Pasquale Temi

Far-Infrared Spitzer Observations of Elliptical Galaxies: Evidence for Extended Diffuse Dust

Far-infrared Spitzer observations of elliptical galaxies are inconsistent with simple steady state models of dust creation in red giant stars and destruction by grain sputtering in the hot interstellar gas at T ~ 107 K. The flux at 24 μm correlates with optical fluxes, suggesting that this relatively hot dust is largely circumstellar. But fluxes at 70 and 160 μm do not correlate with optical fluxes. Elliptical galaxies with similar LB have luminosities at 70 and 160 μm (L70 and L160) that vary over a factor of ~100, implying an additional source of dust unrelated to that produced by ongoing local stellar mass loss. Neither L70/LB nor L160/LB correlate with the stellar age or metallicity. Optical line fluxes from warm gas at T ~ 104 K correlate weakly with L70 and L160, suggesting that the dust may be responsible for cooling this gas. Many normal elliptical galaxies have emission at 70 μm that is extended to 5-10 kpc. Extended far-infrared emission with sputtering lifetimes of ~108 yr is difficult to maintain by mergers with gas-rich galaxies. Instead, we propose that this cold dust is buoyantly transported from reservoirs of dust in the galactic cores, which are supplied by mass loss from stars in the core. Intermittent energy outbursts from AGNs can drive the buoyant outflow.

Evidence of Star Formation in Local S0 Galaxies: Spitzer Observations of the Sauron Sample

We discuss infrared Spitzer observations of early-type galaxies in the SAURON sample at 24, 60, and 170 μm. When compared with 2MASS Ks band luminosities, lenticular (S0) galaxies exhibit a much wider range of mid-to-far-infrared luminosities than elliptical (E) galaxies. Mid- and far-infrared emission from E galaxies is a combination of circumstellar or interstellar emission from local mass-losing red giant stars, dust buoyantly transported from the galactic cores into distant hot interstellar gas and dust accreted from the environment. The source of mid- and far-IR emission in S0 galaxies is quite different and is consistent with low levels of star formation, 0.02-0.2 M ☉ yr–1, in cold, dusty gaseous disks. The infrared 24 μm-70 μm color is systematically lower for (mostly S0) galaxies with known molecular disks. Our observations support the conjecture that cold dusty gas in some S0 galaxies is created by stellar mass loss at approximately the same rate that it is consumed by star formation, so the mass depletion of these disks by star formation will be slow. Unlike E galaxies, the infrared luminosities of S0 galaxies correlate with both the mass of molecular gas and the stellar Hβ spectral index, and all are related to the recent star formation rate (SFR). However, star formation rates estimated from the Hβ-emission-line luminosities L Hβ in SAURON S0 galaxies are generally much smaller. Since L Hβ does not correlate with 24 μm emission from dust heated by young stars, optical emission lines appear to be a poor indicator of SFRs in SAURON S0 galaxies. The absence of Hβ emission may be due to a relative absence of OB stars in the initial mass function or to dust absorption of Hβ emission lines.

Spitzer Observations of Passive and Star-Forming Early-Type Galaxies: An Infrared Color?Color Sequence

We describe the infrared properties of a large sample of early-type galaxies, comparing data from the Spitzer archive with Ks-band emission from the Two Micron All Sky Survey. While most representations of this data result in correlations with large scatter, we find a remarkably tight relation among colors formed by ratios of luminosities in Spitzer-Multiband Imaging Photometer bands (24, 70, and 160 μm) and the Ks band. Remarkably, this correlation among E and S0 galaxies follows that of nearby normal galaxies of all morphological types. In particular, the tight infrared color-color correlation for S0 galaxies alone follows that of the entire Hubble sequence of normal galaxies, roughly in order of galaxy type from ellipticals to spirals to irregulars. The specific star formation rate (SFR) of S0 galaxies estimated from the 24 μm luminosity increases with decreasing K-band luminosity (or stellar mass) from essentially zero, as with most massive ellipticals, to rates typical of irregular galaxies. Moreover, the luminosities of the many infrared-luminous S0 galaxies can significantly exceed those of the most luminous (presumably post-merger) E galaxies. SFRs in the most infrared-luminous S0 galaxies approach 1-10 solar masses per year. Consistently, with this picture we find that while most early-type galaxies populate an infrared red sequence, about 24% of the objects (mostly S0s) are in an infrared blue cloud together with late-type galaxies. For those early-type galaxies also observed at radio frequencies, we find that the far-infrared luminosities correlate with the mass of neutral and molecular hydrogen, but the scatter is large. This scatter suggests that the star formation may be intermittent or that similar S0 galaxies with cold gaseous disks of nearly equal mass can have varying radial column density distributions that alter the local and global SFRs.

