Fri Feb 22

The Origin of Dust in Early-Type Galaxies and Implications for Accretion onto Supermassive Black Holes

Paul Martini (The Ohio State University), Daniel Dicken (IAS, France), Thaisa Storchi-Bergmann(UFGRS, Brazil)
(Submitted on 20 Feb 2013)

We have conducted an archival Spitzer study of 38 early-type galaxies (ETGs) in order to determine the origin of the dust in approximately half of this population. Our sample galaxies generally have good wavelength coverage from 3.6um to 160um, as well as visible-wavelength HST images. We use the Spitzer data to estimate dust masses, or establish upper limits, and find that all of the ETGs with dust lanes in the HST data are detected in all of the Spitzer bands and have dust masses of ~10^{5-6.5} Msun, while galaxies without dust lanes are not detected at 70um and 160um and typically have <10^5 Msun of dust. The apparently dust-free galaxies do have 24um emission that scales with the shorter wavelength flux, yet substantially exceeds the expectations of photospheric emission by approximately a factor of three. We conclude this emission is dominated by hot, circumstellar dust around evolved stars that does not survive to form a substantial interstellar component. The order of magnitude variations in dust masses between galaxies with similar stellar populations rules out a subtantial contribution from continual, internal production in spite of the clear evidence for circumstellar dust. We demonstrate that the interstellar dust is not due to purely external accretion, unless the product of the merger rate of dusty satellites and the dust lifetime is at least an order of magnitude higher than expected. We propose that dust in ETGs is seeded by external accretion, yet the accreted dust is maintained by continued growth in externally-accreted cold gas beyond the nominal lifetime of individual grains. The several Gyr depletion time of the cold gas is long enough to reconcile the fraction of dusty ETGs with the merger rate of gas-rich satellites. As the majority of dusty ETGs are also low-luminosity AGN and likely fueled by this cold gas, their lifetime should similarly be several Gyr.

Comments: ApJ Accepted. 22 pages, 15 figures in emulateapj format
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1302.5124 [astro-ph.CO]

On the evolution and environmental dependence of the star formation rate versus stellar mass relation since z~2

Yusei Koyama (Durham/NAOJ), Ian Smail (Durham), Jaron Kurk (MPE), James E. Geach (McGill),David Sobral (Leiden), Tadayuki Kodama (NAOJ), Fumiaki Nakata (NAOJ), A. M. Swinbank(Durham), Philip N. Best (Edinburgh), Masao Hayashi (NAOJ), Ken-ichi Tadaki (Tokyo)
(Submitted on 21 Feb 2013)

This paper discusses the evolution of the correlation between galaxy star formation rates (SFRs) and stellar mass (M*) over the last ~10 Gyrs, particularly focusing on its environmental dependence. We first present the mid-infrared (MIR) properties of the H-alpha-selected galaxies in a rich cluster Cl0939+4713 at z=0.4. We use wide-field Spitzer/MIPS24um data to show that the optically red H-alpha emitters, which are most prevalent in group-scale environments, tend to have higher SFRs and stronger dust extinction than the majority population of blue H-alpha sources. With a MIR stacking analysis, we also find that the median SFR of H-alpha emitters increases in higher-density environment at z=0.4, and this trend is confirmed for both red and blue galaxies. The trend becomes much less significant when we compare their specific SFR (SSFR), although we find that there still remains a weak, but positive correlation between SSFR and galaxy number density. We then discuss the environmental dependence of the SFR versus M* relation for star-forming galaxies since z~2, by compiling comparable H-alpha-selected galaxy samples in both distant cluster environments (from MAHALO-Subaru) and field environments (from HiZELS). Based on these large, narrow-band selected H-alpha emitter samples, we find that the SFR-M* relation does not show detectable environmental dependence since z~2. Our data also suggest that the SSFR of H-alpha-selected galaxies (at log(M*/Msun)=10) evolves as (1+z)^3, and this evolutionary speed seems to be independent of the environment. Therefore we suggest that the primary star-formation quenching mechanism in cluster environments is a fast-acting process at any time in the history of the Universe since z~2.

Comments: 12 pages, 8 figures, submitted to MNRAS
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1302.5315 [astro-ph.CO]
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