29,688 research outputs found
SDSS surface photometry of M 31 with absorption corrections
The objective of this work is to obtain an extinction-corrected distribution
of optical surface brightness and colour indices of the large nearby galaxy M
31 using homogeneous observational data and a model for intrinsic extinction.
We process the Sloan Digital Sky Survey (SDSS) images in ugriz passbands and
construct corresponding mosaic images, taking special care of subtracting the
varying sky background. We apply the galactic model developed in Tempel et al.
(2010) and far-infrared imaging to correct the photometry for intrinsic dust
effects.
We obtain observed and dust-corrected distributions of the surface brightness
of M 31 and a map of line-of-sight extinctions inside the galaxy. Our
extinction model suggests that either M 31 is intrinsically non-symmetric along
the minor axis or the dust properties differ from those of the Milky Way.
Assuming the latter case, we present the surface brightness distributions and
integral photometry for the Sloan filters as well as the standard UBVRI system.
We find the following intrinsic integral colour indices for M 31: (U-B)_0=0.35;
(B-V)_0=0.86; (V-R)_0=0.63; (R-I)_0=0.53; the total intrinsic
absorption-corrected luminosities of M 31 in the B and the V filters are 4.10
and 3.24 mag, respectively.Comment: 6 pages, 8 figures, accepted for publication in A&A. The
high-resolution zoomable colour image of M 31 can be seen at
http://www.aai.ee/~elmo/m31
Mission: Impossible (Escape from the Lyman Limit)
We investigate the intrinsic opacity of high-redshift galaxies to outgoing
ionising photons using high-quality photometry of a sample of 27
spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep
Field. Our measurement is based on maximum-likelihood fitting of model galaxy
spectral energy distributions-including the effects of intrinsic Lyman-limit
absorption and random realizations of intervening Lyman-series and Lyman-limit
absorption-to photometry of galaxies from space- and ground-based broad-band
images. Our method provides several important advantages over the methods used
by previous groups, including most importantly that two-dimensional sky
subtraction of faint-galaxy images is more robust than one-dimensional sky
subtraction of faint-galaxy spectra. We find at the 3sigma statistical
confidence level that on average no more than 4% of the ionising photons escape
galaxies of redshift 1.9<z<3.5. This result is consistent with observations of
low- and moderate-redshift galaxies but is in direct contradiction to a recent
result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies.
Dividing our sample in subsamples according to luminosity, intrinsic
ultraviolet colour, and redshift, we find no evidence for selection effects
that could explain such discrepancy. Even when all systematic effects are
included, the data could not realistically accomodate any escape fraction value
larger than ~15%.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 8 pages, 4 b/w figures, MNRAS styl
Accurate photometry of extended spherically symmetric sources
We present a new method to derive reliable photometry of extended spherically
symmetric sources from {\it HST} images (WFPC2, ACS/WFC and NICMOS/NIC2
cameras), extending existing studies of point sources and marginally resolved
sources. We develop a new approach to accurately determine intrinsic sizes of
extended spherically symmetric sources, such as star clusters in galaxies
beyond the Local Group (at distances <~ 20 Mpc), and provide a detailed
cookbook to perform aperture photometry on such sources, by determining
size-dependent aperture corrections (ACs) and taking sky oversubtraction as a
function of source size into account. In an extensive Appendix, we provide the
parameters of polynomial relations between the FWHM of various input profiles
and those obtained by fitting a Gaussian profile (which we have used for
reasons of computational robustness, although the exact model profile used is
irrelevant), and between the intrinsic and measured FWHM of the cluster and the
derived AC. Both relations are given for a number of physically relevant
cluster light profiles, intrinsic and observational parameters. AC relations
are provided for a wide range of apertures. Depending on the size of the source
and the annuli used for the photometry, the absolute magnitude of such extended
objects can be underestimated by up to 3 mag, corresponding to an error in mass
of a factor of 15. We carefully compare our results to those from the more
widely used DeltaMag method, and find an improvement of a factor of 3--40 in
both the size determination and the AC.Comment: The paper is accepted for publication in A&A, Section 13
(Observational Techniques, published electronically). The published version
contains one example table per appendix. A version of the paper containing
all tables as well as all data in electronical form are available
http://www.astro.physik.uni-goettingen.de/~galev/panders/Sizes_AC
The Correlations between the Intrinsic Colors and Spectroscopic Metallicities of M31 Globular Clusters
We present the correlations between the spectroscopic metallicities and
ninety-three different intrinsic colors of M31 globular clusters, including
seventy-eight BATC colors and fifteen SDSS and near infrared ugrizK colors. The
BATC colors were derived from the archival images of thirteen filters (from c
to p), which were taken by Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor
Sky Survey with a 60/90 cm f/3 Schmidt telescope. The spectroscopic
metallicities adopted in our work were from literature. We fitted the
correlations of seventy-eight different BATC colors and the metallicities for
123 old confirmed globular clusters, and the result implies that correlation
coefficients of twenty-three colors r>0.7. Especially, for the colors
, , and , the correlation coefficients are r>0.8.
