Richard Ellis

California Institute of Technology, Pasadena, California, United States

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Publications (59)232.33 Total impact

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    ABSTRACT: Deep spectroscopic observations of z~6.5 galaxies have revealed a marked decline with increasing redshift in the detectability of Lyman-alpha emission. While this may offer valuable insight into the end of the reionisation process, it presents a fundamental challenge to the detailed spectroscopic study of the many hundreds of photometrically-selected distant sources now being found via deep HST imaging, and particularly those bright sources viewed through foreground lensing clusters. In this paper we demonstrate the validity of a new way forward via the convincing detection of an alternative diagnostic line, CIII]1909, seen in spectroscopic exposures of two star forming galaxies at z=6.029 and 7.213. The former detection is based on a 3.5 hour X-shooter spectrum of a bright (J=25.2) gravitationally-lensed galaxy behind the cluster Abell 383. The latter detection is based on a 4.2 hour MOSFIRE spectra of one of the most distant spectroscopically confirmed galaxies, GN-108036, with J=25.2. Both targets were chosen for their continuum brightness and previously-known redshift (based on Lyman-alpha), ensuring that any CIII] emission would be located in a favorable portion of the near-infrared sky spectrum. We compare our CIII] and Lyman-alpha equivalent widths in the context of those found at z~2 from earlier work and discuss the motivation for using lines other than Lyman-alpha to study galaxies in the reionisation era.
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    ABSTRACT: Charge Transfer Inefficiency (CTI) due to radiation damage above the Earth's atmosphere creates spurious trailing in images from Charge-Coupled Device (CCD) imaging detectors. Radiation damage also creates unrelated warm pixels, which can be used to measure CTI. This code provides pixel-based correction for CTI and has proven effective in Hubble Space Telescope Advanced Camera for Surveys raw images, successfully reducing the CTI trails by a factor of ~30 everywhere in the CCD and at all flux levels. The core is written in java for speed, and a front-end user interface is provided in IDL. The code operates on raw data by returning individual electrons to pixels from which they were unintentionally dragged during readout. Correction takes about 25 minutes per ACS exposure, but is trivially parallelisable to multiple processors.
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    ABSTRACT: Hubble Space Telescope spectroscopic observations of the nearby type Ia supernova (SN Ia) SN 2011fe, taken on 10 epochs from -13.5 to +41 days relative to B-band maximum light, and spanning the far-ultraviolet (UV) to the near-infrared (IR) are presented. This spectroscopic coverage makes SN 2011fe the best-studied local SN Ia to date. SN 2011fe is a typical moderately-luminous SN Ia with no evidence for dust extinction. Its near-UV spectral properties are representative of a larger sample of local events studied in Maguire et al. (2012). As a result, conclusions inferred from our detailed investigations are likely representative of those for other normal SNe Ia. The near-UV to optical spectra of SN 2011fe are modelled with a Monte Carlo radiative transfer code using the technique of 'abundance tomography', providing tight constraints on the density structure and abundance stratification of the event. SN 2011fe was a relatively weak explosion, with moderate Fe-group yields. Although its density structure is close to the 'standard' SN Ia pure deflagration explosion model W7, an improved model was developed which demonstrates that the ejecta of SN 2011fe have a more pronounced high-velocity tail, typical of a detonation wave affecting the outer layers. This improved model has a lower energy than typical delayed-detonation models. The derived Fe abundance in the outermost layer is consistent with the metallicity of ~0.5 solar at the SN explosion site in M101. Importantly, the spectroscopic rise time of ~19 days is significantly longer than that measured from the early optical light curve, implying a 'dark phase' of ~1-1.5 days. Such an extension in the rise time has significant implications when deducing the properties of the white dwarf and binary system from the early photometric behaviour.
