Main sequence | EPFL Graph Search

In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or dwarf stars. These are the most numerous true stars in the universe and include the Sun. After condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium. During this stage of the star's lifetime, it is located on the main sequence at a position determined primarily by its mass but also based on its chemical composition and age. The cores of main-sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward pressure of gravitational collapse from the overlying layers. The strong dependence of the rate of energy generation

Molecular gas in distant brightest cluster galaxies

Gianluca Castignani

The mechanisms governing the stellar mass assembly and star formation history of brightest cluster galaxies (BCGs) are still being debated. By means of new and archival molecular gas observations we investigate the role of dense megaparsec-scale environments in regulating the fueling of star formation in distant BCGs, through cosmic time. We observed in CO with the IRAM 30 m telescope two star-forming BCGs belonging to SpARCS clusters, namely, 3C 244.1 (z=0.4) and SDSS J161112.65+550823.5 (z=0.9), and compared their molecular gas and star formation properties with those of a compilation of similar to 100 distant cluster galaxies from the literature, including nine additional distant BCGs at z similar to 0.4-3.5. We set robust upper limits of M-H2< 1.0x10(10) M-circle dot and < 2.8x10(10) M-circle dot to their molecular gas content, respectively, and to the ratio of molecular gas to stellar mass M(H-2)/M-star less than or similar to 0.2 and depletion time tau(dep)less than or similar to 40 Myr of the two targeted BCGs. They are thus among the distant cluster galaxies with the lowest gas fractions and shortest depletion times. The majority (64%+/- 15% and 73%+/- 18%) of the 11 BCGs with observations in CO have lower M(H-2)/M-star values and tau(dep), respectively, than those estimated for main sequence galaxies. Statistical analysis also tentatively suggests that the values of M(H-2)/M-star and tau(dep) for the 11 BCGs deviates, with a significance of similar to 2 sigma, from those of the comparison sample of cluster galaxies. A morphological analysis for a subsample of seven BCGs with archival HST observations reveals that 71%+/- 17% of the BCGs are compact or show star-forming components or substructures. Our results suggest a scenario where distant star-forming BCGs assemble a significant fraction similar to 16% of their stellar mass on the relatively short timescale similar to tau(dep), while environmental mechanisms might prevent the replenishment of gas feeding the star formation. We speculate that compact components also favor the rapid exhaustion of molecular gas and ultimately help to quench the BCGs. Distant star-forming BCGs are excellent targets for ALMA and for next-generation telescopes such as the James Webb Space Telescope.

2020

SEEDisCS: I. Molecular gas in galaxy clusters and their large-scale structure: The case of CL1411.1-1148 at z similar to 0.5

Gianluca Castignani, Pascale Jablonka, Damien Louis Alexandre Spérone-Longin

We investigate how the galaxy reservoirs of molecular gas fuelling star formation are transformed while the host galaxies infall onto galaxy cluster cores. As part of the Spatially Extended ESO Distant Cluster Survey (SEEDisCS), we present CO(3-2) observations of 27 star-forming galaxies obtained with the Atacama Large Millimeter Array. These sources are located inside and around CL1411.1-1148 at z=0.5195, within five times the cluster virial radius. These targets were selected to have stellar masses (Mstar), colours, and magnitudes similar to those of a field comparison sample at similar redshift drawn from the Plateau de Bure high-z Blue Sequence Survey (PHIBSS2). We compare the cold gas fraction (mu H2=MH2/Mstar), specific star formation rates (SFR/Mstar) and depletion timescales (tdepl=MH2/SFR) of our main-sequence galaxies to the PHIBSS2 subsample. While the most of our galaxies (63%) are consistent with PHIBSS2, the remainder fall below the relation between mu H2 and Mstar of the PHIBSS2 galaxies at z similar to 0.5. These low-mu H2 galaxies are not compatible with the tail of a Gaussian distribution, hence they correspond to a new population of galaxies with normal SFRs but low gas content and low depletion times (less than or similar to 1 Gyr), absent from previous surveys. We suggest that the star formation activity of these galaxies has not yet been diminished by their low fraction of cold molecular gas.

EDP SCIENCES S A2021

Virgo filaments I. Processing of gas in cosmological filaments around the Virgo cluster

Gianluca Castignani, Pascale Jablonka

It is now well established that galaxies have different morphologies, gas contents, and star formation rates (SFR) in dense environments like galaxy clusters. The impact of environmental density extends to several virial radii, and galaxies appear to be pre-processed in filaments and groups before falling into the cluster. Our goal is to quantify this pre-processing in terms of gas content and SFR, as a function of density in cosmic filaments. We have observed the two first CO transitions in 163 galaxies with the IRAM-30 m telescope, and added 82 more measurements from the literature, thus forming a sample of 245 galaxies in the filaments around the Virgo cluster. We gathered HI-21cm measurements from the literature and observed 69 galaxies with the Nancay telescope to complete our sample. We compare our filament galaxies with comparable samples from the Virgo cluster and with the isolated galaxies of the AMIGA sample. We find a clear progression from field galaxies to filament and cluster galaxies for decreasing SFR, increasing fraction of galaxies in the quenching phase, an increasing proportion of early-type galaxies, and decreasing gas content. Galaxies in the quenching phase, defined as having a SFR below one-third of that of the main sequence (MS), are only between 0% and 20% in the isolated sample, according to local galaxy density, while they are 20%-60% in the filaments and 30%-80% in the Virgo cluster. Processes that lead to star formation quenching are already at play in filaments; they depend mostly on the local galaxy density, while the distance to the filament spine is a secondary parameter. While the HI-to-stellar-mass ratio decreases with local density by an order of magnitude in the filaments, and two orders of magnitude in the Virgo cluster with respect to the field, the decrease is much less for the H-2-to-stellar-mass ratio. As the environmental density increases, the gas depletion time decreases, because the gas content decreases faster than the SFR. This suggests that gas depletion precedes star formation quenching.

EDP SCIENCES S A2021

Virgo filaments I. Processing of gas in cosmological filaments around the Virgo cluster

Gianluca Castignani, Pascale Jablonka

EDP SCIENCES S A2021

Molecular gas in distant brightest cluster galaxies

Gianluca Castignani

2020