thin, sub-parsec Keplerian disk in the nucleus, implying a central binding mass of ~ 4.6*10^7 M_sun. With single dish monitoring of the maser spectra over the past two years, we measured the secular drift of masers near the systemic velocity of the galaxy. The drift rates fall in the range 0.5 to 1.0 km/s/yr. Fitting a warped, thin disk model to these measurements, we determine an angular-diameter distance to NGC5765b of 126 Mpc and determine an H_0 of 66 km/s/Mpc, with ~ 10% uncertainty.
A Cepheid Distance to the Coma Cluster
Samantha Hoffmann (Texas A&M University)
In the current era of "precision cosmology'', measuring the Hubble constant (H0) more accurately and precisely helps to better constrain the properties of dark energy. Cepheid-based distances are a critical step in the Extragalactic Distance Scale recently used to measure H0 with a total uncertainty of only 3.4%. NGC 4921 is located in the heart of the Coma cluster at a distance of about 100 Mpc. We conducted a search for Cepheid variables using the Hubble Space Telescope and discover 28 Cepheids, extending the reach of HST by a factor of 3 relative to previous Cepheid work. Since Coma is in the Hubble flow, this approach eliminates the need for a secondary distance indicator and enables a direct determination of H0 based exclusively on a Cepheid distance. We present preliminary results from this challenging project, including the characteristics of the new Cepheids and a description of a series of simulations with HST images of NGC 5584 used to derive a blending and crowding correction.
The Maser-Calibrated IR Surface Brightness Fluctuation Distance to the Coma Cluster
Joseph Jensen (Utah Valley University)
The discovery of H2O masers in NGC 4258 provided a precise and accurate geometrical distance measurement to a galaxy in which Cepheids have also been measured, making NGC 4258 a popular first rung on the extragalactic distance ladder. Although surface brightness fluctuations (SBF) are usually measured in elliptical galaxies and calibrated as a secondary distance indicator using Cepheids, new IR images of NGC 4258 taken using the new WFC3 camera on HST make a robust measurement of SBF in this galaxy possible for the first time. We have independently calibrated the IR SBF distance scale using the geometrical NGC 4258 maser distance and compare that calibration to optical and IR SBF measurements in the Virgo and Fornax clusters. We then apply the maser calibration to IR SBF measurements of the cD galaxy NGC 4874 in the Coma cluster, yielding a direct SBF distance to Coma and a new measurement of the relative distance between the Virgo and Coma clusters.
Applications of Galactic and LMC Cepheid JHK Period-Luminosity Relations
S. Kanbur, A. Bharadwaj, L. Macri, C. Ngeow, H.P. Singh
Shashi Kanbur (SUNY Oswego)
We use the first results from a near infrared JHK survey of the central region of the Large Magellanic Cloud using the CPAPIR camera at the CTIO 1.5m telescope together with Galactic JHK Cepheid data compiled from the literature to
1) obtain a LMC distance assuming Galactic Cepheid parallaxes compiled from the literature and
2) compare the distances to Galactic Cepheids determined bt various means with that inferred from the recent determination of the LMC distance by Pietrzynski et al.
We consider issues with metallicity, provide an error budget and discuss possible future avenues and problems.
Using High Redshift Standard Candles for Constraining Dark Energy
Anthea King (University of Queensland/University of Copenhagen)
Standard candles have had central roles in major cosmological discoveries, from the use of Cepheid variable stars in the discovery of the expanding universe by Hubble (1929) to the more recent use of type Ia Supernovae in the discovery of the accelerated expansion and `dark energy' (Riess et al.1998; Perlmutter et al. 1999).
Recently, Active galactic nuclei, gamma ray bursts, and certain types of core collapse supernovae have been proposed as potential high redshift standard candle candidates. Due to their high luminosity, they can be used to probe unexplored regions in the expansion history of the universe. Information from these regions can help constrain the properties of dark energy, and in particular, whether it exhibits time-evolving behaviour.
We investigate whether future measurements of high-redshift standard candles will be a powerful probe of dark energy, when compared to other types of planned dark energy measurements.
We consider both linear and piecewise parameterizations of the dark energy equation-of-state, w(z), and assess the optimal redshift distribution a high-redshift standard-candle survey could take to constrain these models.
The more general the form of the dark energy equation-of-state w(z) being tested, the more useful high-redshift standard candles were found to become. For a linear parameterization of w(z), they give only small improvements over planned supernova and baryon acoustic oscillation measurements; a wide redshift range with many low redshift points is optimal to constrain this linear model. However to constrain a general, and thus potentially more informative, form of w(z), having many high-redshift standard candles can significantly improve limits on the nature of dark energy, even compared to the future dark energy experiments currently being planned
Andromeda as distance anchor
Chien-Hsiu Lee (University Observatory Munich)
As the nearest spiral galaxy, M31 plays a key role in the cosmic distance scale. Its proximity enables us to investigate various distance indicators (e.g. Cepheids, eclipsing binaries, tip of red giant branch), while its geometry effect is negligible compared to the Magellanic Clouds. It also serves as a local counter part for the distant spiral galaxies used in the Tully-Fisher relation.