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    ABSTRACT: The Hubble Deep Field provides one of the deepest multiwavelength views of the distant Universe and has led to the detection of thousands of galaxies seen throughout cosmic time. An early map of the Hubble Deep Field at a wavelength of 850 micrometres, which is sensitive to dust emission powered by star formation, revealed the brightest source in the field, dubbed HDF 850.1 (ref. 2). For more than a decade, and despite significant efforts, no counterpart was found at shorter wavelengths, and it was not possible to determine its redshift, size or mass. Here we report a redshift of z = 5.183 for HDF 850.1, from a millimetre-wave molecular line scan. This places HDF 850.1 in a galaxy overdensity at z ≈ 5.2, corresponding to a cosmic age of only 1.1 billion years after the Big Bang. This redshift is significantly higher than earlier estimates and higher than those of most of the hundreds of submillimetre-bright galaxies identified so far. The source has a star-formation rate of 850 solar masses per year and is spatially resolved on scales of 5 kiloparsecs, with an implied dynamical mass of about 1.3 × 10(11) solar masses, a significant fraction of which is present in the form of molecular gas. Despite our accurate determination of redshift and position, a counterpart emitting starlight remains elusive.
    Nature 06/2012; 486(7402):233-6. DOI:10.1038/nature11073 · 42.35 Impact Factor
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    ABSTRACT: We present a method for attaining sub-arcsecond pointing stability during sub- orbital balloon flights, as designed for in the High Altitude Lensing Observatory (HALO) concept. The pointing method presented here has the potential to perform near-space quality optical astronomical imaging at 1-2% of the cost of space-based missions. We also discuss an architecture that can achieve sufficient thermomechanical stability to match the pointing stability. This concept is motivated by advances in the development and testing of Ultra Long Duration Balloon (ULDB) flights which promise to allow observation campaigns lasting more than three months. The design incorporates a multi-stage pointing architecture comprising: a gondola coarse azimuth control system, a multi-axis nested gimbal frame structure with arcsecond stability, a telescope de-rotator to eliminate field rotation, and a fine guidance stage consisting of both a telescope mounted angular rate sensor and guide CCDs in the focal plane to drive a fast-steering mirror. We discuss the results of pointing tests together with a preliminary thermo-mechanical analysis required for sub-arcsecond pointing at high altitude. Possible future applications in the areas of wide-field surveys and exoplanet searches are also discussed.
    Astroparticle Physics 05/2012; 38. DOI:10.1016/j.astropartphys.2012.05.015 · 4.45 Impact Factor
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    ABSTRACT: Gravitational lensing can provide pure geometric tests of the structure of space-time, for instance by determining empirically the angular diameter distance-redshift relation. This geometric test has been demonstrated several times using massive clusters which produce a large lensing signal. In this case, matter at a single redshift dominates the lensing signal, so the analysis is straightforward. It is less clear how weaker signals from multiple sources at different redshifts can be stacked to demonstrate the geometric dependence. We introduce a simple measure of relative shear which for flat cosmologies separates the effect of lens and source positions into multiplicative terms, allowing signals from many different source-lens pairs to be combined. Applying this technique to a sample of groups and low-mass clusters in the COSMOS survey, we detect a clear variation of shear with distance behind the lens. This represents the first detection of the geometric effect using weak lensing by multiple, low-mass systems. The variation of distance with redshift is measured with sufficient precision to constrain the equation of state of the universe under the assumption of flatness, equivalent to a detection of a dark energy component Omega_X at greater than 99% confidence for an equation-of-state parameter -2.5 < w < -0.1. For the case w = -1, we find a value for the cosmological constant density parameter Omega_Lambda = 0.85+0.044-0.19 (68% C.L.), and detect cosmic acceleration (q_0 < 0) at the 98% C.L.. We consider the systematic uncertainties associated with this technique and discuss the prospects for applying it in forthcoming weak-lensing surveys.
    The Astrophysical Journal 11/2011; 749(2). DOI:10.1088/0004-637X/749/2/127 · 6.28 Impact Factor
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    ABSTRACT: There is convincing evidence that the progenitors of some SNe Ia have finally been identified. The subset of "super-Chandra" SNe Ia almost certainly result from the merger of two white dwarfs. These SNe show unburned carbon (i.e. progenitor material) in their outer layers, visible only at early times. Other SNe also show this, and thus might be related, but still others show high velocity CaII and SiII at early times instead. High velocity material has been shown to be related to shells or disks or progenitor material causing overdensities in the SN ejecta. Until now, observing SNe Ia early enough to see this -- within a day or two of explosion -- was an extreme rarity. Now the Palomar Transient Factory is making it commonplace. We plan to take the first census of progenitor material in SNe Ia and test whether Hubble diagram residuals from SNe in different environments are correlated with progenitor signatures. Recent advances in observations and theory may finally allow us to test whether SNe arise from the single-degenerate, double-degenerate, or sub-Chandra scenarios. Finally, these GMOS spectra will be used to trigger our Cycle 18 HST STIS program studying early progenitor and metallicity signatures in the UV.