Meanwhile, we also note that the correlation coefficients (r) approach zero for
, , , and , which are likely to be
independent of metallicity. Similarity, we fitted the correlations of
metallicity and ugrizK colors for 127 old confirmed GCs. The result indicates
that all these colors are metal-sensitive (r>0.7), of which is the
most metal-sensitive color. Our work provides an easy way to simply estimate
the metallicity from colors.Comment: 25 pages, 11 figures, 2 tables, accepted for publication in PASP
Microlensing induced spectral variability in Q2237+0305
We present both photometry and spectra of the individual images of the
quadruple gravitational lens system Q2237+0305. Comparison of spectra obtained
at two epochs, separated by years, shows evidence for significant
changes in the emission line to continuum ratio of the strong ultraviolet
CIV~1549, CIII]~1909 and MgII~2798 lines. The short,
day, light--travel time differences between the sight lines to the
four individual quasar images rule out any explanation based on intrinsic
variability of the source. The spectroscopic differences thus represent direct
detection of microlensing--induced spectroscopic differences in a quasar. The
observations allow constraints to be placed on the relative spatial scales in
the nucleus of the quasar, with the ultra--violet continuum arising in a region
of \la~0.05~{\rm pc} in extent, while the broad emission line material is
distributed on scales much greater than this.Comment: Accepted for Publication in MNRAS. Paper with 11 figure
Galaxy surface photometry
We describe galaxy surface photometry based on fitting ellipses to the
isophotes of the galaxies. Example galaxies with different isophotal shapes are
used to illustrate the process, including how the deviations from elliptical
isophotes are quantified using Fourier expansions. We show how the definitions
of the Fourier coefficients employed by different authors are linked. As
examples of applications of surface photometry we discuss the determination of
the relative disk luminosities and the inclinations for E and S0 galaxies. We
also describe the color-magnitude and color-color relations. When using both
near-infrared and optical photometry, the age-metallicity degeneracy may be
broken. Finally we discuss the Fundamental Plane where surface photometry is
combined with spectroscopy. It is shown how the FP can be used as a sensitive
tool to study galaxy evolution.Comment: 40 pages. Lectures given at the Nordic-Baltic Research Course in
Applied Astrophysical Photometry, held September 1999 at the Moletai
Observatory, Lithuania. Baltic Astronomy, 8, 535 (1999), in press. Note the
year. The paper with Fig. 2, 14 and 15 in original (high) resolution is
available at http://www.astro.ku.dk/~milvang/papers/BA_MJ_J.ps.gz or
http://www.gemini.edu/documentation/preprints/pre58.htm
How the extinction of extragalactic background light affects surface photometry of galaxies, groups and clusters
The faint regions of galaxies, groups and clusters hold important clues about
how these objects formed, and surface photometry at optical and near-infrared
wavelengths represents a powerful tool for studying such structures. Here, we
identify a hitherto unrecognized problem with this technique, related to how
the night sky flux is typically measured and subtracted from astronomical
images. While most of the sky flux comes from regions between the observer and
the target object, a small fraction - the extragalactic background light (EBL)
- comes from behind. We argue that since this part of the sky flux can be
subject to extinction by dust present in the galaxy/group/cluster studied,
standard reduction procedures may lead to a systematic oversubtraction of the
EBL. Even very small amounts of extinction can lead to spurious features in
radial surface surface brightness profiles and colour maps of extended objects.
We assess the likely impact of this effect on a number of topics in
extragalactic astronomy where very deep surface photometry is currently
attempted, including studies of stellar halos, starburst host galaxies, disc
truncations and diffuse intragroup/intracluster light. We argue that EBL
extinction may provide at least a partial explanation for the anomalously red
colours reported for the halos of disc galaxies and the hosts of local
starburst galaxies. EBL extinction effects also mimic truncations in discs with
unusually high dust opacities, but are unlikely to be the cause of such
features in general. Failure to account for EBL extinction can also give rise
to a non-negligible underestimate of intragroup and intracluster light at the
faintest surface brightness levels currently probed. (Abridged)Comment: 15 pages, 10 figures, accepted for publication in MNRA
The two states of Sgr A* in the near-infrared: bright episodic flares on top of low-level continuous variability
In this paper we examine properties of the variable source Sgr A* in the
near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT
observations taken 2004 to 2009. We investigate the variability of Sgr A* with
two different photometric methods and analyze its flux distribution. We find
Sgr A* is continuously emitting and continuously variable in the near-infrared,
with some variability occurring on timescales as long as weeks. The flux
distribution can be described by a lognormal distribution at low intrinsic
fluxes (<~5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a
median flux of approximately 1.1 mJy, but above 5 mJy the flux distribution is
significantly flatter (high flux events are more common) than expected for the
extrapolation of the lognormal distribution to high fluxes. We make a general
identification of the low level emission above 5 mJy as flaring emission and of
the low level emission as the quiescent state. We also report here the
brightest Ks-band flare ever observed (from August 5th, 2008) which reached an
intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27
increase over the median flux of Sgr A*, close to double the brightness of the
star S2, and 40% brighter than the next brightest flare ever observed from
Sgr~A*.Comment: 14 pages, 6 figures, accepted for publication in Ap
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