24 research outputs found
The formation and evolution of galaxies as a function of environment
This thesis investigates the role of environment on galaxy formation and evolution, giving particular focus to the transformation of star forming spirals into passive S0s. The data utilised for this study comes from photometric and spectroscopic observations of galaxies at 0 < z < 1 in different environments from the ESO Distant Cluster Survey. We first study the formation history of (172) cluster ellipticals (Es) and S0s, the oldest types of galaxies in the local universe. We examine their colour-magnitude relation (CMR), and find a very small intrinsic colour scatter. Only 7% of the galaxies are significantly bluer than the CMR. The scarcity of blue S0s indicates that, if they are the descendants of spirals, these were already red when they became S0s. We observe no dependence of the CMR scatter with z or cluster velocity dispersion. This implies that by the time cluster E/S0s achieve their morphology, the vast majority have already joined the red sequence. We estimate the galaxy formation redshift z_F for each cluster and find that while it does not depend on the cluster velocity dispersion, it increases weakly with cluster redshift. This suggests that, at any given z, in order to have a population of fully-formed E and S0s they needed to have formed most of their stars ≃2–4Gyr prior to observation. In other words, the galaxies that already have early-type morphologies also have reasonably-old stellar populations. This is partly a manifestation of the "progenitor bias", but also a consequence of the fact that the vast majority of the E/S0s in clusters (in particular the massive ones) were already red by the time they achieved their morphology. Moreover, E and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. We also find that fainter E/S0s finished forming their stars later, consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Finally, we find that the E/S0s cluster galaxies must have had their star formation truncated over an extended period ∆t ≿1 Gyr.
We then move our focus to the evolution of star-forming galaxies. We investigate the effect of the environment on the transformation of star-forming spirals into passive S0s by studying the properties of the gas and the stars in a sample of 422 emission-line galaxies in different environments. We identify galaxies with kinematical disturbances (in their gas disks), and find that they are more frequent in clusters than in the field. The fraction of kinematically-disturbed galaxies increases with cluster velocity dispersion and decreases with distance from the cluster centre, but remains constant with projected galaxy density. We also studied morphological disturbances in the stellar light, finding that the fraction of morphologically disturbed galaxies is independent of environment. Moreover, there is little correlation between the presence of kinematically-disturbed gas and morphological distortions. For the kinematically-undisturbed galaxies, we find that the cluster and field B-band Tully-Fisher relations are remarkably similar. Additionally, we find that the kinematically-disturbed galaxies show a suppressed specific star formation rate. There is also evidence indicating that the gas disks in cluster galaxies have been truncated, and therefore their star formation is more concentrated than in low-density environments. If spirals are the progenitors of cluster S0s, our findings imply that the physical mechanism transforming cluster galaxies efficiently disturbs the star forming gas and reduces their specific star formation rate. This star-forming gas is either removed more efficiently from the outskirts of the galaxies or it is driven towards the centre (or both). In any case, this makes any remaining star formation more centrally concentrated, helping to build the bulges of S0s. All this evidence, together with the fact that the transformation mechanism does not seem to induce strong morphological disturbances on the galaxies, suggests that the physical processes involved are related to the intracluster medium, with galaxy-galaxy interactions playing only a limited role in clusters. Interestingly, in analogy with the "blue" early-type galaxies found in the CMR study in clusters, we have also found several emission-line E/S0 galaxies with extended rotating star-forming gas disks
The formation and evolution of galaxies as a function of environment
This thesis investigates the role of environment on galaxy formation and evolution, giving particular focus to the transformation of star forming spirals into passive S0s. The data utilised for this study comes from photometric and spectroscopic observations of galaxies at 0 < z < 1 in different environments from the ESO Distant Cluster Survey. We first study the formation history of (172) cluster ellipticals (Es) and S0s, the oldest types of galaxies in the local universe. We examine their colour-magnitude relation (CMR), and find a very small intrinsic colour scatter. Only 7% of the galaxies are significantly bluer than the CMR. The scarcity of blue S0s indicates that, if they are the descendants of spirals, these were already red when they became S0s. We observe no dependence of the CMR scatter with z or cluster velocity dispersion. This implies that by the time cluster E/S0s achieve their morphology, the vast majority have already joined the red sequence. We estimate the galaxy formation redshift z_F for each cluster and find that while it does not depend on the cluster velocity dispersion, it increases weakly with cluster redshift. This suggests that, at any given z, in order to have a population of fully-formed E and S0s they needed to have formed most of their stars ≃2–4Gyr prior to observation. In other words, the galaxies that already have early-type morphologies also have reasonably-old stellar populations. This is partly a manifestation of the "progenitor bias", but also a consequence of the fact that the vast majority of the E/S0s in clusters (in particular the massive ones) were already red by the time they achieved their morphology. Moreover, E and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. We also find that fainter E/S0s finished forming their stars later, consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Finally, we find that the E/S0s cluster galaxies must have had their star formation truncated over an extended period ∆t ≿1 Gyr.