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    ABSTRACT: Weak gravitational lensing causes shear in the images of distant galaxies. Shear statistics can be measured from high-quality astronomical images and then fit to theoretical expectations. Thus constraints can be made on dark matter structures and on dark energy. Code to measure the two-point correlation function (2PCF) including tomography (redshift information) is modified and developed. Code to measure the signal-to-noise peak counts is adapted. The 2PCF and peak counts are measured on a shear catalog derived from the Hubble Space Telescope COSMOS survey. Constraints on cosmological parameters are then derived from the measured 2PCF and from the peak counts. These constraints are combined to break the degeneracy between the matter mass density (phim) and the amplitude of density fluctuations (sigma8), making the overall set of constraints tighter. )
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    ABSTRACT: We present spatially-resolved dynamics for six strongly lensed star-forming galaxies at z=1.7-3.1, each enlarged by a linear magnification factor ~8. Using the Keck laser guide star AO system and the OSIRIS integral field unit spectrograph we resolve kinematic and morphological detail in our sample with an unprecedented fidelity, in some cases achieving spatial resolutions of ~100 pc. With one exception our sources have diameters ranging from 1-7 kpc, star formation rates of 2-40 Msun/yr (uncorrected for extinction) and dynamical masses of 10^(9.7-10.3) Msun. With this exquisite resolution we find that four of the six galaxies display coherent velocity fields consistent with a simple rotating disk model, which can only be recovered with the considerably improved spatial resolution and sampling from the combination of adaptive optics and strong gravitational lensing. Our model fits imply ratios for the systemic to random motion, V sin(i)/sigma, ranging from 0.5-1.3 and Toomre disk parameters Q<1. The large fraction of well-ordered velocity fields in our sample is consistent with data analyzed for larger, more luminous sources at this redshift. Our high resolution data further reveal that all six galaxies contain multiple giant star-forming HII regions whose resolved diameters are in the range 300 pc - 1.0 kpc, consistent with the Jeans length expected in the case of dispersion support. The density of star formation in these regions is ~100 times higher than observed in local spirals; such high values are only seen in the most luminous local starbursts. The global dynamics and demographics of star formation in these HII regions suggest that vigorous star formation is primarily governed by gravitational instability in primitive rotating disks. Comment: 18 pages, 8 figures, submitted to MNRAS
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    ABSTRACT: The quest to discover the most distant galaxies has developed rapidly in the last decade. We are now exploring redshifts of 6 and beyond, when the Universe was less than a billion years old, an epoch when the previously-neutral intergalactic medium was reionized. The continuing discovery of galaxies at progressively higher and higher redshifts has been driven by the availability of large telescopes on the ground and in space, improvements in detector technology, and new search strategies. Over the past 4 years, the Lyman break technique has been shown to be effective in isolating z~6 star-forming i'-drop galaxies through spectroscopic confirmation with large ground-based telescopes (Keck, Gemini and the ESO VLTs). Narrow-band imaging, notably with the wide field of the Subaru telescope, has also produced samples of Lyman-alpha emitters at these redshifts. A The discovery of this i'-drop galaxy population has been used to infer the global star formation rate density at this epoch (z~6), and we are now beginning to constrain the contribution to reionization of the UV flux from these galaxies. Infrared data from the Spitzer Space Telescope has been used to determine the spectral energy distributions (SEDs) from the rest-frame UV to the optical, and constrain the previous star formation histories, masses and ages. The indications are that much of the stellar mass of these galaxies might have formed in vigorous bursts at z>6. The next big advances would be to test the population synthesis modelling of these z~6 galaxies through spectroscopy of the rest-frame optical (rather than crude broad-band SEDs), and also to push the observational horizon for galaxies further to directly explore star formation during the reionization epoch. JWST is likely to have a profound impact on realising these goals. Comment: ASP Conference Series, Vol. 395 (50 years of NRAO, eds A. Bridle, J. Condon & G. Hunt)
