9,164 research outputs found
The Search for Extraterrestrial Intelligence (SETI)
A bibliography of reports concerning the Search for Extraterrestrial Intelligence is presented. Cosmic evolution, space communication, and technological advances are discussed along with search strategies and search systems
A universal ultraviolet-optical colour-colour-magnitude relation of galaxies
Although the optical colour-magnitude diagram of galaxies allows one to
select red sequence objects, neither can it be used for galaxy classification
without additional observational data such as spectra or high-resolution
images, nor to identify blue galaxies at unknown redshifts. We show that adding
the near ultraviolet colour to the optical CMD reveals a tight relation in the
three-dimensional colour-colour-magnitude space smoothly continuing from the
"blue cloud" to the "red sequence". We found that 98 per cent of 225,000
low-redshift (Z<0.27) galaxies follow a smooth surface g-r=F(M,NUV-r) with a
standard deviation of 0.03-0.07 mag making it the tightest known galaxy
photometric relation. There is a strong correlation between morphological types
and integrated NUV-r colours. Rare galaxy classes such as E+A or tidally
stripped systems become outliers that occupy distinct regions in the 3D
parameter space. Using stellar population models for galaxies with different
SFHs, we show that (a) the (NUV-r, g-r) distribution is formed by objects
having constant and exponentially declining SFR with different characteristic
timescales; (b) colour evolution for exponentially declining models goes along
the relation suggesting its weak evolution up-to a redshift of 0.9; (c)
galaxies with truncated SFHs have very short transition phase offset from the
relation thus explaining the rareness of E+A galaxies. This relation can be
used as a powerful galaxy classification tool when morphology remains
unresolved. Its mathematical consequence is the photometric redshift estimates
from 3 broad-band photometric points. This approach works better than most
existing photometric redshift techniques applied to multi-colour datasets.
Therefore, the relation can be used as an efficient selection technique for
galaxies at intermediate redshifts (0.3<Z<0.8) using optical imaging surveys.Comment: 15 pages, 10 figures, accepted to MNRAS. This is an updated version
that addresses referee's remarks. All relations have been recomputed using
Petrosian magnitudes. The best-fitting relations in the electronic form are
available at the project web-page: http://specphot.sai.msu.ru/galaxies
EoR Foregrounds: the Faint Extragalactic Radio Sky
A wealth of new data from upgraded and new radio interferometers are rapidly
improving and transforming our understanding of the faint extra-galactic radio
sky. Indeed the mounting statistics at sub-mJy and uJy flux levels is finally
allowing us to get stringent observational constraints on the faint radio
population and on the modeling of its the various components. In this paper I
will provide a brief overview of the latest results in areas that are
potentially important for an accurate treatment of extra-galactic foregrounds
in experiments designed to probe the Epoch of Reionization.Comment: 8 pages, 2 figures. Invited review at IAU Symposium No. 333 "Peering
towards Cosmic Dawn". This submission includes updated figures wrt the
version published in the proceedings volume (where an error in the plotting
routine produced wrong labels for the y- and x-axis
Metadata enrichment for digital heritage: users as co-creators
This paper espouses the concept of metadata enrichment through an expert and user-focused approach to metadata creation and management. To this end, it is argued the Web 2.0 paradigm enables users to be proactive metadata creators. As Shirky (2008, p.47) argues Web 2.0’s social tools enable “action by loosely structured groups, operating without managerial direction and outside the profit motive”. Lagoze (2010, p. 37) advises, “the participatory nature of Web 2.0 should not be dismissed as just a popular phenomenon [or fad]”. Carletti (2016) proposes a participatory digital cultural heritage approach where Web 2.0 approaches such as crowdsourcing can be sued to enrich digital cultural objects. It is argued that “heritage crowdsourcing, community-centred projects or other forms of public participation”. On the other hand, the new collaborative approaches of Web 2.0 neither negate nor replace contemporary standards-based metadata approaches. Hence, this paper proposes a mixed metadata approach where user created metadata augments expert-created metadata and vice versa. The metadata creation process no longer remains to be the sole prerogative of the metadata expert. The Web 2.0 collaborative environment would now allow users to participate in both adding and re-using metadata. The case of expert-created (standards-based, top-down) and user-generated metadata (socially-constructed, bottom-up) approach to metadata are complementary rather than mutually-exclusive. The two approaches are often mistakenly considered as dichotomies, albeit incorrectly (Gruber, 2007; Wright, 2007) .
This paper espouses the importance of enriching digital information objects with descriptions pertaining the about-ness of information objects. Such richness and diversity of description, it is argued, could chiefly be achieved by involving users in the metadata creation process. This paper presents the importance of the paradigm of metadata enriching and metadata filtering for the cultural heritage domain. Metadata enriching states that a priori metadata that is instantiated and granularly structured by metadata experts is continually enriched through socially-constructed (post-hoc) metadata, whereby users are pro-actively engaged in co-creating metadata. The principle also states that metadata that is enriched is also contextually and semantically linked and openly accessible. In addition, metadata filtering states that metadata resulting from implementing the principle of enriching should be displayed for users in line with their needs and convenience. In both enriching and filtering, users should be considered as prosumers, resulting in what is called collective metadata intelligence
The effect of the environment on the HI scaling relations
We use a volume-, magnitude-limited sample of nearby galaxies to investigate
the effect of the environment on the HI scaling relations. We confirm that the
HI-to-stellar mass ratio anti correlates with stellar mass, stellar mass
surface density and NUV-r colour across the whole range of parameters covered
by our sample (10^9 <M*<10^11 Msol, 7.5 <mu*<9.5 Msol kpc^-2, 2<NUV-r<6 mag).