Spitzer Observations of Transient, Extended Dust in Two Elliptical Galaxies: New Evidence of Recent Feedback Energy Release in Galactic Cores

Spitzer observations of extended dust in two optically normal elliptical galaxies provide a new confirmation of buoyant feedback outflow in the hot gas atmospheres around these galaxies. AGN feedback energy is required to prevent wholesale cooling and star formation in these group-centered galaxies. In NGC 5044 we observe interstellar (presumably PAH) emission at 8 μm out to about 5 kpc. Both NGC 5044 and NGC 4636 have extended 70 μm emission from cold dust exceeding that expected from stellar mass loss. The sputtering lifetime of this extended dust in the ~1 keV interstellar gas, ~107 yr, establishes the time when the dust first entered the hot gas. Evidently the extended dust originated in dusty disks or clouds, commonly observed in elliptical galaxy cores, that were disrupted, heated, and buoyantly transported outward. The surviving central dust in NGC 5044 and NGC 4636 has been disrupted into many small filaments. It is remarkable that the asymmetrically extended 8 μm emission in NGC 5044 is spatially coincident with Hα+[N II] emission from warm gas. A calculation shows that dust-assisted cooling in buoyant hot gas moving out from the galactic core can cool within a few kiloparsecs in ~107 yr, explaining the optical line emission observed. The X-ray images of both galaxies are disturbed. All timescales for transient activity—restoration of equilibrium and buoyant transport in the hot gas, dynamics of surviving dust fragments, and dust sputtering—are consistent with a central release of feedback energy in both galaxies about 107 years ago.

The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions

The mean ages of early-type galaxies obtained from the analysis of optical spectra give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr. Independent age determinations are possible by using infrared spectra (5-21 μm), which we have obtained with the Infrared Spectrograph on Spitzer. This age indicator is based on the collective mass-loss rate of stars, in which mass loss from AGB stars produces a silicate emission feature at 9-12 μm. This feature decreases more rapidly than the shorter wavelength continuum as a stellar population ages, providing an age indicator. From observations of 30 nearby early-type galaxies, 29 show a spectral energy distribution dominated by stars, and one has significant emission from the ISM and is excluded. The infrared age indicators for the 29 galaxies show them all to be old, with a mean age of about 10 Gyr and a standard deviation of only a few Gyr. This is consistent with the ages inferred from the values of M/LB, but is inconsistent with the ages derived from the optical line indices, which can be much younger. All of these age indicators are luminosity weighted and should be correlated, even if multiple-age components are considered. The inconsistency indicates that there is a significant problem with either the infrared and the M/LB ages, which agree, or with the ages inferred from the optical absorption lines.

The Mid-Infrared Spectral Energy Distribution, Surface Brightness, and Color Profiles in Elliptical Galaxies

We combine 2MASS data and Spitzer archival data to study the emission in mid-infrared passbands (1.2-24 μm) from a sample of 18 elliptical galaxies. In general the surface brightness distributions resemble de Vaucouleurs profiles, indicating that most of the emission arises from the photospheres or circumstellar regions of red giant stars. The spectral energy distribution peaks near the H band at 1.6 μm. The half-light or effective radius has a pronounced minimum near the K band (2.15 μm) with a second, less consistent minimum in the 24 μm passband. All sample-averaged radial color profiles λi − λj , where λi < λj (and j≠ 24 μm), have positive slopes within about twice the (K-band) effective radius. Evidently this variation arises because of an increase in stellar metallicity toward the galactic cores. Color profiles K − j all have positive slopes, particularly when j = 5.8 μm, although no obvious absorption feature is observed in spectra of elliptical galaxies near 5.8 μm. This, and the minimum in Re, suggests that the K band may be anomalously luminous in metal-rich stars in galaxy cores. Unusual radial color profiles involving the 24 μm passband may suggest that some 24 μm emission comes from interstellar not circumstellar dust grains.