    Il Nuovo Cimento B 09/2009; 122(9-11). DOI:10.1393/ncb/i2008-10513-y
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    ABSTRACT: Charge Transfer Inefficiency (CTI) due to radiation damage above the Earth's atmosphere creates spurious trailing in Hubble Space Telescope (HST) images. Radiation damage also creates unrelated warm pixels - but these happen to be perfect for measuring CTI. We model CTI in the Advanced Camera for Surveys (ACS)/Wide Field Channel (WFC) and construct a physically motivated correction scheme. This operates on raw data, rather than secondary science products, by returning individual electrons to pixels from which they were unintentionally dragged during readout. We apply our correction to images from the HST COSMOS survey, successfully reducing the CTI trails by a factor of ~30 everywhere in the CCD and at all flux levels. We quantify changes in galaxy photometry, astrometry and shape. The remarkable 97% level of correction is more than sufficient to enable a (forthcoming) reanalysis of downstream science products, and the collection of larger surveys. Comment: MNRAS in press; 14 pages, 11 figures
    Monthly Notices of the Royal Astronomical Society 09/2009; 401(1). DOI:10.1111/j.1365-2966.2009.15638.x · 5.23 Impact Factor
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    ABSTRACT: We have found an unexplained diversity in the UV spectra of intermediate redshift Type Ia supernovae (SNe Ia) which may arise from variations in progenitor metallicity. To understand the implications and any redshift-dependent biases in the use of SNe Ia for measurements of dark energy, we are undertaking a new HST ToO program from July 2009 - June 2010 to gather UV spectra of 35 local SNe Ia, taking advantage of two new local "rolling'' transient searches. Our sample also will be unique in having continuous photometric/spectroscopic coverage from early phases (LCOGT, Lick, Palomar, Keck, HET, VLT). Here, we seek to exploit the rapid ToO mode of Gemini N and S to obtain the required very early-phase confirmation spectra for candidates in 2009B, essential to the success of this program, providing the classification upon which targets can be scheduled at HST. These rare early-phase spectra will also offer new opportunities in studying SNe Ia and their progenitor compositions.
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    ABSTRACT: Richard Ellis, Pedro G Ferreira, Richard Massey and Gisa Weszkalnys return to Príncipe in the International Year of Astronomy to celebrate the 1919 RAS expedition led by Sir Arthur Eddington.
    Astronomy & Geophysics 08/2009; 50(4). DOI:10.1111/j.1468-4004.2009.50412.x · 0.34 Impact Factor
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    ABSTRACT: The cosmic dark ages are the mysterious epoch during which the pristine gas began to condense and ultimately form the first stars. Although these beginnings have long been a topic of theoretical interest, technology has only recently allowed the beginnings of observational insight into this epoch. Many questions surround the formation of stars in metal-free gas and the history of the build-up of metals in the intergalactic medium: (1) What were the properties of the first stellar and galactic sources to form in pristine (metal-free) gas? (2) When did the epoch of Population III (metal-free) star formation take place and how long did it last? (3) Was the stellar initial mass function dramatically different for the first stars and galaxies? These questions are all active areas of theoretical research. However, new observational constraints via the direct detection of Population III star formation are vital to making progress in answering the broader questions surrounding how galaxies formed and how the cosmological properties of the universe have affected the objects it contains.