These scaling relations are also followed by galaxies in the Virgo cluster,
although they are significantly offset towards lower gas content.
Interestingly, the difference between field and cluster galaxies gradually
decreases moving towards massive, bulge-dominated systems. By comparing our
data with the predictions of chemo-spectrophotometric models of galaxy
evolution, we show that starvation alone cannot explain the low gas content of
Virgo spirals and that only ram-pressure stripping is able to reproduce our
findings. Finally, motivated by previous studies, we investigate the use of a
plane obtained from the relations between the HI-to-stellar mass ratio, stellar
mass surface density and NUV-r colour as a proxy for the HI deficiency
parameter. We show that the distance from the `HI gas fraction plane' can be
used as an alternative estimate for the HI deficiency, but only if carefully
calibrated on pre-defined samples of `unperturbed' systems.Comment: Accepted for publication on MNRAS main journal. 11 pages, 6 figures,
1 tabl
Peculiar early-type galaxies in the SDSS Stripe82
We explore the properties of `peculiar' early-type galaxies (ETGs) in the
local Universe, that show (faint) morphological signatures of recent
interactions such as tidal tails, shells and dust lanes. Standard-depth (51s
exposure) multi-colour galaxy images from the Sloan Digital Sky Survey (SDSS)
are combined with the significantly (2 mags) deeper monochromatic images from
the public SDSS Stripe82 to extract, through careful visual inspection, a
robust sample of nearby, luminous ETGs, including a subset of ~70 peculiar
systems. 18% of ETGs exhibit signs of disturbed morphologies (e.g. shells),
while 7% show evidence of dust lanes and patches. The peculiar ETG population
is found to preferentially inhabit low-density environments (outskirts of
clusters, groups or the field). An analysis of optical emission-line ratios
indicates that the fraction of peculiar ETGs that are Seyferts or LINERs
(19.4%) is twice the corresponding values in their relaxed counterparts
(10.1%). LINER-like emission is the dominant type of nebular activity in all
ETG classes, plausibly driven by stellar photoionisation associated with recent
star formation. An analysis of UV-optical colours indicates that, regardless of
the luminosity range being considered, the fraction of peculiar ETGs that have
experienced star formation in the last Gyr is a factor of ~1.5 higher than that
in their relaxed counterparts. The spectro-photometric results strongly suggest
that the interactions that produce the morphological peculiarities also induce
low-level recent star formation which, based on the recent literature, are
likely to contribute a few percent of the stellar mass over the last 1 Gyr. The
catalogue of galaxies that forms the basis of this paper can be obtained at:
http://www.mssl.ucl.ac.uk/~ska/stripe82/skaviraj_stripe82.dat or on request
from the author.Comment: MNRAS in pres
The properties of Brightest Cluster Galaxies in the SDSS DR6 adaptive matched filter cluster catalogue
We study the properties of Brightest Cluster Galaxies (BCGs) drawn from a
catalogue of more than 69000 clusters in the SDSS DR6 based on the adaptive
matched filter technique (AMF, Szabo et al., 2010). Our sample consists of more
than 14300 galaxies in the redshift range 0.1-0.3. We test the catalog by
showing that it includes well-known BCGs which lie in the SDSS footprint. We
characterize the BCGs in terms of r-band luminosities and optical colours as
well as their trends with redshift. In particular, we define and study the
fraction of blue BCGs, namely those that are likely to be missed by either
colour-based cluster surveys and catalogues. Richer clusters tend to have
brighter BCGs, however less dominant than in poorer systems. 4-9% of our BCGs
are at least 0.3 mag bluer in the g-r colour than the red-sequence at their
given redshift. Such a fraction decreases to 1-6% for clusters above a richness
of 50, where 3% of the BCGs are 0.5 mag below the red-sequence. A preliminary
morphological study suggests that the increase in the blue fraction at lower
richnesses may have a non-negligible contribution from spiral galaxies. We show
that a colour selection based on the g-r red-sequence or on a cut at colour u-r
>2.2 can lead to missing the majority of such blue BCGs. We also extend the
colour analysis to the UV range by cross-matching our catalogue with publicly
available data from Galex GR4 and GR5. We show a clear correlation between
offset from the optical red-sequence and the amount of UV-excess. Finally, we
cross-matched our catalogue with the ACCEPT cluster sample (Cavagnolo et al.,
2009), and find that blue BCGs tend to be in clusters with low entropy and
short cooling times. That is, the blue light is presumably due to recent star
formation associated to gas feeding by cooling flows. (abridged)Comment: 15 pages, 15 figures, submitted to MNRA
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