Mid-Infrared Emission from Elliptical Galaxies: Sensitivity to Stellar Age

Mid-infrared observations (3.6-24 μm) of normal giant elliptical galaxies with the Spitzer Space Telescope are consistent with pure populations of very old stars with no evidence of younger stars. Most of the stars in giant elliptical galaxies are old, but the mean stellar age determined from Balmer absorption in optical spectra can appear much younger due to a small admixture of younger stars. The mean stellar age can also be determined from the spectral energy distribution in the mid-infrared, which decreases with time relative to the optical emission and shifts to shorter wavelengths. The observed flux ratios F8 μm/F3.6 μm and F24 μm/F3.6 μm for elliptical galaxies with the oldest Balmer line ages are lower than predicted by recent models of single stellar populations. For elliptical galaxies with the youngest Balmer line ages in our sample, 3-5 Gyr, the flux ratios F24 μm/F3.6 μm are identical to those of the oldest stars. When theoretical mid-IR spectra of old (12 Gyr) and young stellar populations are combined, errors in the F24 μm/F3.6 μm observations are formally inconsistent with a mass fraction of young stars that exceeds ~1%. This is less than the fraction of young stars expected in discussions of recent surveys of elliptical galaxies at higher redshifts. However, this inconsistency between Balmer line ages and those inferred from mid-IR observations must be regarded as provisional until more accurate observations and theoretical spectra become available. Finally, there is no evidence to date that central disks or patches of dust commonly visible in optical images of elliptical galaxies contribute sensibly to the mid-IR spectrum.

Arcus: Exploring the formation and evolution of clusters, galaxies, and stars

Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50Å bandpass with unprecedented sensitivity: effective areas of >450 cm2 and spectral resolution >2500. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned, although there will be capabilities to observe sources such as tidal disruption events or supernovae with a ~3 day turnaround. Following the 2nd year of operation, Arcus will transition to a proposal-driven guest observatory facility.

Arcus: the x-ray grating spectrometer explorer (Conference Presentation)

Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50 A bandpass with unprecedented sensitivity: effective areas of >350 cm^2 and spectral resolution >2500 at the energies of O VII and O VIII for z=0-0.3. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Arcus will be launched into an ~ 7 day 4:1 lunar resonance orbit, resulting in high observing efficiency, low particle background and a favorable thermal environment. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned. The baseline science mission will be completed in <2 years, although the margin on all consumables allows for 5+ years of operation.

The evolution of structure and feedback with Arcus

Arcus is a NASA/MIDEX mission under development in response to the anticipated 2016 call for proposals. It is a freeflying, soft X-ray grating spectrometer with the highest-ever spectral resolution in the 8-51 Å (0.24 – 1.55 keV) energy range. The Arcus bandpass includes the most sensitive tracers of diffuse million-degree gas: spectral lines from O VII and O VIII, H- and He-like lines of C, N, Ne and Mg, and unique density- and temperature-sensitive lines from Si and Fe ions. These capabilities enable an advance in our understanding of the formation and evolution of baryons in the Universe that is unachievable with any other present or planned observatory. The mission will address multiple key questions posed in the Decadal Survey1 and NASA’s 2013 Roadmap2: How do baryons cycle in and out of galaxies? How do black holes and stars influence their surroundings and the cosmic web via feedback? How do stars, circumstellar disks and exoplanet atmospheres form and evolve? Arcus data will answer these questions by leveraging recent developments in off-plane gratings and silicon pore optics to measure X-ray spectra at high resolution from a wide range of sources within and beyond the Milky Way. CCDs with strong Suzaku heritage combined with electronics based on the Swift mission will detect the dispersed X-rays. Arcus will support a broad astrophysical research program, and its superior resolution and sensitivity in soft X-rays will complement the forthcoming Athena calorimeter, which will have comparably high resolution above 2 keV.