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    ABSTRACT: There is a vast menagerie of plausible candidates for the constituents of dark matter, both within and beyond extensions of the Standard Model of particle physics. Each of these candidates may have scattering (and other) cross section properties that are consistent with the dark matter abundance, BBN, and the most scales in the matter power spectrum; but which may have vastly different behavior at sub-galactic "cutoff" scales, below which dark matter density fluctuations are smoothed out. The only way to quantitatively measure the power spectrum behavior at sub-galactic scales at distances beyond the local universe, and indeed over cosmic time, is through probes available in multiply imaged strong gravitational lenses. Gravitational potential perturbations by dark matter substructure encode information in the observed relative magnifications, positions, and time delays in a strong lens. Each of these is sensitive to a different moment of the substructure mass function and to different effective mass ranges of the substructure. The time delay perturbations, in particular, are proving to be largely immune to the degeneracies and systematic uncertainties that have impacted exploitation of strong lenses for such studies. There is great potential for a coordinated theoretical and observational effort to enable a sophisticated exploitation of strong gravitational lenses as direct probes of dark matter properties. This opportunity motivates this white paper, and drives the need for: a) strong support of the theoretical work necessary to understand all astrophysical consequences for different dark matter candidates; and b) tailored observational campaigns, and even a fully dedicated mission, to obtain the requisite data.
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    ABSTRACT: Theories of structure formation in a cold dark matter dominated universe predict that massive clusters of galaxies assemble from the hierarchical merging of lower mass subhalos. Exploiting strong and weak gravitational lensing signals inferred from panoramic Hubble Space Telescope imaging data, we present a high-resolution reconstruction of the mass distribution in the massive, lensing cluster Cl 0024+16 at z = 0.39. Applying galaxy-galaxy lensing techniques we track the fate of dark matter subhalos as a function of projected cluster-centric radius out to 5 Mpc, well beyond the virial radius. We report the first detection of the statistical lensing signal of dark matter subhalos associated with late-type galaxies in clusters. The mass of a fiducial dark matter halo that hosts an early-type L* galaxy varies from M = 6.3+2.7 –2.0 × 1011 M ☉ within r < 0.6 Mpc, 1.3+0.8 –0.6 × 1012 M ☉ within r < 2.9 Mpc, and increases further to M = 3.7+1.4 –1.1 × 1012 M ☉ in the outskirts. The mass of a typical dark matter subhalo that hosts an L* galaxy increases with projected cluster-centric radius in line with expectations from the tidal stripping hypothesis. The mass of a dark matter subhalo that hosts a late-type L* galaxy is 1.06+0.52 –0.41 × 1012 M ☉. Early-type galaxies appear to be hosted on average in more massive dark matter subhalos compared to late-type galaxies. Early-type galaxies also trace the overall mass distribution of the cluster whereas late-type galaxies are biased tracers. We interpret our findings as evidence for the active assembly of mass via tidal stripping in galaxy clusters. The mass function of dark matter subhalos as a function of projected cluster-centric radius is compared with an equivalent mass function derived from clusters in the Millennium Run simulation populated with galaxies using semianalytic models. The shape of the observationally determined mass functions based on an I-band-selected sample of cluster members and lensing data are in agreement with the shapes of the subhalo mass functions derived from the Millennium Run simulation. However, simulated subhalos appear to be more efficiently stripped than lensing observations suggest. This is likely an artifact of comparison with a dark matter only simulation. Future simulations that simultaneously follow the detailed evolution of the baryonic component during cluster assembly will be needed for a more detailed comparison.
    The Astrophysical Journal 03/2009; 693(1):970. DOI:10.1088/0004-637X/693/1/970 · 6.28 Impact Factor
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    ABSTRACT: We have carried out an extensive spectroscopic survey with the Keck and VLT telescopes, targeting lensed galaxies in the background of the massive cluster Abell 68. Spectroscopic measurements are obtained for 26 lensed images, including a distant galaxy at z = 5.4. Redshifts have been determined for 5 out of 7 multiple-image systems. Through a careful modeling of the mass distribution in the strongly lensed regime, we derive a mass estimate of 5.3 × 1014 M☉ within 500 kpc. Our mass model is then used to constrain the redshift distribution of the remaining multiply imaged and singly imaged sources. This enables us to examine the physical properties for a subsample of 7 Lyα emitters at 1.7 z 5.5, whose unlensed luminosities of 1041 ergs s-1 are fainter than similar objects found in blank fields. Of particular interest is an extended Lyα emission region surrounding a highly magnified source at z = 2.6, detected in VIMOS integral field spectroscopy data. The physical scale of the most distant lensed source at z = 5.4 is very small (<300 pc), similar to the lensed z ~ 5.6 emitter reported by Ellis et al. in Abell 2218. New photometric data available for Abell 2218 allow for a direct comparison between these two unique objects. Our survey illustrates the practicality of using lensing clusters to probe the faint end of the z ~ 2-5 Lyα luminosity function in a manner that is complementary to blank-field narrowband surveys.