We then move our focus to the evolution of star-forming galaxies. We investigate the effect of the environment on the transformation of star-forming spirals into passive S0s by studying the properties of the gas and the stars in a sample of 422 emission-line galaxies in different environments. We identify galaxies with kinematical disturbances (in their gas disks), and find that they are more frequent in clusters than in the field. The fraction of kinematically-disturbed galaxies increases with cluster velocity dispersion and decreases with distance from the cluster centre, but remains constant with projected galaxy density. We also studied morphological disturbances in the stellar light, finding that the fraction of morphologically disturbed galaxies is independent of environment. Moreover, there is little correlation between the presence of kinematically-disturbed gas and morphological distortions. For the kinematically-undisturbed galaxies, we find that the cluster and field B-band Tully-Fisher relations are remarkably similar. Additionally, we find that the kinematically-disturbed galaxies show a suppressed specific star formation rate. There is also evidence indicating that the gas disks in cluster galaxies have been truncated, and therefore their star formation is more concentrated than in low-density environments. If spirals are the progenitors of cluster S0s, our findings imply that the physical mechanism transforming cluster galaxies efficiently disturbs the star forming gas and reduces their specific star formation rate. This star-forming gas is either removed more efficiently from the outskirts of the galaxies or it is driven towards the centre (or both). In any case, this makes any remaining star formation more centrally concentrated, helping to build the bulges of S0s. All this evidence, together with the fact that the transformation mechanism does not seem to induce strong morphological disturbances on the galaxies, suggests that the physical processes involved are related to the intracluster medium, with galaxy-galaxy interactions playing only a limited role in clusters. Interestingly, in analogy with the "blue" early-type galaxies found in the CMR study in clusters, we have also found several emission-line E/S0 galaxies with extended rotating star-forming gas disks
GASP XIX: AGN and their outflows at the center of jellyfish galaxies
The GASP survey, based on MUSE data, is unveiling the properties of the gas
in the so-called "jellyfish" galaxies: these are cluster galaxies with
spectacular evidence of gas stripping by ram pressure. In a previous paper, we
selected the seven GASP galaxies with the most extended tentacles of ionized
gas, and based on individual diagnostic diagrams concluded that at least five
of them present clear evidence for an Active Galactic Nucleus. Here we present
a more detailed analysis of the emission lines properties in these galaxies.
Our comparison of several emission line ratios with both AGN and shock models
show that photoionization by the AGN is the dominant ionization mechanism. This
conclusion is strengthened by the analysis of luminosities, the
presence of nuclear iron coronal lines and extended ( kpc) emission line
regions ionized by the AGN in some of these galaxies. From emission line
profiles, we find the presence of outflows in four galaxies, and derive mass
outflow rates, timescales and kinetic energy of the outflows.Comment: 20 pages, 10 Figure
GASP. XVI. Does cosmic web enhancement turn on star formation in galaxies?
Galaxy filaments are a peculiar environment, and their impact on the galaxy
properties is still controversial. Exploiting the data from the GAs Stripping
Phenomena in galaxies with MUSE (GASP), we provide the first characterisation
of the spatially resolved properties of galaxies embedded in filaments in the
local Universe. The four galaxies we focus on show peculiar ionised gas
distributions: Halpha clouds have been observed beyond four times the effective
radius. The gas kinematics, metallicity map and the ratios of emission line
fluxes confirm that they do belong to the galaxy gas disk, the analysis of
their spectra shows that very weak stellar continuum is associated to them.
