ABSTRACT
Massive quiescent compact galaxies have been discovered at high redshifts, associated with rapid compaction and cessation of star formation (SF). In this work, we set out to quantify the ...time-scales in which SF is quenched in compact galaxies at intermediate redshifts. For this, we select a sample of green valley galaxies within the COSMOS field in the midst of quenching their SF at 0.5 < $z$ < 1.0 that exhibit varying degrees of compactness. Based on the Hδ absorption line and the 4000 Å break of coadded zCOSMOS spectra for subsamples of normal-sized and compact galaxies we determine quenching time-scales as a function of compactness. We find that the SF quenching time-scales in green valley compact galaxies are much shorter than in normal-sized ones. In an effort to understand this trend, we use the Illustris simulation to trace the evolution of the SF history, the growth rate of the central super massive black hole (SMBH), and the AGN feedback in compact and normal-sized galaxies. We find that the difference in SF quenching time-scales is due to the mode of the AGN feedback. In the compact galaxies the kinematic-mode is dominant, being highly efficient at quenching the SF by depleting the internal gas. For normal-sized galaxies, the prevailing thermal-mode injects energy into the circumgalactic gas, preventing cold gas accretion and quenching SF via the slower strangulation mechanism. These results are consistent with the violent disc instability and gas-rich mergers scenarios, followed by strong AGN and stellar feedback. Although this kind of event is most expected to occur at $z$ = 2–3, we find evidences that the formation of compact quiescent galaxies can occur at $z$ < 1.
We use SDSS-DR4 photometric and spectroscopic data out to redshift z ~ 0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of 25 fossil groups (FGs), defined as bound systems ...dominated by a single, luminous elliptical galaxy with extended X-ray emission. We examine possible biases introduced by varying the parameters used to define the sample, and the main pitfalls are also discussed. The spatial density of FGs, estimated via the V/V MAX test, is 2.83 X 10-6 h 3 75 Mpc-3 for LX > 0.89 X 1042 h -2 75 erg s-1 consistent with Vikhlinin et al., who examined an X-ray overluminous elliptical galaxy sample (OLEG). We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same data set. These two samples show no differences in the distribution of neighboring faint galaxy density excess, distance from the red sequence in the color-magnitude diagram, and structural parameters such as a 4 and internal color gradients. Furthermore, examination of stellar populations shows that our 25 FGs have similar ages, metallicities, and a-enhancement as the bright field ellipticals, undermining the idea that these systems represent fossils of a physical mechanism that occurred at high redshift. Our study reveals no difference between FGs and field ellipticals, suggesting that FGs might not be a distinct family of true fossils, but rather the final stage of mass assembly in the universe.
We present an analysis of stellar population gradients in 4546 early-type galaxies (ETGs) with photometry in grizYHJK along with optical spectroscopy. ETGs were selected as bulge-dominated systems, ...displaying passive spectra within the SDSS fibers. A new approach is described which utilizes color information to constrain age and metallicity gradients. Defining an effective color gradient, {nabla}, which incorporates all of the available color indices, we investigate how {nabla} varies with galaxy mass proxies, i.e., velocity dispersion, stellar (M ) and dynamical (M dyn) masses, as well as age, metallicity, and Delta *a/Fe. ETGs with M dyn larger than 8.5 X 1010 M have increasing age gradients and decreasing metallicity gradients with respect to mass, metallicity, and enhancement. We find that velocity dispersion and Delta *a/Fe are the main drivers of these correlations. ETGs with 2.5 X 1010 M <= M dyn <= 8.5 X 1010 M show no correlation of age, metallicity, and color gradients with respect to mass, although color gradients still correlate with stellar population parameters, and these correlations are independent of each other. In both mass regimes, the striking anti-correlation between color gradient and Delta *a-enhancement is significant at ~5 Delta *s and results from the fact that metallicity gradient decreases with Delta *a/Fe. This anti-correlation may reflect the fact that star formation and metallicity enrichment are regulated by the interplay between the energy input from supernovae, and the temperature and pressure of the hot X-ray gas in ETGs. For all mass ranges, positive age gradients are associated with old galaxies (>5-7 Gyr). For galaxies younger than ~5 Gyr, mostly at low mass, the age gradient tends to be anti-correlated with the Age parameter, with more positive gradients at younger ages.
Using optical-optical and optical-NIR colors, we analyze the radial dependence of age and metallicity inside massive (M 1010.5 M ), low-redshift (z < 0.1), early-type galaxies (ETGs), residing in ...both high-density group regions and the field. On average, internal color gradients of ETGs are mainly driven by metallicity, consistent with previous studies. However, we find that group galaxies feature positive age gradients, {nabla} t , i.e., a younger stellar population in the galaxy center, and steeper metallicity gradients, compared to the field sample, whose {nabla} t ranges from negative in lower mass galaxies to positive gradients at higher mass. These dependencies yield new constraints on models of galaxy formation and evolution. We speculate that age and metallicity gradients of group ETGs result from (either gas-rich or minor-dry) mergers and/or cold-gas accretion, while field ETGs exhibit the characteristic flatter gradients expected from younger, more metal-rich stars formed inside-out by later gas cooling.
