Emission In Early-type Galaxies Research Articles

Star formation exists in all early-type galaxies – evidence from ubiquitous structure in UV images

ABSTRACT Recent surveys have demonstrated the widespread presence of ultraviolet (UV) emission in early-type galaxies (ETGs), suggesting the existence of star formation in many of these systems. However, potential UV contributions from old and young stars, together with model uncertainties, makes it challenging to confirm the presence of young stars using integrated photometry alone. This is particularly true in ETGs that are fainter in the UV and have red UV-optical colours. An unambiguous way of disentangling the source of the UV is to look for structure in UV images. Optical images of ETGs, which are dominated by old stars, are smooth and devoid of structure. If the UV is also produced by these old stars, then the UV images will share this smoothness, while, if driven by young stars, they will exhibit significant structure. We compare the UV and optical morphologies of 32 ETGs (93 per cent of which are at z < 0.03) using quantitative parameters (concentration, asymmetry, clumpiness, and the Sérsic index), calculated via deep UV and optical images with similar resolution. Regardless of stellar mass, UV-optical colour or the presence of interactions, the asymmetry and clumpiness of ETGs is significantly larger (often by several orders of magnitudes) in the UV than in the optical, while the UV Sérsic indices are typically lower than their optical counterparts. The ubiquitous presence of structure demonstrates that the UV flux across our entire ETG sample is dominated by young stars and indicates that star formation exists in all ETGs in the nearby Universe.

The LOFAR view of massive early-type galaxies: Transition from radio AGN to host emission

We extend the study of the radio emission in early-type galaxies (ETGs) in the nearby Universe (recession velocity < 7500 km s−1) as seen by the 150 MHz Low-Frequency ARray (LOFAR) observations and extend the sample from giant ETGs to massive (∼6 × 1010 − 3 × 1011 M⊙) ETGs (mETGS) with −25 < MK < −23.5. Images from the second data release of the LOFAR Two-metre Sky Survey were available for 432 mETGs, 48% of which are detected above a typical luminosity of ∼3 × 1020 W Hz−1. Most (85%) of the detected sources are compact, with sizes ≲4 kpc. The radio emission of 31 mETGs is extended on scales ranging from 2 to 180 kpc (median 12 kpc). In several cases, it is aligned with the host galaxy. We set a limit of ≲1% to the fraction of remnant or restarted objects, which is ≲16% of the extended sources. We found that the properties of the radio sources are connected with the stellar mass of the ETGs (the median radio power, the fraction of extended radio sources, and the link with the large-scale environment). However, these results only describe statistical trends because the radio properties of sources of similar stellar mass and environment show a large spread of radio properties. These trends break at the lowest host luminosities (MK > −24.5). This effect is strengthened by the analysis of even less massive ETGs, with −23.5 < MK < −21.5. This suggests that at a mass of ∼2 × 1011 M⊙, a general transition occurs from radio emission produced from radio-loud active galactic nuclei (AGN) to processes related to the host galaxy and (or) radio quiet AGN. At this luminosity, a transition in the stellar surface brightness profile also occurs from Sérsic galaxies to those with a depleted stellar core, the so-called core galaxies. This finding is in line with previous results that indicated that only core galaxies host radio-loud AGN.

