M. Clemens

The Herschel Reference Survey: dust in early-type galaxies and across the Hubble Sequence

We present Herschel observations of 62 early-type galaxies (ETGs), including 39 galaxies morphologically classified as S0+S0a and 23 galaxies classified as ellipticals using SPIRE at 250, 350, and 500 mu m as part of the volume-limited Herschel Reference Survey (HRS). We detect dust emission in 24% of the ellipticals and 62% of the S0s. The mean temperature of the dust is \textless T-d \textgreater = 23.9 +/- 0.8 K, warmer than that found for late-type galaxies in the Virgo Cluster. The mean dust mass for the entire detected early-type sample is log M-d = 6.1 +/- 0.1 M-circle dot with a mean dust-to-stellar-mass ratio of log(M-d/M-*) = -4.3 +/- 0.1. Including the non-detections, these parameters are log M-d = 5.6 +/- 0.1 and log(M-d/M-*) = -5.1 +/- 0.1, respectively. The average dust-to-stellar-mass ratio for the early-type sample is fifty times lower, with larger dispersion, than the spiral galaxies observed as part of the HRS, and there is an order-of-magnitude decline in M-d/M-* between the S0s and ellipticals. We use UV and optical photometry to show that virtually all the galaxies lie close to the red sequence yet the large number of detections of cool dust, the gas-to-dust ratios, and the ratios of far-infrared to radio emission all suggest that many ETGs contain a cool interstellar medium similar to that in late-type galaxies. We show that the sizes of the dust sources in S0s are much smaller than those in early-type spirals and the decrease in the dust-to-stellar-mass ratio from early-type spirals to S0s cannot simply be explained by an increase in the bulge-to-disk ratio. These results suggest that the disks in S0s contain much less dust (and presumably gas) than the disks of early-type spirals and this cannot be explained simply by current environmental effects, such as ram-pressure stripping. The wide range in the dust-to-stellar-mass ratio for ETGs and the lack of a correlation between dust mass and optical luminosity suggest that much of the dust in the ETGs detected by Herschel has been acquired as the result of interactions, although we show these are unlikely to have had a major effect on the stellar masses of the ETGs. The Herschel observations tentatively suggest that in the most massive systems, the mass of interstellar medium is unconnected to the evolution of the stellar populations in these galaxies.

The star formation history of early‐type galaxies as a function of mass and environment

Using the third data release of the Sloan Digital Sky Survey (SDSS), we have rigorously defined a volume-limited sample of early-type galaxies in the redshift range 0.005 < z≤ 0.1. We have defined the density of the local environment for each galaxy using a method which takes account of the redshift bias introduced by survey boundaries if traditional methods are used. At luminosities greater than our absolute r-band magnitude cut-off of −20.45, the mean density of environment shows no trend with redshift. We calculate the Lick indices for the entire sample and correct for aperture effects and velocity dispersion in a model-independent way. Although we find no dependence of redshift or luminosity on environment, we do find that the mean velocity dispersion, σ, of early-type galaxies in dense environments tends to be higher than in low-density environments. Taking account of this effect, we find that several indices show small but very significant trends with environment that are not the result of the correlation between indices and velocity dispersion. The statistical significance of the data is sufficiently high to reveal that models accounting only for α-enhancement struggle to produce a consistent picture of age and metallicity of the sample galaxies, whereas a model that also includes carbon enhancement fares much better. We find that early-type galaxies in the field are younger than those in environments typical of clusters but that neither metallicity, α-enhancement nor carbon enhancement are influenced by the environment. The youngest early-type galaxies in both field and cluster environments are those with the lowest σ. However, there is some evidence that the objects with the largest σ are slightly younger, especially in denser environments. Independent of environment both the metallicity and α-enhancement grow monotonically with σ. This suggests that the typical length of the star formation episodes which formed the stars of early-type galaxies decreases with σ. More massive galaxies were formed in faster bursts. We argue that the timing of the process of formation of early-type galaxies is determined by the environment, while the details of the process of star formation, which has built up the stellar mass, are entirely regulated by the halo mass. These results suggest that the star formation took place after the mass assembly and favours an anti-hierarchical model. In such a model, the majority of the mergers must take place before the bulk of the stars form. This can only happen if there exists an efficient feedback mechanism which inhibits the star formation in low-mass haloes and is progressively reduced as mergers increase the mass.