Similarly, the star formation history and luminosity weighted age maps point to
a recent formation of such clouds. The clouds are powered by star formation,
and are characterised by intermediate values of dust absorption. We hypothesise
a scenario in which the observed features are due to "Cosmic Web Enhancement":
we are most likely witnessing galaxies passing through or flowing within
filaments that assist the gas cooling and increase the extent of the star
formation in the densest regions in the circumgalactic gas. Targeted
simulations are mandatory to better understand this phenomenon.Comment: MNRAS in press, 18 pages, 12 figure
Enhanced star formation in both disks and ram pressure stripped tails of GASP jellyfish galaxies
Exploiting the data from the GAs Stripping Phenomena in galaxies with MUSE
(GASP) program, we compare the integrated Star Formation Rate- Mass relation
(SFR-M_ast) relation of 42 cluster galaxies undergoing ram pressure stripping
("stripping galaxies") to that of 32 field and cluster undisturbed galaxies.
Theoretical predictions have so far led to contradictory conclusions about
whether ram pressure can enhance the star formation in the gas disks and tails
or not and until now a statistically significant observed sample of stripping
galaxies was lacking. We find that stripping galaxies occupy the upper envelope
of the control sample SFR-M_ast relation, showing a systematic enhancement of
the SFR at any given mass. The star formation enhancement occurs in the disk
(0.2 dex), and additional star formation takes place in the tails. Our results
suggest that strong ram pressure stripping events can moderately enhance the
star formation also in the disk prior to gas removal.Comment: ApJL in pres
Discovery of ram-pressure stripped gas around an elliptical galaxy in Abell 2670
Studies of cluster galaxies are increasingly finding galaxies with
spectacular one-sided tails of gas and young stars, suggestive of intense
ram-pressure stripping. These so-called "jellyfish" galaxies typically have
late-type morphology. In this paper, we present MUSE observations of an
elliptical galaxy in Abell 2670 with long tails of material visible in the
optical spectra, as well as blobs with tadpole-like morphology. The spectra in
the central part of the galaxy reveals a stellar component as well as ionized
gas. The stellar component does not have significant rotation, while the
ionized gas defines a clear star-forming gas disk. We argue, based on deep
optical images of the galaxy, that the gas was most likely acquired during a
past wet merger. It is possible that the star-forming blobs are also remnants
of the merger. In addition, the direction and kinematics of the one-sided
ionized tails, combined with the tadpole morphology of the star-forming blobs,
strongly suggests that the system is undergoing ram pressure from the
intracluster medium. In summary, this paper presents the discovery of a
post-merger elliptical galaxy undergoing ram pressure stripping.Comment: 6 pages, 5 figures, accepted for publication in ApJ
GASP III. JO36: a case of multiple environmental effects at play?
The so-called jellyfish galaxies are objects exhibiting disturbed morphology,
mostly in the form of tails of gas stripped from the main body of the galaxy.
Several works have strongly suggested ram pressure stripping to be the
mechanism driving this phenomenon. Here, we focus on one of these objects,
drawn from a sample of optically selected jellyfish galaxies, and use it to
validate SINOPSIS, the spectral fitting code that will be used for the analysis
of the GASP (GAs Stripping Phenomena in galaxies with MUSE) survey, and study
the spatial distribution and physical properties of gas and stellar populations
in this galaxy. We compare the model spectra to those obtained with GANDALF, a
code with similar features widely used to interpret the kinematic of stars and
gas in galaxies from IFU data. We find that SINOPSIS can reproduce the
pixel-by-pixel spectra of this galaxy at least as good as GANDALF does,
providing reliable estimates of the underlying stellar absorption to properly
correct the nebular gas emission. Using these results, we find strong evidences
of a double effect of ram pressure exerted by the intracluster medium onto the
gas of the galaxy. A moderate burst of star formation, dating between 20 and
500 Myr ago and involving the outer parts of the galaxy more strongly than the
inner regions, was likely induced by a first interaction of the galaxy with the
intracluster medium. Stripping by ram pressure, plus probable gas depletion due
to star formation, contributed to create a truncated ionized gas disk. The
presence of an extended stellar tail on only one side of the disk, points
instead to another kind of process, likely a gravitational interaction by a
fly-by or a close encounter with another galaxy in the cluster.Comment: ApJ in press, 26 pages, 18 figure
Enhanced star formation in both disks and ram pressure stripped tails of GASP jellyfish galaxies
Exploiting the data from the GAs Stripping Phenomena in galaxies with MUSE
(GASP) program, we compare the integrated Star Formation Rate- Mass relation
(SFR-M_ast) relation of 42 cluster galaxies undergoing ram pressure stripping
("stripping galaxies") to that of 32 field and cluster undisturbed galaxies.