The SAMI Galaxy Survey: kinematics of dusty early-type galaxies

Recently, large samples of visually classified early-type galaxies (ETGs) containing dust have been identified using space-based infrared observations with the Herschel Space Telescope. The presence of large quantities of dust in massive ETGs is peculiar as X-ray halos of these galaxies are expected to destroy dust in 10 Myr (or less). This has sparked a debate regarding the origin of the dust: is it internally produced by asymptotic giant branch (AGB) stars, or is it accreted externally through mergers? We examine the 2D stellar and ionised gas kinematics of dusty ETGs using IFS observations from the SAMI galaxy survey, and integrated star-formation rates, stellar masses, and dust masses from the GAMA survey. Only 8% (4/49) of visually-classified ETGs are kinematically consistent with being dispersion-supported systems. These "dispersion-dominated galaxies" exhibit discrepancies between stellar and ionised gas kinematics, either offsets in the kinematic position angle or large differences in the rotational velocity, and are outliers in star-formation rate at a fixed dust mass compared to normal star-forming galaxies. These properties are suggestive of recent merger activity. The remaining 90% of dusty ETGs have low velocity dispersions and/or large circular velocities, typical of "rotation-dominated galaxies". These results, along with the general evidence of published works on X-ray emission in ETGs, suggest that they are unlikely to host hot, X-ray gas consistent with their low stellar mass when compared to dispersion-dominated galaxies. This means dust will be long lived and thus these galaxies do not require external scenarios for the origin of their dust content.

HIDDEN ACTIVE GALACTIC NUCLEI IN EARLY-TYPE GALAXIES

ABSTRACT We present a stacking analysis of the complete sample of early-type galaxies (ETGs) in the Chandra COSMOS (C-COSMOS) survey, to explore the nature of the X-ray luminosity in the redshift and stellar luminosity ranges and . Using established scaling relations, we subtract the contribution of X-ray binary populations to estimate the combined emission of hot ISM and active galactic nuclei (AGNs). To discriminate between the relative importance of these two components, we (1) compare our results with the relation observed in the local universe for hot gaseous halos emission in ETGs, and (2) evaluate the spectral signature of each stacked bin. We find two regimes where the non-stellar X-ray emission is hard, consistent with AGN emission. First, there is evidence of hard, absorbed X-ray emission in stacked bins including relatively high z (∼1.2) ETGs with average high X-ray luminosity ( ). These luminosities are consistent with the presence of highly absorbed “hidden” AGNs in these ETGs, which are not visible in their optical–IR spectra and spectral energy distributions. Second, confirming the early indication from our C-COSMOS study of X-ray detected ETGs, we find significantly enhanced X-ray luminosity in lower stellar mass ETGs ( ), relative to the local relation. The stacked spectra of these ETGs also suggest X-ray emission harder than expected from gaseous hot halos. This emission is consistent with inefficient accretion onto .

ISM chemistry in metal-rich environments: molecular tracers of metallicity

In this paper we use observations of molecular tracers in metal rich and alpha-enhanced galaxies to study the effect of abundance changes on molecular chemistry. We selected a sample of metal rich spiral and star bursting objects from the literature, and present here new data for a sample of early-type galaxies (ETGs). We conducted the first survey of CS and methanol emission in ETGs, detecting 7 objects in CS, and 5 in methanol emission. We find evidence to support the hypothesis that CS is a better tracer of dense star-forming gas than HCN. We suggest that the methanol emission in these sources is driven by dust mantle destruction due to ionisation from high mass star formation, but cannot rule out shocks dominating in some sources. The derived source averaged CS/methanol column densities and rotation temperatures are similar to those found in normal spiral and starburst galaxies, suggesting dense clouds are little affected by the differences between galaxy types. Finally we used the total column density ratios for our galaxy samples to show for the first time that some molecular tracers do seem to show systematic variations that appear to correlate with metallicity, and that these variations roughly match those predicted by chemical models. Using this fact, the chemical models of Bayet et al. (2012b), and assumptions about the optical depth we are able to roughly predict the metallicity of our spiral and ETG sample, with a scatter of ~0.3 dex. We provide the community with linear approximations to the relationship between the HCN and CS column density ratio and metallicity. Further study will clearly be required to determine if this, or any, molecular tracer can be used to robustly determine gas-phase metallically, but that a relationship exists at all suggests that in the future it may be possible to calibrate a metallicity indicator for the molecular interstellar medium (abridged).