In this work we aim to refine the distance of M31 with different probes, i.e. Cepheids and eclipsing binaries, found in the long-term, high cadence PAndromeda survey. Based on more than 2000 Cepheids discovered in PAndromeda, we will improve the period-luminosity relation of Riess et al. (2011). We also detect ~ 300 eclipsing binaries, where a dozen of them are bright than 20 mag in V, render spectroscopic follow-up and make an independent M31 distance estimate feasible.
Optical to near-infrared light curves of classical Cepheids
Noriyuki Matsunaga (The University of Tokyo)
Infrared properties of classical Cepheids are getting more and more important because of their period-luminosity relation has smaller systematic errors in the infrared compared to the optical. The shapes of their light curves show a systematic trend varying with the wavelength and with the period. This trend can be explained by simple arithmetic considering the bolometric correction and underlying variations of temperature and radius of Cepheids.
FGS Trigonometric Parallaxes for three First Overtone Cepheids: Polaris, DT Cyg, and SU Cas
Edmund Nelan (STScI)
We have used the FGS system on the Hubble Space Telescope to measure
trigonometric parallaxes of three first-overtone Cepheids: Polaris, SU Cas, and DT Cyg. We find statistical agreement between the Hipparcos and FGS parallaxes for SU Cas and DT Cyg, but the FGS parallax of Polaris (6.11 +/- 0.20 mas) is significantly smaller than that measured by Hipparcos (7.54 +/- 0.11 mas). We note that we measured the parallax of Polaris B, and not Polaris itself, which is too bright for the FGS. We find excellent agreement in the FGS proper motion of Polaris B and the Hipparcos proper motion of Polaris, further supporting their well-established physical association. We outline our data analysis technique with an emphasis on our investigation into the FGS vs. Hipparcos discrepancy for Polaris. We strongly rule out recent suggestions that Polaris has a parallax of ~10 mas and the implication that it is a fundamental-mode pulsator; in fact, if the FGS parallax is correct, Polaris may be oscillating in the 2nd overtone.
Searching for Ultra Long Period Cepheids in M31
Chow-Choong Ngeow (National Central University)
The ultra Long Period Cepheids (ULPCs) are those classical Cepheids with pulsation period longer than 80 days. Bird et al (200) proposed that ULPC can be used to determine the Hubble constant in "one-step". ULPCs have been identified in Magellanic clouds, nearby dwarf galaxies and and distant spiral galaxies that being targeted for the SH0ES program. Surprisingly, no ULPC has been identified in the nearest spiral galaxy -- the Andromeda galaxy. In this work, I present our result of the search of ULPC(s) in M31, using the R-band time series data taken from the Palomar Transient Factory (PTF). Eight candidates have been identified, with three of them are likely to be ULPC, other five candidates mostly belong to the long period variables or Mira-type variable. Follow-up observations in VI filters for these eight candidates were done with Palomar 1.5m and Lulin 1.0m telescopes to verify/falsify the ULPC nature of them. Preliminary results based on the VI band light curves analysis will be presented as well.
RR Lyrae variable stars in the Phoenix Dwarf Galaxy
Antonio Ordoñez (University of Florida)
We present the first detailed study of the RR Lyrae variable population in the Local Group dSph/dIrr transition galaxy, Phoenix, using HST/WFPC2 observations of the galaxy. We utilize template light curve fitting routines to obtain best fit light curves for RR Lyrae variables in Phoenix. Our technique has identified 78 highly probable RR Lyrae stars (54 ab-type; 24 c- type) with about 40 additional candidates. We find mean periods for the two populations of ⟨Pab⟩ = 0.60 ± 0.03 days and ⟨Pc⟩ = 0.353 ± 0.002 days. Our analysis of the RR Lab stars yields a mean metallicity of ⟨[Fe/H]⟩ = −1.68±0.06 dex. We also study the chemical enrichment law for Phoenix. We find that our metallicity distribution function is reasonably well fitted by a closed-box model. Finally, we utilize the metallicity-luminosity relation for RR Lyrae stars to calculate the distance modulus to Phoenix. Using the average metallicity, reddening, and average magnitude from our analysis, we derive a distance modulus of (m-M)0 = 23.1±0.1, fully consistent with other distance determinations for Phoenix.
Distances, Reddenings, and Light-Curve Templates of Cepheids and Type II-Plateau Supernovae
Ondrej Pejcha (Princeton University)
We present a method to perform global fits of observed radial velocities and multi-band light curves of variable objects with well-defined photospheres. We apply this method to Cepheids in the Galaxy, LMC, and SMC, and independently to nearby Type II-P supernovae. The model yields consistent relative distances and reddenings and provides light-curve templates for all our photometric bands from near-UV to near-IR, which can be easily generalized to any additional band. The model derives the spectral energy distributions and the derivatives with respect to the temperature, and constrains the mean reddening law toward objects in our samples. Combining our results with stellar atmosphere models, we study the biases in distance and reddening determinations in Cepheids due to the metallicity. We provide a framework for optimally selecting filters that yield the smallest overall errors in Cepheid parameter determination. We eliminate the need of "dilution factors" in the distance determination with Type II-P supernova. We compare our distances to previous Type II-P supernova distance estimates and also to other distance indicators for the same galaxies.