Theoretical predictions have so far led to contradictory conclusions about
whether ram pressure can enhance the star formation in the gas disks and tails
or not and until now a statistically significant observed sample of stripping
galaxies was lacking. We find that stripping galaxies occupy the upper envelope
of the control sample SFR-M_ast relation, showing a systematic enhancement of
the SFR at any given mass. The star formation enhancement occurs in the disk
(0.2 dex), and additional star formation takes place in the tails. Our results
suggest that strong ram pressure stripping events can moderately enhance the
star formation also in the disk prior to gas removal.Comment: ApJL in pres
GASP. XX. From the loose spatially-resolved to the tight global SFR-Mass relation in local spiral galaxies
Exploiting the sample of 30 local star-forming, undisturbed late-type
galaxies in different environments drawn from the GAs Stripping Phenomena in
galaxies with MUSE (GASP), we investigate the spatially resolved Star Formation
Rate-Mass ({\Sigma}SFR-{\Sigma}_star) relation. Our analysis includes also the
galaxy outskirts (up to >4 effective radii, re), a regime poorly explored by
other Integral Field Spectrograph surveys. Our observational strategy allows us
to detect H{\alpha} out to more than 2.7re for 75% of the sample. Considering
all galaxies together, the correlation between the {\Sigma}SFR and
{\Sigma}_star is quite broad, with a scatter of 0.3 dex. It gets steeper and
shifts to higher {\Sigma}_star values when external spaxels are excluded and
moving from less to more massive galaxies. The broadness of the overall
relation suggests galaxy-by-galaxy variations. Indeed, each object is
characterized by a distinct {\Sigma}SFR-{\Sigma}_star relation and in some
cases the correlation is very loose. The scatter of the relation mainly arises
from the existence of bright off-center star-forming knots whose
{\Sigma}SFR-{\Sigma}_star relation is systematically broader than that of the
diffuse component. The {\Sigma}SFR-{\Sigma}tot gas (total gas surface density)
relation is as broad as the {\Sigma}SFR-{\Sigma}_star relation, indicating that
the surface gas density is not a primary driver of the relation. Even though a
large galaxy-by-galaxy variation exists, mean {\Sigma}SFR and {\Sigma}_star
values vary of at most 0.7 dex across galaxies. We investigate the relationship
between the local and global SFR-M_star relation, finding that the latter is
driven by the existence of the size-mass relation.Comment: MNRAS in press, 19 pages, 14 figure
BUDHIES - III : the fate of HI and the quenching of galaxies in evolving environments
In a hierarchical Universe clusters grow via the accretion of galaxies from the field, groups and even other clusters. As this happens, galaxies can lose and/or consume their gas reservoirs via different mechanisms, eventually quenching their star formation. We explore the diverse environmental histories of galaxies through a multiwavelength study of the combined effect of ram-pressure stripping and group 'processing' in Abell 963, a massive growing cluster at z = 0.2 from the Blind Ultra Deep HI Environmental Survey (BUDHIES). We incorporate hundreds of new optical redshifts (giving a total of 566 cluster members), as well as Subaru and XMM-Newton data from LoCuSS, to identify substructures and evaluate galaxy morphology, star formation activity, and HI content (via HI deficiencies and stacking) out to 3 x R-200. We find that Abell 963 is being fed by at least seven groups, that contribute to the large number of passive galaxies outside the cluster core. More massive groups have a higher fraction of passive and HI-poor galaxies, while low-mass groups host younger (often interacting) galaxies. For cluster galaxies not associated with groups we corroborate our previous finding that HI gas (if any) is significantly stripped via ram-pressure during their first passage through the intracluster medium, and find mild evidence for a starburst associated with this event. In addition, we find an overabundance of morphologically peculiar and/or star-forming galaxies near the cluster core. We speculate that these arise from the effect of groups passing through the cluster (post-processing). Our study highlights the importance of environmental quenching and the complexity added by evolving environments.Peer reviewe