Preface: Joint Discussion JD4: Ultraviolet emission in early-type galaxies

The presence of enhanced ultraviolet (UV) emission in early-type galaxies, which are dominated by old stellar populations, has been a puzzle for more than 40 years. The observed UV flux plausibly contains contributions from both evolved stars (traditionally referred to as the 'UV upturn' phenomenon) and young stars. While the UV upturn is well-characterised observationally, a firm understanding of its origin still eludes us. Similarly, while recent results from GALEX and deep optical surveys strongly indicate the presence of small mass fractions of young stars across low and intermediate redshift, the exact mechanism (e.g. stellar mass loss, minor mergers) that drives this star formation is not fully understood. The role of active galactic nuclei in regulating the low-level star formation at late epochs also remains relatively unexplored in galaxy formation models.

THE ROLE OF ENVIRONMENT IN LOW-LEVEL ACTIVE GALACTIC NUCLEUS ACTIVITY: NO EVIDENCE FOR CLUSTER ENHANCEMENT

We use the AMUSE-Virgo and AMUSE-Field surveys for nuclear X-ray emission in early-type galaxies to conduct a controlled comparison of low-level supermassive black hole activity within cluster and field spheroids. While both the Virgo and the Field samples feature highly sub-Eddington X-ray luminosities (log L_x/L_Edd between -8 and -4), we find that after accounting for the influence of host galaxy stellar mass, the field early-type galaxies tend toward marginally greater (0.38+/-0.14 dex) nuclear X-ray luminosities, at a given black hole mass, than their cluster counterparts. This trend is qualitatively consistent with the field black holes having access to a greater reservoir of fuel, plausibly in the form of cold gas located near the nucleus. We are able to rule out at high confidence the alternative of enhanced X-ray activity within clusters. Presuming nuclear X-ray emission correlates with the total energy and momentum output of these weakly accreting black holes, this indicates that low-level active galactic nucleus feedback is not generally stronger within typical cluster galaxies than in the field. These results confirm that for most cluster early-type galaxies (i.e., excluding brightest cluster galaxies) direct environmental effects, such as gas stripping, are more relevant in quenching star formation.

A VERY LARGE ARRAY RADIO SURVEY OF EARLY-TYPE GALAXIES IN THE VIRGO CLUSTER

We present the results of a 8.4 GHz Very Large Array radio survey of early-type galaxies extracted from the ACS Virgo Cluster Survey. The aim of this survey is to investigate the origin of radio emission in early-type galaxies and its link with the host properties in an unexplored territory toward the lowest levels of both radio and optical luminosities. Radio images, available for all 63 galaxies with BT < 14.4, show the presence of a compact radio source in 12 objects, with fluxes spanning from 0.13 to 2700 mJy. The remaining 51 galaxies, undetected at a flux limit of ~0.1 mJy, have radio luminosities L < 4 10E18 W/Hz . The fraction of radio-detected galaxies are a strong function of stellar mass, in agreement with previous results: none of the 30 galaxies with stellar mass M(star) < 1.7 10E10 M(sun) is detected, while 8 of the 11 most massive galaxies have radio cores. There appears to be no simple relation between the presence of a stellar nucleus and radio emission. A multiwavelength analysis of the active galactic nucleus (AGN) emission, combining radio and X-ray data, confirms the link between optical surface brightness profile and radio loudness in the sense that the bright core galaxies are associated with radio-loud AGNs, while non-core galaxies host radio-quiet AGNs. Not all radio-detected galaxies have a X-ray nuclear counter part (and vice-versa). A complete census of AGNs (and supermassive black holes, SMBHs) thus requires observations, at least, in both bands. Nonetheless, there are massive galaxies in the sample, expected to host a large SMBH (M(BH) ~ 10E8 M(sun)), whose nuclear emission eludes detection despite their proximity and the depth and the spatial resolution of the available observations. Most likely this is due to an extremely low level of accretion onto the central SMBH.