The Herschel Virgo Cluster Survey - II. Truncated dust disks in H I-deficient spirals

By combining Herschel-SPIRE observations obtained as part of the Herschel Virgo Cluster Survey with 21 cm Hi data from the literature, we investigate the role of the cluster environment on the dust content of Virgo spiral galaxies. We show for the first time that the extent of the dust disk is significantly reduced in Hi-deficient galaxies, following remarkably well the observed “truncation” of the Hi disk. The ratio of the submillimetre-to-optical diameter correlates with the Hi-deficiency, suggesting that the cluster environment is able to strip dust as well as gas. These results provide important insights not only into the evolution of cluster galaxies but also into the metal enrichment of the intra-cluster medium.

Spitzer irs spectra of virgo early type galaxies: detection of stellar silicate emission

We present high signal-to-noise ratio Spitzer Infrared Spectrograph observations of 17 Virgo early-type galaxies. The galaxies were selected from those that define the color-magnitude relation of the cluster, with the aim of detecting the silicate emission of their dusty, mass-losing evolved stars. To flux calibrate these extended sources, we have devised a new procedure that allows us to obtain the intrinsic spectral energy distribution and to disentangle resolved and unresolved emission within the same object. We have found that 13 objects of the sample (76%) are passively evolving galaxies with a pronounced broad silicate feature that is spatially extended and likely of stellar origin, in agreement with model predictions. The other four objects (24%) are characterized by different levels of activity. In NGC 4486 (M87), the line emission and the broad silicate emission are evidently unresolved, and, given also the typical shape of the continuum, they likely originate in the nuclear torus. NGC 4636 shows emission lines superposed on extended (i.e., stellar) silicate emission, thus pushing the percentage of galaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central unresolved region. A more detailed analysis of our sample, with updated models, will be presented in a forthcoming paper.

The Herschel Virgo Cluster Survey – VIII. The Bright Galaxy Sample★

We describe the Herschel Virgo Cluster Survey and the first data that cover the complete survey area (four 4 x 4 deg2 regions). We use these data to measure and compare the global far-infrared properties of 78 optically bright galaxies that are selected at 500 mu m and detected in all five far-infrared bands. We show that our measurements and calibration are broadly consistent with previous data obtained by the IRAS, ISO, Spitzer and Planck. We use SPIRE and PACS photometry data to produce 100-, 160-, 250-, 350- and 500-mu m cluster luminosity distributions. These luminosity distributions are not power laws, but peaked, with small numbers of both faint and bright galaxies. We measure a cluster 100500 mu m far-infrared luminosity density of 1.6(7.0) +/- 0.2 x 10(9) L Mpc(-3). This compares to a cluster 0.42.5 mu m optical luminosity density of 5.0(20.0) x 10(9) L Mpc(-3), some 3.2(2.9) times larger than the far-infrared. A typical photon originates from an optical depth of 0.4 +/- 0.1. Most of our sample galaxies are well fitted by a single modified blackbody (beta= 2), leading to a mean dust mass of log M-Dust= 7.31 M and temperature of 20.0 K. We also derive both stellar and atomic hydrogen masses from which we calculate mean values for the star-to-gas (atomic) and gas (atomic)-to-dust mass ratios of 15.1 and 58.2, respectively. Using our derived dust, atomic gas and stellar masses, we estimate cluster mass densities of 8.6(27.8) x 106, 4.6(13.9) x 108 and 7.8(29.7) x 109 M Mpc-3 for dust, atomic gas and stars, respectively. These values are higher than those derived for field galaxies by factors of 39(126), 6(18) and 34(129), respectively. In the above, the luminosity/mass densities are given using the whole sample with the values in brackets using just those galaxies that lie between 17 and 23 Mpc. We provide a data table of flux densities in all the Herschel bands for all 78 bright Virgo Cluster galaxies.

