196 research outputs found
Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators
Context: The CARMENES survey is a high-precision radial velocity (RV)
programme that aims to detect Earth-like planets orbiting low-mass stars.
Aims: We develop least-squares fitting algorithms to derive the RVs and
additional spectral diagnostics implemented in the SpEctrum Radial Velocity
Analyser (SERVAL), a publicly available python code.
Methods: We measured the RVs using high signal-to-noise templates created by
coadding all available spectra of each star.We define the chromatic index as
the RV gradient as a function of wavelength with the RVs measured in the
echelle orders. Additionally, we computed the differential line width by
correlating the fit residuals with the second derivative of the template to
track variations in the stellar line width.
Results: Using HARPS data, our SERVAL code achieves a RV precision at the
level of 1m/s. Applying the chromatic index to CARMENES data of the active star
YZ CMi, we identify apparent RV variations induced by stellar activity. The
differential line width is found to be an alternative indicator to the commonly
used full width half maximum.
Conclusions: We find that at the red optical wavelengths (700--900 nm)
obtained by the visual channel of CARMENES, the chromatic index is an excellent
tool to investigate stellar active regions and to identify and perhaps even
correct for activity-induced RV variations.Comment: 13 pages, 13 figures. A&A in press. Code is available at
https://github.com/mzechmeister/serva
Three Different Types of Galaxy Alignment within Dark Matter Halos
Using a large galaxy group catalogue based on the Sloan Digital Sky Survey
Data Release 4 we measure three different types of intrinsic galaxy alignment
within groups: halo alignment between the orientation of the brightest group
galaxies (BGG) and the distribution of its satellite galaxies, radial alignment
between the orientation of a satellite galaxy and the direction towards its
BGG, and direct alignment between the orientation of the BGG and that of its
satellites. In agreement with previous studies we find that satellite galaxies
are preferentially located along the major axis. In addition, on scales r < 0.7
Rvir we find that red satellites are preferentially aligned radially with the
direction to the BGG. The orientations of blue satellites, however, are
perfectly consistent with being isotropic. Finally, on scales r < 0.1 \Rvir, we
find a weak but significant indication for direct alignment between satellites
and BGGs. We briefly discuss the implications for weak lensing measurements.Comment: 4 pages, 4 figures, ApJL accepte
Spatial and kinematic alignments between central and satellite halos
Based on a cosmological N-body simulation we analyze spatial and kinematic
alignments of satellite halos within six times the virial radius of group size
host halos (Rvir). We measure three different types of spatial alignment: halo
alignment between the orientation of the group central substructure (GCS) and
the distribution of its satellites, radial alignment between the orientation of
a satellite and the direction towards its GCS, and direct alignment between the
orientation of the GCS and that of its satellites. In analogy we use the
directions of satellite velocities and probe three further types of alignment:
the radial velocity alignment between the satellite velocity and connecting
line between satellite and GCS, the halo velocity alignment between the
orientation of the GCS and satellite velocities and the auto velocity alignment
between the satellites orientations and their velocities. We find that
satellites are preferentially located along the major axis of the GCS within at
least 6 Rvir (the range probed here). Furthermore, satellites preferentially
point towards the GCS. The most pronounced signal is detected on small scales
but a detectable signal extends out to 6 Rvir. The direct alignment signal is
weaker, however a systematic trend is visible at distances < 2 Rvir. All
velocity alignments are highly significant on small scales. Our results suggest
that the halo alignment reflects the filamentary large scale structure which
extends far beyond the virial radii of the groups. In contrast, the main
contribution to the radial alignment arises from the adjustment of the
satellite orientations in the group tidal field. The projected data reveal good
agreement with recent results derived from large galaxy surveys. (abridged)Comment: accepted for publication in Ap
The H alpha Galaxy Survey VI. Star-forming companions of nearby field galaxies
We present a search for star-forming satellite galaxies that are close enough
to their parent galaxies to be considered analogues of the Magellanic Clouds.
