Search arXiv⌕ Search
Home/Archive/arXiv

arXiv · 1004.2041

Star Formation Signatures in Optically Quiescent Early-type Galaxies

Samir Salim·R. Michael Rich
View original source ↗Read source document ↗Export metadata ↓

Abstract

In recent years argument has been made that a high fraction of early-type galaxies in the local universe experience low levels (< 1 M_sun/yr) of star formation (SF) that causes strong excess in UV flux, yet leaves the optical colors red. Many of these studies were based on GALEX imaging of SDSS galaxies (z~0.1), and were thus limited by its 5" FWHM. Poor UV resolution left other possibilities for UV excess open, such as the old populations or an AGN. Here we study high-resolution far-ultraviolet HST/ACS images of optically quiescent early-type galaxies with strong UV excess. The new images show that three-quarters of these moderately massive (~5x10^10 M_sun) early-type galaxies shows clear evidence of extended SF, usually in form of wide or concentric UV rings, and in some cases, striking spiral arms. SDSS spectra probably miss these features due to small fiber size. UV-excess early-type galaxies have on average less dust and larger UV sizes (D>40 kpc) than other green-valley galaxies, which argues for an external origin for the gas that is driving the SF. Thus, most of these galaxies appear `rejuvenated' (e.g., through minor gas-rich mergers or IGM accretion). For a smaller subset of the sample, the declining SF (from the original internal gas) cannot be ruled out. SF is rare in very massive early-types (M_* > 10^11 M_sun), a possible consequence of AGN feedback. In addition to extended UV emission, many galaxies show a compact central source, which may be a weak, optically inconspicuous AGN.

Explore related subjects

astro-ph.CO

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Samir Salim, R. Michael Rich. 2010-04-12. Star Formation Signatures in Optically Quiescent Early-type Galaxies. https://doi.org/10.1088/2041-8205%2F714%2F2%2Fl290

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

$ν$Gundam: a unified framework for point-redshift and PDF-aware spatial correlation functions

Context. Photometric-redshift uncertainties redistribute galaxy pairs along the line of sight (LOS), diluting spatial clustering measurements. Propagating full redshift probability density functions (PDFs) through standard pair-count estimators remains computationally demanding. Aims. We present $ν$Gundam, a unified framework to propagate radial PDFs at the pair-count level while retaining a common interface for exact, compressed, and stochastic projected-clustering calculations. Methods. The code implements four modes: exact empirical-PDF integration, Gaussian-mixture compression, Monte Carlo (MC) sampling, and a quantile-compression method. All modes accumulate pair counts on a resolved ($r_\mathrm{p},π$) grid, from which $ξ(r_\mathrm{p},π)$ and $w_\mathrm{p}(r_\mathrm{p};Π)$ are reconstructed. We validate these with DESI galaxies and CosmoDC2 mocks. Results. The PDF-aware modes produce nearly indistinguishable LOS redistribution kernels, while point-photo-$z$ estimates discard probabilistic pair information. Radial uncertainty mainly redistributes the clustering signal to larger $π$; the depth required to recover the truth projection scales with the pairwise radial dispersion. For practical settings, MC, quantile, and Gaussian-mixture compression reproduce the exact result with fractional RMS residuals below 5%. Applied to DESI Bright Galaxy Survey samples, MC sampling recovers spectroscopic clustering shapes and relative amplitudes for $r_\mathrm{p}\gtrsim0.8\,h^{-1}\mathrm{Mpc}$ using sample-dependent recovery depths of ~400 and ~$180\,h^{-1}\mathrm{Mpc}$ for red and blue galaxies. Conclusions. Projected clustering with redshift PDFs is a pair-redistribution problem. When propagated consistently, the signal is recovered accurately and efficiently. The integration depth is sample dependent and should be part of the measurement definition rather than a fixed choice.

astro-ph.CO↗
arXiv2026

Parity-odd CMB Correlations induced by a Chiral Gravitational Wave Background

We investigate weak lensing effects of a chiral gravitational wave background (GWB) as a new mechanism for generating parity-odd correlations in the cosmic microwave background (CMB). Accounting for both photon deflection and polarization rotation, we derive the resulting parity-odd EB and TB angular power spectra. For representative GWB energy density spectra, we find parameter choices that provide good fits to existing measurements. This mechanism provides a potential observational probe of chiral GWB, which can arise in a wide range of theoretical scenarios.

astro-ph.CO↗
arXiv2026

Dark Energy Survey Year 6 Results: improved mitigation of spatially varying observational systematics with masking for the MagLim++ lens sample

As photometric surveys reach unprecedented statistical precision, systematic uncertainties increasingly dominate large-scale structure probes relying on galaxy number density. Defining the final survey footprint is critical, as it excludes regions affected by artefacts or suboptimal observing conditions. For galaxy clustering, spatially varying observational systematics, such as seeing, are a leading source of bias. Template maps of contaminants are used to derive spatially dependent corrections, but extreme values may fall outside the applicability range of mitigation methods, compromising correction reliability. The complexity and accuracy of systematics modelling depend on footprint conservativeness, with aggressive masking enabling simpler, robust mitigation. We present a unified approach to define the DES Y6 JOINT footprint, integrating observational systematics templates and artefact indicators that degrade mitigation performance. This removes extreme values from an initial SEED footprint, leading to the final JOINT footprint. By evaluating the DES Y6 lens sample MagLim++ on this footprint, we enhance the Iterative Systematics Decontamination (ISD) method, detecting non-linear systematic contamination and improving correction accuracy. While the mask's impact on clustering is less significant than systematics decontamination, it remains non-negligible, comparable to statistical uncertainties in certain w(theta) scales and redshift bins. Supporting coherent analyses of galaxy clustering and cosmic shear, the final footprint spans 4,031.04 deg2, setting the basis for DES Y6 1x2pt, 2x2pt, and 3x2pt analyses. This work highlights how targeted masking strategies optimise the balance between statistical power and systematic control in Stage-III and -IV surveys.

astro-ph.CO↗