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) - I. The UV luminosity function of the central 12 sq. deg

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) is a complete blind survey of the Virgo cluster covering similar to 40 sq. deg in the far UV (FUV, lambda(eff) = 1539 angstrom, Delta lambda = 442 angstrom) and similar to 120 sq. deg in the near UV (NUV, lambda(eff) = 2316 angstrom, Delta lambda = 1060 angstrom). The goal of the survey is to study the ultraviolet (UV) properties of galaxies in a rich cluster environment, spanning a wide luminosity range from giants to dwarfs, and regardless of prior knowledge of their star formation activity. The UV data will be combined with those in other bands (optical: NGVS; far-infrared - submm: HeViCS; HI: ALFALFA) and with our multizone chemo-spectrophotometric models of galaxy evolution to make a complete and exhaustive study of the effects of the environment on the evolution of galaxies in high density regions. We present here the scientific objectives of the survey, describing the observing strategy and briefly discussing different data reduction techniques. Using UV data already in-hand for the central 12 sq. deg we determine the FUV and NUV luminosity functions of the Virgo cluster core for all cluster members and separately for early-and late-type galaxies and compare it to the one obtained in the field and other nearby clusters (Coma, A1367). This analysis shows that the FUV and NUV luminosity functions of the core of the Virgo clusters are flatter (alpha similar to -1.1) than those determined in Coma and A1367. We discuss the possible origin of this difference.

FIR colours and SEDs of nearby galaxies observed with Herschel

We present infrared colours (in the 25-500 mu m spectral range) and UV to radio continuum spectral energy distributions of a sample of 51 nearby galaxies observed with SPIRE on Herschel. The observed sample includes all morphological classes, from quiescent ellipticals to active starbursts. Active galaxies have warmer colour temperatures than normal spirals. In ellipticals hosting a radio galaxy, the far-infrared (FIR) emission is dominated by the synchrotron nuclear emission. The colour temperature of the cold dust is higher in quiescent E-S0a than in star-forming systems probably because of the different nature of their dust heating sources (evolved stellar populations, X-ray, fast electrons) and dust grain properties. In contrast to the colour temperature of the warm dust, the f350/f500 index sensitive to the cold dust decreases with star formation and increases with metallicity, suggesting an overabundance of cold dust or an emissivity parameter beta <2 in low metallicity, active systems.

PACS photometry of the

We present Herschel/PACS 100 and 160 mu m integrated photometry for the 323 galaxies in the Herschel Reference Survey (HRS), a K-band, volume-limited sample of galaxies in the local Universe. Once combined with the Herschel/SPIRE observations already available, these data make the HRS the largest representative sample of nearby galaxies with homogeneous coverage across the 100-500 mu m wavelength range. In this paper, we take advantage of this unique data set to investigate the properties and shape of the far-infrared/submillimetre spectral energy distribution in nearby galaxies. We show that, in the stellar mass range covered by the HRS (8 less than or similar to log (M-*/M-circle dot) less than or similar to 12), the far-infrared/submillimetre colours are inconsistent with a single modified blackbody having the same dust emissivity index beta for all galaxies. In particular, either beta decreases or multiple temperature components are needed, when moving from metal-rich/gas-poor to metal-poor/gas-rich galaxies. We thus investigate how the dust temperature and mass obtained from a single modified blackbody depend on the assumptions made on beta. We show that, while the correlations between dust temperature, galaxy structure and star formation rate are strongly model dependent, the dust mass scaling relations are much more reliable, and variations of beta only change the strength of the observed trends.