VLBA parallaxes across the Milky Way and the distance to NGC 4258
Mark Reid (Harvard-Smithsonian CfA)
The VLBA now achieves unprecedented astrometric accuracy that approaches micro-arcsecond levels. Parallax distances to young stars have been measured across the Milky Way, providing the most accurate distance to the Pleiades cluster, locations and properties of spiral arms, and the best estimates of the fundamental parameters Ro (distance to the center) and Theta_o (circular rotation speed). Extragalactic distances by rotational parallax have been estimated for M33 in the Local Group. Finally, the distance to the megamaser galaxy NGC 4258, which is well beyond the Local Group, has been measured to 3% accuracy, providing strong constraints on the extragalactic distance scale (via Cepheids) and for Ho.
Local Host Environment of Type Ia Supernovae and Cosmological Biases
Mickael Rigault (Physikalisches Institut Universität Bonn)
The use of Type Ia supernovae (SNe Ia) as distance indicators has proven to be a powerful technique for measuring the dark energy equation of state. However, recent studies have highlighted potential biases correlated with properties of their global and local host environments. These effects are sufficient in size to induce systematic errors in cosmological measurements.
I will present the results of a study of host-galaxy regions in close proximity to SNe Ia from the Nearby Supernova factory (Rigault et al 2013). We find a significant offset in SNe Ia standardized magnitude between those from locally star-forming regions and those from passive regions. As the mass of a galaxy is related to its star formation activity, I also show that this bias naturally explains the known – but so far not understood – mass-step effect. Since the fraction of SNe Ia from passive regions is expected to decrease with redshift, such a magnitude bias introduces a bias in distance measurements using these standardizable candles.
Systematic errors are today dominating the error budget of cosmological constant measurements (such as w). In this context, local host information of SNe Ia are a valuable handle to better understand this probe. In the near future, this information will enable us to improve their use as standardizable candles. As an example, I will show that a simple environmental selection of SNe Ia allows a significant reduction of the dispersion of the Hubble diagram by 30%.
Updates the Union Supernova Ia Compilation
David Rubin (Florida State University)
High-redshift supernovae observed with the Hubble Space Telescope (HST) are crucial for constraining any time variation in dark energy. In a forthcoming paper (Rubin+, in prep), we will present a cosmological analysis incorporating these SNe, with improved HST calibrations, and improved photometry. We also present a new Bayesian framework for cosmological analysis of supernova light-curve fits.
Type Ia Supernovae and Evidence for a Phantom Dark Energy Equation of State
Daniel Shafer (University of Michigan)
Some recent observations provide > 2-sigma evidence for phantom dark energy -- a value of the dark energy equation of state less than the cosmological-constant value of -1. We focus on constraining the equation of state by combining current data from the most mature geometrical probes of dark energy: Type Ia supernovae (SNe Ia), baryon acoustic oscillations (BAO), and the cosmic microwave background (CMB). The combined data are consistent with w = -1 for the Union2.1 SN sample, though they present moderate (~ 1.9-sigma) evidence for a phantom value when either the SNLS3 or PS1 SN sample is used instead. We study the dependence of the constraints on the redshift, stretch, color, and host galaxy stellar mass of SNe, but we find no unusual trends. In contrast, the constraints strongly depend on any external H_0 prior: a higher adopted value for the direct measurement of the Hubble constant (H_0 > 71 km/s/Mpc) leads to > 2-sigma evidence for phantom dark energy. Given Planck data, we can therefore make the following statement at 2-sigma confidence: either the SNLS3 and PS1 data have systematics that remain unaccounted for or the Hubble constant is below 71 km/s/Mpc; else the dark energy equation of state is indeed phantom.
The cosmic distance scale from strong gravitational lens time delays
Tommaso Treu (UCSB)
Strong lensing gravitational time delays are a powerful way to measure the cosmic distance scale independent of other more traditional techniques. After reviewing the basic principles of the method, I will illustrate recent and dramatic improvements in the measurement of the time-delays themselves, and in the modeling of the gravitational potential and line of sight effect, with specific examples, and present the resulting cosmological inference. I will then describe current efforts to improve the precision and accuracy of the method by collecting larger samples of suitable systems and identify, control, and reduce systematic uncertainties (known and unknown unknowns).
The Cepheid Period-Luminosity Relation in M31 Based on the PHAT Survey
Rachel Wagner-Kaiser (University of Florida)
Using Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) and Wide Field Camera 3 (WFC3) observations from the Panchromatic Hubble Andromeda Treasury (PHAT), we present new period-luminosity relations for Cepheid variables in M31 in the visual and IR bands. From several ground-based studies, we correlate Cepheid positions with the PHAT photometry to derive new Period-Luminosity and Wesenheit Period-Luminosity relations in HST's NIR and visual filters. We then derive a new distance modulus to M31 in agreement with previous studies. We find a significant difference between ground-based and HST Period-Luminosity relations with the same Cepheids, suggesting adverse affects from blending and contamination in ground-based Cepheid observations.