    The Astrophysical Journal 12/2008; 662(2):781. DOI:10.1086/517875 · 6.28 Impact Factor
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    ABSTRACT: We present results from a decimetric radio survey undertaken with the Very Large Array as part of a longer term goal to intercompare star formation and dust extinction diagnostics on a galaxy-by-galaxy basis for a representative sample of nearby galaxies. For our survey field, Selected Area 57, star formation rates derived from 1.4 GHz luminosities are compared with earlier nebular emission-line and ultraviolet (UV) continuum diagnostics. We find broad correlations, over several decades in luminosity, between the Hα, UV continuum, and 1.4 GHz diagnostics. However, the scatter in these relations is found to be larger than observational errors, with offsets between the observed relations and those expected assuming constant star formation histories and luminosity-independent extinction models. We investigate the physical origin of the observed relations and conclude that the discrepancies between different star formation diagnostics can only be partly explained by simple models of dust extinction in galaxies. These models cannot by themselves explain all the observed differences, introducing the need for temporally varying star formation histories and/or more complex models of extinction to explain the entire data set.
    The Astrophysical Journal 12/2008; 558(1):72. DOI:10.1086/322451 · 6.28 Impact Factor
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    ABSTRACT: We present a three-dimensional cosmic shear analysis of the Hubble Space Telescope COSMOS survey, the largest ever optical imaging program performed in space. We have measured the shapes of galaxies for the telltale distortions caused by weak gravitational lensing and traced the growth of that signal as a function of redshift. Using both 2D and 3D analyses, we measure cosmological parameters Ωm, the density of matter in the universe, and σ8, the normalization of the matter power spectrum. The introduction of redshift information tightens the constraints by a factor of 3 and also reduces the relative sampling (or "cosmic") variance compared to recent surveys that may be larger but are only two-dimensional. From the 3D analysis, we find that σ8(Ωm/0.3)0.44 = 0.866 at 68% confidence limits, including both statistical and potential systematic sources of error in the total budget. Indeed, the absolute calibration of shear measurement methods is now the dominant source of uncertainty. Assuming instead a baseline cosmology to fix the geometry of the universe, we have measured the growth of structure on both linear and nonlinear physical scales. Our results thus demonstrate a proof of concept for tomographic analysis techniques that have been proposed for future weak-lensing surveys by a dedicated wide-field telescope in space.
    The Astrophysical Journal Supplement Series 12/2008; 172(1):239. DOI:10.1086/516599 · 14.14 Impact Factor
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    ABSTRACT: We present the first high-redshift Hubble diagram for Type II-P supernovae (SNe II-P) based on five events at redshift up to z ~ 0.3. This diagram was constructed using photometry from the Canada-France-Hawaii Telescope Supernova Legacy Survey and absorption-line spectroscopy from the Keck Observatory. The method used to measure distances to these supernovae is based on recent work by Hamuy & Pinto and exploits a correlation between the absolute brightness of SNe II-P and the expansion velocities derived from the minimum of the Fe II λ5169 P Cygni feature observed during the plateau phases. We present three refinements to this method that significantly improve the practicality of measuring the distances of SNe II-P at cosmologically interesting redshifts. These are an extinction correction measurement based on the V-I colors at day 50, a cross-correlation measurement for the expansion velocity, and the ability to extrapolate such velocities accurately over almost the entire plateau phase. We apply this revised method to our data set of high-redshift SNe II-P and find that the resulting Hubble diagram has a scatter of only 0.26 mag, thus demonstrating the feasibility of measuring the expansion history, with present facilities, using a method independent of that based on supernovae of Type Ia.
    The Astrophysical Journal 12/2008; 645(2):841. DOI:10.1086/504413 · 6.28 Impact Factor