Our search technique relied on the detection of the satellites in
continuum-subtracted narrow-band H alpha imaging of the central galaxies, which
removes most of the background and foreground line-of-sight companions, thus
giving a high probability that we are detecting true satellites. The search was
performed for 119 central galaxies at distances between 20 and 40 Mpc, although
spatial incompleteness means that we have effectively searched 53 full
satellite-containing volumes. We find only 9 probable star-forming satellites,
around 9 different central galaxies, and 2 possible satellites. After
incompleteness correction, this is equivalent to 0.17/0.21 satellites per
central galaxy. The Small Magellanic Cloud is just below the median values of
both star formation rate and R-band luminosity of the 9 probable satellites.
The Large Magellanic Cloud, however, has a higher R-band luminosity than any of
the 9 and is only exceeded in star formation rate by the one satellite that
appears to be undergoing a tidally-induced starburst. Thus the Milky Way
appears to be quite unusual, both in having two star-forming satellite galaxies
and in the high luminosity of the Large Magellanic Cloud.Comment: 13 pages, accepted for publication in A&
Probing the Intrinsic Shape and Alignment of Dark Matter Haloes using SDSS Galaxy Groups
We study the three-dimensional and projected shapes of galaxy groups in the
Sloan Digital Sky Survey Data Release 4, and examine the alignment between the
orientation of the central galaxy and the spatial distribution of satellite
galaxies. The projected ellipticity of a group is measured using the moments of
the discrete distribution of its member galaxies. We infer the
three-dimensional and projected axis ratios of their dark matter haloes by
comparing the measured ellipticity distributions with those obtained from Monte
Carlo simulations of projected, triaxial dark matter haloes with different axis
ratios. We find that the halo shape has a strong dependence on the halo mass.
While the haloes of low-mass groups are nearly spherical, those of massive
groups tend to be prolate. For groups containing at least four members, the
statistical distribution of their measured ellipticities does not have a strong
dependence on the colors of their central galaxies. Our analysis further shows
that the average three-dimensional axis ratio for haloes with is about , resulting in a
projected axis ratio of . Our results for the alignment between the
orientation of the central galaxy of a group and the distribution of their
satellite galaxies are in broad agreement with those obtained by Yang et al.
The distribution of satellite galaxies preferentially aligns with the major
axis of the central galaxy, with a clear dependence on both halo mass and
galaxy colors. (abridged)Comment: 13 pages, 10 figures and 2 tables. Accepted for publication in MNRA
Chapter 6. Primary Production, Cycling of Nutrients, Surface Layer and Plankton.
RADIALES (IEO)Versión del edito
The CARMENES search for exoplanets around M dwarfs. Line-by-line sensitivity to activity in M dwarfs
Radial velocities (RVs) measured from high-resolution stellar spectra are
routinely used to detect and characterise orbiting exoplanet companions. The
different lines present in stellar spectra are created by several species,
which are non-uniformly affected by stellar variability features such as spots
or faculae. Stellar variability distorts the shape of the spectral absorption
lines from which precise RVs are measured, posing one of the main problems in
the study of exoplanets. In this work we aim to study how the spectral lines
present in M dwarfs are independently impacted by stellar activity. We used
CARMENES optical spectra of six active early- and mid-type M dwarfs to compute
line-by-line RVs and study their correlation with several well-studied proxies
of stellar activity. We are able to classify spectral lines based on their
sensitivity to activity in five M dwarfs displaying high levels of stellar
activity. We further used this line classification to compute RVs with
activity-sensitive lines and less sensitive lines, enhancing or mitigating
stellar activity effects in the RV time series. For specific sets of the least
activity-sensitive lines, the RV scatter decreases by ~ 2 to 5 times the
initial one, depending on the star. Finally, we compare these lines in the
different stars analysed, finding the sensitivity to activity to vary from star
to star. Despite the high density of lines and blends present in M dwarf
stellar spectra, we find that a line-by-line approach is able to deliver
precise RVs. Line-by-line RVs are also sensitive to stellar activity effects,
and they allow for an accurate selection of activity-insensitive lines to
mitigate activity effects in RV. However, we find stellar activity effects to
vary in the same insensitive lines from star to star.Comment: Accepted for publication in A&
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