Environmental differences in the stellar populations of early-type galaxies are explored using principal component analysis (PCA), focusing on differences between elliptical galaxies in Hickson ...Compact Groups (HCGs) and in the field. The method is model-independent and purely relies on variations between the observed spectra. The projections (PC1, PC2) of the observed spectra on the first and second principal components reveal a difference with respect to environment, with a wider range in PC1 and PC2 in the group sample. We define a spectral parameter (ζ≡ 0.36PC1 − PC2) which simplifies this result to a single number: field galaxies have a very similar value of ζ, whereas HCG galaxies span a wide range in this parameter. The segregation is found regardless of the way the input spectral energy distributions (SEDs) are presented to PCA (i.e. changing the spectral range; using uncalibrated data; subtracting the continuum or masking the SED to include only the Lick spectral regions). Simple models are applied to give physical meaning to the PCs. We obtain a strong correlation between the values of ζ and the mass fraction in younger stars, so that some group galaxies present a higher fraction of them, implying a more complex star formation history in groups. Regarding 'dynamically related' observables such as a
4 or velocity dispersion, we find a correlation with PC3, but not with either PC1 or PC2. PCA is more sensitive than other methods based on a direct analysis of observables such as the structure of the surface brightness profile or the equivalent width of absorption lines. The latter do not reveal any significant variation between field and compact group galaxies. Our results imply that the presence of young stars only amounts to a fraction of a per cent in its contribution to the total variance, reflecting the power of PCA as a tool to extract small variations in the spectra from unresolved stellar populations.
Galaxies are found to obey scaling relations between a number of observables. These relations follow different trends at the low- and high-mass ends. The processes driving the curvature of scaling ...relations remain uncertain. In this Letter, we focus on the specific family of early-type galaxies, deriving the star formation histories of a complete sample of visually classified galaxies from Sloan Digital Sky Survey DR7 over the redshift range 0.01 < z < 0.025, covering a stellar mass interval from 10 super(9) to 3 x 10 super(11) M sub(middot in circle). Our sample features the characteristic "knee" in the surface brightness versus mass distribution at Mlow * ~ 3 x 10 super(10) M sub(middot in circle). We find a clear difference between the age and metallicity distributions of the stellar populations above and beyond this knee, which suggests a sudden transition from a constant, highly efficient mode of star formation in high-mass galaxies, gradually decreasing toward the low-mass end of the sample. At fixed mass, our early-type sample is more efficient in building up the stellar content at early times in comparison to the general population of galaxies, with half of the stars already in place by redshift z ~ 2 for all masses. The metallicity-age trend in low-mass galaxies is not compatible with infall of metal-poor gas, suggesting instead an outflow-driven relation.
We derive the fundamental plane (FP) relation for a sample of 1430 early- type galaxies in the optical (r band) and the near-infrared (K band), by combining SDSS and UKIDSS data. With such a large, ...homogeneous data set, we are able to assess the dependence of the FP on the wave band. Our analysis indicates that the FP of luminous early-type galaxies is essentially wave band-independent, with its coefficients increasing at most by 8% from the optical to the NIR. This finding fits well into a consistent picture in which the tilt of the FP is not driven by stellar populations but results from other effects, such as nonhomology. In this framework, the optical and NIR FPs require more massive galaxies to be slightly more metal-rich than less massive ones, and to have highly synchronized ages, with an age variation per decade in mass smaller than a few percent.
The Crab Nebula is the brightest TeV gamma-ray source in the sky and has been used for the past 25 years as a reference source in TeV astronomy, for calibration and verification of new TeV ...instruments. The High Altitude Water Cherenkov Observatory (HAWC), completed in early 2015, has been used to observe the Crab Nebula at high significance across nearly the full spectrum of energies to which HAWC is sensitive. HAWC is unique for its wide field of view, nearly 2 sr at any instant, and its high-energy reach, up to 100 TeV. HAWC's sensitivity improves with the gamma-ray energy. Above ∼1 TeV the sensitivity is driven by the best background rejection and angular resolution ever achieved for a wide-field ground array. We present a time-integrated analysis of the Crab using 507 live days of HAWC data from 2014 November to 2016 June. The spectrum of the Crab is fit to a function of the form . The data is well fitted with values of = 2.63 0.03, β = 0.15 0.03, and when E0 is fixed at 7 TeV and the fit applies between 1 and 37 TeV. Study of the systematic errors in this HAWC measurement is discussed and estimated to be 50% in the photon flux between 1 and 37 TeV. Confirmation of the Crab flux serves to establish the HAWC instrument's sensitivity for surveys of the sky. The HAWC all-sky survey will be the deepest survey of the northern sky ever conducted in the multi-TeV band.
After four years during which only the spectacular light echo was showing continuous and rapid evolution while the central star was nearly constant, in autumn 2006 V838 Mon began a sequence of events ...which profoundly altered its spectroscopic and photometric behavior: (a) an eclipse of the B3V companion, characterized by the disappearance and reappearance of the B3V companion from optical spectra, and an eclipse-like lightcurve of ~70 day duration and $\Delta B \sim 1.15$ mag, $\Delta V \sim 0.55$ mag, $\Delta R_{\rm C} \sim 0.10$ mag maximum depth; (b) a large increase in intensity of the FeII and FeII emission lines, and the appearance in emission for the first time since the 2002 outburst of Hα and higher Balmer series lines. While the FeII and FeII lines maintained a very sharp and unresolved profile, the Hα developed into a wide and structured profile, characterized by a sharp central reversal at the same velocity as one of the CO radio emission components. The disappearance of the B3V companion is equally well explained by a grazing eclipse from the outbursting L-type supergiant or by an eclipse from a dust cloud characterized by $E_{B-V}=0.55$ and $R_{V}=3.1$. We believe the flaring of the emission lines occurred at a similar time as the B3V eclipse just by chance.
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