Early Type Galaxies in the Mid Infrared: a new flavor to their stellar populations

The mid infrared emission of early type galaxies traces the presence of intermediate age stellar populations as well as even tiny amounts of ongoing star formation. Here we discuss high S/N Spitzer IRS spectra of a sample of Virgo early type galaxies, with particular reference to NGC 4435. We show that, by combining mid infrared spectroscopic observations with existing broad band fluxes, it is possible to obtain a very clean picture of the nuclear activity in this galaxy.

Dust emission in early-type galaxies: The mid-infrared view

We present mid-infrared (MIR) maps for a sample of 18 early-type galaxies observed at 4.5, 6.7 and 15 µm with the ISOCAM instrument on board the ISO satellite with a 6 �� spatial resolution. We model the Spectral Energy Distribution (SED) of these galaxies using the stellar evolutionary synthesis model P ´ EGASE and we derive the MIR excess over the stellar component. We then explore the nature of this excess in terms of dust and Polycyclic Aromatic Hydrocarbon molecules (PAHs). We find that out of 18 galaxies, 10 show excess at 6.7 µm (due to the presence of PAH features) and 14 show excess at 15 µm (due to the presence of warm dust). In two galaxies, where a more complete wavelength coverage exists, an excess around 9.7 µm is seen (presumably due to silicate dust emission), while two other galaxies are totally devoid of dust. We also examine the morphology of the galaxies in these wavelengths by plotting the azimuthally averaged radial profiles as well as the MIR color profiles. We find that for the majority of the galaxies the 4.5 µm emission is well described by a de Vaucouleurs profile. The 6.7 µm and 15 µm emission is smoothly distributed throughout the galaxy while only a few galaxies show MIR emission which is more concentrated close to the center. Two dwarf galaxies in our sample show patchy distributions of the MIR emission while two other galaxies show edge-on disks. With color-color diagrams we delineate the regions occupied by late-type and early-type galaxies. Finally we show that the MIR excess found in strong radio galaxies like NGC 4486 (M87) can be explained by synchrotron emission.

Extended X-Ray Emission from Early-Type Galaxies: Comparison with Optical

In this poster we display the results from a detailed analysis of the distribution of the X-ray emission in early type galaxies. Two major results have come out of the analysis so far: a)The surface brightness radial profiles of isolated elliptical and SO galaxies are smoothly decreasing functions of radius out to a Rmax (similar to the optical radius). Outside Rmax a flattening in the slope is observed, although the exact shape of the profiles at large radii, where the data are poorest, cannot be determined at present.b)For R«Rmax, the X-ray and optical surface brightness profiles are similar (the nuclear region could be an exception). At larger radii, the X-ray profile could be flatter (NGC 4649 and NGC 4472 northern sector) or steeper (NGC 4472 southern sector) than the optical profile, or have a similar shape (NGC 4636).It is likely that the different profiles reflect the action of slightly different environments and/or a different ambient density around each galaxy, combined with the “history” of each galaxy. The tail in NGC 4472 could be the result of the motion of this galaxy in a dense intracluster medium. The X-ray deficiency in NGC 4649 could be due to either ram pressure stripping or the action of wind in the outer regions of the galaxy.

A statistical analysis of the Einstein normal galaxy sample. I - Spiral and irregular galaxies

The results of a statistical analysis of 48 spiral and irregular galaxies observed with the Einstein Observatory are reported. It is found that the X-ray luminosity is not directly correlated with the mass of the galaxies, but is strongly correlated with the blue luminosity. This suggests that most X-ray sources, including low-mass binaries, are binary systems belonging to both old disk and young arm Population I components. A real bulge population of X-ray sources exists that can contribute significantly to the X-ray luminosity of early-type spirals. The X-ray emission from a sample of 29 relatively isolated, normal elliptical and S0 galaxies is also studied, and the results are compared to those for the above sample to investigate the origin of the X-ray emission in early-type galaxies and the possible emission mechanisms. The influence of a powerful radio source and the onset of nuclear activity are also examined.