Herschel

The Herschel Virgo Cluster Survey (HeViCS) is the deepest, confusion-limited survey of the Virgo Cluster at far-infrared (FIR) wavelengths. The entire survey at full depth covers similar to 55 deg(2) in five bands (100-500 mu m), encompassing the areas around the central dominant elliptical galaxies (M87, M86 and M49) and extends as far as the NW cloud, the W cloud and the Southern extension. The survey extends beyond this region with lower sensitivity so that the total area covered is 84 deg(2). In this paper we describe the data, the data acquisition techniques and present the detection rates of the optically selected Virgo Cluster Catalogue (VCC). We detect 254 (34 per cent) of 750 VCC galaxies found within the survey boundary in at least one band and 171 galaxies are detected in all five bands. For the remainder of the galaxies we have measured strict upper limits for their FIR emission. The population of detected galaxies contains early as well as late types although the latter dominate the detection statistics. We have modelled 168 galaxies, showing no evidence of a strong synchrotron component in their FIR spectra, using a single-temperature modified blackbody spectrum with a fixed emissivity index (beta = 2). A study of the chi(2) distribution indicates that this model is not appropriate in all cases, and this is supported by the FIR colours which indicate a spread in beta = 1-2. Statistical comparison of the dust mass and temperature distributions from 140 galaxies with chi(2)(d.o.f.=3) \textless 7.8 (95 per cent confidence level) shows that late types have typically colder, more massive dust reservoirs; the early-type dust masses have a mean of log[\textless M \textgreater/M-circle dot] = 6.3 +/- 0.3, while for late types log[\textless M \textgreater/M-circle dot] = 7.1 +/- 0.1. The late-type dust temperatures have a mean of \textless T \textgreater = 19.4 +/- 0.2 K, while for the early types, \textless T \textgreater = 21.1 +/- 0.8 K. Late-type galaxies in the cluster exhibit slightly lower dust masses than those in the field, but the cluster environment seems to have little effect on the bulk dust properties of early types. In future papers we will focus more on the scientific analysis of the catalogue (e.g. measuring FIR luminosity functions, dust mass functions and resolved gas and dust properties).

Reference Survey – far-infrared/submillimetre colours as tracers of dust properties in nearby galaxies

Context. Using Herschel data from the Open Time Key Project the Herschel Virgo Cluster Survey (HeViCS) , we investigated the relationship between the metallicity gradients expressed by metal abundances in the gas phase as traced by the chemical composition of HII regions, and in the solid phase, as traced by the dust-t o-gas mass ratio. Aims. We derived the radial gradient of the dust-to-gas mass ratio for all galaxies observed by HeViCS whose metallicity gradients are available in the literature. They are all late type Sbc ga laxies, namely NGC4254, NGC4303, NGC4321, and NGC4501. Methods. We fitted PACS and SPIRE observations with a single-temperat ure modified blackbody, inferred the dust mass, and calculated two dimensional maps of the dust-to-gas mass ratio, with the total mass of gas from available HI and CO maps. HI moment-1 maps were used to derive the geometric parameters of the galaxies and extract the radial profiles. We examined di fferent dependencies on metallicity of the CO-to-H2 conversion factor (XCO ), used to transform the 12 CO observations into the amount of molecular hydrogen. Results. We found that in these galaxies the dust-to-gas mass ratio radial profile is extremely sensitive to choice of the XCO value, since the molecular gas is the dominant component in the inner parts. We found that for three galaxies of our sample, namely NGC4254, NGC4321, and NGC4501, the slopes of the oxygen and of the dust-to-gas radial gradients agree up to∼0.6-0.7R25 using XCO values in the range 1/3-1/2 Galactic XCO . For NGC4303 a lower value of XCO∼0.1× 10 20 is necessary. Conclusions. We suggest that such low XCO values might be due to a metallicity dependence of XCO (from close to linear for NGC4254, NGC4321, and NGC4501 to superlinear for NGC4303),especially in the radial regions RG <0.6-0.7R25 where the molecular gas dominates. On the other hand, the outer regions, where the atomic gas component is dominant, are less affected by the choice of XCO , and thus we cannot put constraints on its value.